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Aims: The aim of this study was to determine the effectiveness of natural (NatZeo) and acid-treated (AcidZeo) zeolites in increasing the nitrogen (N) recovery of sugarcane grown under conditions highly conducive for N losses. Methods: This glasshouse trial replicated the pedoclimatic conditions typical of the Australian sugarcane industry in the Wet Tropics to evaluate the capacity and economic feasibility of using NatZeo and AcidZeo to reduce leaching and increase plant and soil N recovery in a highly permeable sandy-loam soil. Nitrogen fertiliser was applied at two rates (84 and 120 kg N ha−1) as 15N-enriched urea. Results: Compared with the unamended treatments, AcidZeo led to significantly higher average fertiliser N recoveries in both the soil (+24%) and plant (+54%), which resulted in significantly lower overall N losses (−22%). However, the significantly higher capacity of AcidZeo in retaining ions limited the availability of potassium and sulphur to the plants and led to a 4% yield reduction. The use of NatZeo led to lower abatements of N losses (−6%) but increased N recoveries in the plant (+22%). NatZeo did not result in as much cation immobilisation, which, combined with the increased plant/soil N recovery, resulted in the highest yield of the trial (+8% compared with the unamended treatments). The economic analysis indicated that only NatZeo could potentially increase farmer's income when applied in 20 cm wide strips around the fertiliser band. Conclusions: This study highlights that zeolites have potential to improve the environmental sustainability and profitability of sugarcane cropping systems. [ABSTRACT FROM AUTHOR]

Background and aims: Deep roots are needed to allow uptake of nitrogen (N) and water available in the deeper soil layers, to help tolerate increasingly extreme climates. Yet few studies in the field have been able to identify genetic differences in deep roots and how this relates to N and water uptake. This study aimed to identify the relationship between deep roots and tolerance to drought, and how this varies by genotype and with differing N fertilization. Methods: We grew 14 diverse genotypes of winter wheat in a semi-field facility in Denmark, in 2019 and 2020, with a soil depth gradient and a rain-out shelter to create a water stress. We used minirhizotron tubes reaching to 2.5 m depth to quantify differences in deep roots. We applied isotope tracers (15N and 2H labelled water) at 1.6-1.8 m at anthesis to assess differences in root function. Grain and straw 13C were used to assess drought stress. Results: We found differences in deep roots between genotypes, and slightly less deep root growth when more N was applied. Deep roots were correlated with grain yield, uptake of deep-placed tracers of water and N, and tolerance to drought. Genotypes with deeper roots had the biggest decrease in water stress and increase in grain yield, when their roots had access to deeper soil. Conclusion: Deeper roots were related to drought tolerance and increased yields. This suggests that deep rooting should be considered in future breeding efforts for more climate resilient crops. [ABSTRACT FROM AUTHOR]

Background and aims: Understanding the fate of nitrogen (N) in maize (Zea mays L.) intercropped with tropical grass is essential to develop sustainable fertilization recommendations and mitigate greenhouse gas emissions. To test the hypothesis that intercropping forage grass with maize improves yield and fertilizer N recovery in the system, this study evaluated the recovery of N applied to maize intercropped with forage grass grown as a double crop after soybean [Glycine max (L.) Merrill]. Methods: A two–year field experiment was conducted. In each season maize was intercropped with either Guinea grass (Megathyrsus maximus; syn. Panicum maximum) or ruzigrass (Urochloa ruziziensis; syn. Brachiaria ruziziensis) in southeastern Brazil in the presence or absence of N fertilization, followed by a soybean crop. 15N–enriched ammonium sulfate was used to trace the fate of the applied N. Results: Total recovery of fertilizer N by maize differed with season (≈ 35 and 22%) and was greater in the grain than vegetative fractions. Uptake of fertilizer N by intercropped forage grass was less than maize but substantial (≈ 11–18%) After maize harvest more than 60% of the N applied remained in the system, partly in forage grass and maize residues but mostly in soil. Up to 12% of the N applied was recovered by soybean grown in succession. Conclusion: The introduction of either Guinea grass or ruzigrass intercropped with maize can decrease N losses from fertilizer and increase recovery by soybean grown after maize, particularly when maize yield and N uptake are low. [ABSTRACT FROM AUTHOR]

Background and aims: Root distribution over the soil profile is important for crop resource uptake. Using machine learning (ML), this study investigated whether measured square root of planar root length density (Sqrt_pRLD) at different soil depths were related to uptake of isotope tracer (15N) and drought stress indicator (13C) in wheat, to reveal root function. Methods: In the RadiMax semi-field root-screening facility 95 winter wheat genotypes were phenotyped for root growth in 2018 and 120 genotypes in 2019. Using the minirhizotron technique, root images were acquired across a depth range from 80 to 250 cm in May, June, and July and RL was extracted using a convolutional neural network. We developed ML models to explore whether the Sqrt_pRLD estimates at different soil depths were predictive of the uptake of deep soil nitrogen - using deep placement of 15N tracer as well as natural abundance of 13C isotope. We analyzed the correlations to tracer levels to both a parametrized root depth estimation and an ML approach. We further analyzed the genotypic effects on root function using mediation analysis. Results: Both parametrized and ML models demonstrated clear correlations between Sqrt_pRLD distribution and resource uptake. Further, both models demonstrated that deep roots at approx. 150 to 170 cm depth were most important for explaining the plant content of 15N and 13C isotopes. The correlations were higher in 2018. Conclusions: The results demonstrated that, parametrized models and ML-based analysis provided complementary insight into the importance of deep rooting for water and nitrogen uptake. [ABSTRACT FROM AUTHOR]

Background and aims: Increased atmospheric nitrogen (N) deposition and climate warming can both exert strong, cumulative effects on N losses from ecosystems. However, given their potential interactions with interannual variability in environmental conditions or other factors, it is uncertain how effectively the comparison of short- vs. long-term results from N addition and warming field experiments can reveal these cumulative effects. Our aim was to compare the short-term versus long-term effects of N addition and warming on aboveground and belowground N retention while controlling for interannual variation in other factors.We added 15N tracer to N addition and warming plots in a northern temperate grass-dominated old field in early spring, then assessed the amount of the added 15N recovered aboveground and belowground at the peak of the growing season. We controlled for interannual variation in background environmental conditions by comparing new N addition and warming plots (3 years old) to those of an existing field experiment (16 years old).Both N addition and warming significantly increased aboveground and belowground 15N tracer retention. However, there were no significant interactions between either of the treatments and plot age for 15N tracer retention, nor for plant production. The lack of interactions with plot age corresponded with the continued dominance of two non-native grass species in the plots.Our results highlight the potentially important role of dominant species that resist replacement in buffering against the effects of global change on cumulative, long-term changes in ecosystem N retention.Methods: Increased atmospheric nitrogen (N) deposition and climate warming can both exert strong, cumulative effects on N losses from ecosystems. However, given their potential interactions with interannual variability in environmental conditions or other factors, it is uncertain how effectively the comparison of short- vs. long-term results from N addition and warming field experiments can reveal these cumulative effects. Our aim was to compare the short-term versus long-term effects of N addition and warming on aboveground and belowground N retention while controlling for interannual variation in other factors.We added 15N tracer to N addition and warming plots in a northern temperate grass-dominated old field in early spring, then assessed the amount of the added 15N recovered aboveground and belowground at the peak of the growing season. We controlled for interannual variation in background environmental conditions by comparing new N addition and warming plots (3 years old) to those of an existing field experiment (16 years old).Both N addition and warming significantly increased aboveground and belowground 15N tracer retention. However, there were no significant interactions between either of the treatments and plot age for 15N tracer retention, nor for plant production. The lack of interactions with plot age corresponded with the continued dominance of two non-native grass species in the plots.Our results highlight the potentially important role of dominant species that resist replacement in buffering against the effects of global change on cumulative, long-term changes in ecosystem N retention.Results: Increased atmospheric nitrogen (N) deposition and climate warming can both exert strong, cumulative effects on N losses from ecosystems. However, given their potential interactions with interannual variability in environmental conditions or other factors, it is uncertain how effectively the comparison of short- vs. long-term results from N addition and warming field experiments can reveal these cumulative effects. Our aim was to compare the short-term versus long-term effects of N addition and warming on aboveground and belowground N retention while controlling for interannual variation in other factors.We added 15N tracer to N addition and warming plots in a northern temperate grass-dominated old field in early spring, then assessed the amount of the added 15N recovered aboveground and belowground at the peak of the growing season. We controlled for interannual variation in background environmental conditions by comparing new N addition and warming plots (3 years old) to those of an existing field experiment (16 years old).Both N addition and warming significantly increased aboveground and belowground 15N tracer retention. However, there were no significant interactions between either of the treatments and plot age for 15N tracer retention, nor for plant production. The lack of interactions with plot age corresponded with the continued dominance of two non-native grass species in the plots.Our results highlight the potentially important role of dominant species that resist replacement in buffering against the effects of global change on cumulative, long-term changes in ecosystem N retention.Conclusion: Increased atmospheric nitrogen (N) deposition and climate warming can both exert strong, cumulative effects on N losses from ecosystems. However, given their potential interactions with interannual variability in environmental conditions or other factors, it is uncertain how effectively the comparison of short- vs. long-term results from N addition and warming field experiments can reveal these cumulative effects. Our aim was to compare the short-term versus long-term effects of N addition and warming on aboveground and belowground N retention while controlling for interannual variation in other factors.We added 15N tracer to N addition and warming plots in a northern temperate grass-dominated old field in early spring, then assessed the amount of the added 15N recovered aboveground and belowground at the peak of the growing season. We controlled for interannual variation in background environmental conditions by comparing new N addition and warming plots (3 years old) to those of an existing field experiment (16 years old).Both N addition and warming significantly increased aboveground and belowground 15N tracer retention. However, there were no significant interactions between either of the treatments and plot age for 15N tracer retention, nor for plant production. The lack of interactions with plot age corresponded with the continued dominance of two non-native grass species in the plots.Our results highlight the potentially important role of dominant species that resist replacement in buffering against the effects of global change on cumulative, long-term changes in ecosystem N retention. [ABSTRACT FROM AUTHOR]

Aims: We investigated the role of plants and their plant-derived carbon in shaping the microbial community that decomposes substrates and traced the return of nutrients from decomposition back to plant shoots in order to understand the importance of plants for ecosystem element cycling. Methods: We performed a greenhouse experiment having plant communities with and without arbuscular mycorrhizal fungi (AMF) and ingrowth cores that held different 15N labeled substrates. We determined the microbial community structure using molecular sequencing and the net assimilation of plant carbon into soil microorganisms using a 13CO2 pulse and 13C measurements of microbial biomarkers. We determined the return of nitrogen back to the shoots using the 15N signal, which was provided from the decomposition of the substrate added to the ingrowth cores. Results: We observed that the microbial community composition in the ingrowth cores and their net 13C assimilation depended on the presence of AMF and the added substrate. Both plant communities had similar 15N uptake into their shoots, but the net N uptake cost was significantly lower in presence of AMF. In the presence of AMF also lower net N uptake cost was observed for the decomposition of plant-derived and microorganism-derived substrates compared to inorganic nitrogen suggesting that AMF actively controls the decomposer comunity and their carbon demand. Conclusion: Our results identify for the first time a functional overlap of soil microorganisms as identical substrate is decomposed by different microorganisms suggesting functional redundancy of microbial communities. In consequence a better understanding of ecosystem element cycling can only be achieved when the whole plant-microorganism-organic matter-soil continuum is investigated. [ABSTRACT FROM AUTHOR]

Aims: Half of field crop nitrogen (N) is often derived soil organic matter (SOM) mineralization, yet we do not fully understand the extent to which plant genotypic differences influence SOM mineralization dynamics across different soil N contexts. We explored the effects of rapeseed (Brassica napus) genotypic diversity on N uptake from organic and inorganic N sources. Methods: In a greenhouse study, we applied dual 15N labeled ammonium-nitrate fertilizer to examine N uptake patterns of rapeseed in different N environments. Ten varieties were grown in a full factorial experiment with four treatments, including combinations of high and low N fertilizer and SOM. Results: We found limited varietal differences in total biomass or N uptake across soil environments. Across all varieties, SOM was an important, additive N source even as N fertilizer availability increased. High SOM/High Fertilizer treatment plants obtained 64% of N from SOM, while plants grown with High SOM/Low Fertilizer obtained 89% of total N from SOM. Under low SOM availability, the high fertilizer addition increased overall N uptake from SOM by 42% relative to the low N fertilizer treatment. Conclusions: Integrating plant reliance on SOM-N sources into crop breeding and NUE estimates has potential to improve crop productivity and improve overall system N use efficiency. [ABSTRACT FROM AUTHOR]

Aims: To assess how biochar addition in rainfed conservation agriculture affects short-term transformation, plant uptake, retention of nitrogen (N) in soil, and nitrous oxide (N2O) fluxes in a tropical Arenosol planted to maize. Methods: A ten-day in situ15N pool dilution and N cycling experiment, using tracer amounts (0.1 g m-2) of 15N labeled ammonium (15NH4+), nitrate (15NO3-) or 15N-urea, was carried out seven weeks after planting of maize (Zea mays L.) under conservation agriculture in Zambia, using planting basins without (CA) and with pigeon-pea biochar (BC) addition (4 t ha-1). Results: Pigeon-pea biochar increased soil NO3- concentration, gross nitrification rate, 15N recovery in extractable soil NO3-, and soil moisture. However, effects of biochar on soil N retention and plant N uptake were not significant. Likewise, biochar did not affect N2O fluxes. Conclusions: At low dosage, pigeon pea biochar has a positive effect on gross nitrification rate but does not affect short-term N retention in soil, N2O fluxes, nor does it help increasing the uptake of N by maize. [ABSTRACT FROM AUTHOR]

The enigma of soil nitrogen balance sheets originally described by Allison in 1955 has endured for 65 years. Originally the accounting showed a positive balance between N gains and losses, which indicated a failure to measure all losses, widely suspected of being gaseous N emissions. Measurement of organic, inorganic and gaseous N forms derived from 15N-enriched nitrate added to soils under both anaerobic and aerobic conditions in small- to medium-scale short-term experiments in closed vessels without plants, demonstrated that it is possible to achieve a complete 15N balance. A recent partial N balance at the global scale of the three main cereal crops over a 50-year time span showed a negative balance between N gains and losses, indicating a failure to measure all N gains or to accurately measure net changes in total soil N to a sufficient soil depth. Therefore, if the enigma is to be resolved in a realistic agronomic setting, fundamental decisions need to be made regarding the approach to be adopted, the cereal crop to be studied, the nature of the measurements required and their temporal and spatial scales. [ABSTRACT FROM AUTHOR]

Increased grain yields are frequently obtained when a cereal follows a grain legume in sequence compared with a cereal-cereal rotation. The biotic (disease break) and abiotic (N supply) components of the observed benefits are identified and methods for differentiating the components are discussed. Annual measurements of the N balance of grain legume-cereal rotations are extremely variable when measured over short time periods and are therefore not useful as indicators of cropping system N sustainability. While measurement of long-term changes in total soil N is a valid index of N sustainability, this approach is impractical. We suggest an alternative avenue for the assessment of N sustainability by using simulation modelling, after validation, which takes climatic and biophysical parameters into account. [ABSTRACT FROM AUTHOR]

Aims: Cover crops (CC) have been widely used to improve soil quality and sustain agricultural productivity. However, the turnover of carbon (C) and nitrogen (N) from different species and parts of CCs incorporated into the soil under Northern European conditions remains unclear. Methods: In this study, we examined mineralization of C and N from isolated shoots and roots of two leguminous and two non-leguminous CCs labeled with 15N tracers in a 100-day incubation experiment at 10 °C. Results: After 100 days incubation, the average net N mineralization was 33% from the leguminous CCs and 20% from the non-legumes. Net N mineralization from shoots was 1–15% (% of the N input) higher than that from the corresponding roots, and the net C mineralization accounted for 53–62% and 18–39% of the total C applied with CC shoots and roots, respectively. The C/N ratio of plant residues was a good predictor for net N mineralization, while the lignin concentration was a good predictor for C mineralization from both roots and shoots. Roots of both legume and non-legume CCs caused net immobilization of (unlabeled) soil N, whereas no significant soil N immobilization occurred from shoots with low C/N ratios. Conclusions: Our results indicate that different CC characteristics (C/N ratio and lignin) control the turnover of residue N and C, respectively, in the soil. Our results further showed very limited interactions with soil N turnover (no priming effects) when materials with low C/N ratios were applied to soil. [ABSTRACT FROM AUTHOR]

Aims: More sustainable agricultural systems, which contribute to C sequestration and biological N fixation, require accurate quantification of plant C and N inputs into soils. This has to be conducted under field conditions, as there are serious shortcomings to pot-based experiments, which have dominated studies on rhizodeposition estimation in the past. Therefore, amounts of below-ground plant C, N, and especially the rhizodeposition of peas were quantified in the field more accurately, with particular emphasis on their transfer into different soil compartments during vegetation. Methods: Pea (Pisum sativum L.) plants were labelled with multiple pulses of 13C-glucose and 15N urea using the cotton wick method. Plants were harvested at four dates depending on plant development. Representative soil samples were taken at harvest with a special drilling tool, considering the spatial distribution of pea roots. Results: Our results indicate that the quantity of C and N rhizodeposition did not change between flowering and maturity. About one third of the C and N rhizodeposits were present in microbial biomass and extractable C or the inorganic N pool of soil. When comparing this field study to a former pot experiment, a higher root-to-shoot ratio was detected; also the rhizodeposition-to-root ratio was altered in the field, questioning the assumption that results from pot experiments can be transferred to field conditions. Conclusions: Due to a higher below-ground C (BGC) and N (BGN) input compared to pot experiments, studies aiming at quantifying BGC and BGN input will have to be conducted under field conditions. [ABSTRACT FROM AUTHOR]

Aims: Niche complementarity theory explains how species coexist by using different resources. Two pathways to partition resource have been demonstrated: classical niche differentiation and plasticity in resource use. We aimed to determine N-uptake patterns in three Tibetan Plateau grassland species, and to examine how N-partitioning is driven by neighbor interactions. Methods: We conducted a transplantation experiment using ten plant communities, each comprising a different combination of Kobresia humilis, Stipa aliena, and Saussurea superba. Soil was sprayed uniformly with a mixture of (NH4)2SO4, KNO3, and glycine (C2H5NO2) (1:1:1 by mass of N, each containing one form of 15N) after growing for 45 days. Results: Across three species, the N-uptake pattern was NO3− > NH4+ > glycine (NO3−: 58.47%; NH4+: 26.91%; glycine: 14.62%). Neighbor presence had species-specific effects on 15N recovery. Kobresia humilis took up more 15N-NO3− when it was in competition with other species, whereas Stipa aliena and Saussurea superba took up more 15N-NH4+ and 15N-glycine, respectively. Conclusions: Plasticity in N resource utilization of the three species was limited. The species competed for N resources proportionally to the availability of these sources, and tended to partition niches. These findings provide important insights into how plant species grow together in alpine grasslands. [ABSTRACT FROM AUTHOR]

Background and aims: Elevated atmospheric CO2 (eCO2) can affect microbial decomposition of native soil organic carbon (SOC) via enhanced root exudation and rhizosphere activity. Few studies have examined the effect of eCO2 on the decomposition of newly-added crop residues, which are important to understand below-ground C changes. A soil microcosm experiment was conducted to examine whether eCO2 would enhance the rhizosphere effects on the decomposition of crop residues.Methods: White lupin (Lupinus albus L. cv. Kiev) was grown for 34 or 62 days under ambient CO2 (aCO2, 400 μmol mol−1) or eCO2 (800 μmol mol−1) in a low-C (2.0 mg g−1) soil which was amended with or without dual 13C and 15N labelled wheat, field pea or canola crop residues. An isotopic tracing technique was adopted to partition residue-derived CO2 from total below-ground CO2 efflux. Two independent groups of data were analysed statistically at either Day 34 or 62.Results: The presence of white lupin increased the decomposition of all residues at Day 34. This positive rhizosphere effect on residue decomposition decreased and was even reversed at Day 62, probably due to depletion of labile C, or microbial N limitation, or rhizosphere acidification. The eCO2-induced decomposition depended on residue type at Day 34. Specifically, when compared to aCO2, eCO2 did not affect the decomposition of canola residue, increased that of field pea residue by 13.5% but decreased wheat straw decomposition by 7.4%. However, residue decomposition was, on average, 13% higher under eCO2 at Day 62, which was correlated positively with the increase in rhizosphere extractable C (P < 0.01).Conclusions: Elevated CO2 generally increased residue decomposition in the rhizosphere, but this effect was mediated by residue type at Day 34. Enhanced residue decomposition under legumes at eCO2 may favour C turnover and the release of residue N. [ABSTRACT FROM AUTHOR]

Aims: Peatland porewaters frequently contain large concentrations of dissolved organic nitrogen (DON), but small concentrations of inorganic N (NH4+ and NO3−). Our aim was to test whether peatland plants, with low tissue N concentrations (0.7 to 1.7%), have the ability to take up organic forms of N, specifically glycine.Methods: We applied dual-labeled (13C and 15N) glycine to two peatlands: an ombrotrophic bog in eastern Canada and a New Zealand restiad bog undergoing restoration. We compared the δ13C and δ15N values of control and treated above and below-ground plant tissues to establish whether the glycine had been taken up.Results: At the Canadian bog, the moss, shrub and sedge species treated with glycine showed significant increases in δ15N, above and below-ground, and there was evidence of a significant change in δ13C in shrub roots. At the New Zealand peatland, above- and below-ground tissues of the two restiad species and the roots of the sedge species showed significantly increased δ15N, while only the sedge roots showed a significant increase in δ13C.Conclusions: All but one of the nine plant species examined showed significant uptake of 15N in foliage and all roots showed increases in 15N. Variations in uptake appear to be partially related to the plant type and root characteristics. Large variations in C, 13C, N and 15N concentration within treatments precluded strong evidence of whether uptake included intact glycine. [ABSTRACT FROM AUTHOR]

Background and aims: Soil nutrient dynamics are affected by root-microbe interactions and plant development. We investigated the influence of plant growth stage and arbuscular mycorrhiza fungi (AMF) on carbon (C) and nitrogen (N) rhizodeposition and the transfer into the microbial biomass (MB).Methods: Pea varieties (Pisum sativum L.) with (Frisson) and without mycorrhiza (P2) were 13C-15N-labelled and harvested at 45, 63, 71, and 95 days after sowing. Mycorrhization, MB, total C, N, 13C, 15N were determined in plant and soil compartments to calculate C and N derived from rhizodeposition (CdfR, NdfR).Results: Total CdfR increased until pea maturity, NdfR until end of flowering. Their relative contribution steadily decreased over time, accounting for 4-10% of total plant C and N at harvest. Rhizodeposition contributed between 1 and 6% to MB C and N, although 20% of the rhizodeposits were discovered in the MB. Frisson released more NdfR than P2 but it was not possible to accurately estimate AMF effects on C and N due to differences in biomass partitioning.Conclusions: CdfR followed an even flow from early growth until senescence. NdfR flow ceased after flowering possibly due to N relocation within the plant. Rhizodeposits contribute very little to MB in our study. [ABSTRACT FROM AUTHOR]

Background and aims: Legumes integrated in crop rotations are intended to improve crop nitrogen (N) supply and yield. In conservation agriculture (CA) systems under low input conditions on highly weathered tropical soils, experimental evidence for these benefits is lacking. To understand the mechanisms and evaluate the impact of the legume N on the subsequent crop, an in-depth study on N dynamics in the soil-plant system was conducted.Methods: In Madagascar, a CA based crop rotation with the perennial forage legume Stylosanthes guianensis (stylo) and upland rice (rice/stylo - stylo - rice/stylo) was established under three fertilization regimes. In addition, rice was grown in a non-CA bare fallow rotation without fertilizer. Over the three years N2 fixed in stylo shoots, the incorporation of stylo shoot (mulch) N into soil N pools and mulch N uptake by rice was quantified using 15N techniques and mulch and stylo root residue decomposition was investigated in a litterbag study.Results: N2 fixed in stylo shoots ranged from 96 to 122 kg N ha−1. Between 50 to 70% of stylo mulch and root residues decomposed during the third cropping season. Without fertilizer, grain yield of rice after the fallow with stylo was about 70% greater than after bare fallow, corresponding to 11 kg N ha−1 greater N uptake. Recoveries of stylo mulch N after rice harvest were on average 64% in soil, with about 3% in each of the microbial and mineral N pools, with 39% on the soil surface, and 6% in the rice crop. The N input via stylo seed, leaf litter and belowground N totalled about three times the amount of N contained in stylo mulch, which usually is considered as major rice N source.Conclusions: Legumes, like stylo, can improve crop N supply and yield in low input CA cropping systems on highly weathered tropical soils. To explain the impact and mechanisms involved requires a consideration of all legume-N components beyond the mulch N present at the onset of the rice-cropping season. [ABSTRACT FROM AUTHOR]

Background and aims: Natural abundance of the stable nitrogen (N) isotope 15N can elucidate shifts in plant N acquisition and ecosystem N cycling following disturbance events. This study examined the potential relationship between foliar δ 15N and depth of plant N acquisition (surface organic vs. mineral soil) and nitrification as conifer stands develop following stand-replacing wildfire. Methods: We measured foliar δ 15N along an 18-site chronosequence of jack pine ( Pinus banksiana) stands, 1 to 72 years in age post-wildfire. Foliar δ 15N was compared to total δ 15N of the organic (Oe + Oa) and mineral (0–15 cm) soil horizons, and organic horizon N mineralization and nitrification as functions of total mineralization. Results: Foliar δ 15N declined with stand age, yet wildfire effects were heterogeneous. Jack pine seedlings on burned, mineral soil patches in the youngest stand were significantly more enriched than those on unburned, organic patches ( P = 0.007). High foliar values in the youngest stands relative to mineral-horizon δ 15N indicate that nitrification also likely contributed to seedling enrichment. Conclusions: Our results suggest jack pine seedlings on burned patches obtain N from the mineral soil with potentially high nitrification rates, whereas seedlings on unburned patches and increasingly N-limited, mature jack pine acquire relatively more N from organic horizons. [ABSTRACT FROM AUTHOR]

Background and Aims: Quantitative information on the fate and efficiency of nitrogen (N) fertiliser applied to coarse textured highly calcareous soils in semi-arid farming systems is scarce but, as systems intensify, is essential to support sustainable agronomic management decisions Method: A glasshouse study was undertaken to trace the fate of N fertiliser applied to wheat ( Triticum aestivum L. cv Yitpi) grown on a reconstructed profile (0 to 600 mm) of a grey highly calcareous (>35% CaCO) sandy loam soil. Two watering treatments were applied (drier and wetter) equivalent to low (decile 2, 179 mm) and medium (decile 5, 234 mm) growing season rainfall for a location with typical semi-arid environment in southern Australia. 15 N-labelled urea fertiliser (35.4 mg N/pot) was applied in a split application - at sowing and 70 days after sowing, followed by immediate watering or watering after delay of 1 week. Results: Recovery of N fertiliser in grain (30 to 52%) was greater for the wetter treatment, and when water was applied immediately following fertiliser application. It was also similar for N applied at sowing and N applied during crop growth. Overall, more than 40% of the urea fertiliser N remained in the soil at anthesis, largely in the top 100 mm, indicating little movement of fertiliser N down the profile even with application of water. Losses of urea fertiliser N (13-24%) were considered relatively small given the highly calcareous nature of the soil; and were significantly greater from N applied during growth compared to at sowing, particularly where watering did not immediately follow application. There was no effect of fertiliser N on grain yield due to sufficient available N in soil at sowing (139mgN/pot), but N concentration and DM of stubbles was increased. Watering treatment did not affect shoot dry matter production up to anthesis, although root weight was higher in the wetter treatment, and grain yield was 9% greater. Conclusions: It is concluded that the potential for N losses from urea applied to highly calcareous coarse textured soils in semi-arid agricultural areas appears relatively low. Further, where there are relatively large amounts of plant available N present at sowing, a strategy of delayed or withheld applications of N to manage economic risk may have minimal effects on grain production in seasons with drier than average rainfall. [ABSTRACT FROM AUTHOR]

There is increasing interest in the ecology of tree-based intercropping systems, particularly alley cropping, in temperate settings. Shoot pruning of the tree component is an integral practice to alley cropping. Shoot pruning likely results in an input of belowground C and N to soil, with potential, but unknown impacts on the time pattern of N availability. We evaluated the impacts of pruning of Amorpha fruticosa L. on (a) its root dynamics; (b) mineral N and microbial N, and (c) the fate of root-derived N and its importance in determining N availability. For this, we conducted repeated partial pruning and foliar labeling with N under field conditions in Georgia, USA. Pruning altered root dynamics by initially decreasing biomass and later preventing increases, likely due to root death. Pruning modified the time pattern of mineral N by substantially increasing concentrations during the 3 months following pruning. N labeling indicated that root-derived N was an important source of this extra N and that pruning caused the availability of root-derived N to microbes to be extended until the following growing season. Greater concentrations of mineral N, a greater pool of microbial N and prolonged microbial access and storage of root-derived N with pruning has the potential to positively impact system productivity. [ABSTRACT FROM AUTHOR]

Grazing ruminants urinate and deposit N onto pastoral soils at rates up to 1,000 kg ha, with most of this deposited N present as urea. In urine patches, nitrous oxide (NO) emissions can increase markedly. Soil derived CO fluxes can also increase due to priming effects.While NO fluxes are affected by temperature, no studies have examined the interaction of pasture plants, urine and temperature on NO fluxes and the associated CO fluxes. We postulated the response of NO emissions to bovine urine application would be affected by plants and temperature. Dairy cattle urine was collected, labelled with N, and applied at 590 kg N ha to a sub-tropical soil,with and without pasture plants at 11°, 19°, and 23°C. Over the experimental period (28 days), 0.2% (11°C with plants) to 2.2% (23°C with plants) of the applied N was emitted as NO. At 11°C, plants had no effect on cumulative NO-N fluxes, whereas at 23°C, the presence of plants significantly increased the flux, suggesting plant-derived C supply affected the NO producing microbes. In contrast, a significant urine application effect on the cumulative CO flux was not affected by varying temperature from 11-23°C or by growing plants in the soil. This study has shown that plants and their responses to temperature affect NO emissions from ruminant urine deposition. The results have significant implications for forecasting and understanding the effect of elevated soil temperatures on NO emissions and CO fluxes from grazed pasture systems. [ABSTRACT FROM AUTHOR]

The rapid expansion of intensively farmed vegetable fields has substantially contributed to the total NO emissions from croplands in China. However, to date, the mechanisms underlying this phenomenon have not been completely understood. To quantify the contributions of autotrophic nitrification, heterotrophic nitrification, and denitrification to NO production from the intensive vegetable fields and to identify the affecting factors, a N tracing experiment was conducted using five soil samples collected from adjacent fields used for rice-wheat rotation system (WF), or for consecutive vegetable cultivation (VF) for 0.5 (VF1), 6 (VF2), 8 (VF3), and 10 (VF4) years. Soil was incubated under 50% water holding capacity (WHC) at 25°C for 96 h after being labeled with NHNO or NHNO. The average NO emission rate was 24.2 ng N kg h in WF soil, but it ranged from 69.6 to 507 ng N kg h in VF soils. Autotrophic nitrification, heterotrophic nitrification and denitrification accounted for 0.3-31.4%, 25.4-54.4% and 22.5-57.7% of the NO emissions, respectively. When vegetable soils were moderately acidified (pH, 6.2 to ≥ 5.7), the increased NO emissions resulted from the increase of both the gross autotrophic and heterotrophic nitrification rates and the NO product ratio of autotrophic nitrification. However, once severe acidification occurred (as in VF4, pH ≤ 4.3) and salt stress increased, both autotrophic and heterotrophic nitrification rates were inhibited to levels similar to those of WF soil. The enhanced NO product ratios of heterotrophic nitrification (4.84‰), autotrophic nitrification (0.93‰) and denitrification processes were the most important factors explaining high NO emission in VF4 soil. Data from this study showed that various soil conditions (e.g., soil salinity and concentration of NO or NH) could also significantly affect the sources and rates of NO emission. [ABSTRACT FROM AUTHOR]

Internal nitrogen (N) storage and remobilization processes support seasonal growth (flowering/fructification and subsequent leaf development) in particular in early spring, when soil temperatures are unfavourable for adequate N uptake. Storage nitrogen mobilization in young citrus trees was studied under two contrasting N supplies; high N (HN) and low N dose (LN) in the critical period of flowering and fruit set. N labelling technique was used to distinguish N derived from internal remobilization from that taken up by the roots. Regardless N supply, the greatest N remobilization took place from the beginning of the vegetative activity until flowering. Low N availability significantly increased (+14%) N retranslocation at the end of June drop agreeing with the hypothesis that reserve mobilization depends on soil N availability during flowering and fruit set. At the end of fruit drop, N remobilization contributed up to 70% and 61% of total N of young organs for LN and HN, respectively. Remobilized N was mainly recovered in abscised organs of both HN and LN trees and to a lesser extent in new flush leaves; however a greater percentage partitioned to abscised organs of LN as a consequence of the greater remobilization rate and the increased fruit abscission. Old leaves of LN remobilized significantly higher N, while woody organs and root system did not show differences between HN and LN supplied trees. The results presented in this paper demonstrate that the amount of N remobilized by young citrus plants depends on external N availability. Thus, low N application rates in early stages (flowering and fruit set) lead to higher translocation of N stored during the previous cycle to developing new organs. [ABSTRACT FROM AUTHOR]

The integration of multipurpose legumes into low-input tropical agricultural systems is needed because they are a nitrogen (N) input through symbiotic fixation. The drought-tolerant cover legume canavalia ( Canavalia brasiliensis) has been introduced for use either as forage or as a green manure into the crop-livestock system of the Nicaraguan hillsides. To evaluate its impact on the subsequent maize crop, an in-depth study on N dynamics in the soil-plant system was conducted. Microplots were installed in a 6-year old field experiment with maize-canavalia rotation. Direct and indirect N-labelling techniques were used to determine N uptake by maize from canavalia residues and canavalia-fed cows' manure compared to mineral fertilizer. Litter bags were used to determine the N release from canavalia residues. The incorporation of N from the amendment into different soil N pools (total N, mineral N, microbial biomass) was followed during the maize cropping season. Maize took up an average of 13.3 g N m, within which 1.0 g N m was from canavalia residues and 2.6 g N m was from mineral fertilizer, corresponding to an amendment N recovery of 12% and 32%, respectively. Recoveries in maize would probably be higher at a site with lower soil available N content. Most of the amendment N remained in the soil. Mineral N and microbial N were composed mainly of N derived from the soil. Combined total N recovery in maize and soil at harvest was highest for the canavalia residue treatment with 98% recovery, followed by the mineral fertilizer treatment with 83% recovery. Despite similar initial enrichment of soil microbial and mineral N pools, the indirect labelling technique failed to assess the N fertilizer value of mineral and organic amendments due to a high N mineralization from the soil organic matter. [ABSTRACT FROM AUTHOR]

Cheatgrass ( Bromus tectorum L.), an invasive annual grass, is displacing native species and causing increased fire frequency in the Great Basin of the southwestern United States. Growth and nitrogen uptake patterns by cheatgrass were examined in a greenhouse study using soils from sites with the same soil type but different fire histories: 1) an area that burned in 1999 that is now completely invaded with cheatgrass (CG); 2) an area that has not burned recently and is now dominated by Wyoming big sagebrush ( Artemisia tridentatassp. wyomingensis Beetle and Young) and Sandberg's bluegrass ( Poa secunda J. Presl) (WBS); and 3) a Wyoming big sagebrush area that burned in August of 2008 just prior to soil collection (NB). Cheatgrass seedlings had higher leaf numbers, height and mass in the NB soil. Ammonium-N mobilized by fire in the NB soil had significantly enriched N than soils from CG or WBS sites and this pattern was reflected in the isotopic signatures of the plants. Fire-mobilized mineral N accounted for only 58% of N taken up by cheatgrass in the NB soil, suggesting fire enhanced the ability of cheatgrass to assimilate more recalcitrant soil organic N. [ABSTRACT FROM AUTHOR]

Nitrogen (N) rhizodeposition by cowpea ( Vigna unguiculata (L.) Walp) is potentially a large N source in cropping systems of Sub-Saharan Africa. A field experiment was conducted to measure cowpea N rhizodeposition under the conditions of the Sudano-Sahelian zone using direct N labelling techniques to trace the amount of deposition and its transfer to associated and subsequent crops. Half of the total cowpea crop N was located below-ground at plant maturity, which exceeded 20 kg N ha when intercropped with millet. Only 15% of the below-ground cowpea N was recovered in roots, while 85% was found in the rhizodeposited pools. The experiment demonstrated that direct below-ground N transfer occurred from cowpea to millet in intercrop at a rate of 2 kg N ha over the growing season. Forty percent of the 25 kg below-ground N that the cowpea crop left at harvest were identifiable in the top 0.30 m soil in the beginning of the next planting season 7 months later; a pool still present at the end of that second season. Thus, the subsequent crop of millet ( Pennisetum glaucum (L.) R. Br.) only recovered 2.5 kg N ha from the below-ground cowpea pre-crop N during this growth season. The role and potential of cowpea as N provider has been underestimated in the past by ignoring the large proportion of N contained in its rhizodeposits. However, information is needed to determine how losses of the rhizodeposited N can be minimized to fully harness the potential of cowpea as N provider in agro-ecosystems of the region. [ABSTRACT FROM AUTHOR]

The objectives were to develop a screening technique to evaluate the tetraploid germplasm of Paspalum notatum Flüggé (bahiagrass) for potential rate of root depth development (RRDD), estimate the variability for RRDD among hybrids, and analyze the association between RRDD and nitrogen (N) uptake. First, a screening technique was developed based on the evaluation of two clones grown in clear acrylic columns of different sizes, filled with mineral or organic substrates. The RRDD response was determined to be constant across substrates, and tube sizes. Second, differences in RRDD among thirteen clones were compared by growing plants in acrylic columns filled with mineral substrate. Genetic variability was identified among the bahiagrass germplasm for RRDD. Greater RRDD values resulted in greater root and shoot mass, indicating that RRDD was related to early vigor. Finally, to evaluate the relation between RRDD and N uptake, labeled N (N) was injected either at the depth of the deepest root of each clone or at the maximum depth of all clones. Rapid root depth development was a determining factor for rapid access and uptake of nitrogen present in deep soil layers. This relationship might explain why clones exhibiting rapid root penetration also showed greater early vigor. [ABSTRACT FROM AUTHOR]

Current research shows that glyphosate herbicides can increase nitrogen exudation by plant roots and, as a consequence, favor pathogenic fungal colonization in soil. In Brazilian agroecosystems, the main herbicides used in conservationist systems for glyphosate substitution are glufosinate and paraquat. However, no studies have been developed that to evaluate the effects of these herbicides on nitrogen exudation by roots. This work was carried out with the objective of evaluating the exudation of nitrogen (N) compounds and root detachment after the application of glyphosate, glufosinate-ammonium or paraquat on Brachiaria decumbens Stapf. The ammonium concentration in the plant’s tissues and the nitrogen (N) recovery in the plant-solution system were also evaluated. The nitrogen left by the soil-solution system and the ammonium concentration in the plant’s tissues were increased after glyphosate or glufosinate application, but they were not modified by paraquat. Nitrogen compound exudation and root detachment were increased by the desiccation of Brachiaria decumbens Stapf by the use of the following herbicides: glyphosate, glufosinate-ammonium and paraquat. This increase demonstrated that the nitrogen exudation was not affected by a particular herbicide’s mode of action. [ABSTRACT FROM AUTHOR]

Dinitrogen fixation by cyanobacteria is of particular importance for the nutrient economy of cold biomes, constituting the main pathway for new N supplies to tundra ecosystems. It is prevalent in cyanobacterial colonies on bryophytes and in obligate associations within cyanolichens. Recent studies, applying interspecific variation in plant functional traits to upscale species effects on ecosystems, have all but neglected cryptogams and their association with cyanobacteria. Here we looked for species-specific patterns that determine cryptogam-mediated rates of N2 fixation in the Subarctic. We hypothesised a contrast in N2 fixation rates (1) between the structurally and physiologically different lichens and bryophytes, and (2) within bryophytes based on their respective plant functional types. Throughout the survey we supplied 15N-labelled N2 gas to quantify fixation rates for monospecific moss, liverwort and lichen turfs. We sampled fifteen species in a design that captures spatial and temporal variations during the growing season in Abisko region, Sweden. We measured N2 fixation potential of each turf in a common environment and in its field sampling site, in order to embrace both comparativeness and realism. Cyanolichens and bryophytes differed significantly in their cyanobacterial N2 fixation capacity, which was not driven by microhabitat characteristics, but rather by morphology and physiology. Cyanolichens were much more prominent fixers than bryophytes per unit dry weight, but not per unit area due to their low specific thallus weight. Mosses did not exhibit consistent differences in N2 fixation rates across species and functional types. Liverworts did not fix detectable amounts of N2. Despite the very high rates of N2 fixation associated with cyanolichens, large cover of mosses per unit area at the landscape scale compensates for their lower fixation rates, thereby probably making them the primary regional atmospheric nitrogen sink. [ABSTRACT FROM AUTHOR]

Cutting strategy effect on N2 fixation and distribution of fixed N above and below ground in red clover ( Trifolium pratense L.) and mixed red clover/perennial ryegrass ( Lolium perenne L.) green manure leys was quantified in field experiments including in situ mezotrons and microplots. Symbiotically fixed N in clover, transfer of fixed N to grass in the mixed stands and the fate of 15N contained in mulch were estimated by isotope dilution. Below ground clover-derived N was estimated by leaf labelling. Total N2 fixation was estimated by correcting fixed N in plant shoots with plant-derived N below ground and recycled N from mulch. The total N2 fixation was larger in harvested and mulched stands (average 45 g m−2) than in the intact stands (32 g m−2). Of the fixed N, 53% (intact), 46% (harvested) and 60% (mulched) was found below ground. The average recycling of N in mulch was 21% and contributed 13.7% (pure clover) and 2.2% (mixed) of the clover N in the regrowth. Recycling of N did not decrease N2 fixation in the mulched compared with harvested stands. The results indicate that cutting regime should be considered when estimating total amounts of N fixed by green manure leys. [ABSTRACT FROM AUTHOR]

Savannas cover about 20% of the Earth’s land area and 50% of Africa. As an indispensable component of savanna, grasses play an important role in these ecosystems. A better understanding of grass productivity and its controlling factors in savanna ecosystems could therefore be a key to understand the functioning of savannas and predict savanna responses to future climatic changes. In this study, a stable isotope fertilization experiment was conducted to determine how factors limiting grass production in savannas differ across regional climate gradients. The study was conducted on the geomorphically homogenous Kalahari Transect (KT), which offers an ideal setting to study nutrient and vegetation dynamics independently of confounding soil effects. The results show that the grasses assimilated the added fertilizer at all the sites but they did not respond to nitrogen fertilization for both dry and wet years, and at both dry and wet ends of the Transect. Although prior studies have proposed a switch between water and nitrogen limitations between arid and mesic savannas, our results suggest that nitrogen availability may not limit grass productivity across the whole KT. Thus, although the traditional classifications as nutrient poor (broad-leaf) and nutrient rich (fine-leaf) savanna ecosystems may still be useful, it does not necessarily imply the existence of nitrogen limitation in the nutrient poor area; in fact, it is more likely that the herbaceous species found in the more humid sites (nutrient poor sites) are already adapted to lower nitrogen availability. [ABSTRACT FROM AUTHOR]

A 3-year field lysimeter experiment was performed to determine transformations of 15N-labeled cauliflower ( Brassica oleracea) residues incorporated into lysimeter topsoil in a potato ( Solanum tuberosum)/cauliflower rotation. Only the potato crop received 150 kg mineral N ha−1y−1. Cauliflower yields were high (12–13 t fresh matter ha−1), and N returned to the soil represented 51% of the aboveground plant N uptake. The 15N recovery by the potato/cauliflower rotation began at 46%, then decreased sharply to 12 and 6% for the second and third year, respectively. The cumulative 15N leaching rate was only 3%; 63% remained in the soil 3 years after incorporation. Soil N mineralization rates described by a parallel first-order kinetic model predicted 27, 7 and 6% of residual N lost annually during the first, second and third year, respectively. Thus, a potato/cauliflower rotation with moderate N fertilization optimizes N recovery of crop residues and can control leaching loss efficiently. [ABSTRACT FROM AUTHOR]

N fertilizer labelled with 15N was used to follow the accumulation and distribution of N applied at different times after planting Eucalyptus grandis W Hill ex Maiden seedlings in south-eastern Queensland, Australia. The first application was made to selected trees at 0.12 y after planting (T0), and treated trees were harvested at intervals after. Further labelled applications were made on different sets of trees 0.50, 0.74, 1.30 and 1.49 years after T0; T0.5, T0.74, T1.30 and T1.49 respectively. After 0.5 y there was no significant difference in growth between T0 trees and unfertilized controls, but after 1 y above-ground biomass and N content of the controls was only 30% and 39% of the fertilized trees respectively ( P <0.05). At later applications controls were not significantly different from fertilized trees up to 1 y later, but after 2 y above-ground biomass and N content was reduced ( P <0.05). Growth reductions occurred after leaves present on the tree at the time of fertilizer application were shed. Leaf biomass and N content increased steadily to age 1.5 y and then stabilized at about 2.5 kg tree-1 and 35 g tree−1 respectively, new growth balanced by litter fall. N content in woody material (stems+branches) increased steadily, equaling that of the leaves at 3.5 y. N derived from fertilizer and the proportion of applied N recovered peaked at 1 y and then decreased. These relationships were not affected by time of application. In T0 trees N in above-ground parts derived from fertilizer increased steadily to 42% of the total 0.5 y after application. It then decreased rapidly after leaf fall commenced while total N content increased. When observations ceased 3.5 y after planting, less than 5% of the N in the trees was derived from fertilizer. N uptake was two-phased. In the first year mineral N from the fertilizer was taken up by the trees. In subsequent years fertilizer contributed to the tree growth but the N taken up came from a different source. [ABSTRACT FROM AUTHOR]

In a lysimeter experiment with juvenile beech trees ( Fagus sylvatica L.) we studied the development of depth gradients of soil organic matter (SOM) composition and distribution after soil disturbance. The sampling scheme applied to the given soil layers (0–2 cm, 2–5 cm, 5–10 cm and 10–20 cm) was crucial to study the subtle reformation of SOM properties with depth in the artificially filled lysimeters. Due to the combination of physical SOM fractionation with the application of 15N-labelled beech litter and 13C-CPMAS NMR spectroscopy we were able to obtain a detailed view on vertical differentiation of SOM properties. Four years after soil disturbance a significant decrease of the mass of particulate OM (POM) with depth could be found. A clear depth distribution was also shown for carbon (C) and nitrogen (N) within the SOM fractions related to bulk soil. The mineral fractions <63 µm clearly dominated C storage (between 47 to 60% of bulk soil C) and N storage (between 68 to 86% of bulk soil N). A drastic increase in aliphatic C structures concomitant to decreasing O/N-alkyl C was detected with depth, increasing from free POM to occluded POM. Only a slight depth gradient was observed for 13C but a clear vertical incorporation of 15N from the applied labelled beech litter was demonstrated probably resulting from faunal and fungal incorporation. We clearly demonstrated a significant reformation of a SOM depth profile within a very short time of soil evolution. One important finding of this study is that especially in soils with reforming SOM depth gradients after land-use changes selective sampling of whole soil horizons can bias predictions of C and N dynamics as it overlooks a potential development of gradients of SOM properties on smaller scales. [ABSTRACT FROM AUTHOR]

The effects of 15N-labelled green manure (GM) application on rice yield and N uptake were investigated and compared with those of inorganic fertilizer (IF) and no fertilizer (NF) application. Nine GMs (white clover, Chinese milk vetch, hairy vetch, crimson clover, oats, rye, Italian ryegrass, white mustard, lacy phacelia) were either incorporated into or surface-applied on paddy soil. Among the nine GM treatments, the rice yield and N uptake tended to be higher in the white clover and hairy vetch treatments. The rice N uptake efficiency from the applied N source was highest in the incorporated hairy vetch (56%), followed by IF (52%), and incorporated white clover (44%), whereas for the other treatments there were no significant differences. The mineral fertilizer equivalent values of hairy vetch and white clover in case of the incorporation were 108% and 84%, respectively. Therefore, hairy vetch and white clover seem to be the most efficient organic N source as alternatives to inorganic N fertilizers. The N uptake efficiency of rice and residual N in soil derived from GM were higher in the incorporation treatments than that in the surface-applied treatments, indicating that N losses from the soil-plant system were lower with the incorporation. [ABSTRACT FROM AUTHOR]

In many temperate ecosystems, rates of atmospheric nitrogen deposition remain high over winter despite decreased agricultural activity over this season. The extent to which this nitrogen is accessible for plant growth over the following growing season may depend strongly on uptake by plants and soil microorganisms from late fall through early spring, when the majority of aboveground plant tissue has senesced. We added Ca(15NO3)2 (5 atom %15N) at a rate of 2 g m−2 of N (corresponding to 100 mg 15N m−2) to the surface of plots in a temperate old field during either late fall, winter, spring melt or early spring. We quantified the recovery of excess 15N in the soil microbial biomass and soil extracts following spring melt and in aboveground plant tissue at the peak of the plant growing season. Nitrate additions had no significant effect on total aboveground plant biomass, relative species abundance or percent tissue nitrogen. However, mean excess 15N in aboveground plant tissue varied significantly among treatments, with values of 8.1, 2.6, 0.3 and 7.3 mg m−2 for late fall, winter, spring melt and early spring addition plots, respectively. Corresponding values of excess 15N were 3.1, 1.4 and 0.2 mg m−2 in microbial biomass, and 0.17, 0.07 and 0.03 mg m−2 in soil extracts, for late fall, winter and spring melt addition plots, respectively. Overall, these results indicate that nitrogen retention from late fall through early spring may depend highly on plant uptake in this system, and that only a small fraction of the nitrogen that accumulates in the winter snow pack may be available to plants. [ABSTRACT FROM AUTHOR]

Intercropping of upland rice with short-duration grain legumes has major advantages in increasing crop yields and soil productivity. However, the contribution of arbuscular mycorrhizas, the common mutualistic symbiosis between most crops and mycorrhizal fungi, is not fully understood in intercropping systems. We assayed the contribution of inoculation of the arbuscular mycorrhizal fungus (AMF) Glomus caledonium on nutrient acquisition and biomass yield. Using the method of plastic film and nylon net partition and tracing 15N transferred between the intercropped upland rice ( Oryza sativa ssp. Japonica Nipponbare) and mung bean ( Vigna radiata L. Chuanyuan), we compared the intercropping, with separation of the whole root systems by a plastic film, with and without a barrier of nylon net to allow penetration of the fungal hyphae. Intercropping significantly improved the formation of arbuscular mycorrhizas, particularly in the upland rice roots. The improved formation of mycorrhizas by the intercropping increased total P uptake by 57% in rice, total P and N acquisition by 65% and 64% respectively in mung bean, and nodulation by 54% in mung bean. The percentage of total 15N transfer from mung bean to rice leaves was increased from 5.4% to 15.7% by inoculation with AMF. In contrast, there was only 2.7% of 15N transfer from rice to mung bean and no AMF effect on N transfer. It is concluded that cereal and legume crop intercropping increase mycorrhiza formation, which in turn improves nodulation, N and P acquisition and N transfer in the legumes. [ABSTRACT FROM AUTHOR]

15N labeled ammonium, glycine or glutamic acid was injected into subarctic heath soil in situ, with the purpose of investigating how the nitrogen added in these pulses was subsequently utilized and cycled in the ecosystem. We analyzed the acquisition of 15N label in mycorrhizal and non-mycorrhizal plants and in soil microorganisms, in order to reveal probable differences in acquisition patterns between the two functional plant types and between plants and soil microorganisms. Three weeks after the label addition, with the 15N-forms added with same amount of nitrogen per square meter, we analyzed the 15N-enrichment in total soil, in soil K2SO4 (0.5 M) extracts and in the microbial biomass after vacuum-incubation of soil in chloroform and subsequent K2SO4 extraction. Furthermore the 15N-enrichment was analyzed in current years leaves of the dominant plant species sampled three, five and 21 days after label addition. The soil microorganisms had very high 15N recovery from all the N sources compared to plants. Microorganisms incorporated most 15N from the glutamic acid source, intermediate amounts of 15N from the glycine source and least 15N from the NH4 + source. In contrast to microorganisms, all ten investigated plant species generally acquired more 15N label from the NH4 + source than from the amino acid sources. Non-mycorrhizal plant species showed higher concentration of 15N label than mycorrhizal plant species 3 days after labeling, while 21 days after labeling their acquisition of 15N label from amino acid injection was lower than, and the acquisition of 15N label from NH4 injection was similar to that of the mycorrhizal species. We conclude that the soil microorganisms were more efficient than plants in acquiring pulses of nutrients which, under natural conditions, occur after e.g. freeze–thaw and dry–rewet events, although of smaller size. It also appears that the mycorrhizal plants in the short term may be less efficient than non-mycorrhizal plants in nitrogen acquisition, but in a longer term show larger nitrogen acquisition than non-mycorrhizal plants. However, the differences in 15N uptake patterns may also be due to differences in leaf longevity and woodiness between plant functional groups. [ABSTRACT FROM AUTHOR]

We have measured the uptake capacity of nitrogen (N) and potassium (K) from different soil depths by injecting 15N and caesium (Cs; as an analogue to K) at 5 and 50 cm soil depth and analysing the recovery of these markers in foliage and buds. The study was performed in monocultures of 40-year-old pedunculate oak ( Quercus robur), European beech ( Fagus sylvatica) and Norway spruce ( Picea abies (L.) Karst.) located at an experimental site in Palsgård, Denmark. The markers were injected as a solution through plastic tubes around 20 trees of each species at either 5 or 50 cm soil depth in June 2003. After 65 days foliage and buds were harvested and the concentrations of 15N and Cs analysed. The recovery of 15N in the foliage and buds tended to be higher from 5 than 50 cm soil depth in oak whereas they where similar in spruce and beech after compensation for differences in immobilization of 15N in the soil. In oak more Cs was recovered from 5 than from 50 cm soil depth whereas in beech and spruce no difference could be detected. Out of the three investigated tree species, oak was found to have the lowest capacity to take up Cs at 50 cm soil depth compared to 5 cm soil depth also after compensating for differences in discrimination against Cs by the roots. The uptake capacity from 50 cm soil depth compared with 5 cm was higher than expected from the root distribution except for K in oak, which can probably be explained by a considerable overlap of the uptake zones around the roots and mycorrhizal hyphae in the topsoil. The study also shows that fine roots at different soil depths with different physiological properties can influence the nutrient uptake of trees. Estimates of fine root distribution alone may thus not reflect the nutrient uptake capacity of trees with sufficient accuracy. Our study shows that deep-rooted trees such as oak may have lower nutrient uptake capacity at deeper soil layers than more shallow-rooted trees such as spruce, as we found no evidence that deep-rooted trees obtained proportionally more nutrients from deeper soil layers. This has implications for models of nutrient cycling in forest ecosystems that use the distribution of roots as the sole criterion for predicting uptake of nutrients from different soil depths. [ABSTRACT FROM AUTHOR]

Effects of light on the short term competition for organic and inorganic nitrogen between maize and rhizosphere microorganisms were investigated using a mixture of amino acid, ammonium and nitrate under controlled conditions. The amount and forms of N added in the three treatments was identical, but only one of the three N forms was labeled with 15N. Glycine was additionally labeled with 14C to prove its uptake by maize and incorporation into microbial biomass in an intact form. Maize out-competed microorganisms for $$^{15} {\text{NO}}_3^ - $$ during the whole experiment under low and high light intensity. Microbial uptake of 15N and 14C was not directly influenced by the light intensity, but was indirectly related to the impact the light intensity had on the plant. More $$^{15} {\text{NH}}_4^ + $$ was recovered in microbial biomass than in plants in the initial 4 h under the two light intensities, although more 15N-glycine was incorporated into microbial biomass than in plants in the initial 4 h under low light intensity. Light had a significant effect on $$^{15} {\text{NO}}_3^ - $$ uptake by maize, but no significant effects on the uptake of $$^{15} {\text{NH}}_4^ + $$ or 15N-glycine. High light intensity significantly increased plant uptake of $$^{15} {\text{NO}}_3^ - $$ and glycine 14C. Based on 14C to 15N recovery ratios of plants, intact glycine contributed at least 13% to glycine-derived nitrogen 4 h after tracer additions, but it contributed only 0.5% to total nitrogen uptake. These findings suggest that light intensity alters the competitive relationship between maize roots and rhizosphere microorganisms and that C4 cereals such as maize are able to access small amounts of intact glycine. We conclude that roots were stronger competitor than microorganisms for inorganic N, but microorganisms out competed plants during a short period for organic N, which was mineralized into inorganic N within a few hours of application to the soil and was thereafter available for root uptake. [ABSTRACT FROM AUTHOR]

A pot experiment was conducted to determine the effects of the application of 13C (1.256 atom%) and 15N (1.098 atom%) dual-labeled maize residue compost (MRC) on the nitrogen and carbon uptake by radish, komatsuna, and chingensai as compared with the effect of inorganic fertilizer (IF). The vegetables were grown over three consecutive growing seasons over 4 months; compost was applied at the rate of 24 g kg–1 soil. Nonlabeled nitrogen fertilizer was applied to the compost treatments in the second and third crops to compare the effects of blends of compost with N fertilizer to fertilizer alone. The N uptake and yield of vegetables were significantly higher with the recommended inorganic N treatment. The vegetables took up significantly ( P < 0.05) lower amounts of N from MRC than from IFs during the three cultivations. The values of the N uptake derived by fertilizer application to the plant exhibited significant differences among different vegetables. Nitrogen recovered by komatsuna and chingensai from MRC was 7.3 (6.6%), 2.7 (1.8%), and 2.3, (1.7%) in the first, second, and third crops, respectively. Radish, komatsuna, and chingensai recovered significant amounts of C from MRC in the first and second crops, with negligible C recovery in the third crop. The initial loss of fertilizer C in soil at the first crop indicates that the microbial decomposition decoupled substantial amounts of 13C/15N-labeled compounds early in plant development, thus giving the microorganisms a preemptive competitive advantage in the acquisition of easily available 13C/15N-labeled substrates. It is concluded that a combination of compost and inorganic N did not supply sufficient plant-available N to increase vegetables yields or N uptake over those of fertilizer alone. The data suggested that higher productivity of vegetables might be achieved after the accumulation of a certain amount of residual compost N. [ABSTRACT FROM AUTHOR]

In situ 15N-labelling was used to provide a quantitative assessment of the total contribution of lupin ( Lupinus angustifolius) to below-ground (BG) N accumulation during a growing season under field conditions, and to directly trace the fate of the lupin BG N in the next season, including quantifying the N benefit from lupin to a following wheat ( Triticum aestivum) crop. The experiments were conducted at two sites, both experiencing a semi-arid Mediterranean-type climate in the wheat-growing region of Western Australia but with differing soil types, a deep sand (Moora) and a sand-over-clay shallow duplex soil (East Beverley, EB). Lupin shoot and root dry matter and total plant N accumulation, proportional dependence on nitrogen fixation and grain yield were greater at the deep sand site than the duplex soil site, although there was a similar proportion of shoot N to estimated total BG N at both sites. The proportion of total plant BG N decreased from the vegetative stage (42–51%) to peak biomass (25–39%) and maturity (23–34%). From 56–67% of BG N on the deep sand and 74–86% on the duplex soil was not recovered in coarse roots (>2 mm) or as soluble N, but was present in the insoluble organic N fraction. There was evidence for cycling of lupin root-derived N into soil microbial biomass and soluble organic N during lupin growth (by the late vegetative stage), but no evidence for leaching of legume derived BG N during the lupin season. Estimates of fixed N input BG were at least four times greater if based on total lupin BG N rather than on N recovered in coarse roots (>2 mm). There were no apparent losses of lupin BG N during the summer fallow period subsequent to lupin harvest at either site. Also, immediately prior to sowing of wheat there were similar proportions of lupin BG N in the inorganic (20–25%) and microbial biomass (6–9%) pools at both sites, with the majority of BG N detected in the <2 mm fraction of the soil column. However, the proportion of residual lupin BG N estimated to benefit the aboveground wheat biomass was relatively low, 10% on the deep sand and only 3% on the shallow duplex. Some (14%) residual lupin BG N was leached as nitrate to 1 m on the deep sand compared to 8% of residual lupin BG N leached to the clay layer (0.3 m) on the shallow duplex. About 27% of the residual lupin BG N on the deep sand at Moora had apparently mineralised by the end of the succeeding wheat season (i.e. recovered either in the wheat shoots, as inorganic N in the soil profile or as leached nitrate) compared to only 12% at EB. There was an unaccounted for large loss of residual lupin BG N (50%) from the duplex soil at EB during the wheat season, postulated to be chiefly via denitrification. At both sites after the wheat season a substantial proportion (32–55%) of legume derived BG N was still present as residual insoluble organic N, considered to be an important contribution to structural and nutritional long-term sustainability of these soils. [ABSTRACT FROM AUTHOR]

The uptake of nutrients from deep soil layers has been shown to be important for the long-term nutrient sustainability of forest soils. When modelling nutrient uptake in forest ecosystems, the nutrient uptake capacity of trees is usually defined by the root distribution. However, this leads to the assumption that roots at different soil depths have the same capacity to take up nutrients. To investigate if roots located at different soil depths differ in their nutrient uptake capacity, here defined as the nutrient uptake rate under standardized conditions, a bioassay was performed on excised roots (<1 mm) of eight oak trees ( Quercus robur L.). The results showed that the root uptake rate of 86Rb+ (used as an analogue for K+) declined with increasing soil depth, and the same trend was found for $$ {\text{NH}}^{ + }_{4} $$ . The root uptake rate of $$ {\text{H}}_{2} {\text{PO}}^{ - }_{4} $$ , on the other hand, did not decrease with soil depth. These different physiological responses in relation to soil depth indicate differences in the oak roots, and suggest that fine roots in shallow soil layers may be specialized in taking up nutrients such as K+ and $$ {\text{NH}}^{ + }_{4} $$ which have a high availability in these layers, while oak roots in deep soil layers are specialized in taking up other resources, such as P, which may have a high availability in deep soil layers. Regardless of the cause of the difference in uptake trends for the various nutrients, these differences have consequences for the modelling of the soil nutrient pool beneath oak trees and raise the question of whether roots can be treated uniformly, as has previously been done in forest ecosystem models. [ABSTRACT FROM AUTHOR]

It has been demonstrated that plant roots can take up small amounts of low-molecular weight (LMW) compounds from the surrounding soil. Root uptake of LMW compounds have been investigated by applying isotopically labelled sugars or amino acids but not labelled organic matter. We tested whether wheat roots took up LMW compounds released from dual-labelled (13C and 15N) green manure by analysing for excess 13C in roots. To estimate the fraction of green manure C that potentially was available for root uptake, excess 13C and 15N in the primary decomposers was estimated by analysing soil dwelling Collembola that primarily feed on fungi or microfauna. The experimental setup consisted of soil microcosm with wheat and dual-labelled green manure additions. Plant growth, plant N and recoveries of 13C and 15N in soil, roots, shoots and Collembola were measured at 27, 56 and 84 days. We found a small (<1%) but significant uptake of green manure derived 13C in roots at the first but not the two last samplings. About 50% of green manure C was not recovered from the soil-plant system at 27 days and additional 8% was not recovered at 84 days. Up to 23% of C in collembolans derived from the green manure at 56 days (the 27 days sampling was lost). Using a linear mixing model we estimated that roots or root effluxes provided the main C source for collembolans (54−79%). We conclude that there is no solid support for claiming that roots assimilated green manure derived C due to very small or no recoveries of excess 13C in wheat roots. During the incubation the pool of green manure derived C available for root uptake decreased due to decomposition. However, the isotopic composition in Collembola indicated that there was a considerable fraction of green manure derived C in the decomposer system at 56 days thus supporting the premise that LMW compounds containing C from the green manure was released throughout the incubation. [ABSTRACT FROM AUTHOR]

While fertigation can increase fertilizer use efficiency, there is an uncertainly as to whether the fertilizer should be introduced at the beginning of the irrigation or at the end, or introduced during irrigation. Our objective was to determine the effect of different fertigation schemes on nitrogen (N) uptake and N use efficiency (NUE) in cotton plants. A pot experiment was conducted under greenhouse conditions in year 2004 and 2005. According to the application timing of nitrogen (N) fertilizer solution and water (W) involved in an irrigation cycle, four nitrogen fertigation schemes [nitrogen applied at the beginning of the irrigation cycle (N–W), nitrogen applied at the end of the irrigation cycle (W–N), nitrogen applied in the middle of the irrigation cycle (W–N–W) and nitrogen applied throughout the irrigation cycle (N&W)] were employed in a completely randomized design with four replications. Cotton was grown in plastic containers with a volume of 84 l, which were filled with a clay loam soil and fertilized with 6.4 g of N per pot as unlabeled and 15N-labeled urea for 2004 and 2005, respectively. Plant total dry matter (DM) and N content in N–W was significantly higher than in N&W in both seasons, but these were not consistent for W–N and W–N–W treatments. In year 2005, a significantly higher nitrogen derived from fertilizer (NDFF) for the whole plant was found in W–N and N–W than that in W–N–W and N&W. Fertigation scheme had a consistent effect on total NUE: N–W had the highest NUE for the whole plant, but this was not significantly different from W–N. Treatments W–N and W–N–W had similar total NUE, and N&W had the lowest total NUE. After harvesting, the total residual fertilizer N in the soil was highest in W–N, lowest in N–W, but this was not significantly different from N&W and W–N–W treatments. Total residual NO3–N in the soil in N&W and W–N treatments was 20.7 and 21.2% higher than that in N–W, respectively. The total 15N recovery was not statistically significant between the four fertigation schemes. In this study, the fertigation scheme N–W (nitrogen applied at the beginning of an irrigation cycle) increased DM accumulation, N uptake and NUE of cotton. This study indicates that Nitrogen application at the beginning of an irrigation cycle has an advantage on N uptake and NUE of cotton. Therefore, NUE could be enhanced by optimizing fertilization schemes with drip irrigation. [ABSTRACT FROM AUTHOR]

The distribution of fine roots and external ectomycorrhizal mycelium of three species of trees was determined down to a soil depth of 55 cm to estimate the relative nutrient uptake capacity of the trees from different soil layers. In addition, a root bioassay was performed to estimate the nutrient uptake capacity of Rb+ and NH by these fine roots under standardized conditions in the laboratory. The study was performed in monocultures of oak ( Quercus robur L .), European beech ( Fagus sylvatica L .) and Norway spruce [ Picea abies (L.) Karst.] on sandy soil in a tree species trial in Denmark. The distribution of spruce roots was found to be more concentrated to the top layer (0–11 cm) than that of oak and beech roots, and the amount of external ectomycorrhizal mycelia was correlated to the distribution of the roots. The uptake rate of [86Rb+] by oak roots declined with soil depth, while that of beech or spruce roots was not influenced by soil depth. In modelling the nutrient sustainability of forest soils, the utilization of nutrient resources in deep soil layers has been found to be a key factor. The present study shows that the more shallow-rooted spruce can have a similar capacity to take up nutrients from deeper soil layers than the more deeply rooted oak. The distribution of roots and mycelia may therefore not be a reliable parameter for describing nutrient uptake capacity by tree roots at different soil depths. [ABSTRACT FROM AUTHOR]

The vertical pattern of root length densities (RLD) of fine roots (<2 mm in diameter) and nitrogen (N) uptake potential were determined at different altitudes (1,900, 2,400, and 3,000 m a.s.l.) of a tropical montane forest in order to improve our knowledge about the depth distribution of nutrient uptake in this ecosystem. At higher altitudes, precipitation rate and frequency of fog were higher than at lower altitudes while mean annual air temperature decreased with increasing altitude. Soils were always very acid with significantly lower pH at a depth of 0.0–0.3 m in mineral soil at 3,000 m (2.8–2.9) than at 1,900 and 2,400 m (3.1–3.5). The vertical distribution of RLD was very similar both during the dry and the rainy season. During the dry season the percentage of root length in the organic layer increased from 51% at 1,900 m to 61% at 2,400 m and 76% at 3,000 m. At 3,000 m, RLD was markedly higher in the upper 0.05 m than in the remaining organic layer, whereas at 1,900 m and 2,400 m RLD were similar in all depths of the organic layer. In mineral soil, RLD decreased to a greater degree with increasing soil depth at the upper two study sites than at 1,900 m. The relative N uptake potential from different soil layers (RNUP) was determined by 15N enrichment of leaves after application of 15N enriched ammonium sulphate at various soil depths. RNUP closely followed fine root distribution confirming the shallower pattern of nutrient uptake at higher altitudes. RNUP was very similar for trees, shrubs and herbs, but shallower for saplings which obtained N only from the organic layer at both altitudes. Liming and fertilizing (N, P, K, Mg) of small patches in mineral soil had no significant impact on fine root growth. We conclude that the more superficial nutrient uptake ability at higher altitudes may be partly related to increased nutrient input from canopy by leaching. However, the specific constraints for root growth in the mineral soil of tropical montane forests warrant further investigations. [ABSTRACT FROM AUTHOR]

Though the potential of plants to take up organic N (e.g., amino acids) is well established, the true significance of organic N acquisition to plant N nutrition has not yet been quantified under field conditions. Here we demonstrate that organic N contributes significantly to the annual N uptake of three dominant plant species ( Kobresia humilis, Saussurea superba and Stipa aliena) of alpine meadows on the Tibet Plateau, China. This was achieved by using double-labelled (14C and 15N) algae as a source for slow and continuous release of amino acids, and tracing both labels in the above- and below-ground plant biomass. Four months after addition of algae, between 0.5% and 2.6% of 14C and 5% and 14% of 15N from added algae were recovered in the plants, which translate into an uptake of organic N between 0.3 mg N m−2 and 1.5 mg N m−2. The calculated contribution of organic N to total N uptake was estimated to range between 21% and 35% for K. humilis, and between 13% and 21% for S. aliena and S. superba, respectively, implying that organic N uptake by grassland plants is quantitatively significant under field conditions in the studied alpine meadows. This finding has important ecological implications with regard to competition for organic N between microorganisms and plant roots. [ABSTRACT FROM AUTHOR]

In this paper we discuss the use of isotope ratios as indicators of organic production. Few studies have investigated the influence of plant nutrition on the isotopic signatures of plants. As plant nutrition is often significantly different between integrated and organic production systems the isotope ratios in the plants may reflect this. Plant samples from a 2-year field-experiment were analyzed for 15N, 13C and 34S content of the bulk-material and 18O-content of the leaf water. In this experiment cabbages ( Brassica oleracea v. capitata f. alba cv. Rolly), onions ( Allium cepa cv. Alisa Craig), lettuces ( Lactuca sativa v. capitata cv. Ponchito) and Chinese cabbage ( Brassica pekinesis cv. Parkin) were cultivated according to good agricultural practices for integrated and organic production. No differences in the δ 34S and δ 18O values of the plants grown under the two production systems were observed. The organically produced vegetables were significantly enriched in 15N and depleted in 13C compared to those grown under the integrated system. [ABSTRACT FROM AUTHOR]

Pastures in southern Chile are composed of forage grasses and associated legumes. Apart from native grass species, in many areas perennial ryegrass (Lolium perenneL.) and white clover (Trifolium repensL.) have been introduced. It is thought that most of the N in these pastures is derived from the symbiosis between rhizobium bacteria and the clover. However, various factors can limit the biological nitrogen fixation (BNF) potential. Soils of Chiloé (southern Chile) are derived from volcanic ash (Andisols), and show high acidity and phosphorus (P) retention capacity and there is often a low availability of exchangeable cations and, sometimes, micronutrients, which together with high aluminium (Al) concentrations can inhibit BNF potential by nodulated legumes. An experiment was carried out on an Andisol of Chiloé, on a permanent pasture, to evaluate the influence of these fertility factors grass and legume productivity and on the BNF contribution to white clover. Treatments included two rates of lime (0 and 4 Mg ha-1) and eight fertilisation treatments: 1. Complete (macronutrients in kg ha-1: N-50, P2O5-180, K2O-100, MgO-70, S-50, and micronutrients); six other treatments with complete fertilisation but, respectively, without N, P, K, Mg, S and micronutrients, and a control without fertiliser application. The experiment was conducted over 2 years and soil fertility parameters, grass and clover biomass production were evaluated and the BNF contribution to the clover was estimated using15N isotope dilution technique. Potassium was found to be the most limiting element for dry matter production, especially for the clover. Liming increased soil pH values and Ca and Mg contents, strongly reduced Al saturation and increased dry matter production. The proportional contribution of BNF to the white clover was high, reaching 80% of total N accumulated by the plants and not significantly affected by addition of lime or fertiliser. However, the accumulation of dry matter and total N, and the total N derived from BNF by the clover were over three times higher in the complete fertilisation treatments than in the control in both years and in the limed and unlimed treatments. The results of this study indicate that pasture productivity can be improved by liming and fertilisation which favour growth of the legume and ryegrass to the detriment of the native grasses. The increase in the proportion of ryegrass and clover should result in improved quality of the available forage, and in the case of the clover, stimulates an increase in the N contribution from BNF thus dispensing with the need for N fertiliser. [ABSTRACT FROM AUTHOR]