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Biological nitrogen (N) fixation, an important pathway of N, inputs from the atmosphere to Earth's ecosystems, is well demonstrated to decline under N input. However, it remains unclear why N fixers sustain N fixation in many forests under high atmospheric N deposition. To address this knowledge gap, we analyzed the response of the diazotroph community to low N loads (short-term and low N addition; 3-year N addition at the rates of 25-50 kg N ha -1 year -1 ) vs high loads (chronic and high N addition; 9-year N addition at the rate of 150 kg N ha -1 year -1 ) in forest soils using high-throughput sequencing. Rates of N fixation decreased under low and high N loads (by 13%-27% and 10%-12%, respectively). Richness and alpha diversity (ACE and Chao1) of the soil diazotroph community decreased under low but not high N loads. Approximately 67.1%-74.4% of the nifH gene sequences at the OTU level overlapped between the control and low N loads, but only 52.0%-53.6% of those overlapped between the control and high N loads, indicating a larger shift of diazotroph community composition under high N loads. Low N loads increased soil NH 4 + concentrations, which decreased diazotroph community richness, diversity, and N fixation rates, whereas the increased soil NH 4 + concentrations under high N loads did not have negative impacts on the structure and function of the diazotroph community. These findings indicate that diazotrophs sustain N fixation under high N deposition via adjustment of their community composition in forest soils., Importance: This study examined the changes in soil diazotroph community under different loads of simulated N deposition and analyzed its relationship with N fixation rates in in five forests using high-throughput sequencing. The magnitudes of N fixation rates reduced by low N loads were higher than those by high N loads. Low N loads decreased richness and diversity of diazotroph community, whereas diazotroph community structure remained stable under high N loads. Compared with low N loads, high N loads resulted in a less similarity and overlap of nifH gene sequences among the treatments and a larger adjustment of diazotroph community. Low N loads increased soil NH4+ concentrations, which decreased diazotroph community richness, diversity, and N fixation rates, whereas the increased soil NH4+ under high N loads did not have negative impacts on diazotroph community structure and N fixation. Based on these findings, it is urgently needed to incorporate the loads of N deposition and the composition of diazotroph community into terrestrial N-cycling models for accurate understanding of N inputs in forest ecosystems., Competing Interests: The authors declare no conflict of interest.

Background: Nitrogen is considered the most limiting nutrient element for herbivorous insects. To alleviate nitrogen limitation, insects have evolved various symbiotically mediated strategies that enable them to colonize nitrogen-poor habitats or exploit nitrogen-poor diets. In frugivorous tephritid larvae developing in fruit pulp under nitrogen stress, it remains largely unknown how nitrogen is obtained and larval development is completed., Results: In this study, we used metagenomics and metatranscriptomics sequencing technologies as well as in vitro verification tests to uncover the mechanism underlying the nitrogen exploitation in the larvae of Bactrocera dorsalis. Our results showed that nitrogenous waste recycling (NWR) could be successfully driven by symbiotic bacteria, including Enterobacterales, Lactobacillales, Orbales, Pseudomonadales, Flavobacteriales, and Bacteroidales. In this process, urea hydrolysis in the larval gut was mainly mediated by Morganella morganii and Klebsiella oxytoca. In addition, core bacteria mediated essential amino acid (arginine excluded) biosynthesis by ammonium assimilation and transamination., Conclusions: Symbiotic bacteria contribute to nitrogen transformation in the larvae of B. dorsalis in fruit pulp. Our findings suggest that the pattern of NWR is more likely to be applied by B. dorsalis, and M. morganii, K. oxytoca, and other urease-positive strains play vital roles in hydrolysing nitrogenous waste and providing metabolizable nitrogen for B. dorsalis., (© 2022. The Author(s).)

Background: Excessive nitrogen (N) fertilization in glasshouse fields greatly increases N loss and fossil-fuel energy consumption resulting in serious environmental risks. Microbial inoculants are strongly emerging as potential alternatives to agrochemicals and offer an eco-friendly fertilization strategy to reduce our dependence on synthetic chemical fertilizers. Effects of a N-fixing strain Pseudomonas protegens CHA0-ΔretS-nif on ginger plant growth, yield, and nutrient uptake, and on earthworm biomass and the microbial community were investigated in glasshouse fields in Shandong Province, northern China., Results: Application of CHA0-ΔretS-nif could promote ginger plant development, and significantly increased rhizome yields, by 12.93% and 7.09%, respectively, when compared to uninoculated plants and plants treated with the wild-type bacterial strain. Inoculation of CHA0-ΔretS-nif had little impact on plant phosphorus (P) acquisition, whereas it was associated with enhanced N and potassium (K) acquisition by ginger plants. Moreover, inoculation of CHA0-ΔretS-nif had positive effects on the bacteria population size and the number of earthworms in the rhizosphere. Similar enhanced performances were also found in CHA0-ΔretS-nif-inoculated ginger plants even when the N-fertilizer application rate was reduced by 15%. A chemical N input of 573.8 kg ha -1 with a ginger rhizome yield of 1.31 × 10 5  kg ha -1 was feasible., Conclusions: The combined application of CHA0-ΔretS-nif and a reduced level of N-fertilizers can be employed in glasshouse ginger production for the purpose of achieving high yields while at the same time reducing the inorganic-N pollution from traditional farming practices. © 2021 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry., (© 2021 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.)

Many insects feed on a low-nitrogen diet, and the origin of their nitrogen supply is poorly understood. It has been hypothesized that some insects rely on nitrogen-fixing bacteria (diazotrophs) to supplement their diets. Nitrogen fixation by diazotrophs has been extensively studied and convincingly demonstrated in termites, while evidence for the occurrence and role of nitrogen fixation in the diet of other insects is less conclusive. Here, we summarize the methods to detect nitrogen fixation in insects and review the available evidence for its occurrence (focusing on insects other than termites). We distinguish between three aspects of nitrogen fixation investigations: (i) detecting the presence of potential diazotrophs; (ii) detecting the activity of the nitrogen-fixing enzyme; and (iii) detecting the assimilation of fixed nitrogen into the insect tissues. We show that although evidence from investigations of the first aspect reveals ample opportunities for interactions with potential diazotrophs in a variety of insects, demonstrations of actual biological nitrogen fixation and the assimilation of fixed nitrogen are restricted to very few insect groups, including wood-feeding beetles, fruit flies, leafcutter ants, and a wood wasp. We then discuss potential implications for the insect's fitness and for the ecosystem as a whole. We suggest that combining these multiple approaches is crucial for the study of nitrogen fixation in insects, and argue that further demonstrations are desperately needed in order to determine the relative importance of diazotrophs for insect diet and fitness, as well as to evaluate their overall impact on the ecosystem., (© 2019 Institute of Zoology, Chinese Academy of Sciences.)

Nitrogen (N) deposition and biological N fixation (BNF) are main external N inputs into terrestrial ecosystems. However, few studies have simultaneously quantified the contribution of these two external N inputs to global NPP and consequent C sequestration. Based on literature analysis, we estimated new net primary production (NPP) due to external N inputs from BNF and N deposition and the consequent C sinks in global boreal, temperate and tropical forest biomes via a stoichiometric scaling approach. Nitrogen-induced new NPP is estimated to be 3.48 Pg C yr -1 in global established forests and contributes to a C sink of 1.83 Pg C yr -1 . More specifically, the aboveground and belowground new NPP are estimated to be 2.36 and 1.12 Pg C yr -1 , while the external N-induced C sinks in wood and soil are estimated to be 1.51 and 0.32 Pg C yr -1 , respectively. BNF contributes to a major proportion of N-induced new NPP (3.07 Pg C yr -1 ) in global forest, and accounts for a C sink of 1.58 Pg C yr -1 . Compared with BNF, N deposition only makes a minor contribution to new NPP (0.41 Pg C yr -1 ) and C sinks (0.25 Pg C yr -1 ) in global forests. At the biome scale, rates of N-induced new NPP and C sink show an increase from boreal forest towards tropical forest, as mainly driven by an increase of BNF. In contrast, N deposition leads to a larger C sink in temperate forest (0.11 Pg C yr -1 ) than boreal (0.06 Pg C yr -1 ) and tropical forest (0.08 Pg C yr -1 ). Our estimate of total C sink due to N-induced new NPP approximately matches an independent assessment of total C sink in global established forests, suggesting that external N inputs by BNF and atmospheric deposition are key drivers of C sinks in global forests., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Legumes belong to the most important crops worldwide. They increase soil fertility due their ability to establish symbiotic associations with soil microorganisms, known as rhizobia, capable of fixing nitrogen from the atmosphere. However, they are frequently exposed to abiotic stress conditions in particular drought. Such adverse conditions impair the biological nitrogen fixation (BNF) and depend largely on the legume. Therefore, two peanut cultivars with contrasting tolerance to drought, namely the more tolerant EC-98 and the sensitive Granoleico, were investigated to elucidate the relative contribution of BNF to the tolerance to drought. The tolerant cultivar EC-98 sustained growth and BNF similar to the control condition despite the reduced water potential and photosynthesis, suggesting the functioning of distinct metabolic pathways that contributed to enhance the tolerance. The biochemical and metabolomics approaches revealed that nodules from the tolerant cultivar accumulated trehalose, proline and gamma-aminobutyric acid (GABA), metabolites with known function in protecting against drought stress. The amide metabolism was severely affected in nodules from the sensitive cultivar Granoleico as revealed by the low content of asparagine and glutamine in the drought stressed plants. The sensitive cultivar upon rehydration was unable to re-establish a metabolism similar to well-watered plants. This was evidenced by the low level of metabolites and, transcripts and specific activities of enzymes from the carbon (sucrose synthase) and nitrogen (glutamine synthetase) metabolism which decreased below the values of control plants. Therefore, the increased content of metabolites with protective functions under drought stress likely is crucial for the full restoration upon rehydration. Smaller changes of drought stress-related metabolites in nodule are another trait that contributes to the effective control of BNF in the tolerant peanut cultivar (EC-98)., (Copyright © 2017 Elsevier B.V. All rights reserved.)

The indiscriminate use of chemical fertilizers has contributed to the deterioration of the biological, physical and chemical properties of the soil, resulting in the loss of its productive capacity. For this reason, the use of biofertilizers has emerged as a technological alternative. The objective of this research was to develop a suitable liquid fermentation system and encystment for the multiplication of Azotobacter chroococcum AC1 strain, a bacterium employed in a biofertilizer formulation produced at present by CARPOICA, Colombia. Sequential statistical designs were used to determine the conditions in the fermentation system. The interaction between agitation, aeration and pH was evaluated on the viable biomass (CFU/ml) of AC1. In addition, the encystment ability of the strain was evaluated using two encystment agents and the potential plant growth-promoting rhizobacteria (PGPR) activity was assessed by different techniques, such as nitrogen fixation by ARA, phosphate solubilization by the phospho-molybdenum-blue reaction and indolic compound production by colorimetric reaction using the Salkowski reagent. Results showed significant effects (p<0.05) on the viable biomass in the three conditions (pH, aeration and agitation) tested individually, in one dual interaction and one tripartite interaction, were demonstrated to have a positive effect on the response variable aeration and agitation. The addition of the two encystment agents evaluated, AE01 and AE02, demonstrated the ability of AC1 to form cysts under stress conditions. Likewise, fermentation and encystment conditions did not affect the biological activities tested., (Copyright © 2017 Asociación Argentina de Microbiología. Publicado por Elsevier España, S.L.U. All rights reserved.)

Cenchrus spinifex is an invasive plant found in large areas of northern China. In this study, we focused on analysis of the effects of C. spinifex on soil nitrogen and plant nitrogen pools in Horqin sandy grassland. In addition, a pot experiment with 15 N tracing techniques was designed to study the biological nitrogen fixation ability of C. spinifex, compared with two native grasses, Elymus dahuricus and Agropyron cristatum. The total soil nitrogen pool in C. spinifex invaded-area increased significantly by 47.5% and 20.8%, and the soil ammonium nitrogen pool decreased significantly by 25.6% and 25.2%, compared with those in bare and native plant Roegneria kamoji areas, respectively. The plant shoot nitrogen pool decreased significantly by 18.7% in C. spinifex compared with native plant R. kamoji. Atom% 15 N, atom% 15 N excess and atom% 15 N weighting excess of C. spinifex were all significantly lower than those of E. dahuricus and A. cristatum. The nitrogen use efficiencies of C. spinifex and E. dahuricus were 48.5% and 47.0%, respectively, and no significant difference was observed. Ndfa of C. spinifex accounted for 60.2%, when growing together with E. dahuricus. These results suggested that the characteristics on the high efficient use for nitrogen of this invasive weed might an ecological adaptation mechanism, leading to successful colonization and spread in Horqin Steppe.

Tropical savannas are hypothesized to be hot spots of nitrogen-fixer diversity and activity because of the high disturbance and low nitrogen characteristic of savanna landscapes. Here we compare the abundances of nitrogen-fixing and non-fixing trees in both tropical savannas and tropical forests under climatically equivalent conditions, using plant inventory studies across 566 plots in South America and Africa. A single factor, aridity, explained 19-54% of the variance in fixer abundance, and unexpectedly was more important than fire frequency, biome, and continent. Nitrogen fixers were more abundant in arid environments; as a result, African savannas, which tend to be drier, were richer in nitrogen fixers than South American savannas. Fixer abundance converged on similar levels in forests in both continents. We conclude that climate plays a greater role than fire in determining the distribution of nitrogen fixers across tropical savanna and forest biomes., (© 2016 by the Ecological Society of America.)

The symbiosis between α nitrogen (N2)-fixing Proteobacteria (family Rhizobiaceae) and legumes belonging to the Fabaceae (a single phylogenetic group comprising three subfamilies: Caesalpinioideae, Mimosoideae and Papilionoideae) results in the formation of a novel root structure called a nodule, where atmospheric N2 is fixed into NH3(+). In the determinate type of nodules harbored by Rhizobium-nodulated Fabaceae species, newly synthesized NH3(+) is finally converted into allantoin (C4H6N4O3) and allantoic acid (C4H8N4O4) (ureides) through complex pathways involving at least 20 different enzymes that act synchronously in two types of nodule cells with contrasting ultrastructure, including the tree nodule cell organelles. Newly synthesized ureides are loaded into the network of nodule-root xylem vessels and transported to aerial organs by the transpirational water current. Once inside the leaves, ureides undergo an enzymatically driven reverse process to yield NH4(+) that is used for growth. This supports the role of ureides as key nitrogen (N)-compounds for the growth and yield of legumes nodulated by Rhizobium that grow in soils with a low N content. Thus, a concrete understanding of the mechanisms underlying ureide biogenesis and catabolism in legumes may help agrobiologists to achieve greater agricultural discoveries. In this review we focus on the transmembranal and transorganellar symplastic and apoplastic movement of N-precursors within the nodules, as well as on the occurrence, localization and properties of enzymes and genes involved in the biogenesis and catabolism of ureides. The synthesis and transport of ureides are not unique events in Rhizobium-nodulated N2-fixing legumes. Thus, a brief description of the synthesis and catabolism of ureides in non-legumes was included for comparison. The establishment of the symbiosis, nodule organogenesis and the plant's control of nodule number, synthesis and translocation of ureides via feed-back inhibition mechanisms are also reviewed., (Copyright © 2016 Elsevier GmbH. All rights reserved.)

Soil amendments with biochar to improve soil fertility and increase soil carbon stocks have received some high-level attention. Physical and chemical analyses of amended soils and biochars from various feedstocks are reported, alongside some evaluations of plant growth promotion capabilities. Fewer studies investigated the soil microbiota and their potential to increase cycling and mobilization of nutrients in biochar-amended soils. This review is discussing the latest findings in the bacterial contribution to cycling and mobilizing nitrogen, phosphorus, and sulfur in biochar-amended soils and potential contributions to plant growth promotion. Depending on feedstock, pyrolysis, soil type, and plant cover, changes in the bacterial community structure were observed for a majority of the studies using amplicon sequencing or genetic fingerprinting methods. Prokaryotic nitrification largely depends on the availability of ammonium and can vary considerably under soil biochar amendment. However, denitrification to di-nitrogen and in particular, nitrous oxide reductase activity is commonly enhanced, resulting in reduced nitrous oxide emissions. Likewise, bacterial fixation of di-nitrogen appears to be regularly enhanced. A paucity of studies suggests that bacterial mobilization of phosphorus and sulfur is enhanced as well. However, most studies only tested for extracellular sulfatase and phosphatase activity. Further research is needed to reveal details of the bacterial nutrient mobilizing capabilities and this is in particular the case for the mobilization of phosphorus and sulfur., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Biological nitrogen fixation (BNF) in woody plants is often investigated using foliar measurements of δ(15) N and is of particular interest in ecosystems experiencing increases in BNF due to woody plant encroachment. We sampled δ(15) N along the entire N uptake pathway including soil solution, xylem sap and foliage to (1) test assumptions inherent to the use of foliar δ(15) N as a proxy for BNF; (2) determine whether seasonal divergences occur between δ(15) Nxylem sap and δ(15) Nsoil inorganic N that could be used to infer variation in BNF; and (3) assess patterns of δ(15) N with tree age as indicators of shifting BNF or N cycling. Measurements of woody N-fixing Prosopis glandulosa and paired reference non-fixing Zanthoxylum fagara at three seasonal time points showed that δ(15) Nsoil inorganic N varied temporally and spatially between species. Fractionation between xylem and foliar δ(15) N was consistently opposite in direction between species and varied on average by 2.4‰. Accounting for these sources of variation caused percent nitrogen derived from fixation values for Prosopis to vary by up to ∼70%. Soil-xylem δ(15) N separation varied temporally and increased with Prosopis age, suggesting seasonal variation in N cycling and BNF and potential long-term increases in BNF not apparent through foliar sampling alone., (© 2014 John Wiley & Sons Ltd.)

Some beneficial plant-interacting bacteria can biologically fix N2 to plant-available ammonium. Biological nitrogen fixation (BNF) is an important source of nitrogen (N) input in agriculture and represents a promising substitute for chemical N fertilizers. Diazotrophic bacteria have the ability to develop different types of root associations with different plant species. Among the highest rates of BNF are those measured in legumes nodulated by endosymbionts, an already very well documented model of plant-diazotrophic bacterial association. However, it has also been shown that economically important crops, especially monocots, can obtain a substantial part of their N needs from BNF by interacting with associative and endophytic diazotrophic bacteria, that either live near the root surface or endophytically colonize intercellular spaces and vascular tissues of host plants. One of the best reported outcomes of this association is the promotion of plant growth by direct and indirect mechanisms. Besides fixing N, these bacteria can also produce plant growth hormones, and some species are reported to improve nutrient uptake and increase plant tolerance against biotic and abiotic stresses. Thus, this particular type of plant-bacteria association consists of a natural beneficial system to be explored; however, the regulatory mechanisms involved are still not clear. Plant N status might act as a key signal, regulating and integrating various metabolic processes that occur during association with diazotrophic bacteria. This review will focus on the recent progress in understanding plant association with associative and endophytic diazotrophic bacteria, particularly on the knowledge of the N networks involved in BNF and in the promotion of plant growth., (© The Author 2014. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

UNLABELLED: Diazotrophic bacteria can reduce atmospheric nitrogen into ammonia enabling bioavailability of the essential element. Many diazotrophs closely associate with plant roots increasing nitrogen availability, acting as plant growth promoters. These associations have the potential to reduce the need for costly synthetic fertilizers if they could be engineered for agricultural applications. However, despite the importance of diazotrophic bacteria, genetic tools are poorly developed in a limited number of species, in turn narrowing the crops and root microbiomes that can be targeted. Here, we report optimized protocols and plasmids to manipulate phylogenetically diverse diazotrophs with the goal of enabling synthetic biology and genetic engineering. Three broad-host-range plasmids can be used across multiple diazotrophs, with the identification of one specific plasmid (containing origin of replication RK2 and a kanamycin resistance marker) showing the highest degree of compatibility across bacteria tested. We then demonstrated modular expression by testing seven promoters and eleven ribosomal binding sites using proxy fluorescent proteins. Finally, we tested four small molecule inducible systems to report expression in three diazotrophs and demonstrated genome editing in Klebsiella michiganensis M5al. ONE-SENTENCE SUMMARY: In this study, broad-host plasmids and synthetic genetic parts were leveraged to enable expression tools in a library of diazotrophic bacteria.

Marine biological nitrogen fixation (BNF) is crucial for introducing "new nitrogen" into the oceans. Over the past 30 years, numerous laboratory and on-board culture experiments have been conducted studying the effects of nutrients such as total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), and dissolved iron (DFe) on marine diazotrophs such as Braarudosphaera bigelowii (B. bigelowii), Trichodesmium, Crocosphaera and noncyanobacterial diazotrophs (NCDs). Most studies concluded that elevated dissolved inorganic nitrogen levels inhibit nitrogen fixation in Trichodesmium, promote its growth, and have minimal effect on B. bigelowii. The impact on NCDs is unclear. Moreover, elevated dissolved inorganic phosphorus (DIP) levels can promote individual growth, population growth, and nitrogen fixation in most diazotrophs in P-limited marine environments. Dissolved organic phosphorus is a potential phosphorous source for diazotrophs in low-DIP environments. Elevated DFe can promote population growth and nitrogen fixation in diazotrophs in Fe-limited marine environments. At present, most diazotrophs have yet to achieve pure culture. Moreover, the effect of nutrients on diazotrophs is mainly limited to the study of a single nutrient, which cannot accurately reflect the actual Marine environment where diazotrophs live. As a result, our understanding of the effect of nutrients on diazotrophs is still insufficient. Future research focusing on the issues above and the development of innovative technologies and methodologies to investigate the impact of marine BNF is highly recommended, which will allow for a more precise assessment of the impact of marine BNF on global primary productivity while providing a scientific foundation for rational evaluation of ocean CO2 uptake and emissions. [ABSTRACT FROM AUTHOR]

Improving biological nitrogen fixation (BNF) in cereal crops is a long-sought objective; however, no successful modification of cereal crops showing increased BNF has been reported. Here, we described a novel approach in which rice plants were modified to increase the production of compounds that stimulated biofilm formation in soil diazotrophic bacteria, promoted bacterial colonization of plant tissues and improved BNF with increased grain yield at limiting soil nitrogen contents. We first used a chemical screening to identify plant-produced compounds that induced biofilm formation in nitrogen-fixing bacteria and demonstrated that apigenin and other flavones induced BNF. We then used CRISPR-based gene editing targeting apigenin breakdown in rice, increasing plant apigenin contents and apigenin root exudation. When grown at limiting soil nitrogen conditions, modified rice plants displayed increased grain yield. Biofilm production also modified the root microbiome structure, favouring the enrichment of diazotrophic bacteria recruitment. Our results support the manipulation of the flavone biosynthetic pathway as a feasible strategy for the induction of biological nitrogen fixation in cereals and a reduction in the use of inorganic nitrogen fertilizers.

Free‐living N2 fixation (FNF) is an important source of bioavailable nitrogen for forests. Though increasing plant species diversity (PSD) benefits soil nitrogen accumulation, how it impacts FNF rate has not been explored.Forty‐five plots covering a PSD gradient were selected in a subtropical karst forest, southwest China. FNF rates in leaf litter and soil were measured using acetylene reduction assay calibrated with a 15N2 fixation method. Diazotrophic community, litter and soil properties were determined as well.The nifH gene abundance was not significantly altered in litter but was significantly decreased in soil by higher PSD. Diazotrophic community's Shannon diversity was significantly increased by higher PSD in both litter and soil. Increasing PSD inhibited litter FNF rate via aggravating phosphorus limitation and decreasing the relative abundance of Betaproteobacteria and Deltaproteobacteria. For the mineral soil horizon, increasing PSD suppressed FNF rate via decreasing nifH gene abundance caused by elevated N availability and lowered phosphorus and iron availability or through decreasing the relative abundance of Deltaproteobacteria but increasing that of Betaproteobacteria.Our findings, for the first time, reveal the microbial and abiotic controls on FNF in litter and soil in response to PSD, and hence benefit the prediction of nitrogen input via biological N2 fixation under changes in PSD. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

ABSTRACT The objective of this work was to assess the effects of A. brasilense inoculation on seeds on the yield performance of maize and black oat as a function of nitrogen rate over a 3-year succession. The experiment was conducted from 2015 to 2018 under field conditions. Maize was grown for three growing seasons in rotation with two seasons of black oat at the same experimental site. A randomized block design with a split-plot arrangement was used. Main plots consisted of A. brasilense inoculation (inoculated and uninoculated plants) and subplots of four nitrogen rates (0, 40, 80, and 120 kg ha−1). Maize plants were evaluated for biometric parameters, yield, and yield components at flowering. Black oat plants were analyzed for dry matter yield at flowering. The inoculation of A. brasilense in maize and black oat seeds in succession does not promote productivity nor does it favor the nitrogen fertilization of these cultures.

The objective of this work was to assess the effects of A. brasilense inoculation on seeds on the yield performance of maize and black oat as a function of nitrogen rate over a 3-year succession. The experiment was conducted from 2015 to 2018 under field conditions. Maize was grown for three growing seasons in rotation with two seasons of black oat at the same experimental site. A randomized block design with a split-plot arrangement was used. Main plots consisted of A. brasilense inoculation (inoculated and uninoculated plants) and subplots of four nitrogen rates (0, 40, 80, and 120 kg ha-1). Maize plants were evaluated for biometric parameters, yield, and yield components at flowering. Black oat plants were analyzed for dry matter yield at flowering. The inoculation of A. brasilense in maize and black oat seeds in succession does not promote productivity nor does it favor the nitrogen fertilization of these cultures. [ABSTRACT FROM AUTHOR]

The great challenge of reducing soil nutrient depletion and assuring agricultural system productivity in low-income countries caused by limited synthetic fertilizer use necessitates local and cost-effective nutrient sources. We estimated the changes of the nitrogen budget of agricultural systems in the East African Community from 1961 to 2018 to address the challenges of insufficient nitrogen inputs and serious soil nitrogen depletion in agricultural systems of the East African Community region. Results showed that total nitrogen input increased from 12.5 kg N ha-1yr-1 in the 1960s to 21.8 kg N ha-1yr-1 in the 2000s and 27 kg N ha-1yr-1 in the 2010s. Total nitrogen crop uptake increased from 12.8 kg N ha-1yr-1 in the 1960s to 18.2 kg N ha-1yr-1 in the 2000s and 21.8 kg N ha-1yr-1 in the 2010s. Soil nitrogen stock increased from -2.0 kg N ha-1yr-1 in the 1960s to -0.5 kg N ha-1yr-1 in the 2000s and 0.3 kg N ha-1yr-1 in the 2010s. Our results allow us to substantiate for the first time that soil nitrogen depletion decreases with increasing input of nitrogen in agricultural systems of the East African Community region. This suggests that increases in nitrogen inputs through biological nitrogen fixation and animal manure are the critical nitrogen management practices to curb soil nitrogen depletion and sustain agricultural production systems in the East African Community region in order to meet food demand for a growing population. [ABSTRACT FROM AUTHOR]

This study aims to explore the possibility of a reduced application of inorganic nitrogen (N) fertiliser on the yield, yield qualities, and biological nitrogen fixation (BNF) of the hydroponic common bean (Phaseolus vulgaris L.), without compromising plant performance, by utilizing the inherent ability of this plant to symbiotically fix N2. Until the flowering stage, plants were supplied with a nutrient solution containing N-concentrations of either a, 100%, conventional standard-practice, 13.8 mM; b, 75% of the standard, 10.35 mM; or c, 50% of the standard, 6.9 mM. During the subsequent reproductive stage, inorganic-N treatments b and c were decreased to 25% of the standard, and the standard (100% level) N-application was not altered. The three different inorganic-N supply treatments were combined with two different rhizobia strains, and a control (no-inoculation) treatment, in a two-factorial experiment. The rhizobia strains applied were either the indigenous strain Rhizobium sophoriradicis PVTN21 or the commercially supplied Rhizobium tropici CIAT 899. Results showed that the 50–25% mineral-N application regime led to significant increases in nodulation, BNF, and fresh-pod yield, compared to the other treatment, with a reduced inorganic-N supply. On the other hand, the 75–25% mineral-N regime applied during the vegetative stage restricted nodulation and BNF, thus incurring significant yield losses. Both rhizobia strains stimulated nodulation and BNF. However, the BNF capacity they facilitated was suppressed as the inorganic-N input increased. In addition, strain PVTN21 was superior to CIAT 899—as 50–25% N-treated plants inoculated with the former showed a yield loss of 11%, compared to the 100%-N-treated plants. In conclusion, N-use efficiency optimises BNF, reduces mineral-N-input dependency, and therefore may reduce any consequential negative environmental consequences of mineral-N over-application. [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]

Despite its pivotal feedback to carbon cycling, representing the dynamic response of vegetation to nitrogen limitation is a key challenge for simulating the terrestrial carbon sink with land models. Here, we explore a representation of this dynamic response of vegetation to nitrogen limitation with a novel representation of biological nitrogen fixation and nitrogen cycling in the Canadian Land Surface Scheme Including Biogeochemical Cycles. First, we assess how incorporating the dynamic response of vegetation to nitrogen limitation via biological nitrogen fixation influences the response to CO2 and nitrogen fertilization experiments, comparing simulations against observation‐based estimates from meta‐analyses. This evaluates whether the underlying mechanisms are realistically represented. Second, we assess how incorporating the dynamic response of vegetation to nitrogen limitation via biological nitrogen fixation affects simulated terrestrial carbon sequestration over the 20th and early 21st century, examining the effects of global change drivers (CO2, nitrogen deposition, climate, and land‐use change) acting both individually and concurrently. Including nitrogen cycling reduces the terrestrial carbon sink driven by elevated CO2 over the historical period. Representing the dynamic response of vegetation to nitrogen limitation via biological nitrogen fixation increases the estimate of the present‐day terrestrial carbon sink by 0.2 Pg C yr−1 (because elevated CO2 intensifies nitrogen limitation, which drives the upregulation of biological nitrogen fixation, alleviating nitrogen limitation). Our results highlight the importance of the dynamic response of vegetation to nitrogen limitation for realistically projecting the future terrestrial carbon sink under global change with land models. Key Points: The dynamic response of vegetation to nitrogen limitation is critical for simulating the terrestrial carbon sink with land modelsIt determines the response of terrestrial carbon and nitrogen cycling to CO2, nitrogen deposition, and other global change driversElevated CO2 intensifies nitrogen limitation, which drives the upregulation of biological nitrogen fixation, alleviating nitrogen limitation [ABSTRACT FROM AUTHOR]

The objective of this study was to evaluate the agronomic efficiency of rhizobia strains already approved or in the selection stage for cowpea, in the Recôncavo da Bahia region, Brazil. Two field experiments were performed in the municipalities of Cruz das Almas and Maragogipe, Bahia, Brazil. A randomized block design was used with seven nitrogen sources and four replicates. Nitrogen sources consisted of the strains UFRB FA51B1, UFRB BA72C2-1, UFLA 03-164T, UFLA 03-84, INPA 03-11B, and two controls without inoculation, one with mineral nitrogen and another without. All the strains nodulated cowpea. The efficiency of the strains was determined by the number of nodules, nodule dry matter, total dry matter, grain yield, nitrogen accumulation in shoots and grain, and relative efficiency. The strain UFLA 03-164T can be recommended for biomass production, green manure, and promotion of grain yields in both soils. In Maragogipe, the UFLA 03-84 and INPA 03-11B strains can be recommended for biomass production, green manure, and promotion of grain yields. The UFLA 03-164T strain showed great potential to promote growth and grain yield in the two municipalities studied. The strain INPA 03-11B can be recommended for Maragogipe soil. UFLA 03-84 can also be recommended to increase grain yield in Cruz das Almas. The return on investment based on the value-cost ratio were positive for UFLA 03-164T and UFLA 03-84 strains, showing that these strains are economically feasible and can replace the fertilization with mineral nitrogen in the cultivation of cowpea. [ABSTRACT FROM AUTHOR]

This study aims to explore the possibility of a reduced application of inorganic nitrogen (N) fertiliser on the yield, yield qualities, and biological nitrogen fixation (BNF) of the hydroponic common bean (Phaseolus vulgaris L.), without compromising plant performance, by utilizing the inherent ability of this plant to symbiotically fix N2. Until the flowering stage, plants were supplied with a nutrient solution containing N-concentrations of either a, 100%, conventional standard-practice, 13.8 mM; b, 75% of the standard, 10.35 mM; or c, 50% of the standard, 6.9 mM. During the subsequent reproductive stage, inorganic-N treatments b and c were decreased to 25% of the standard, and the standard (100% level) N-application was not altered. The three different inorganic-N supply treatments were combined with two different rhizobia strains, and a control (no-inoculation) treatment, in a two-factorial experiment. The rhizobia strains applied were either the indigenous strain Rhizobium sophoriradicis PVTN21 or the commercially supplied Rhizobium tropici CIAT 899. Results showed that the 50–25% mineral-N application regime led to significant increases in nodulation, BNF, and fresh-pod yield, compared to the other treatment, with a reduced inorganic-N supply. On the other hand, the 75–25% mineral-N regime applied during the vegetative stage restricted nodulation and BNF, thus incurring significant yield losses. Both rhizobia strains stimulated nodulation and BNF. However, the BNF capacity they facilitated was suppressed as the inorganic-N input increased. In addition, strain PVTN21 was superior to CIAT 899—as 50–25% N-treated plants inoculated with the former showed a yield loss of 11%, compared to the 100%-N-treated plants. In conclusion, N-use efficiency optimises BNF, reduces mineral-N-input dependency, and therefore may reduce any consequential negative environmental consequences of mineral-N over-application.

Biological nitrogen fixation (BNF) is what makes soybean economically viable. However, the launch of cultivars, with a high-yield potential and abiotic factors that reduce BNF, raised doubts about the need for nitrogen (N) fertilizers for soybeans. The objective of the research was to evaluate the effects of nitrogen supply, in two application times in furrow starter fertilization and top-dressing. The experimental design was in the 3 × 6 factorial scheme. The treatments consisted of three rates of N at in furrow starter fertilization (0, 10, and 20 kg ha−1 of N), and six rates of N in top-dressing, applied at the soybean R1 stage (0, 30, 60, 90, 120, and 150 kg ha−1 of N). The highest rate in furrow starter fertilization of 20 kg ha−1 of N resulted in the increase in the leaf area index (LAI) and specific leaf area (SLA), which are attributes that maximize the absorption of light energy, thus being able to positively influence yield. However, the BNF demonstrated that the strains of the selected Bradyrhizobium are efficient and capable of supplying the required N because the supplementation of nitrogen in furrow and/or top-dressing did not result in significant differences in the yield in relation to inoculated soybean. [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]

The bacteria capable of fixing atmospheric nitrogen were isolated from cassava cultivated soils of Vietnam. The potential isolates were identified by analyzing the 16S rRNA gene and by morphological, biochemical, cultural characteristics. The selected isolates were assigned to the species Bacillus sp. DQT2 M17, Bacillus subtilis DTAN6 M17, and Bacillus megaterium DSHB I8. The effect of culture conditions on the nitrogen-fixing activity of three selected isolates were studied and the obtained results showed that the highest amount of accumulated ammonia was detected after 6 days of incubation at 35 °C, pH 7.0 with sucrose as a carbon source. The selected strains could be exploited as inoculants for microbial fertilizer production. [ABSTRACT FROM AUTHOR]

Biological nitrogen fixation (BNF) refers to a microbial mediated process based upon an enzymatic “Nitrogenase” conversion of atmospheric nitrogen (N2) into ammonium readily absorbable by roots. N2-fixing microorganisms collectively termed as “diazotrophs” are able to fix biologically N2 in association with plant roots. Specifically, the symbiotic rhizobacteria induce structural and physiological modifications of bacterial cells and plant roots into specialized structures called nodules. Other N2-fixing bacteria are free-living fixers that are highly diverse and globally widespread in cropland. They represent key natural source of nitrogen (N) in natural and agricultural ecosystems lacking symbiotic N fixation (SNF). In this review, the importance of Azotobacter species was highlighted as both important free-living N2-fixing bacteria and potential bacterial biofertilizer with proven efficacy for plant nutrition and biological soil fertility. In addition, we described Azotobacter beneficial plant promoting traits (e.g., nutrient use efficiency, protection against phytopathogens, phytohormone biosynthesis, etc.). We shed light also on the agronomic features of Azotobacter that are likely an effective component of integrated plant nutrition strategy, which contributes positively to sustainable agricultural production. We pointed out Azotobacter based-biofertilizers, which possess unique characteristics such as cyst formation conferring resistance to environmental stresses. Such beneficial traits can be explored profoundly for the utmost aim to research and develop specific formulations based on inoculant Azotobacter cysts. Furthermore, Azotobacter species still need to be wisely exploited in order to address specific agricultural challenges (e.g., nutrient deficiencies, biotic and abiotic constraints) taking into consideration several variables including their biological functions, synergies and multi-trophic interactions, and biogeography and abundance distribution.

Biological nitrogen fixation (BNF) refers to a microbial mediated process based upon an enzymatic "Nitrogenase" conversion of atmospheric nitrogen (N2) into ammonium readily absorbable by roots. N2-fixing microorganisms collectively termed as "diazotrophs" are able to fix biologically N2 in association with plant roots. Specifically, the symbiotic rhizobacteria induce structural and physiological modifications of bacterial cells and plant roots into specialized structures called nodules. Other N2-fixing bacteria are free-living fixers that are highly diverse and globally widespread in cropland. They represent key natural source of nitrogen (N) in natural and agricultural ecosystems lacking symbiotic N fixation (SNF). In this review, the importance of Azotobacter species was highlighted as both important free-living N2-fixing bacteria and potential bacterial biofertilizer with proven efficacy for plant nutrition and biological soil fertility. In addition, we described Azotobacter beneficial plant promoting traits (e.g., nutrient use efficiency, protection against phytopathogens, phytohormone biosynthesis, etc.). We shed light also on the agronomic features of Azotobacter that are likely an effective component of integrated plant nutrition strategy, which contributes positively to sustainable agricultural production. We pointed out Azotobacter based-biofertilizers, which possess unique characteristics such as cyst formation conferring resistance to environmental stresses. Such beneficial traits can be explored profoundly for the utmost aim to research and develop specific formulations based on inoculant Azotobacter cysts. Furthermore, Azotobacter species still need to be wisely exploited in order to address specific agricultural challenges (e.g., nutrient deficiencies, biotic and abiotic constraints) taking into consideration several variables including their biological functions, synergies and multi-trophic interactions, and biogeography and abundance distribution. [ABSTRACT FROM AUTHOR]

Maize has a high nitrogen (N) demand; as a result, more sustainable alternatives are needed to reduce demand for mineral fertilizers. The hypothesis is that the agronomic performance of maize genotypes diverges in their potential when subjected to different supplies of N. This study aimed to evaluate and characterize the agronomic performance of maize genotypes submitted to topdressing nitrogen fertilization and inoculation with Azospirillum brasilense and select the best genotypes within each management. An experiment was conducted in the second season of 2017, in Jaboticabal-SP (Brazil), using 48 maize genotypes in a randomized block design. Treatments consisted of: 1) application of 140 kg ha-1 nitrogen, using urea as mineral fertilizer; 2) A. brasilense inoculation via soil, at a rate of 600 mL ha-1, as biological fertilization. Grain yield and agronomic traits were evaluated. The data were subjected to analysis of variance (F-Test), means were compared by Scott-Knott test at 5% probability, and multivariate statistical analysis was performed by principal component analysis. The maize genotypes showed contrasting agronomic performance in relation to the nitrogen supplies evaluated. Inoculation with A. brasilense via soil has a significant effect on increasing maize grain yield, making it a viable and more sustainable alternative in the supply of N. The choice of the genotype is an essential factor for the successful use of mineral nitrogen fertilization or A. brasiliense inoculation. [ABSTRACT FROM AUTHOR]

Background and Aims The tidal flats on which mangrove plants grow tend to have low soil nitrogen contents because nitrogen-containing litter is repeatedly washed offshore by ebb tides. Under such circumstances, it is unclear how mangrove plants acquire the nitrogen required to support their vigorous growth. In the present work, chemical and biological characteristics of diazotrophy around mangrove plant roots were surveyed under natural conditions to elucidate mangrove–diazotroph relationships. Methods Soil nitrogenase activity of a representative mangrove plant, Rhizophora stylosa , which has a broad geographical distribution, was measured using the acetylene reduction assay at forest, tree and prop root scales. In addition, diazotrophic community composition was compared between rhizosphere and bulk soil based on sequencing of nifH genes. Key Results Soil nitrogenase activity was high near prop roots, and this pattern was enhanced as soil live root content increased. At the forest scale, we observed high soil nitrogenase activity (acetylene-reducing activity) inside the forest (the highest value was 90.9 µmol C2H2 min−1 cm−3, average 46.8 ± 18.2 µmol C2H2 min−1 cm−3). Rates decreased sharply from the forest to the tidal flat (range 1.2–22.2 µmol C2H2 min−1 cm−3, average 7.9 ± 4.5 µmol C2H2 min−1 cm−3). The nifH operational taxonomic unit composition differed significantly among forest and tree rhizospheres and the bulk soil (P < 0.0001). Conclusions Our results suggest that the accumulation of diazotrophs around R. stylosa mangrove trees enhances the supply of biologically fixed nitrogen to the mangrove roots. This supply is especially important when the soil naturally contains little nitrogen. This nitrogen acquisition system may be a key process that explains the high productivity of mangrove ecosystems. [ABSTRACT FROM AUTHOR]

Sugarcane is a crop of great economic, social, and environmental relevance in Brazil. The country is the largest sugar producer and the second largest bioethanol producer in the world. The goal of this study was to evaluate the efficiency of a sugarcane inoculant composed of five diazotrophic bacterial strains, as well as nitrogen fertilization of two sugarcane varieties. Two experiments were carried out on two varieties using an experimental design composed of complete randomized blocks in a factorial of two varieties and three treatments with four replicates. The treatments can be described as: inoculation with the consortium of five diazotrophic strains, or N fertilization with 120 kg ha−1, and one control treatment. The following parameters were then evaluated: stem yield, accumulation of total dry matter, nitrogen content, quality of the sugarcane juice, and 15N natural abundance on flag-leaves. Inoculation and N fertilization on the Sapucaia plantation promoted increases of stem yield equivalent to 22.3 and 26.5 Mg ha−1 in the RB867515 variety, in comparison to the control, respectively. Inoculation and N fertilizer used for the Coruripe plantation increased stem yield of 38.0 and 42.4 Mg ha−1, respectively, with the RB867515 variety, while RB72454 showed increases of 16.7 and 37.5 Mg ha−1, both compared to the control. Biological nitrogen fixation was not affected by the treatments, however, both treatments increased the total recoverable sugar yield. Benefits from inoculation appeared to promote plant growth due to the plant–bacteria interaction.

The nitrogen fertilisation management should focus on increasing fertilisation efficiency in favour of a more sustainable and productive agriculture. Thus, this study aimed to evaluate the effect of inoculation with Azospirillum brasilense, associated to N doses and sources, in Savannah conditions, on agronomic efficiency (AE), recovery of applied nitrogen (RAN), physiological efficiency (PE), agrophysiological efficiency (AGREC), nitrogen use efficiency (NUE), nitrogen harvest index (NHI) and wheat grain yield. The experiment was developed in a Rhodic Hapludox soil under no-tillage system. The experimental design was a randomised complete block design with four replicates arranged in a 2 × 5×2 factorial scheme: Two N sources [(urea and urea with inhibitor of the enzyme NBPT (N – (n – butyl thiophosphoric) Triamide))], five N doses applied in top-dressing (0, 50, 100, 150 and 200 kg ha−1), with and without inoculation with A. brasilense. The increase in N doses reduced the nitrogen fertilisation efficiency. No significant differences were observed among the N sources. Inoculation increased the AE of nitrogen sources and can increase RAN. The increase in N dose up to 140 kg ha−1 associated with inoculation increased wheat grain yield. [ABSTRACT FROM AUTHOR]

Biological nitrogen (N) fixation (BNF), an important source of N in terrestrial ecosystems, plays a critical role in terrestrial nutrient cycling and net primary productivity. Currently, large uncertainty exists regarding how nutrient availability regulates terrestrial BNF and the drivers responsible for this process. We conducted a global meta‐analysis of terrestrial BNF in response to N, phosphorus (P), and micronutrient (Micro) addition across different biomes (i.e, tropical/subtropical forest, savanna, temperate forest, grassland, boreal forest, and tundra) and explored whether the BNF responses were affected by fertilization regimes (nutrient‐addition rates, duration, and total load) and environmental factors (mean annual temperature [MAT], mean annual precipitation [MAP], and N deposition). The results showed that N addition inhibited terrestrial BNF (by 19.0% (95% confidence interval [CI]: 17.7%‒20.3%); hereafter), Micro addition stimulated terrestrial BNF (30.4% [25.7%‒35.3%]), and P addition had an inconsistent effect on terrestrial BNF, i.e., inhibiting free‐living N fixation (7.5% [4.4%‒10.6%]) and stimulating symbiotic N fixation (85.5% [25.8%‒158.7%]). Furthermore, the response ratios (i.e., effect sizes) of BNF to nutrient addition were smaller in low‐latitude (<30°) biomes (8.5%‒36.9%) than in mid‐/high‐latitude (≥30°) biomes (32.9%‒61.3%), and the sensitivity (defined as the absolute value of response ratios) of BNF to nutrients in mid‐/high‐latitude biomes decreased with decreasing latitude (p ≤ 0.009; linear/logarithmic regression models). Fertilization regimes did not affect this phenomenon (p > 0.05), but environmental factors did affect it (p < 0.001) because MAT, MAP, and N deposition accounted for 5%‒14%, 10%‒32%, and 7%‒18% of the variance in the BNF response ratios in cold (MAT < 15°C), low‐rainfall (MAP < 2,500 mm), and low‐N‐deposition (<7 kg ha−1 year−1) biomes, respectively. Overall, our meta‐analysis depicts a global pattern of nutrient impacts on terrestrial BNF and indicates that certain types of global change (i.e., warming, elevated precipitation and N deposition) may reduce the sensitivity of BNF in response to nutrient enrichment in mid‐/high‐latitude biomes. [ABSTRACT FROM AUTHOR]

Symbiotic biological nitrogen fixation (BNF) is a complex process that involves rhizobia, a diverse group of α and β-proteobacteria bacteria, and legume species. Benefits provided by BNF associated with legume trees in tropical environments include improvements to efficiency of nitrogen (N) use, increase of soil carbon sequestration, stabilization of soil organic matter, decrease of soil penetration resistance, and improvement of soil fertility. All these benefits make BNF a crucial ecosystem service to the sustainability of tropical agriculture. Due to the importance of this ecological process and the high diversity of rhizobia, these bacteria have been extensively characterized worldwide. Currently, over 400 species of rhizobia are known, distributed into seven families. In the humid tropics, Leucaena leucocephala, Acacia mangium, Gliricidia sepium, and Clitoria fairchildiana are four of the most common species used by family farmers to create sustainable agricultural systems. These four legumes perform symbiosis with different groups of rhizobia. Exploring BNF could help to enable sustainable intensification of agriculture in the humid tropics, mainly because it can increase N use efficiency in an environment where N is a limiting factor to plant growth.

The current study aimed to verify the effect of the inoculant constituted by diazotrophic bacteria applied in different ways, associated to different nitrogen (N)-fertilizer doses, on the yield and N utilization efficiency of sugarcane ratoon. The use of inoculant did not affect the N content in leaves, the chlorophyll content, tiller number, stalk diameter, or any of the other assessed technological attributes. However, the use of inoculant associated to doses of 60 and 90 kg N ha-1 promoted increase in stalk yield, dry matter, and N accumulation and efficiency of N fertilizer, and, on the other hand, the association of inoculant with 120 kg N ha-1 caused reduction of these parameters. [ABSTRACT FROM AUTHOR]

Sugarcane is a crop of great economic, social, and environmental relevance in Brazil. The country is the largest sugar producer and the second largest bioethanol producer in the world. The goal of this study was to evaluate the efficiency of a sugarcane inoculant composed of five diazotrophic bacterial strains, as well as nitrogen fertilization of two sugarcane varieties. Two experiments were carried out on two varieties using an experimental design composed of complete randomized blocks in a factorial of two varieties and three treatments with four replicates. The treatments can be described as: inoculation with the consortium of five diazotrophic strains, or N fertilization with 120 kg ha−1, and one control treatment. The following parameters were then evaluated: stem yield, accumulation of total dry matter, nitrogen content, quality of the sugarcane juice, and15N natural abundance on flag-leaves. Inoculation and N fertilization on the Sapucaia plantation promoted increases of stem yield equivalent to 22.3 and 26.5 Mg ha−1in the RB867515 variety, in comparison to the control, respectively. Inoculation and N fertilizer used for the Coruripe plantation increased stem yield of 38.0 and 42.4 Mg ha−1, respectively, with the RB867515 variety, while RB72454 showed increases of 16.7 and 37.5 Mg ha−1, both compared to the control. Biological nitrogen fixation was not affected by the treatments, however, both treatments increased the total recoverable sugar yield. Benefits from inoculation appeared to promote plant growth due to the plant–bacteria interaction. [ABSTRACT FROM PUBLISHER]

Atmospheric nitrogen (N) deposition is thought to accelerate ecological succession, causing a loss of diversity in species-rich dune grasslands and hampering restoration goals. We tested whether elevated atmospheric N deposition results in faster accumulation of soil C and soil N, using three high-resolution chronosequences of up to 162 years in coastal sand dunes with contrasting N deposition and soil base status (high N deposition calcareous and acidic dunes in Luchterduinen, the Netherlands (LD) and low N deposition calcareous dunes in Newborough, UK (NB)). We also used the process model CENTURY to evaluate the relative contribution of N deposition, climate, and soil pH. In contrast to our hypothesis we found that accumulation of soil C and N was greatest at the low N deposition site NB. Model simulations indicated a negative interaction between high N deposition and symbiotic N 2 fixation. From this we conclude that high N deposition suppresses and replaces N 2 fixation as a key N source. High N deposition led to lower soil C:N only in the early stages of succession (< 20 years). The data also revealed accelerated acidification at high N deposition, which is a major concern for restoration of dune grasslands. More data are needed from acidic dunes from low N deposition areas to assess pH effects on soil C and N pools. Therefore, while N accumulation in soils may not be an issue, both acidification and plant community change due to elevated availability of mineral N remain major conservation problems. Restoration in degraded dune grasslands should focus on maintaining habitat suitability, rather than N removal from soil pools. [ABSTRACT FROM AUTHOR]

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The objective of this study was to evaluate the agronomic characteristics of corn seed inoculated with Azospirillum brasilense, grown on black oat and ryegrass straw, and managed under different grazing strategies and doses of nitrogen. The experiment was conducted in Santa Maria, Rio Grande do Sul State, Brazil, during two agricultural seasons (2012/2013 and 2013/2014) in a randomized, complete block design with three replications. In the winter period, black oat and ryegrass straw were managed at different grazing heights by sheep (0.30, 0.20, 0.10 m, conventional grazing, and no grazing) with three doses of nitrogen (0, 50, and 100 kg ha-1, with or without inoculation by A. brasilense). We used the hybrid Pioneer (P1630H®) in 2012 and the hybrid Agroeste (AS 1551®) in 2013. The height of corn plants was greater when they were grown on black oat and ryegrass straw, and the absence of grazing favored productivity. Under drought conditions, the application of nitrogen to the pasture favored corn development, increasing plant height, ear height, and stem diameter. Inoculation with A. brasilense had a positive effect on the characteristics of yield and productivity of corn, independent of growing season and hybrid used.