Search

Aims: Inoculation with climate-adapted rhizobia is able to increase legume productivity in drought-prone regions of Sub-Saharan Africa. Enhanced nodulation might additionally affect plant-soil interactions and control rhizosphere carbon (C) and nitrogen (N) pools. Methods: We investigated inoculation effects on nodulation and biological N2 fixation (BNF) of Vigna unguiculata and consequent effects on C and N pools in two Namibian soils. Three treatments (Bradyrhizobium sp.1–7 inoculant, non-inoculated, N-fertilised with 50 kg N ha−1) were applied in rhizoboxes at 45% and 20% maximum water holding capacity. Nodule development was photo-documented, and rhizobia-DNA sequences were identified. BNF was assessed by δ15N enrichment, and organic C and N pools were analysed in bulk and root adherent soil. Results: Plant growth initially enhanced mineral N losses from the rhizosphere at flowering stage (6 weeks growth), but led to a re-increase of N, and organic C contents after ripening (10 weeks). Inoculation had no effect on nodulation and soil C and N pools, indicating that both soils contained sufficient indigenous rhizobia to allow effective nodulation. However, the inoculant strain was more competitive in establishing itself in the root nodules, depending on the local conditions, showing a need for regional adjustment of inoculation strategies. Conclusion: Water stress was the main limitation for nodulation and, in combination with soil type, substantially affected rhizosphere and bulk soil C and N contents. The temporally enhanced rhizodeposition after ripening could be able to maintain soil C and N pools after legume cultivation. [ABSTRACT FROM AUTHOR]

Background and aims: Invasive plants often alter soil abiotic and biotic conditions which can benefit their own growth while harming native species. The impacts on native species may persist as legacy effects after the invasive species has been controlled and removed. This study focused on the Critically Endangered Cumberland Plain Woodland (CPW) vegetation in Australia, where we examined the soil impacts, and their associated legacy effects, associated with the invasion of African olive (Olea europaea subsp. cuspidata). Methods: Seedlings of O. europaea subsp. cuspidata and two native legume species, Acacia implexa and Indigofera australis, were grown in different sterilised and unsterilised soils: uninvaded CPW soil, restored CPW soil where O. europaea subsp. cuspidata was removed approximately 20 years ago, and soil from sites still under O. europaea subsp. cuspidata invasion. We characterised nitrogen-fixing rhizobia in the root nodules of seedlings of the two legumes using next-generation sequencing (NGS) barcoding. Results: Olea europaea subsp. cuspidata did not appear to condition the soil to favour its own growth and grew best in uninvaded CPW soil. The performance of both native legume species, however, was negatively impacted when grown in sterilised and unsterilised invaded soils, relative to their growth in CPW and restored soils. The soils from invaded sites affected the associations between both legume species and their rhizobium mutualists. Nodulation was higher in CPW and restored soils than in invaded soils, indicating that the availability of rhizobia was negatively impacted by O. europaea subsp. cuspidata. This was confirmed by a negative link between nodulation and the abundance of rhizobia that were characteristic of invaded soils. Conclusion: Our findings demonstrate that O. europaea subsp. cuspidata invasion affects the availability of microbial mutualists for native legumes in the CPW. The soil conditions created by O. europaea subsp. cuspidata do not benefit its own performance and these impacts do not persist as legacy effects 20 years after the removal of the invader. [ABSTRACT FROM AUTHOR]

Background and Aims: Nitrospirillum amazonense strain CBAmC is a nitrogen-fixing bacterium that has been used for sugarcane inoculation. Despite its beneficial effects, however, little is known about the establishment and colonization of this microorganism in its inoculated host plant. Furthermore, alternative inoculation methods need to be tested in the sugarcane crop. The objective of the present work was to quantify and visualize CBAmC in sugarcane cultivar RB867515 after using three inoculation methods. Methods: Utilizing one node as propagation material, three bacterial inoculation methods were tested under greenhouse conditions. For up to 60 days after inoculation (DAI), qPCR was used to quantify the bacterial loads in the plant or substrate while FISH-CLSM was used to confirm the establishment and colonization profile of CBAmC in different plant tissues. Results: FISH-CLSM indicated that the CBAmC strain could establish itself on different tissues of sugarcane, especially roots. qPCR revealed variable bacterial loads in the plants inoculated according to the three methods tested. In addition, little or no bacterial loads were detected in the substrate a few days after inoculation, regardless of the method used. Conclusions: N. amazonense CBAmC proves to be a bacterium that preferentially associates with the surface of the root tissues of sugarcane propagated by one node, instead of colonizing internal plant tissues or remaining in the soil. The inoculation method consisting of immersion of the mini-setts in bacterial suspension proved to be more efficient than the inoculation methods of applying the suspension onto the post-sprouting substrate or to the lowest leaf. [ABSTRACT FROM AUTHOR]

Aims: The improvement of biological nitrogen fixation (BNF) is beneficial to improving legume yield and soil fertility. Thus, the potential of a white-rot fungus Ceriporia lacerata HG2011 in promoting soybean BNF and yield along with the mechanisms was investigated. Methods: Fungal metabolites and the effects of C. lacerata HG2011 on rhizobial growth, chemotaxis, and biofilm formation were investigated through pure culture. Soybeans inoculated with this fungus were grown in the climatic cabinet, greenhouse, and field for measuring nodulation parameters, BNF-related enzyme activity, BNF, and yield. Results: Although C. lacerata HG2011 had no significant influence on the growth of the rhizobium Ensifer fredii S2-1, the rhizobial chemotaxis and biofilm formation were improved by this fungus broth in pure culture. C. lacerata HG2011 dwelled on the root surface, increased root tips and surface area, produced the nodulation-associated metabolites (phytohormones, flavonoids, and low-molecular-weight organic acids) and hydrolases (cellulase, pectinase, protease, and phosphatase), mobilized soil P and Fe, and stimulated the activities of plant BNF-related enzymes (nitrogenase, glutamine synthetase, and dehydrogenase). Compared with fungus-free control plants, C. lacerata HG2011 improved nodulation and increased %Ndfa by 3.61–4.64%, BNF by 29.26–40.68%, nutrients uptake (N, P, and K) by 4.38–19.60%, and mean grain yield by 10.55% in the fields. Conclusion: C. lacerata HG2011 promoted rhizobial chemotaxis as well as soybean nodulation and BNF via releasing phytohormones and improving mineral nutrition. Thus, the fungal inoculant significantly increased soybean yield in both greenhouse and fields, showing a promising application in legume cultivation. [ABSTRACT FROM AUTHOR]

Background and aims: Straw amendment can increase nitrogen fixation in paddy field, however, the efficiency of carbon sources with different biochemical properties to enhance N2 fixation and nitrogen fixation activity is still unclear. Methods: A 15N2-labelling system was used in the field environment to determine biological nitrogen fixation (BNF) under the addition of three kinds of straw. The nifH gene (DNA) and nifH RNA gene (cDNA) of soil were amplified by real-time fluorescent quantitative PCR. The diversity and community composition of nitrogen fixing microorganisms were studied by high-throughput sequencing. The study is expected to reveal how different carbon sources impact biological nitrogen fixation and its mechanism in the paddy field system. Results: Results showed that the absolute abundance diazotrophs in the treatment of mature stage wheat straw (MWS) was 4.88 times as high as that in the CK treatment, but jointing stage wheat straw (JWS) and poplar branch (PB) did not induce any significant changes. Straw amendment had no impact on cyanobacteria abundance. The proportion of N2 fixation increased by MWS was 2.07 times, but which was much lower than the increase proportion of the heterotrophic diazotrophs, leading to a decrease of diazotrophic nitrogen fixation activity. Conclusions: Mature wheat straw addition increased biologically fixed nitrogen in paddy field by increasing the number of heterotrophic nitrogen fixing bacteria. The results indicated that to increase biological N2 fixation in paddy system, straws with low nitrogen content and high C/N ratio were recommended. [ABSTRACT FROM AUTHOR]

Purpose: Belowground carbon (C) allocation for nitrogen (N) acquisition plays a crucial role in determining primary productivity and plant competitiveness in legume-grass mixtures, but beyond modeling and qualitative assessments, this remains poorly understood, especially with regard to drought stress and interspecific interactions. Methods: We grew a legume (Trifolium repens) and a grass (Lolium perenne) in monocultures and as a 50:50 mixture (with same plant density), at 70% and 50% soil water holding capacity representing non-drought and drought conditions, for 104 days in a growth chamber experiment. By using continuous 13CO2 labelling and 15N pulse soil-labelling, we analyzed how drought and interspecific interaction affected belowground C allocation (including root biomass, root respiration and rhizodeposition) and N acquisition through soil N uptake and biological N fixation. Results: Drought increased belowground C allocation per unit of N acquisition in the legume, but not in the grass. Drought significantly reduced biological N fixation in the legume, so that the legume allocated relatively more C to take up soil N. Interspecific competition increased belowground C allocation per unit of N acquisition, which could be attributed to a reduction in biological N fixation by the legume and an increased abundance of the grass. Conclusions: We highlight that drought and interspecific competition for N strongly alter C allocation towards biological N fixation and soil N uptake. Our measurements provide important process-based information to improve modeling drought effects on productivity and composition in legume-grass mixtures. [ABSTRACT FROM AUTHOR]

Background and Aims: Nitrogen (N2) fixation through endophytic diazotrophs within the non-nodulated plants has been considered a novel source of N inputs in terrestrial ecosystems. However, little is known about the composition and diversity of endophytic diazotrophs within the non-nodulated plants, especially when compared with the diazotrophs in nodulated plants. Methods: High-throughput nifH amplification sequencing was conducted to characterize endophytic diazotrophs within different tissues (leaf, stem, and root) of non-nodulated plants (Salix rehderiana Schneid, S. ernestii Schneid, and Populus purdomii Rehd.) and nodulated plants (Astragalus mahoshanicus and Hippophae rhamnoides L.) which dominate in the newly formed N-limited glacier floodplain in the eastern Qinghai-Tibetan Plateau. Results: We found endophytic diazotrophs, primarily Proteobacteria, in all tissue types of non-nodulated plants and leaf and stem of nodulated plants. The dominant genera were Bradyrhizobium, Ideonella, Azotobacter, and Geobacter. In comparison, diazotrophs within the nodule of leguminous Astragalus mahoshanicus and actinorhizal Hippophae rhamnoides were predominantly of genera Rhizobium and Frankia, respectively. Further, the community composition of diazotrophs was structured by plant species (P = 0.001), tissue types (P = 0.001), and nutrient concentrations (P = 0.001). Leaves of non-nodulated plants exhibited higher Chao1 indices and lower Simpson and Shannon indices than their stem and root, but these were not significantly different among different tissue types of nodulated plants. Conclusions: Endophytic diazotrophs might be prevalent in pioneer plants occurring in the N-limited habitat and have a high degree of host specificity; these findings could help to understand the ability of those pioneer plants to acquire N. [ABSTRACT FROM AUTHOR]

Aims: To evaluate the occurrence, the characterization and identity of nodulating bacteria in symbiosis with Mimosa spp. in the ultramafic massif of Barro Alto, Goiás state, Brazil. Methods: Nodules from field grown M. somnians and M. claussenii were sampled for bacteria isolation and in situ detection using microscopy. Isolates were characterized for their nodulation capacity on M. pudica and common bean, and their tolerance to Ni in culture medium. Bacteria were also partially identified by their 16S rRNA gene sequences. In addition, recA, gyrB, nodC and nifH genes from five representative isolates were sequenced for phylogenetic studies. Results: In situ detection indicated the exclusive presence of Paraburkholderia sp. within the nodules. This identification was confirmed for most of the isolates by the analysis of their 16S rRNA gene sequences. All isolates identified as Paraburkholderia sp. were able to effectively nodulate M. pudica, but those tested in common bean produced ineffective nodules. Inoculation tests showed that these bacteria improved M. pudica growth in the absence of nitrogen. None of the isolates showed Ni tolerance. The concatenated 16S rRNA, recA and gyrB, and the nodC and nifH genes sequences demonstrated that the five selected isolates were closely related to P. atlantica. Conclusions: This is the first report of Paraburkholderia present in nodules from Mimosa plants naturally growing in ultramafic soils. These results suggest that this symbiosis may be a factor to be considered as part of programs to restore ultramafic soils in Barro Alto areas degraded by mining activity. [ABSTRACT FROM AUTHOR]

Aims: Most studies that quantified the biological N fixation (BNF) and partial N balances of soybean have ignored the belowground structures. Our objectives were to evaluate the contribution of belowground structures to the partial N balance of soybean and to identify the origin (soil or BNF) and source (apparent remobilization or current gain during the seed-filling period) of seed N. Methods: Biomass, BNF, and N uptake coming from different N sources and origins, including belowground structures, were quantified in a two-year field study involving two soybean genotypes (MG IV and V) and two water availability conditions (rainfed and irrigated). Results: The inclusion of BNF-derived N present in belowground components (which averaged +12 kg N ha−1 at R7) changed the results of the partial N balances from negative to positive. BNF was the main origin of seed N, accounting for 73 and 79% of seed N under water stressed and non-stressed conditions, respectively. Regarding the seed N source, apparent remobilization was the main contributor to seed N under water stress, whereas current N gain was the main contributor to seed N in unstressed plants. Conclusions: We conclude that i) the root system retains a relevant proportion of the atmospheric N fixed during the crop cycle and should be included in the partial N balance estimations; and ii) BNF is the main origin of seed N, even under contrasting growing conditions. [ABSTRACT FROM AUTHOR]

Aims: Biological nitrogen fixation (BNF), a function performed by diazotrophic microbes, plays an essential role in nitrogen (N) bioavailability in terrestrial ecosystems. However, little is known about the effects of meadow degradation on soil BNF and diazotrophic communities in alpine meadows. Methods: We investigated changes in soil BNF and their potential drivers in alpine meadows along a degradation gradient on the Tibetan Plateau (non-degraded, lightly degraded, moderately degraded, and severely degraded meadows) using real-time quantitative PCR and amplicon sequencing. Results: Soil BNF rates decreased significantly along the meadow degradation gradient with a range of 17.34–79.84 nmol C2H4 g−1 dry soil d−1 across all sites. The highest BNF rate in the non-degraded meadow was 1.5–4.6-fold higher than that in the degraded meadows. The abundance and diversity of diazotrophs measured by nifH abundance and Shannon diversity was also decreased in the degraded meadows, accompanied by decreases in plant biomass, soil moisture, and nutrient content (C, N). Soil BNF rate was correlated with plant biomass, soil nutrient content, and diazotrophic abundance (including Nostoc, Scytonema, Rhodopseudomonas, and unidentified genera within the Rhizobiales and Proteobacteria). The community composition of diazotrophs differed markedly among sites with different levels of degradation, with both autotrophic (Cyanobacteria) and heterotrophic (Proteobacteria) diazotrophs contributing significantly to BNF. The plant functional groups, particularly the sedge family, were the primary drivers of soil BNF rates via mediating soil moisture, nutrient content (dissolved organic C and N), nifH gene abundance, and diazotrophic community composition. Conclusions: Our results reveal the main drivers of decreased BNF during alpine meadow degradation and emphasize the importance of plant functional groups in shaping the diazotrophic community and regulating the BNF rate. This information can be applied to the restoration of degraded meadow ecosystems. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the role nutrients play in arbuscular mycorrhizal fungal improvement of biological nitrogen fixation (BNF) by Bradyrhizobium sp. and increases in Pratylenchus thornei (Sher & Allen) population density in mung bean (Vigna radiata (L.) R. Wilczek). Methods: A glasshouse experiment was conducted on mung bean with 26 factorial treatments of AMF, rhizobia, P. thornei, nitrogen (N), phosphorus (P) and zinc (Zn) in a pasteurised vertisol. Variates of biomass, yield, nodulation, natural abundance δ15N, mycorrhizal colonisation, nutrients in the plant shoot and P. thornei multiplication were assessed at 6 and 11 weeks. Results: The combination of AMF and P improved BNF in the shoots at 6 weeks, while AMF alone improved BNF and nodulation greater than the addition of P at 11 weeks. Inoculation with AMF increased the shoot concentrations of P and Zn greater than fertilisation with either nutrient alone. Seed yield and biomass were similar when AMF or P was each applied alone with no further increase when combined. Rhizobia increased seed yield greater than the addition of N. Inoculation with AMF and rhizobia increased yield and biomass greater than rhizobia alone, and to a higher level than inoculation with AMF and fertiliser N. Pratylenchus thornei populations in the roots increased with AMF, but the addition of N, P and Zn decreased them. Conclusions: AMF increase supply of P to mung bean, improving BNF by Bradyrhizobium, yield and crop nutrition while reducing fertiliser inputs. Active management to ensure effective AMF colonisation and reduced P. thornei reproduction is warranted. [ABSTRACT FROM AUTHOR]

Background: We collated estimates of the percentage of legume N derived from atmospheric N2 (%Ndfa) for 14 major grain legumes and then analysed and aggregated the data to derive average values for different crops and regions/countries. The effects of cultivation year and whether data collected from experimental plots were relevant to crops growing in farmers' fields were examined. Scope: A total of 5374 %Ndfa estimates (observations) were sourced, 4205 from field experiments and 1169 on-farm measurements collected from farmer-grown crops. The largest number of reports (82) and %Ndfa estimates (1391) were for soybean. Conclusions: The %Ndfa estimates for each legume species were consistent across years, except for soybean in North America. For some species estimates were also similar across geographic regions. There were no significant differences (P > 0.05) between estimates of %Ndfa derived from experimental plots and farmer-grown legume crops for nine of the 10 crops evaluated. Three distinct groups were identified with statistically-similar average %Ndfa values with associated standard deviations, namely: pigeonpea, faba bean and lupin – 74±11.8 %; groundnut, green and black gram, cowpea, chickpea, field pea, lentil, vetches and Bambara groundnut – 62±13.4 %; common bean – 38±11.1 %. There were three distinct different regional groupings for soybean: Brazil – 78±6.3 %; North America, Argentina, Asia, Africa and Oceania – 61±14.0 %; Europe – 44±13.8 %. Our findings provide more certainty and simplify the challenge of using field-scale measures of legume %Ndfa to estimate country-to-global inputs of fixed N from grain legumes. [ABSTRACT FROM AUTHOR]

Background and aims: Biological nitrogen fixation (BNF) in paddy systems is impacted by nitrogen application levels and irrigation strategies, but the extent to which these factors influence BNF and its distribution in soil and rice is largely unclear. This study investigates this influence. Methods: An airtight, transparent growth chamber based 15N-labelling system was used to investigate how different nitrogen application levels (0, 125, 187.5 and 250 kg N ha−1) and irrigation strategies (flooding irrigation or intermittent irrigation) impact the amount of BNF and its distribution in soil and rice. Results: Nitrogen application at 125–250 kg N ha−1 reduced the amount of BNF by 81–86%. The inhibition effect of nitrogen application on BNF at a soil depth of 1–15 cm was greater than that at 0–1 cm. Relative to the continuous flooding irrigation, intermittent irrigation enhanced rice growth and promoted the transfer of fixed nitrogen from 0-1 cm soil layer to rice, but it did not change the total amount of BNF. Conclusions: This study indicated that BNF supplied little nitrogen for rice production at the high nitrogen application levels, but the intermittent irrigation could promote utilization of biologically fixed nitrogen. [ABSTRACT FROM AUTHOR]

Aims: Comparing different winter legume species by plant traits that determine their contributions to ecosystems as service crops. Since root biomass determines soil organic matter formation and biological nitrogen fixation (BNF) increases soil organic nitrogen stock, those plant traits are more suitable than shoot biomass to select service crops. Methods: We evaluated shoot and root biomass and BNF (using 15N natural abundance method) of 10 winter legume service crop species under field conditions, in two consecutive years. Results: The legumes significantly varied in shoot (3.4 to 9.1 Mg ha−1 year−1) and root biomass (0.8 to 1.3 Mg ha−1 year−1) and in the proportion and the amount of biologically fixed nitrogen (36–61% and 43–121 kg of N ha−1, respectively). Whereas shoot biomass varied between years, root and BNF were rather constant determining a low correlation between the variables. Among the evaluated species, Trifolium pratense and T. alexandrinum were the top-ranked species for the evaluated conditions, showing the largest amount of root biomass (~ 1.3 Mg ha−1 year−1) and BNF (~ 112 kg of N ha−1 year−1). Conclusions: Selecting service crops species aimed to increase soil organic matter based only on shoot biomass may be insufficient. As a consequence, we highlight the importance of performing breeding programs aimed to increase, in addition to crop yield or shoot biomass production, other plant traits such as root biomass or biological nitrogen fixation that provide key regulating and supporting ecosystem services. [ABSTRACT FROM AUTHOR]

Aims: Late-successional trees have a relatively high growth rate at low temperature, which is important for their competitive advantage over mid-successional trees. However, we have a limited understanding of the underlying mechanism for such an advantage.We transplanted two mid-successional tree seedlings (Castanopsis chinensis and Schima superba) and two late-successional species (Machilus chinensis and Cryptocarya chinensis) from Dinghushan (subtropical forest) to South China Normal University. The seedlings were incubated in pots with the same soil and routinely managed with regular watering and weeding. After 7 years of seedling culture, five pots of each tree species were selected for greenhouse cultivation to simulate the effect of low temperature (16 °C) on the studied tree species. A control treatment (ambient temperature) was also established.After 3 years, low temperatures changed the bacterial and diazotrophic communities, decreased the relative abundance of Actinobacteria in the mid-successional species Schima superba and Castanopsis chinensis, and increased the relative abundance of Rubrivivax in the late-successional species Machilus chinensis and Cryptocarya chinensis. In the low-temperature treatment, late-successional M. chinensis and Cryptocarya chinensis increased N fixation and net N mineralization rates compared with those in mid-successional S. superba and Castanopsis chinensis. Meanwhile, the relative abundance of the fungal community changed insignificantly, indicating that bacteria play a major regulatory role in nutrient cycling at low temperatures.Our results suggest that shifts in the bacterial community and changes in functional group structures may play a key role in soil nitrogen cycling during the succession of evergreen broadleaf forest ecosystems at low temperatures during the dry season.Methods: Late-successional trees have a relatively high growth rate at low temperature, which is important for their competitive advantage over mid-successional trees. However, we have a limited understanding of the underlying mechanism for such an advantage.We transplanted two mid-successional tree seedlings (Castanopsis chinensis and Schima superba) and two late-successional species (Machilus chinensis and Cryptocarya chinensis) from Dinghushan (subtropical forest) to South China Normal University. The seedlings were incubated in pots with the same soil and routinely managed with regular watering and weeding. After 7 years of seedling culture, five pots of each tree species were selected for greenhouse cultivation to simulate the effect of low temperature (16 °C) on the studied tree species. A control treatment (ambient temperature) was also established.After 3 years, low temperatures changed the bacterial and diazotrophic communities, decreased the relative abundance of Actinobacteria in the mid-successional species Schima superba and Castanopsis chinensis, and increased the relative abundance of Rubrivivax in the late-successional species Machilus chinensis and Cryptocarya chinensis. In the low-temperature treatment, late-successional M. chinensis and Cryptocarya chinensis increased N fixation and net N mineralization rates compared with those in mid-successional S. superba and Castanopsis chinensis. Meanwhile, the relative abundance of the fungal community changed insignificantly, indicating that bacteria play a major regulatory role in nutrient cycling at low temperatures.Our results suggest that shifts in the bacterial community and changes in functional group structures may play a key role in soil nitrogen cycling during the succession of evergreen broadleaf forest ecosystems at low temperatures during the dry season.Results: Late-successional trees have a relatively high growth rate at low temperature, which is important for their competitive advantage over mid-successional trees. However, we have a limited understanding of the underlying mechanism for such an advantage.We transplanted two mid-successional tree seedlings (Castanopsis chinensis and Schima superba) and two late-successional species (Machilus chinensis and Cryptocarya chinensis) from Dinghushan (subtropical forest) to South China Normal University. The seedlings were incubated in pots with the same soil and routinely managed with regular watering and weeding. After 7 years of seedling culture, five pots of each tree species were selected for greenhouse cultivation to simulate the effect of low temperature (16 °C) on the studied tree species. A control treatment (ambient temperature) was also established.After 3 years, low temperatures changed the bacterial and diazotrophic communities, decreased the relative abundance of Actinobacteria in the mid-successional species Schima superba and Castanopsis chinensis, and increased the relative abundance of Rubrivivax in the late-successional species Machilus chinensis and Cryptocarya chinensis. In the low-temperature treatment, late-successional M. chinensis and Cryptocarya chinensis increased N fixation and net N mineralization rates compared with those in mid-successional S. superba and Castanopsis chinensis. Meanwhile, the relative abundance of the fungal community changed insignificantly, indicating that bacteria play a major regulatory role in nutrient cycling at low temperatures.Our results suggest that shifts in the bacterial community and changes in functional group structures may play a key role in soil nitrogen cycling during the succession of evergreen broadleaf forest ecosystems at low temperatures during the dry season.Conclusions: Late-successional trees have a relatively high growth rate at low temperature, which is important for their competitive advantage over mid-successional trees. However, we have a limited understanding of the underlying mechanism for such an advantage.We transplanted two mid-successional tree seedlings (Castanopsis chinensis and Schima superba) and two late-successional species (Machilus chinensis and Cryptocarya chinensis) from Dinghushan (subtropical forest) to South China Normal University. The seedlings were incubated in pots with the same soil and routinely managed with regular watering and weeding. After 7 years of seedling culture, five pots of each tree species were selected for greenhouse cultivation to simulate the effect of low temperature (16 °C) on the studied tree species. A control treatment (ambient temperature) was also established.After 3 years, low temperatures changed the bacterial and diazotrophic communities, decreased the relative abundance of Actinobacteria in the mid-successional species Schima superba and Castanopsis chinensis, and increased the relative abundance of Rubrivivax in the late-successional species Machilus chinensis and Cryptocarya chinensis. In the low-temperature treatment, late-successional M. chinensis and Cryptocarya chinensis increased N fixation and net N mineralization rates compared with those in mid-successional S. superba and Castanopsis chinensis. Meanwhile, the relative abundance of the fungal community changed insignificantly, indicating that bacteria play a major regulatory role in nutrient cycling at low temperatures.Our results suggest that shifts in the bacterial community and changes in functional group structures may play a key role in soil nitrogen cycling during the succession of evergreen broadleaf forest ecosystems at low temperatures during the dry season. [ABSTRACT FROM AUTHOR]

Aims: The Sub-Boreal Pine-Spruce xeric-cold (SBPSxc) biogeoclimatic region in British Columbia, Canada is characterized by weakly-developed soils, thin organic forest floor and limited plant-available nitrogen. Yet, lodgepole pine trees are thriving in this region unaffected by these nitrogen-limitations, which led us to hypothesize that endophytic nitrogen-fixing bacteria could be playing a potential role in sustaining pine tree growth. Methods: We evaluated these endophytes in a yearlong greenhouse experiment with their native host (lodgepole pine) for in planta nitrogen-fixation (15N isotope dilution assay) and growth-promotion (length and biomass). These endophytes were also evaluated with a foreign host native to the SBPSxc region (hybrid white spruce) in another yearlong greenhouse trial. Results: Each bacterial strain considerably enhanced seedling length and biomass of pine and spruce along with fixing significant amount of nitrogen from atmosphere (15–50%). Notably, Caballeronia sordidicola HP-S1r strain fixed 49–50% of the host nitrogen from atmosphere and enhanced seedling length and biomass by up to 1.5-fold and 4-fold, respectively. Conclusions: Endophytic bacteria could be playing a crucial role in the survival of lodgepole pine trees in the SBPSxc region by providing them with significant amounts of fixed nitrogen. Their effectiveness with a foreign host (hybrid white spruce) shows the lack of plant x microbe specificity, indicating their potential role in supporting the growth of multiple boreal forest trees. [ABSTRACT FROM AUTHOR]

Aims: Biological nitrogen (N) fixation (BNF) driven by diazotrophs plays an essential role in reducing N fertilizer input in acidic soils. Amendments improve the acidic soil environment and increase crop N utilization; however, whether their effects on soil BNF and diazotrophs contribute to crop N utilization remains unclear.A 5-year field trial was implemented to assess the effects of the sole or joint application of organic and inorganic amendments (straw, manure and alkaline slag) on soil BNF rates, canola N utilization, and diazotrophic communities of acidic bulk and rhizosphere soils.Amendments, especially joint application of organic and inorganic amendments, significantly increased canola biomass, N utilization, soil BNF rates, and diazotrophic abundance. There were positive correlations among BNF rates, N uptake and physiological N use efficiency of canola. Both amendments and canola planting significantly affected the structure of diazotrophic communities, with the stronger effects in the joint application of organic and inorganic amendments. Influences of soil C/N, SOC and diazotrophic α-diversity on BNF rates in rhizosphere soils were greater than in bulk soils. The dominant genus affecting BNF rates differed between bulk and rhizosphere soils. Rhizosphere BNF rates were positively correlated with the number of nodes, total edges, and positive correlations of edges of rhizosphere diazotrophic network, but not in bulk soils.The improved soil BNF by organic and inorganic amendments in acidic soils have positive effects on crop N utilization, and the mechanisms underlying the contribution of diazotrophs to BNF rates differ in bulk and rhizosphere soils.Methods: Biological nitrogen (N) fixation (BNF) driven by diazotrophs plays an essential role in reducing N fertilizer input in acidic soils. Amendments improve the acidic soil environment and increase crop N utilization; however, whether their effects on soil BNF and diazotrophs contribute to crop N utilization remains unclear.A 5-year field trial was implemented to assess the effects of the sole or joint application of organic and inorganic amendments (straw, manure and alkaline slag) on soil BNF rates, canola N utilization, and diazotrophic communities of acidic bulk and rhizosphere soils.Amendments, especially joint application of organic and inorganic amendments, significantly increased canola biomass, N utilization, soil BNF rates, and diazotrophic abundance. There were positive correlations among BNF rates, N uptake and physiological N use efficiency of canola. Both amendments and canola planting significantly affected the structure of diazotrophic communities, with the stronger effects in the joint application of organic and inorganic amendments. Influences of soil C/N, SOC and diazotrophic α-diversity on BNF rates in rhizosphere soils were greater than in bulk soils. The dominant genus affecting BNF rates differed between bulk and rhizosphere soils. Rhizosphere BNF rates were positively correlated with the number of nodes, total edges, and positive correlations of edges of rhizosphere diazotrophic network, but not in bulk soils.The improved soil BNF by organic and inorganic amendments in acidic soils have positive effects on crop N utilization, and the mechanisms underlying the contribution of diazotrophs to BNF rates differ in bulk and rhizosphere soils.Results: Biological nitrogen (N) fixation (BNF) driven by diazotrophs plays an essential role in reducing N fertilizer input in acidic soils. Amendments improve the acidic soil environment and increase crop N utilization; however, whether their effects on soil BNF and diazotrophs contribute to crop N utilization remains unclear.A 5-year field trial was implemented to assess the effects of the sole or joint application of organic and inorganic amendments (straw, manure and alkaline slag) on soil BNF rates, canola N utilization, and diazotrophic communities of acidic bulk and rhizosphere soils.Amendments, especially joint application of organic and inorganic amendments, significantly increased canola biomass, N utilization, soil BNF rates, and diazotrophic abundance. There were positive correlations among BNF rates, N uptake and physiological N use efficiency of canola. Both amendments and canola planting significantly affected the structure of diazotrophic communities, with the stronger effects in the joint application of organic and inorganic amendments. Influences of soil C/N, SOC and diazotrophic α-diversity on BNF rates in rhizosphere soils were greater than in bulk soils. The dominant genus affecting BNF rates differed between bulk and rhizosphere soils. Rhizosphere BNF rates were positively correlated with the number of nodes, total edges, and positive correlations of edges of rhizosphere diazotrophic network, but not in bulk soils.The improved soil BNF by organic and inorganic amendments in acidic soils have positive effects on crop N utilization, and the mechanisms underlying the contribution of diazotrophs to BNF rates differ in bulk and rhizosphere soils.Conclusion: Biological nitrogen (N) fixation (BNF) driven by diazotrophs plays an essential role in reducing N fertilizer input in acidic soils. Amendments improve the acidic soil environment and increase crop N utilization; however, whether their effects on soil BNF and diazotrophs contribute to crop N utilization remains unclear.A 5-year field trial was implemented to assess the effects of the sole or joint application of organic and inorganic amendments (straw, manure and alkaline slag) on soil BNF rates, canola N utilization, and diazotrophic communities of acidic bulk and rhizosphere soils.Amendments, especially joint application of organic and inorganic amendments, significantly increased canola biomass, N utilization, soil BNF rates, and diazotrophic abundance. There were positive correlations among BNF rates, N uptake and physiological N use efficiency of canola. Both amendments and canola planting significantly affected the structure of diazotrophic communities, with the stronger effects in the joint application of organic and inorganic amendments. Influences of soil C/N, SOC and diazotrophic α-diversity on BNF rates in rhizosphere soils were greater than in bulk soils. The dominant genus affecting BNF rates differed between bulk and rhizosphere soils. Rhizosphere BNF rates were positively correlated with the number of nodes, total edges, and positive correlations of edges of rhizosphere diazotrophic network, but not in bulk soils.The improved soil BNF by organic and inorganic amendments in acidic soils have positive effects on crop N utilization, and the mechanisms underlying the contribution of diazotrophs to BNF rates differ in bulk and rhizosphere soils. [ABSTRACT FROM AUTHOR]

Background and aims: Herbaspirillum seropedicae is a Plant Growth Promoting Bacterium (PGPB) that colonizes crops of economic interest, especially grasses. Here, we aimed to characterize the association of H. seropedicae with the model dicot Arabidopsis thaliana and its effect on plant growth. The molecular pathways activated during the association were monitored by comparative proteomics. Methods: Arabidopsis thaliana seedlings were inoculated with H. seropedicae and grown for 28 days. Plant growth promotion was analyzed by quantifying the total leaf area and the fresh and dry biomass. The endophytic bacterial population was quantified by counts of colony-forming units (CFU), and the bacterial location in plant tissues was determined by epifluorescence microscopy. Changes in the proteomic profile of the plants were investigated by ESI-LC-MS/MS. Results: Herbaspirillum seropedicae colonized the roots of A. thaliana, but was absent from the shoots. The association promoted plant growth significantly. The proteomic analysis revealed several protein groups regulated in the roots and shoots during the association, with an emphasis on the photosynthesis and phenylpropanoid biosynthesis. Conclusions: Herbaspirillum seropedicae colonizes A. thaliana roots and promotes its growth, leading to an increase in biomass and the modulation of specific proteomic responses. These findings open new perspectives into the study of the molecular basis of the positive interaction between H. seropedicae and host plants, both monocots and dicots. [ABSTRACT FROM AUTHOR]

Aims: The evaluation of 15N abundance of N derived from biological N2 fixation (BNF) in legume shoots (‘BS’ value) is essential for quantifying BNF inputs to field-grown legumes. The aim of this study was to investigate the impact of shading, development stage of soybean (Glycine max) and rhizobium strain on the ‘BS’ value.Methods: Soybean plants were grown in pots of autoclaved sand/perlite mixture in the open field. Plants were harvested at weekly intervals from 46 days after planting (DAP) to 75 DAP. All material was analysed for N and 15N abundance. ‘BS’ was calculated assuming 50% of seed N was translocated to the shoots.Results: ‘BS’ was stable until 60 DAP but subsequently increased for the three strains tested. Nodule efficiency (N2 fixed g DM nodule−1) was greatly increased by shading and was significantly different between Bradyrhizobium species. ‘BS’ was greatly increased by shading.Conclusion: We recommend that ‘BS’ should be evaluated on plants of the same development stage and light intensity as those where BNF is quantified in the field. Different Bradyrhizobium strains make a large impact on ‘BS’ and may lead to considerable errors in estimation of BNF inputs to plants with high %N derived from BNF. [ABSTRACT FROM AUTHOR]

Background and aims: The addition of biochar to soil may offer a chance to mitigate climate change by increasing soil carbon stocks, improving soil fertility and enhancing plant growth. The impacts of biochar in cold environments with limited microbial activity are still poorly known.Methods: In order to understand to what extent different types and application rates of biochar affect carbon (C) and nitrogen (N) fluxes in boreal forests, we conducted a field experiment where two different spruce biochars (pyrolysis temperatures 500 °C and 650 °C) were applied at the rate of 0, 5 and 10 t ha−1 to Pinus sylvestris forests in Finland.Results: During the second summer after treatment, soil CO2 effluxes showed no clear response to biochar addition. Only in June, the 10 t ha−1 biochar (650 °C) plots had significantly higher CO2 effluxes compared to the control plots. The pyrolysis temperature of biochar did not affect soil CO2 effluxes. Soil pH increased in the plots receiving 10 t ha−1 biochar additions. Biochar treatments had no significant effect on soil microbial biomass and biological N fixation. Nitrogen mineralization rates in the organic layer tended to increase with the amount of biochar, but no statistically significant effect was detected.Conclusions: The results suggest that wood biochar amendment rates of 5-10 t ha−1 to boreal forest soil do not cause large or long-term changes in soil CO2 effluxes or reduction in native soil C stocks. Furthermore, the results imply that biochar does not adversely affect soil microbial biomass or key N cycling processes in boreal xeric forests, at least within this time frame. Thus, it seems that biochar is a promising tool to mitigate climate change and sequester additional C in boreal forest soils. [ABSTRACT FROM AUTHOR]

Background and aims: To evaluate the influence of soil type on the symbiosis between Mimosa spp. and rhizobia.Methods: A greenhouse experiment was carried out with trap plants using seeds of six species of Mimosa and soils from three different locations in central Brazil: Posse, Brasília and Cavalcante. Plant dry biomass and number of nodules were measured after four months. Symbiotic bacteria were isolated from nodules and their molecular identification was performed. Three housekeeping genes (16S rRNA, recA and gyrB) plus the nodC and nifH symbiotic genes were used to determine the identity of the symbionts and to reconstruct the phylogenetic relationships among the isolated nitrogen-fixing bacteria.Results: Rhizobia from the Betaproteobacterial genus Paraburkholderia (former Burkholderia) and the Alphaproteobacterial genus Rhizobium were isolated from different species of Mimosa. As in previous studies, the phylogenies of their symbiosis-essential genes, nodC and nifH, were broadly congruent with their core housekeeping genes (16S rRNA, recA and gyrB), which suggests limited or no horizontal gene transfer. Edaphic factors such as pH and fertility influenced the occurrence of these unrelated rhizobial types in the nodules on these Mimosa spp.Conclusions: Mimosa species have the ability to associate with different types of rhizobia (α- and β-proteobacteria), suggesting low specificity between host and bacterium in experimental conditions. Soil factors such as pH, nitrogen and fertility seem to favour the predominance of certain types of rhizobia, thus influencing the establishment of symbiotic relationships. [ABSTRACT FROM AUTHOR]

Aims: Successful inoculation of legume crops with rhizobia depends on dominating nodule occupancy with highly efficient strains. The aim of this study was to develop a rapid and reliable conventional PCR methodology to specifically detect an elite Bradyrhizobium strain in root nodule extracts from soil-grown cowpea plants. Methods: The draft genome sequence of Bradyrhizobium pachyrhizi BR 3262 was compared to the closely related strain PAC 48. BR 3262-specific regions were selected to design specific primer pairs, which were tested with respect to PCR amplification specificity and efficiency on extracted DNA, bacterial cells and root nodules from cowpea plants grown under gnotobiotic conditions and in soil. Results: Eleven designed primer pairs were specific for BR 3262 amplification and two of them (pairs 2645 and 2736) were highly sensitive and selected for further analyses. Experiments with gnotobiotic and soil-grown plants showed that both primer pairs were suitable to reliably determine nodule occupancy and confirmed the competitiveness of strain BR 3262 in natural soil. Conclusions: Primer pairs 2645 and 2736 are novel tools to accompany the fate of strain BR 3262 in inoculation experiments of cowpea in soil. This strategy should be applicable to other rhizobium/legume symbioses in the field. [ABSTRACT FROM AUTHOR]

Aims: Anthropogenic nitrogen deposition can provide legumes with a cheap source of nitrogen relative to symbiotic nitrogen fixation, leading to the potential breakdown of this critical symbiosis. Here, the effects of nitrogen deposition were tested on a native symbiosis between legumes and rhizobia. Methods: Deposition rates, soil nitrogen concentration, and plant nitrogen isotopic composition were quantified along a predicted deposition gradient in California. Acmispon strigosus seedlings were exposed to fertilization spanning nitrogen concentrations observed in the plant's California range. Both wild and experimental plants from pristine and nitrogen polluted sites were tested using rhizobial strains that varied in nitrogen fixation. Results: Deposition intensity was tightly correlated with nitrogen concentration in soils. The growth benefits of rhizobial nodulation were dramatically reduced by even modest levels of mineral nitrogen, and all Acmispon lines failed to form root nodules at high nitrogen concentrations. Conclusions: Our dataset suggests that anthropogenic deposition has greatly increased soil nitrogen concentrations in Southern California leading to significantly reduced benefits of rhizobial symbiosis. If nitrogen deposition increases continue, plant host mortality and a total collapse of the symbiosis could result. [ABSTRACT FROM AUTHOR]

Background and scope: Mangroves provide several ecosystem goods and services to society. However, mangroves are frequently subjected to land conversion, overharvesting, and pollution including increased nitrogen (N) availability. Aiming to provide useful information to predict effects of N enrichment on mangroves, we evaluated literature data on N transformation rates in sediment, dissolved N (DN) fluxes across the sediment-water interface, and natural abundance of N stable isotopes (δN) in the sediment-plant system in conserved mangroves and those subjected to anthropogenic N enrichment. Conclusions: Mangroves and terrestrial tropical forests exhibit a great overlap in rates of biological N fixation (BNF) and denitrification and nitrous oxide flux rates. Mangroves can be highly efficient users of DN forms from tidal waters, related to rapid plant uptake and an efficient conservation of DN in sediment by microbial activity. The main factors limiting N transformation rates in mangrove sediment are inorganic DN availability and microbial immobilization. The δN data indicated that fringe forests exhibit higher N transformation rates in sediment and higher N losses to atmosphere, compared to other mangrove types. Except for BNF, all other N transformation and flux rates seem to be intensified by increasing N availability. [ABSTRACT FROM AUTHOR]

Background and aims: The genus Burkholderia is highly diverse and may mediate plant growth via several mechanisms. The current study evaluates the role of strains from four Burkholderia species in maize growth promotion. Methods: Twenty three strains of diazotrophic Burkholderia species were applied to two maize genotypes (SHS5050 and BRS4157) under greenhouse and field conditions. Strains were tested for growth promotion in greenhouse during 40 days. Strain 41 from B. silvatlantica was used under field conditions. Parameters evaluated: biomass accumulation, N and root measurements in pots and grain yield, N % and N in the grains under field conditions. Results: Eleven strains were evaluated under soil substrate. The inoculation of the hybrid SHS5050 showed root biomass improvement using three strains, namely M130, M209 and PPe7 . The highest acetylene reduction activity was observed in strain 41 from B. silvatlantica. Strain 41 from B. silvatlantica improved plant dry mass on both genotypes. This strain produced 31 % more aerial tissue and 7 % more root dry mass in the BRS4157 variety than it did in the control. Both genotypes planted at Seropédica, RJ, produced 7 % more grains under field conditions and the variety produced 10.9 % more grains in comparison to the non-inoculated control. Conclusion: Strain 41 from B. silvatlantica showed beneficial plant-growth promotion of maize under greenhouse and field experiments specially in a low fertility soil. [ABSTRACT FROM AUTHOR]

Background: Gluconacetobacter diazotrophicus is a nitrogen-fixing bacterium initially isolated from sugarcane plants. Endophytic diazotrophs have the potential to fix nitrogen, but the extent of nitrogen fixation is variable and is dependent on the sugarcane genotype and other factors. Exploration of colonization of G. diazotrophicus in other plant species will provide a possibility for the development of nitrogen fixation potential in new host plants. Aims: The aim of the present study is to measure the efficiency of G. diazotrophicus colonization in different sorghum genotypes, which is an important precondition for substantial nitrogen fixation in sorghum plants. Methods: Using root-dip inoculation, G. diazotrophicus wild-type PAL5 was introduced into five grain and two sweet sorghum genotypes. The bacterium colonization in sorghum plants was assessed by the polymerase chain reaction using specific DNA primers to G. diazotrophicus, by analyzing and visualizing the expression of reporter gene uid that marked G. diazotrophicus and by reisolating the bacterium from plant tissues. The sugar content of each sorghum genotype was also measured. Results: G. diazotrophicus was detected in all sorghum genotypes tested, and the bacterium was detected in roots, stems, and leaves of sorghum genotypes via polymerase chain reaction (PCR) analysis. Analysis of the uid reporter gene expression and visualization of embedded samples also indicated full colonization of the bacterium in sorghum plants. The colonization efficiency varied among different plant genotypes and organs, with the number of bacteria ranging from 1.08 × 10 to 7.16 × 10 per g fresh tissue. Colonization of the bacterium was higher in sweet sorghum genotypes than in grain genotypes, and a positive correlation ( r = 0.82, p = 0.025) was found between sucrose content of the plants and bacterium colonization efficiency. Conclusion: The nitrogen-fixing bacterium, G. diazotrophicus, can be introduced into different types of sorghum, and the bacterium can be detected in different organs of the plants. Successful and high levels of colonization are an important step to further explore this bacterium for biological nitrogen fixation in sorghum. [ABSTRACT FROM AUTHOR]

Aims: Herbaspirillum spp. has been described as bacteria that colonizes not only maize roots but also stems and leaves. This study aimed to access the growth-promoting effect of 21 strains of Herbaspirillum on maize plants. This study is composed by 12 strains of the H. seropedicae, 5 strains of the H. rubrisubalbicans and 4 strains of the H. frisingense. Methods: Bacterial selection was performed in two phases, consisting on planting the BRS4157 maize variety and hybrid SHS5050 in the greenhouse, with sterile and non-sterile substrate and in the field. After 40 days, the greenhouse plants were harvested and the dry matter accumulation of the aerial part and roots were measured. The best-performing strain was tested in field trials treated with 40 and 80 kg ha of N fertilizer, parallel to the same treatments without inoculation. In this trial, the grain yield was evaluated as well as the contribution of biological nitrogen fixation (BNF), using the technique of natural abundance of N. Results: The results showed that H. seropedicae ZAE94 was the best strain under controlled conditions and its application as a field inoculant increased maize yield up to 34 %, depending on the plant genotype. The BNF quantification revealed that 37 % BNF-derived plant nitrogen in the hybrid SHS5050 inoculated with the H. seropedicae strain ZAE94. Conclusions: The difference in response to nitrogen as well as to inoculation depends on the selected plant genotype and the bacterial inoculant. H. seropedicae ZAE94 can promote plant growth and contribute biologically with fixed nitrogen in the maize hybrid SHS5050. [ABSTRACT FROM AUTHOR]

Background: Legume plants can establish symbiotic interactions with rhizobia to form nodules, in which atmosphere dinitrogen (N ) is reduced to ammonium (NH) as a nitrogen (N) source for legume plants. This process is called biological nitrogen fixation (BNF). It is widely believed that BNF in legumes is phosphorus (P)-demanding and might be limited by P availability.Here, we first discuss the overlooked role of plant P-acquisition/−use strategies in maintaining legume nodulation and BNF, then summarize the identified genes or regulatory mechanisms supporting this. Secondly, we synthesize emerging evidences showing how microbial strategies and soil management promote legume nodulation and BNF. Finally, we propose studying legume nodulation and BNF in a plant-soil-microbe system under low-P conditions.Such a system perspective of plant, soil, and microbial strategies will provide valuable insights into mechanisms underlying nodulation under nutrient-limited soil conditions and how to explore belowground interactions for sustainable development in terrestrial ecosystems.Scope: Legume plants can establish symbiotic interactions with rhizobia to form nodules, in which atmosphere dinitrogen (N ) is reduced to ammonium (NH) as a nitrogen (N) source for legume plants. This process is called biological nitrogen fixation (BNF). It is widely believed that BNF in legumes is phosphorus (P)-demanding and might be limited by P availability.Here, we first discuss the overlooked role of plant P-acquisition/−use strategies in maintaining legume nodulation and BNF, then summarize the identified genes or regulatory mechanisms supporting this. Secondly, we synthesize emerging evidences showing how microbial strategies and soil management promote legume nodulation and BNF. Finally, we propose studying legume nodulation and BNF in a plant-soil-microbe system under low-P conditions.Such a system perspective of plant, soil, and microbial strategies will provide valuable insights into mechanisms underlying nodulation under nutrient-limited soil conditions and how to explore belowground interactions for sustainable development in terrestrial ecosystems.Conclusions: Legume plants can establish symbiotic interactions with rhizobia to form nodules, in which atmosphere dinitrogen (N ) is reduced to ammonium (NH) as a nitrogen (N) source for legume plants. This process is called biological nitrogen fixation (BNF). It is widely believed that BNF in legumes is phosphorus (P)-demanding and might be limited by P availability.Here, we first discuss the overlooked role of plant P-acquisition/−use strategies in maintaining legume nodulation and BNF, then summarize the identified genes or regulatory mechanisms supporting this. Secondly, we synthesize emerging evidences showing how microbial strategies and soil management promote legume nodulation and BNF. Finally, we propose studying legume nodulation and BNF in a plant-soil-microbe system under low-P conditions.Such a system perspective of plant, soil, and microbial strategies will provide valuable insights into mechanisms underlying nodulation under nutrient-limited soil conditions and how to explore belowground interactions for sustainable development in terrestrial ecosystems. [ABSTRACT FROM AUTHOR]

Background: In a recent framework, Raven et al. New Phytologist 217: 1420-1427. (2018) considered carbon cost of acquiring phosphorus by mycorrhizal and non-mycorrhizal plants.We broaden their conceptual framework by incorporating belowground carbon allocation for both nitrogen and phosphorus acquisition in conditions of nutrient co-limitation and shifts in nutrient limitation, symbiotic associations with nitrogen-fixing bacteria, and nutrient mining via rhizosphere priming. We introduce a new parameter: carbon efficiency for nutrient acquisition (CENA) defined as the amount of nutrient acquisition per unit carbon allocated belowground.We explain how CENA increases with increased nutrient availability, and how it reaches a plateau when the increased availability of one limiting nutrient leads to the emergence of limitation by another nutrient. We describe how the relationship between CENA and mycorrhizal plants may be less steep compared to non-mycorrhizal plants so that CENA may be higher for mycorrhizal plants when nutrient availability is low and vice versa. In contrast, the CENA of nitrogen-fixing plants would be independent of soil nitrogen availability as long as biological nitrogen fixation meets plant nitrogen demand, but it would increase with increased soil phosphorus availability. The CENA would be more affected by soil nitrogen and phosphorus locked in organic matter or insoluble forms if plants perform nutrient mining strategies, but would be more sensitive to soil nitrogen and phosphorus availabilities if plants rely on nutrient scavenging strategies.The updated conceptual frameworks would provide better understanding of how plants optimize belowground carbon allocation for nutrient acquisition that is affected by perturbations in nutrient availability.Scope: In a recent framework, Raven et al. New Phytologist 217: 1420-1427. (2018) considered carbon cost of acquiring phosphorus by mycorrhizal and non-mycorrhizal plants.We broaden their conceptual framework by incorporating belowground carbon allocation for both nitrogen and phosphorus acquisition in conditions of nutrient co-limitation and shifts in nutrient limitation, symbiotic associations with nitrogen-fixing bacteria, and nutrient mining via rhizosphere priming. We introduce a new parameter: carbon efficiency for nutrient acquisition (CENA) defined as the amount of nutrient acquisition per unit carbon allocated belowground.We explain how CENA increases with increased nutrient availability, and how it reaches a plateau when the increased availability of one limiting nutrient leads to the emergence of limitation by another nutrient. We describe how the relationship between CENA and mycorrhizal plants may be less steep compared to non-mycorrhizal plants so that CENA may be higher for mycorrhizal plants when nutrient availability is low and vice versa. In contrast, the CENA of nitrogen-fixing plants would be independent of soil nitrogen availability as long as biological nitrogen fixation meets plant nitrogen demand, but it would increase with increased soil phosphorus availability. The CENA would be more affected by soil nitrogen and phosphorus locked in organic matter or insoluble forms if plants perform nutrient mining strategies, but would be more sensitive to soil nitrogen and phosphorus availabilities if plants rely on nutrient scavenging strategies.The updated conceptual frameworks would provide better understanding of how plants optimize belowground carbon allocation for nutrient acquisition that is affected by perturbations in nutrient availability.Results: In a recent framework, Raven et al. New Phytologist 217: 1420-1427. (2018) considered carbon cost of acquiring phosphorus by mycorrhizal and non-mycorrhizal plants.We broaden their conceptual framework by incorporating belowground carbon allocation for both nitrogen and phosphorus acquisition in conditions of nutrient co-limitation and shifts in nutrient limitation, symbiotic associations with nitrogen-fixing bacteria, and nutrient mining via rhizosphere priming. We introduce a new parameter: carbon efficiency for nutrient acquisition (CENA) defined as the amount of nutrient acquisition per unit carbon allocated belowground.We explain how CENA increases with increased nutrient availability, and how it reaches a plateau when the increased availability of one limiting nutrient leads to the emergence of limitation by another nutrient. We describe how the relationship between CENA and mycorrhizal plants may be less steep compared to non-mycorrhizal plants so that CENA may be higher for mycorrhizal plants when nutrient availability is low and vice versa. In contrast, the CENA of nitrogen-fixing plants would be independent of soil nitrogen availability as long as biological nitrogen fixation meets plant nitrogen demand, but it would increase with increased soil phosphorus availability. The CENA would be more affected by soil nitrogen and phosphorus locked in organic matter or insoluble forms if plants perform nutrient mining strategies, but would be more sensitive to soil nitrogen and phosphorus availabilities if plants rely on nutrient scavenging strategies.The updated conceptual frameworks would provide better understanding of how plants optimize belowground carbon allocation for nutrient acquisition that is affected by perturbations in nutrient availability.Conclusions: In a recent framework, Raven et al. New Phytologist 217: 1420-1427. (2018) considered carbon cost of acquiring phosphorus by mycorrhizal and non-mycorrhizal plants.We broaden their conceptual framework by incorporating belowground carbon allocation for both nitrogen and phosphorus acquisition in conditions of nutrient co-limitation and shifts in nutrient limitation, symbiotic associations with nitrogen-fixing bacteria, and nutrient mining via rhizosphere priming. We introduce a new parameter: carbon efficiency for nutrient acquisition (CENA) defined as the amount of nutrient acquisition per unit carbon allocated belowground.We explain how CENA increases with increased nutrient availability, and how it reaches a plateau when the increased availability of one limiting nutrient leads to the emergence of limitation by another nutrient. We describe how the relationship between CENA and mycorrhizal plants may be less steep compared to non-mycorrhizal plants so that CENA may be higher for mycorrhizal plants when nutrient availability is low and vice versa. In contrast, the CENA of nitrogen-fixing plants would be independent of soil nitrogen availability as long as biological nitrogen fixation meets plant nitrogen demand, but it would increase with increased soil phosphorus availability. The CENA would be more affected by soil nitrogen and phosphorus locked in organic matter or insoluble forms if plants perform nutrient mining strategies, but would be more sensitive to soil nitrogen and phosphorus availabilities if plants rely on nutrient scavenging strategies.The updated conceptual frameworks would provide better understanding of how plants optimize belowground carbon allocation for nutrient acquisition that is affected by perturbations in nutrient availability. [ABSTRACT FROM AUTHOR]

Background and aims: Acacia senegal, a leguminous dryland tree, is economically and ecologically important to sub-Saharan Africa. Water-use efficiency (WUE) and biological nitrogen fixation (BNF) are fundamental to plant productivity and survival. We quantify provenance differences in WUE, BNF, photosynthesis, biomass and gum arabic production from A. senegal assessing genetic improvement potential. Methods: Using stable isotope ratios, we determined WUE (δC) and BNF (δN) from provenances of mature A. senegal in field-trials (Senegal), sampling leaves at the beginning (wet) and end (dry) of the rainy season. Seedling provenance trials (UK) determined photosynthesis, and biomass and δC in relation to water table. Environmental data were characterised for all provenances at their sites of origin. Results: Provenances differed in both δC and δN. Gum yield declined with increasing WUE. Virtually no BNF was detected during the dry season and seedlings and mature trees may have different WUE strategies. Wind speed and soil characteristics at provenance origin were correlated with isotope composition and gum production. Conclusion: Provenance differences suggest that selection for desirable traits, e.g., increased gum production, may be possible. As ecological strategies relate to native locality, the environmental conditions at plantation site and provenance origin are important in assessing selection criteria. [ABSTRACT FROM AUTHOR]

Background and aims: Biochar is produced from the pyrolysis of organic materials, and when buried in soil can act as a long term soil carbon (C) store. Evidence suggests that biochar can also increase crop yields, reduce nutrient leaching and increase biological nitrogen fixation in leguminous plants. However, the potential for increasing biological N fixation in agroecosystems is poorly understood, with inconsistent reports of root nodulation following biochar application. Therefore, the aim of this study was to determine the effect of biochar application rate and time since application on nodulation and nitrogenase activity in nodules of clover grown in a temperate agricultural soil. Methods: We used replicated field plots with three biochar application rates (0, 25 and 50 t ha). Three years after biochar amendment, the plots were further split and fresh biochar added at two different rates (25 and 50 t ha) resulting in double-loaded reapplications of 25 + 25 and 50 + 50 t ha. Results: Three years after biochar application, there was no significant difference in the total number of root nodules between biochar-amended and unamended soil, regardless of the application rate. However, despite clover root nodules being of a similar number and size the level of nitrogenase activity of individual nodules in biochar-amended soil was significantly higher than in unamended soil. Reapplication of biochar resulted in decreased nodulation, although the rate of nitrogenase activity per nodule remained unaffected. Conclusion: In the short term, biochar influences root nodule number and localised N fixation per nodule; however, total nitrogenase activity for the whole root system remained unaffected by the application rate of biochar or time since its application. These results emphasise the importance of long-term field studies, with a variety of applications rates for determining the influence of biochar applications on N-fixing organisms and in providing data that can meaningfully inform agronomic management decisions and climate change mitigation strategies. [ABSTRACT FROM AUTHOR]

To identify active diazotrophs in sugarcane, 16S rRNA and nifH transcript analyses were applied. This should help to better understand the basis of the biological nitrogen fixation (BNF) activity of a high nitrogen fixing sugarcane variety. A field experiment using the sugarcane variety RB 867515 was conducted in Seropédica, RJ, Brazil, receiving the following treatments: unfertilised and fertilised controls without inoculation, unfertilised with inoculation. The five-strain mixture developed by EMBRAPA-CNPAB was used as inoculum. Root and leaf sheath samples were harvested in the third year of cultivation to analyse the 16S rRNA and nifH transcript diversity. In addition to nifH expression from Gluconacetobacter spp. and Burkholderia spp ., a wide diversity of nifH sequences from previously uncharacterised Ideonella/Herbaspirillum related phylotypes in sugarcane shoots as well as Bradyrhizobium sp. and Rhizobium sp. in roots was found. These results were confirmed using 16S cDNA analysis. From the inoculated bacteria, only nifH transcripts from G. diazotrophicus and B. tropica were detected in leaf sheaths and roots. Known as well as yet uncultivated diazotrophs were found active in sugarcane roots and stems using molecular analyses. Two strains of the inoculum mix were identified at the late summer harvest. [ABSTRACT FROM AUTHOR]

Background and aims: Gluconacetobacter diazotrophicus is a nitrogen-fixing endophytic bacterium isolated from sugarcane, rice, elephant grass, sweet potato, coffee, and pineapple. These plants have high level of asparagine, which promotes microbial growth and inhibits nitrogenase activity. The regulation of intracellular concentrations of this amino acid is essential for growth and biological nitrogen fixation (BNF) in this diazotroph; however its asparagine metabolic pathway has not yet been clearly established. Methods: The work reported here is the first to demonstrate the use of an alternative route for asparaginyl-tRNA (Asn-tRNA) and asparagine formation in an endophytic nitrogen-fixing bacterium by using in silico and in vitro analysis. Results: The indirect route involves transamidation of incorrectly charged tRNA via GatCAB transamidase. Nitrogenase activity was completely inhibited by 20 mM Asn in LGI-P medium, which in contrast promotes protein synthesis and microbial growth. Conclusions: The analysis carried out in this work shows that intracellular levels of asparagine regulate the expression of nitrogenase nifD gene (GDI0437), suggesting that the presence of an alternative route to produce asparagine might give the G. diazotrophicus a tighter control over cell growth and BNF, and may be of importance in the regulation of the endophytic plant-microbe interaction. [ABSTRACT FROM AUTHOR]

Diazotrophic species in the genus Herbaspirillum (e.g. H. frisingense, H. rubrisubalbicans and H. seropedicae) associate with several economically important crops in the family Poaceae, such as maize ( Zea mays), Miscanthus, rice ( Oryza sativa), sorghum ( Sorghum bicolor) and sugarcane ( Saccharum sp.), and can increase their growth and productivity by a number of mechanisms, including nitrogen fixation. Hence, the improvement and use of these plant growth-promoting bacteria could provide economic and environmental benefits. We review the colonization processes of host plants by Herbaspirillum spp., including histological aspects and molecular mechanisms involved in these interactions, which may be epiphytic, endophytic, and even occasionally pathogenic. Herbaspirillum can recognize plant signals that modulate the expression of colonization traits and plant growth-promoting factors. Although a large proportion of herbaspirilla remain rhizospheric and epiphytic, plant-associated species in this genus are noted for their ability to colonize the plant internal tissues. Endophytic colonization by herbaspirilla begins with the attachment of the bacteria to root surfaces, followed by colonization at the emergence points of lateral roots and penetration through discontinuities of the epidermis; this appears to involve bacterial envelope structures, such as lipopolysaccharide (LPS), exopolysaccharide (EPS), adhesins and the type three secretion system (T3SS), but not necessarily the involvement of cell wall-degrading enzymes. Intercellular spaces are then rapidly occupied, proceeding to colonization of xylem and the aerial parts of the host plants. The response of the host plant includes both the recognition of the bacteria as non-pathogenic and the induction of systemic resistance to pathogens. Phytohormone signaling cascades are also activated, regulating the plant development. This complex molecular communication between some Herbaspirillum spp. and plant hosts can result in plant growth-promotion. [ABSTRACT FROM AUTHOR]

Background and aims: Rice cultivars, in combination with diazotrophic bacteria, can obtain variable contributions from Biological Nitrogen Fixation (BNF). Plant genetic controls and the genotype of the bacterial strains might regulate in particular ways the plant responses during these associations. Some of this regulation is likely to occur during the first stages of bacterial infection and plant colonization, and some of the early plant responses might involve ethylene signaling, as previously reported for sugarcane. The aim of this work was to investigate whether rice early responses to inoculation with beneficial diazotrophic bacteria and the expression of ethylene receptors (ERs) by the host are dependent on plant and bacterial genotypes. Methods: Bacterial colonization, lateral root development and expression of ERs were measured in seedlings of two rice cultivars, IR42 and IAC4440, which, respectively, are known to have higher and lower BNF capabilities. Inoculation experiments were performed with two strains of bacteria: Azospirillum brasilense sp245 and Burkholderia kururiensis M130. Results: Cultivar IR42, which has a relatively high BNF capability, was more colonized by the bacteria early after inoculation in comparison with cv. IAC 4440, which has a lower BNF capability. However, although both cultivars showed a significant increase in lateral root numbers 10 days after inoculation with A. brasilense sp245, plants inoculated with B. kururiensis M130 did not. In addition, a differential ER expression pattern was observed between IR42 and IAC4440 in response to inoculation with the two strains. The expression of ERs was higher in the more BNF-effective cultivar IR42, than in the less BNF-effective cv. IAC4440, especially when cv. IR42 was inoculated with A. brasilense sp245. Moreover, plants associated with B. kururiensis M130 showed a different ER expression profile compared to those inoculated with A. brasilense sp245, with the latter exhibiting greater increases of ER mRNA levels three days after bacterial inoculation. Conclusions: We have shown that two rice genotypes contrasting in their BNF capability responded differentially to early inoculation with different strains of diazotrophic bacteria. The two rice genotypes were colonized with different bacterial numbers during their early association with two strains of diazotrophic bacteria, and the two strains had different effects on the promotion of lateral root development early after inoculation. They also triggered distinct ER expression patterns in the two rice cultivars. These data thus suggest that rice ethylene responses to beneficial diazotrophic bacteria are controlled by both plant and bacterial genotypes. [ABSTRACT FROM AUTHOR]

Temperature, water, salinity, sodicity, acidity and nutrient disorders are major abiotic stresses that can affect legume growth or the establishment and function of the legume- Rhizobium symbiosis. We have examined the literature where the application of the 15N isotope dilution methodology permits the effect of individual abiotic stresses to be independently and quantitatively separated into plant growth-mediated and BNF (biological N2 fixation)-mediated components. The response of the symbiosis to a particular stress depends on a host of factors, including legume genotype, cultivar, Rhizobium inoculant, climatic conditions, and the duration, timing and severity of the stress. Published data are analysed in terms of the above variables and their interactions. As a general rule, severe stress inhibits both legume dry matter (DM) and the proportional dependence of the legume on BNF as a source of N. The symbiosis is resilient to low to moderate stress, but there may still be a penalty on legume DM. Gaps in knowledge are identified, and general guidelines on the identification and amelioration of abiotic stresses are provided. [ABSTRACT FROM AUTHOR]

Understanding native communities is a crucial step for the management of biological nitrogen fixation, since they may be either a source of efficient strains or a limiting factor when efficient strains need to be introduced. This work aimed to evaluate the density, diversity and efficiency of Leguminosae nodulating bacterial (LNB) communities and their component strains in soils under various land use systems (LUSs): pristine forest, agriculture, pasture, agroforestry, young secondary forest, and old secondary forest,. The LNB communities were trapped from these soils by using the promiscuous host siratro under controlled conditions. We also studied their relationships with physical and chemical attributes of the soil. Agroforestry and agriculture soil samples induced the highest number of nodules in siratro, while forest soil samples induced the lowest number of nodules. No relationship was found between LNB and Leguminosae species diversity in the LUSs. The soil chemical variables that were most related to differences in nodule number and shoot dry matter weight of plants inoculated with soil suspensions of the LUSs were, respectively: Ca2+, Mg2+, base saturation, exchangeable bases and Cu2+; and pH, cation exchange capacity, B, Cu2+ and clay. Although, LNB communities from all LUSs were efficient under controlled and similar conditions, they were found to be composed of strains with variable efficiency: inefficient, efficient, highly efficient and superior efficiency. Efficient strains occurred at the highest frequency in all LUSs. The isolated strains presented similar and new sequences that were phylogenetically related to well known LNB genera in α-and β-Proteobacteria. Unusual genera in these branches, as well as in other branches, which are probably endophytic bacteria, were also isolated from nodules. These data support siratro as a useful trap species to study the LNB biodiversity of diverse ecosystems in tropical soils. The fact that the highest diversity and nodulation were seen in managed systems such as agriculture and agroforestry suggests a high resilience of LNB communities to changes in land use after deforestation in a region where large forest areas are still preserved and can be a source of propagules. [ABSTRACT FROM AUTHOR]

Bacteria of the genus Azospirillum are considered to be plant-growth promoting bacteria (PGPR) and stimulate plant growth directly either by synthesising phyto-hormones or by promoting nutrition by the process of biological nitrogen fixation (BNF). Although this genus extensively studied, the effects of inoculation and the possible BNF contribution of the Azospirillum amazonense specie are very scarce. The aim of this study was to isolate, characterise and evaluate auxin production and nitrogenase activity of this species and to select, by inoculation of rice plants, promising isolates based on their ability to improve plant growth, yield and the BNF contribution. One hundred and ten isolates obtained from rice were characterised and grouped according to colony features. Forty-two isolates, confirmed as A. amazonense by the fluorescent in situ hybridization (FISH) technique, were tested for auxin production and nitrogenase activity in vitro. Subsequently plant growth promotion related to plant nutrition effect was evaluated, in vitro and in greenhouse experiments. The BNF contribution to plant growth was evaluated using the 15N isotope dilution technique. All A. amazonense strains tested produced indoles, but only 10% of them showed high production, above 1.33 μM mg protein−1. The nitrogenase activity also was variable and only 9% of isolates showed high nitrogenase activity and the majority (54%) exhibited a low potential. The inoculation of selected strains in rice under gnotobiotic conditions reduced the growth of root and aerial part when compared to the control, showing the negative effects of excess of phytohormone accumulation in the medium. However, in the greenhouse experiment, inoculation of strains of A. amazonense increased grain dry matter accumulation (7 to 11.6%), the number of panicles (3 to 18.6%) and nitrogen accumulation at grain maturation (3.5 to 18.5%). BNF contributions up to 27% were observed for A. amazonense Y2 (wild type strain). The data presented here is the first report that the PGPR effect of A. amazonense for rice plants grown under greenhouse conditions is mainly due the BNF contribution as measured by 15N isotope dilution technique, in contrast to the hormonal effect observed with other Azospirillum species studied. [ABSTRACT FROM AUTHOR]

Drosera villosa var. villosa A. St.-Hil is a carnivorous plant that grows in Brazilian flooded soils very poor on nutrients, including low levels of N. Under these conditions, the plant shows vigorous growth, low root number, low number of captured prey (less than 50%) and a great assemblage of bacteria associated with the roots and leaves that grow in N-free medium. These preliminary results have led us to investigate the number of colony forming units (log CFU) in the roots (rhizosphere and endorhizosphere) and leaves (phyllosphere and endophyllosphere) of D. villosa var. villosa by the tenfold serial dilution technique in two N-free culture media. The results showed that the phyllosphere had 6.65 log CFU g dry leaf−1 and the rhizosphere 6.47 log CFU g dry soil−1, with the lowest value detected in the endophyllosphere (4.39 log CFU g dry leaf−1). Sixty-three different bacteria morphotypes were isolated from the surface and interior of the roots and leaves and the amplification of the DNA with specific primers detected the nifH gene in 34 of those strains. The DNAs of the 34 strains were compared by the BOX-PCR technique and a great diversity was observed, with the bacteria clustering at a final level of similarity of only 12%. The strains were also submitted to the partial sequencing of the 16S rRNA gene and several genera of N2-fixing bacteria were detected, including Bacillus, Burkholderia, Methylobacterium, Paenibacillus, Pseudomonas and Sphingomonas. [ABSTRACT FROM AUTHOR]

It is well described that the beneficial interactions between plants and bacteria are genotype and site specific. Brazilian sugarcane varieties can obtain up to 70% of their nitrogen requirement from biological nitrogen fixation (BNF), and this contribution is related to the Brazilian breeding and selection processes, by example of the variety SP70-1143. In this study the effect of two inoculation mixtures containing diazotrophic bacteria in our earlier pot experiment was evaluated with two sugarcane varieties, a known responder, SP70-1143, and a newly selected variety, SP81-3250, to investigate the sugarcane genotype effect and the role of the mixtures. The sugarcane varieties SP70-1143 and SP81-3250 were grown under commercial field conditions at three sites with contrasting soil types: an Alfisol, an Oxisol and an Ultisol that means a low, medium and high natural fertility respectively. The stem yield and BNF contribution in response to bacterial inoculation were influenced by the strain combinations in the inoculum, the plant genotype, and the soil type and nitrogen fertilization, confirming the genetic and environmental influence in PGP-bacteria interactions. Inoculation effects on the BNF contribution and stem yield increased in the variety SP70-1143 grown in the Alfisol without nitrogen fertilization for three consecutive crops, and it was equivalent to the annual nitrogen fertilization. The plants grown in the Oxisol showed small increases in the productivity of the variety SP70-1143, and in the Ultisol the sugarcane plants presented even decreases in the stem productivity due to inoculation with diazotrophic bacteria mixtures. The results demonstrate the feasibility of the inoculation technology using diazotrophic bacteria in micropropagated sugarcane varieties grown in soils with low to medium levels of fertility. In addition, the results also indicated that specific plant – bacteria – environment combinations are needed to harness the full benefits of BNF. [ABSTRACT FROM AUTHOR]

A field experiment was carried out to quantify biological nitrogen fixation (BNF) using the 15N isotope natural abundance method in maize ( Zea mays L.)/faba bean ( Vicia faba L.) and wheat ( Triticum aestivum L.)/faba bean intercropping systems. Faba bean was yielding more in the maize/faba bean intercropping, but not in the wheat/faba bean intercropping. Biomass, grain yield and N acquisition of faba bean were significantly increased when intercropped with maize, and decreased significantly with wheat, irrespective of N-fertilizer application, indicating that the legume could gain or lose productivity in an intercropping situation. There was yield advantage of maize/faba bean intercropping, but no in wheat/faba bean intercropping. The grain yield of the faba bean intercropped with maize was greater than that of faba bean monoculture due to increases of the stems per plant and the pods per stem of faba bean. N fertilization inhibited N fixation of faba bean in maize/faba bean and wheat/faba bean intercropping and faba bean monoculture. The responses of different cropping systems to N-fertilizer application, however, were not identical, with competitive intercropping (wheat/faba bean) being more sensitive than facilitative intercropping (maize/faba bean). Intercropping increased the percentage of N derived from air (%Ndfa) of the wheat/faba bean system, but not that of the maize/faba bean system when no N fertilizer was applied. When receiving 120 kg N/ha, however, intercropping did not significantly increase %Ndfa either in the wheat/faba bean system or in the maize/faba bean system in comparison with faba bean in monoculture. The amount of shoot N derived from air (Ndfa), however, increased significantly when intercropped with maize, irrespective of N-fertilizer application. Ndfa decreased when intercropped with wheat, albeit not significantly at 120 kg N/ha. Ndfa was correlated more closely with dry matter yield, grain yield and competitive ratio, than with %Ndfa. This indicates that that total dry matter yield (sink strength), not %Ndfa, was more critical for the legume to increase Ndfa. The results suggested that N fixation could be improved by yield maximization in an intercropping system. [ABSTRACT FROM AUTHOR]

Since the 1970s the area under sugarcane in Brazil has increased from 2 million to over 5 million ha (M ha), and it is expected to pass the 7 M ha mark in 2007. More than half of the cane is harvested to produce bioethanol as a fuel for light vehicles. The distilleries produce approximately 13 L of distillery waste (vinasse) for each litre of ethanol produced. In the 1980s there was considerable concern over the long-term effects of the disposal of this material (containing about 1% carbon and high in K) on cane yields if it was applied to the field. At the same time there was a growing movement to abandon the practice of pre-harvest burning and some research was showing that some Brazilian varieties of sugar cane were able to obtain significant contributions of N from plant-associated biological nitrogen fixation (BNF). For these reasons an experiment was installed on a cane plantation in the state of Pernambuco, NE Brazil to investigate the long-term effects of vinasse and N fertiliser additions and the practice of pre-harvest burning on crop and sugar yield, soil fertility parameters, N balance and soil C stocks. The results showed that over a 16-year period, trash conservation (abandonment of burning) increased cane yields by 25% from a mean of 46 to 58 Mg ha−1. Vinasse applications (80 m3 ha−1 crop−1) increased mean cane and sugar yield by 12 to 13% and the application of 80 kg N ha−1 as urea increased cane yields by 9%, but total sugar yield by less than 6% (from 7.0 to 7.4 Mg ha−1 crop−1). The total N balance for the soil/plant system when only the surface 20 cm of the soil was considered was positive in plots where no N fertiliser was added. However, the data indicated that during the 16 years of the study considerable quantities of soil organic matter were accumulated below 20 cm depth such that the N balance considering the soil to 60 cm depth was strongly positive, except where N fertiliser was added. The data indicated that there were considerable BNF inputs to the system, which was consistent with its low response to N fertiliser and low N fertiliser-use-efficiency. There were no significant effects of vinasse or urea addition, or trash conservation on soil C stocks, although the higher yields proportioned by trash conservation had potentially significant benefits for increased mitigation of CO2 emissions where the main use of the cane was for bioethanol production. [ABSTRACT FROM AUTHOR]

It has previously been reported that endophytic diazotrophic bacteria contribute significantly to the nitrogen budgets of some graminaceous species. In this study the contribution of biological nitrogen fixation to the N-budget of a South African sugarcane cultivar was evaluated using 15N natural abundance, acetylene reduction and 15N incorporation. Plants were also screened for the presence of endophytic diazotrophic bacteria using acetylene reduction and nifH-gene targeted PCR with the pure bacterial strains. 15N natural abundance studies on field-grown sugarcane indicated that the plants did not rely extensively on biological nitrogen fixation. Furthermore, no evidence was found for significant N2-fixation or nitrogenase activity in field-grown or glasshouse-grown plants using 15N incorporation measurements and acetylene reduction assays. Seven endophytic bacterial strains were isolated from glasshouse-grown and field-grown plants and cultured on N-free medium. The diazotrophic character of these seven strains could not be confirmed using acetylene reduction and PCR screening for nifH. Thus, although biological nitrogen fixation may occur in South African sugarcane varieties, the contribution of this N-source in the tested cultivar was not significant. [ABSTRACT FROM AUTHOR]

We investigated the effects of three plant growth promoting rhizobacteria (PGPR), on Biological Nitrogen Fixation (BNF), nodulation and growth promotion by soybean (Glycine max) var. Osumi plants. The strains, Aur 6, Aur 9 and Cell 4, belong toPsedomonas fluorescens, Chryseobacterium balustinumandSerratia fonticola, respectively. Inoculation modes for the PGPRs andSinorhizobium fredii(carried out through irrigation), were examined. In the first mode, PGPRs andS. frediiwere co-inoculated. In the second mode, we first inoculatedS. frediiand after the PGPRs, which were added 5 or 10 days later (each inoculation being an independent treatment). In the third mode, the PGPRs were inoculated first, and theS. frediiwas inoculated 5 days later. We also included treatments inoculated with only the PGPRs (one PGPR per treatment) and only withS. fredii. Plants were maintained in a greenhouse under controlled environmental conditions, and were sampled 3 months after sowing. The results obtained showed the effects of the inoculation sequence. The most significant effects on growth parameters (stem plus leaf weight and fresh root weight) were found when inoculations with PGPR andS. frediiwere at different times or when we inoculated only with PGPR and the plants were watered with nitrogen. Co-inoculation had no positive effects on any parameter, probably due to competition between the PGPR andS. fredii. Our results indicate that the inoculation modes with PGPR and rhizobia play a very important role in the effects produced. Thus, although plant growth promoting rhizobacteria may interact synergistically with root-nodulating rhizobia, plant growth promoting rhizobacteria selected for one crop should be assessed for potentially hazardous effects on other crops before being used as inoculants. [ABSTRACT FROM AUTHOR]

Asymbiotic, associative or symbiotic biological N2 fixation (BNF), is a free and renewable resource, which should constitute an integral part of sustainable agro-ecosystems. Yet there has been a rapid increase in use of fertiliser N and a parallel decline in the cultivation of leguminous plants and BNF, especially in the developed world. Fertilisers have boosted crop yields, but intensive agricultural systems have increasingly negative effects on the atmospheric and aquatic environments. BNF, either alone or in combination with fertilisers and animal manures, may prove to be a better solution to supply nitrogen to the cropping systems of the future. This review focuses on the potential benefit of BNF on the environment especially on soil acidification, rhizosphere processes and plant CO2 fixation. As fertiliser N has supplanted BNF in agriculture the re-substitution of BNF is considered. What is the consequence of fertiliser N production on energy use? The effect of fertiliser use on the release of the greenhouse gas CO2 is estimated at approximately 1% of the global anthropogenic emission of CO2. The role of BNF on nitrogen cycling, ammonia volatilisation, N2O emission and NO3 leaching suggests that BNF is less likely than fertilisers to cause losses during pre-cropping and cropping. Sometimes however the post-harvest losses may be greater, due to the special qualities of legume residues. Nevertheless, legumes provide other `ecological services' including improved soil structure, erosion protection and greater biological diversity. [ABSTRACT FROM AUTHOR]

Approximately forty years after commercial cropping of soybean in Brazil began, the total area under this crop has reached over 13 M ha with a mean productivity of 2400 kg ha-1. Soybean varieties introduced from the USA and varieties rescued from early introductions in Brazilian territory were part of the Brazilian soybean-breeding programme which spread the crop from high to low latitudes. Disease-resistance, pest-resistance, tolerance to low fertility soils, as well as production of plants with pods sufficiently high above the ground for efficient mechanical harvesting, were all aims of the programme. Although BNF was not explicitly considered as a trait for selection in the breeding/selection programme, maximisation of biological nitrogen fixation (BNF) was favoured by conducting selection and breeding trials on soils low in N, in which the seeds were inoculated with efficient Bradyrhizobium inoculants but without N fertiliser application. Several efficient imported Bradyrhizobium strains were found to be unable to compete with native soil micro-flora and other previously-introduced Bradyrhizobium strains. Surprisingly, after being in the soil for many years one or two of these strains had become more competitive while maintaining their high BNF capacity. Today, these strains are included amongst the recommended Brazilian inoculants and have promoted significant improvements in grain yields. The breeding of soybeans in conditions that made grain yield highly dependent on BNF, and the continuous attention paid to the selection of Bradyrhizobium strains appropriate for the newly released varieties, have been the main contributors to today's high yields and their great benefit to the Brazilian economy. There seems to be no reason why this ongoing research programme should not serve as an appropriate model to improve BNF inputs to grain legumes in other countries of the world. [ABSTRACT FROM AUTHOR]

Nitrogen-fixing bacteria are able to enter into roots from the rhizosphere, particularly at the base of emerging lateral roots, between epidermal cells and through root hairs. In the rhizosphere growing root hairs play an important role in symbiotic recognition in legume crops. Nodulated legumes in endosymbiosis with rhizobia are amongst the most prominent nitrogen-fixing systems in agriculture. The inoculation of non-legumes, especially cereals, with various non-rhizobial diazotrophic bacteria has been undertaken with the expectation that they would establish themselves intercellularly within the root system, fixing nitrogen endophytically and providing combined nitrogen for enhanced crop production. However, in most instances bacteria colonize only the surface of the roots and remain vulnerable to competition from other rhizosphere micro-organisms, even when the nitrogen-fixing bacteria are endophytic, benefits to the plant may result from better uptake of soil nutrients rather than from endophytic nitrogen fixation. Azorhizobium caulinodans is known to enter the root system of cereals, other non-legume crops and Arabidopsis, by intercellular invasion between epidermal cells and to internally colonize the plant intercellularly, including the xylem. This raises the possibility that xylem colonization might provide a non-nodular niche for endosymbiotic nitrogen fixation in rice, wheat, maize, sorghum and other non-legume crops. A particularly interesting, naturally occurring, non-nodular xylem colonising endophytic diazotrophic interaction with evidence for endophytic nitrogen fixation is that of Gluconacetobacter diazotrophicus in sugarcane. Could this beneficial endophytic colonization of sugarcane by G. diazotrophicus be extended to other members of the Gramineae, including the major cereals, and to other major non-legume crops of the World? [ABSTRACT FROM AUTHOR]