15N Natural Abundance Research Articles

Development and validation of a model for early prediction of residual feed intake in beef cattle using plasma biomarkers

Identification of plasma biomarkers for feed efficiency in growing beef cattle offers a promising opportunity for developing prediction models to improve precision feeding strategies. However, these models must accurately predict feed efficiency at early stages of fattening. Our study aimed to evaluate the reliability of candidate biomarkers previously identified in late-fattening cattle when analyzed during early fattening stages and to develop diet-specific prediction equations for residual feed intake (RFI). From a total of 364 Charolais bulls across 7 cohorts, we selected 64 animals with extreme RFI values. The animals were fed either a corn‑ or grass-silage diets. These animals were chosen from 4 out of available 7 cohorts. Animals from three cohorts (24 high-RFI and 24 low-RFI, having a mean RFI difference of 1.48 kg/d) were used for biomarker confirmation and prediction model training. Animals from a fourth cohort (8 high-RFI and 8 low-RFI, having a mean RFI difference of 0.98 kg/d) were used for model external validation. Blood samples were collected at the beginning of the feed efficiency test (333±20 days), and plasma underwent targeted metabolomic for 630 metabolites, natural abundance of 15N (δ15N), insulin, and IGF-1 analysis. Seven previously identified plasma biomarkers for RFI in late-fattening beef cattle still kept their capability for discriminating low and high RFI animals when analyzed during early fattening stages (P < 0.05). Among these confirmed biomarkers, five were common for both grass- and corn-fed animals (creatinine, β-alanine, triglyceride TG18:0_34:2, symmetric dimethyl-arginine and phosphatidylcholine PC aa C30:2) while two were diet-specific (insulin-like growth factor 1 for grass silage-based diet, and isoleucine for corn silage-based diet. No new plasma biomarkers of RFI were identified at early-fattening stages (false discovery rate, FDR>0.05). Prediction models were developed based on seven confirmed RFI biomarkers analyzed during early-fattening. Two logistic regression models incorporating creatinine and either IGF-1 (for grass silage-based diet) or PC aa C30:2 (for corn silage-based diet) effectively distinguished between high and low-RFI animals with high sensitivity and specificity (area under the curve, AUC > 0.80). The biomarkers used in the models showed moderate to high repeatability between early and late fattening stages (45 < r <65). The models were successfully externally validated, with more than 85% of animals from the fourth cohort correctly classified. Once validated in larger cohorts and utilizing cost-effective and rapid analytical methods, these models could support precision feeding and breeding programs, aiming to reduce the cost of raising beef cattle.

An efficiently biological nitrogen fixation of non-native Lotus tenuis justifies its key role in the flooding Pampas (Argentina)

Forage production is a major challenge in the Salado River basin, Argentina (also denominated “Flooding Pampas”), the largest livestock raising area. The successful naturalization of the European legume Lotus tenuis (native from Mediterranean area) has been of productive and environmental relevance to the region. The main objective of this work was to quantify the efficiency of L. tenuis through its ability to fix nitrogen biologically and thus strengthening their strategic role in the region. The 15N natural abundance method was used to evaluate the Biological Nitrogen Fixation (BNF) of L. tenuis in soil plots. Non-leguminous weed species in the same plots were utilized as reference plants. Simultaneously, evaluations were carried out in the greenhouse and the relative reproducibility of the field experiments. The results showed that L. tenuis promotion was accompanied by an increase in forage quality improving the Crude Protein (CP) content and the Digestibility of Dry Matter (DDM). Moreover, its contribution to N inputs to the system through BNF in symbiosis with native rhizobia was about 80%, a highly relevant percentage for these constrained agroecosystems where the presence of native legumes is very scarce. In conclusion, this study supports the hypothesis that L. tenuis promotion plays a strategic role in ecosystem sustainability in the Salado River basin, especially in soil constrained areas. Together with previously published data, including information on ruminant management, the results obtained contribute to supporting the idea that the naturalized legume plays a strategic role in the sustainability of marginal agricultural soils.

Evidence for the translocation of fixed N in the N2-fixing lichen Stereocaulon vesuvianum.

The fruticose lichen Stereocaulon vesuvianum is among the most abundant and widespread lichens in upland Britain. It typically produces cephalodia (nodules) that contain the cyanobacterium Stigonema, which can fix atmospheric nitrogen. However, over much of England, Wales, and southern Scotland S. vesuvianum no longer produces cephalodia and does not fix nitrogen, a morphological change linked to elevated atmospheric nitrogen deposition. This provided a unique opportunity to compare the 15N natural abundance signatures in N2-fixing and non-N2-fixing lichen populations, keeping in mind that fixed nitrogen has a 15N content close to that of atmospheric N2 while, in comparison, several components of atmospheric combined N (e.g. nitrate and ammonium in precipitation) tend to be 15N depleted. We found that in N2-fixing samples, there was a steep gradient in 15N relative abundance in the terminal 15 mm of thallus branches (pseudopodetia), changing from 15N depleted tissues at 10-15 mm below the tips to values close to that of atmospheric N2 at the apices while in non-N2-fixing samples thallus branches were uniformly 15N depleted. The 15N gradient in N2-fixing material could not be explained by the presence of cephalodia since these are more abundant towards branch bases. The data provide the first evidence in lichens of translocation of recently fixed N to sink regions of active growth and production of asexual reproductive propagules, bringing lichens into line with N source-sink relationships in N2-fixing plant symbioses.

Incorporation of the 15N-labeled simulated arthropod rain in the soil food web.

Direct trophic links between aboveground and belowground animal communities are rarely considered in food web models. Most invertebrate animals inhabiting aboveground space eventually become prey of soil predators and scavengers forming a gravity-driven spatial subsidy to detrital food webs, but its importance remains unquantified. We used laboratory-grown 15N-labeled Collembola to trace the incorporation of arthropod rain into soil food webs. Live or euthanized Collembola were supplemented once to field mesocosms in the amount equivalent to the mean daily input of the arthropod rain (19mg d.w. m-2). After the addition of live Collembola, the isotopic label was found most often in predatory Trombidiformes (83% of samples) and Mesostigmata mites (85%), followed by Araneae (58%), Chilopoda (45%), and Coleoptera (29%). Among non-predatory groups, the isotopic label was recorded in Thysanoptera (27%), Collembola (24%), and Oribatida (18%). The 15N-label was also detected in Symphyla, Formicidae, Diplura, Diplopoda, Opiliones, Diptera, Hemiptera, Oligochaeta, and Nematoda. There was a positive correlation between natural 15N abundance and the frequency of the isotopic label among predators, but not among decomposers. In the non-replicated treatment, in which dead collembolans were added, the label was found in predators and decomposers in approximately equal proportions (21-25%). Unlike other forms of the aboveground subsidy (such as leaflitter, frass, or honeydew) that are primarily processed by microorganisms, arthropod rain is assimilated directly by the animals. The high frequency of consumption of the aboveground subsidy suggests that it plays a significant role in maintaining the abundance of soil predators.

Monocropping and Intercropping of Maize with Six Food Legumes at Malkerns in Eswatini: Their Effects on Plant Growth, Grain Yield and N2 Fixation, Measured using the 15N Natural Abundance and Ureide Techniques

Intercropping of legumes and cereals has many benefits to both plant partners. In this study, the effect of legume-maize intercropping on plant growth, grain yield and N2 fixation of six legumes was assessed using the 15N natural abundance and ureide techniques. For this, a field experiment involving six legume species and two cropping systems was established at the Malkerns Research Station, Eswatini during the 2017/2018 cropping season. Based on the 15N isotopic and ureide analysis, the six test legumes respectively obtained 39.06 – 70.19% and 16.46 – 55.79% of their N nutrition from symbiosis. The amounts of N-fixed ranged from 12.66 to 66.57 kg ha−1. In general, high amount of N-fixed by legumes correlated strongly with greater shoot dry matter accumulation (r = 0.7981; p < 0.001) and high grain yield (r = 0.5905; p < 0.001), indicating the importance of N2 fixation in plant growth and reproduction. Legumes grown under monocropping recorded higher plant growth, symbiotic performance and grain yield when compared to those grown in mixed culture with maize. However, shoot %Ndfa was much higher under intercropping than sole cropping due to competition by cereal and legume for soil N. Components of maize yield were similar for the two cropping systems. The %N derived from fixation and %relative ureide-N abundance were significantly correlated (r = 0.4005; p < 0.001), indicating that the 15N natural abundance technique and the ureide method were complementary in measuring N2 fixation in the test legumes. These results have provided some insights on the impact of cropping system on plant growth, symbiotic performance and grain yield of six selected legumes.

Nitrogen-15 natural abundance is robust to quantify nitrogen transfer from clover to grass in temporary grassland

Nitrogen (N) transfer from clover to grass may present a significant input of symbiotically fixed clover N in grasslands. We determined grass N transferred from clover over two years using both 15N natural abundance (NA) and 15N labelling (LAB) methods. These methods have not previously been compared under identical environmental and management conditions. We established a model clover-grass mixture in a field experiment that had been fertilised with no, organic, or mineral N for three decades. With the NA method we used procedures differing in proxies that either represented the δ15N of the N sources clover N and plant available soil N while LAB procedures differed in labelling strategies and/or calculation. Different proxies in NA had little impact on the proportion of grass N transferred from clover. The δ15N of the two N sources differed significantly under all fertilisation treatments, by 4.0‰–5.9‰ under zero and organic fertilisation, though often less than 2‰ under mineral fertilisation. The by-treatment average proportion of N transferred from clover ranged from 27% to 55% of grass N for all NA procedures, and from 21% to 68% for all LAB procedures. The LAB procedures were affected by temporally non-uniform 15N enrichment of clover roots. For both methods, all treatments, and both years, about 44% of grass N was transferred from clover. As a result, from 1.5 to 6.3 g N m−2 a−1 of clover N was transferred to grass. The NA method was found to be robust and can be applied to determine the N transfer in temporary grasslands established across a range of fertilisation treatments, including under moderate mineral N fertilisation. The NA procedure that agreed best with the average results from all NA and LAB procedures requires only the δ15N of the shoots of clover and grass growing in the mixture.

Recent progress on solid-state cross-polarization (CP)/magic angle spinning (MAS) NMR measurements: a tool for investigating structural changes in stimuli-responsive organic compounds

Abstract Over the years, cross-polarization/magic angle sample spinning (CP/MAS) NMR has proven to be valuable for obtaining detailed structural information in solid-state materials. A notable advantage of this technique is that it does not require a well-organized arrangement in the sample such as single crystals. Consequently, even materials with low crystallinity such as amorphous substances can be analyzed to obtain structural information. In recent years, research on externally stimuli-responsive molecules utilizing crystal phase transitions has attracted attention. Certain aryl disilane and oligosilane compounds exhibit external stimulus responsiveness even in the crystalline state due to the hindered rotation of the Si–Si σ bond and sparse crystal packing. Understanding the structural alternations linked to changes in physical properties is crucial for investigation of the responsiveness. High-resolution CP/MAS NMR spectra are particularly suitable for analyzing crystalline polymorphs, crystal phase transitions, and solid-state conformations. In this report, we present the results of natural abundance 13C, 29Si, and 15N CP/MAS NMR experiments conducted on externally stimuli-responsive disilane-bridged molecules. These measurements provide information on the structure before and after crystal phase transitions, as well as the amorphous state. A comprehensive analysis of our findings is essential for elucidating alternations in the structure and arrangement of crystalline materials under external stimuli.

Symbiotic N nutrition, carbon accumulation, and water-use efficiency in species of the genus Aspalathus native to the Cape fynbos, South Africa

Low soil moisture and N in the fynbos are a major constraint to plant growth and N2 fixation in legumes, including Aspalathus species. The 15N and 13C natural abundance techniques were used to study N nutrition and plant water relations in different Aspalathus species sampled from 13 study sites in the fynbos. The results showed that A. acuminata shoots recorded lower δ15N values at Kalbaskraal, Worcester Langerug and Roundebosch, resulting in higher %Ndfa at those sites relative to Elim, which registered greater δ15N values. However, A. hispida shoots showed the lowest δ15N at Elim and Dutoitskloof than Yzerfontein, and Silwerstroomstrand, resulting in high %Ndfa at Elim. Of A. cordata, A. astroites, A. hispida, and A. wildenowiana collected from Dutoitskloof differed in δ15N and %Ndfa. A. cordata and A. astroites shoots showed lower δ15N, leading to higher %Ndfa than A. wildenowiana, which exhibited greater shoot δ15N. Of four Aspalathus species (A. cordata, A. carnosa, A. capensis and A. juniperina) sampled from Silvermine, A. cordata revealed the lowest δ15N and hence the highest %Ndfa. Conversely, A. carnosa recorded the highest shoot δ15N and the lowest %Ndfa. At Kokrivier, shoot δ15N was much greater in A. zeyheri and lowest in A. incurvifolia, resulting in higher %Ndfa in the latter than the former. The Aspalathus species differed in water-use efficiency or shoot δ13C values, which ranged from −30.35‰ to −26.93‰. Rainfall, soil moisture deficit and altitude were the major determinants of shoot δ13C values, but how these factors influence shoot δ13C of plants remains to be determined.

Traceability of atmospheric ammonia in a suburban area of the Beijing-Tianjin-Hebei region

Ammonia (NH3) is one of the most important sources that have been linked to the formation of PM2.5. Therefore, it is important to study the source contributions to atmospheric NH3 for air pollution control. Here we used 15N natural abundance (expressed by δ15N) values to quantify the source contributions to atmospheric NH3 in the Beijing-Tianjin-Hebei (BTH) region, which suffers from the country's worst air pollution. Results showed that from 2017 to 2019, the annual mean δ15N-NH3 value at the livestock site (−27.5 ± 6.0 ‰) was lower than at cropland (−20.7 ± 6.0 ‰) and rural residential sites (−22.1 ± 7.4 ‰), while their concentrations were the opposite. Seasonal mean δ15N-NH3 values were the highest in winter and lowest in summer, whereas monthly mean δ15N-NH3 values were the highest in January and lowest in June. The isotope mixing model results showed that agricultural sources account for 64.5 ± 13.5 % of year-round total NH3 emissions, while industrial and other sources contributed 27.4 and 8.1 %, respectively. However, the contribution of industrial sources was higher than that of agricultural sources in January. Our results indicated that the contribution of agricultural sources has decreased after the implementation of air pollution control policies in this region suggesting that NH3 abatement from agricultural sources is effective. However, further refinement of agricultural emission abatement measures will be required, accompanied by a greater focus on controlling winter non-agricultural sources.

Residual effects of composted sewage sludge on nitrogen cycling and plant metabolism in a no-till common bean-palisade grass-soybean rotation.

In the context of increasing population and decreasing soil fertility, food security is one of humanity's greatest challenges. Large amounts of waste, such as sewage sludge, are produced annually, with their final disposal causing environmental pollution and hazards to human health. Sludge has high amounts of nitrogen (N), and, when safely recycled by applying it into the soil as composted sewage sludge (CSS), its residual effect may provide gradual N release to crops. A field study was conducted in the Brazilian Cerrado. The aims were to investigate the residual effect of successive applications of CSS as a source of N in the common bean (Phaseolus vulgaris L. cv. BRS Estilo)-palisade grass (Urochloa brizantha (A.Rich.) R.D. Webster)-soybean (Glycine max L.) rotation under no-tillage. Additionally, N cycling was monitored through changes in N metabolism; the efficiency of biological N2 fixation (BNF) and its implications for plant nutrition, development, and productivity, was also assessed. The experiment consisted of a randomized complete block design comparing four CSS rates (10, 15, 20, and 25 Mg ha-1, wet basis) to a control treatment (without adding mineral or organic fertilizer) over two crop years. Multiple plant and soil analyses (plant development and crop yield, Falker chlorophyll index (FCI), enzymatic, biochemical, 15N natural abundance, was evaluated, root and shoot N accumulation, etc.) were evaluated. Results showed that CSS: i) maintained adequate N levels for all crops, increasing their productivity; ii) promoted efficient BNF, due to the stability of ureide metabolism in plants and increased protein content; iii) increased the nitrate content and the nitrate reductase activity in soybean; iv) affected urease activity and ammonium content due to changes in the plant's urea metabolism; v) increased N accumulation in the aerial part of palisade grass. Composted sewage sludge can be used as an alternative source to meet crops' N requirements, promoting productivity gains and N cycling through forage and improving N metabolism.

Greater differentiation in nitrogen sources is essential for co-occurring epiphytes under low-nitrogen canopy conditions

Acquiring enough nitrogen (N) to support their life processes in nutrient-poor canopies is a crucial challenge for epiphytes. When different epiphytes coexist in the same environment, they may use different strategies to acquire N. In this study, we employed stable isotope analysis to explore N acquisition and differentiation among co-occurring epiphytes in a subtropical forest in Southwest China. We found that functional group was the most important factor influencing N and 15N natural abundance (δ15N) of epiphytes, and that different epiphyte groups used different N acquisition, uptake, and utilization pathways. Our analysis of the enrichment factors suggested that biological nitrogen fixation, phorophytes, canopy soil and ground soil could participate in N acquisition of epiphyte groups to varying degrees. However, epiphytes were more affected by precipitation and other canopy N sources than soil. Our structural equation models (SEMs) further showed that the potential N sources of plants and substrates affected throughfall and stemflow, thereby influenced N acquisition and preference of N forms in epiphytes. Overall, our findings indicate that there is differentiation in N sources among co-occurring epiphytes.

Nitrogen fixation among pea and lentil varieties in the Northern Great Plains

AbstractPulse crops, including lentil (Lens culinaris Medik.) and pea (Pisum sativum L.), can improve the sustainability of Northern Great Plains cropping systems, largely through biological N fixation. Greater N fixation amounts can help producers to increase yield while decreasing N fertilizer inputs for the following crop. There may be potential to breed greater N‐fixing pulse varieties, yet little is known about varietal differences in N fixation. Nitrogen fixation of pea and lentil varieties was quantified at two sites in Montana from 2019 to 2021 using an N difference approach and the 15N natural abundance method. Riveland and CDC Richlea were frequently high N‐fixing lentil varieties, both fixing ca. 130 kg N ha−1 in the site‐year with the most favorable growing conditions. No pea variety had consistently greater N fixation than others, despite N fixation ranging from 88 to 135 kg N ha−1 in one site‐year among varieties. Nitrogen fixation by lentil had an inverse relationship with days to flowering but was not correlated with days to maturity. Nitrogen fixation by pea was positively correlated with days to maturity but was not correlated with days to flowering. Breeding lentil and pea for high N fixation by selecting high N‐fixing varieties is likely difficult, as varieties performed differently under variable environmental conditions. Breeding efforts based on traits, such as days to flowering, could be more successful. There were more positive correlations between N fixation parameters and seed yield for pea than for lentil, suggesting that breeding for greater yields could increase N fixation for pea but not lentil.

Dynamic Structure of Organic Compounds in Solution According to NMR Data and Quantum Mechanical Calculations: IV. Benzamide

To study the structure and dynamics of nitrogen-containing compounds, NMR parameters with directly involved nitrogen can provide valuable structure information. However, this information can only be obtained using15N-enriched compounds due to low natural abundance of 15N and extremely short relaxation of 14N nuclei. For synthesis of these compounds from 15N-ammonium salts, 15N-enriched benzamides often used as intermediates. In the present work, we study the dynamic structure of benzamide caused by two independent factors: hindered internal rotation of the NH2 group around the C(O)-N bond and the amide group as a whole relative to the benzene ring. This deeper knowledge of mechanism and parameters of these processes in amides is important for meaningful interpretation and prediction of the biological activity of aromatic amides in living systems, the strength and conformation of its supramolecular complexes with lanthanide and actinide ions. Double enriched [2H5, 15N]benzamide was synthesized to avoid unwanted superposition of intense aromatic multiplet on the amide signals in the 1H NMR spectra. In the 1H spectrum of this compound observed only intense signals of amide protons, which allowed accurate quantitative characterization of the parameters of the dynamic processes under study. The experimental data obtained is in good agreement with our results of simulation by quantum molecular dynamics techniques.

Lentil nitrogen fixation response to fertilizer and inoculant in the northern Great Plains

AbstractLentil (Lens culinarisMedikus) production in the semiarid northern Great Plains of the United States has increased dramatically over the past two decades. Lentil in rotation provides agroecosystem benefits of more efficient water use, pest cycle disruption, and biological nitrogen (N) fixation. Increasing N fixation could alleviate soil acidification and groundwater impairment, decrease N fertilizer costs, and increase lentil seed yield. Despite widespread farmer adoption of lentil in the region, little is known about the benefits of fertilizer or inoculant type on N fixation. The aim of this study was to determine how nutrients (potassium (K), sulfur (S), and foliar‐applied micronutrients) and rhizobial inoculant types (seed‐coat powder and granular) influence N fixation of lentil. The study was conducted at two field sites in Montana from 2019 to 2021. Fixed N amounts were calculated using both an N difference approach and15N natural abundance method. N fixation was highly responsive to climatic conditions and soil characteristics. The amount of N fixed did not respond to K fertilization, likely because soil test K levels were sufficient. In a moderately dry year at a site with low soil sulfate‐S, fertilizer application of 5.6 kg S ha−1increased N fixed by 40%. Inoculated lentil fixed more N than uninoculated lentil in two site‐years, but there were no differences in N fixed between inoculant types. Inoculation response was not related to field cropping histories with legumes. The study shows that S fertilization and rhizobial inoculation have potential to increase lentil N fixation amounts in the northern Great Plains.

Interplant carbon and nitrogen transfers mediated by common arbuscular mycorrhizal networks: beneficial pathways for system functionality.

Arbuscular mycorrhizal fungi (AMF) are ubiquitous in soil and form nutritional symbioses with ~80% of vascular plant species, which significantly impact global carbon (C) and nitrogen (N) biogeochemical cycles. Roots of plant individuals are interconnected by AMF hyphae to form common AM networks (CAMNs), which provide pathways for the transfer of C and N from one plant to another, promoting plant coexistence and biodiversity. Despite that stable isotope methodologies (13C, 14C and 15N tracer techniques) have demonstrated CAMNs are an important pathway for the translocation of both C and N, the functioning of CAMNs in ecosystem C and N dynamics remains equivocal. This review systematically synthesizes both laboratory and field evidence in interplant C and N transfer through CAMNs generated through stable isotope methodologies and highlights perspectives on the system functionality of CAMNs with implications for plant coexistence, species diversity and community stability. One-way transfers from donor to recipient plants of 0.02-41% C and 0.04-80% N of recipient C and N have been observed, with the reverse fluxes generally less than 15% of donor C and N. Interplant C and N transfers have practical implications for plant performance, coexistence and biodiversity in both resource-limited and resource-unlimited habitats. Resource competition among coexisting individuals of the same or different species is undoubtedly modified by such C and N transfers. Studying interplant variability in these transfers with 13C and 15N tracer application and natural abundance measurements could address the eco physiological significance of such CAMNs in sustainable agricultural and natural ecosystems.

Stable C and N isotope abundances in water-extractable organic matter from air-dried soils as potential indices of microbially utilized organic matter

Stable carbon (C) and nitrogen (N) isotopes (13C and 15N) in water-extractable organic matter (WEOM) derived from air-dried soils may be applicable to elucidate the microbial decomposition of soil organic matter (SOM), which is crucial in terrestrial C cycles. A total of 40 soil samples were collected from a depth of 0–6 cm from a temperate broadleaved forest in Japan with vegetation succession from grassland approximately 150 years ago. Those soil samples were air-dried before the water extraction process and organic matter analysis. The C and N concentrations of WEOM were &amp;lt;3.6% of those of the bulk soil and were positively correlated with those of the bulk soil at a p-value of &amp;lt; 0.01. A positive correlation between the two fractions (i.e., WEOM and bulk soils) was also found for natural 13C and 15N abundances (δ13C and δ15N; p &amp;lt; 0.01). However, the C/N ratio of WEOM was slightly correlated with that of bulk soils, exhibiting a narrow range of values of ~10. Thus, those features of the WEOM were similar to the well-known features of microbial biomass. The δ13C and δ15N enrichments in WEOM relative to bulk soil, the difference in stable isotope abundances between bulk SOM and WEOM were negatively and positively correlated, respectively, with the concentrations of organo-mineral complexes and short-range order minerals (non-crystalline oxyhydroxides of aluminum and iron, allophane, imogolite, and allophane-like constituents), which play significant roles in SOM stabilization in soils. These relationships suggest that the stable isotopic enrichments in WEOM can be a good indicator of the microbial utilization of soil C and N under different substrate availabilities, which are crucial to SOM decomposition and decomposability substantially varying from local to global scales.

An assessment of symbiotic N nutrition in species of the genus Aspalathus endemic to the Cape Floristic Region of South Africa

Nitrogen fixation is important for the growth and yield of nodulated legumes. In this study, the 15N natural abundance technique was used to assess the symbiotic N nutrition of 15 Aspalathus species sampled from 13 locations. The results showed that all 15 species met part of their N requirements from N2 fixation, as shown by the different levels of symbiotic dependency. Seven species, for example, derived 54 to 77% of their N nutrition from symbiosis, nine obtained 36 to 49% from fixation, and five species derived ˂ 20% of their N from the atmosphere. More specifically, A. acuminata and A. clada both from Worcester Langerug derived 76.5% and 72.1% N, respectively, from symbiosis, A. acuminata from Rondebosch obtained 68.4% N, A. acuminata from Kalbaskraal 59.5% N, A. aculeata from Mamesbury 56.8% N, A. divericata from Penhill 56.6% N and A. cordata from Dutoitkloof 53.9% N from the atmosphere. In contrast, A. zeyheri sampled from Kokrivier derived only 4.3% of its N from symbiosis, followed by A. capensis (13.3%) and A. carnosa (14.6%) both from Silvermine. The C/N values found for the Aspalathus species in this study were unusually high for a legume, and could suggest an ecological adaptation to the low N soils of the Cape fynbos as under such N-poor plants metabolically switch from N-containing storage compounds to N-free storage molecules such as sucrose and glycerol, glucosides, galactoses and disaccharides.

Demystifying the agronomic and environmental N performance of grain legumes across contrasting soil textures of central Germany

Grain legume cultivation has multifaceted benefits in farmland and its on-going expansion in Europe contributes to the targeted self-sufficiency in protein production for human and livestock nutrition. However, there is limited information on key environmental processes such as the nitrogen (N) cycle. To address this knowledge gap, N input and output pathways were investigated among six grain legumes across two experimental farms (Gladbacherhof and Oberfeld) in central Germany, between 2020 and 2022. Percentage of N derived from atmospheric fixation (%Ndfa), was estimated at peak biomass production stage through the ¹⁵N natural abundance method, and ranged from 8.9% to 93.9%. Grain legumes at Oberfeld were found to have a higher %Ndfa (median 75.4%) than at Gladbacherhof (54.5%) with winter varieties i.e., winter pea (Pisum sativum) and winter faba (Vicia faba), found to be more efficient at acquiring Ndfa in comparison to soya (Glycine max). At Gladbacherhof, the N export in the form of harvested grain was higher than the N input resulting from biological N fixation while at Oberfeld, an opposite trend was observed.The potential of winter legumes to reduce nitrate leaching compared to plots which were left fallow over winter and then cropped with spring legumes, was measured. Nitrate leaching was found to be low at both sites, ranging from 1.1 to 3.4 kg N ha⁻¹ at Gladbacherhof, and from 1.0 to 1.5 kg N ha⁻¹ at Oberfeld, with no significant differences across treatments. N₂O emissions were measured from November 2020 to October 2021. Daily emissions averaged 0.033 mg N₂O-N m⁻¹ h⁻¹ whereas the cumulative emissions ranged from 0.8 to 1.6 kg N₂O-N ha⁻¹ (November 2020 – April 2021), and from 0.7 to 2.1 kg N₂O-N ha⁻¹ (April 2021 – October 2021).Selecting for grain legume varieties that can fix considerable amounts of atmospheric N₂ can be an important criterion for inclusion in crop rotations. However, the drawbacks of N losses through nitrate leaching and nitrous oxide emissions, as well as N exports through grains, should be accounted for, to ensure efficient field-scale N management.

Overestimated nitrogen loss from denitrification for natural terrestrial ecosystems in CMIP6 Earth System Models

Denitrification and leaching nitrogen (N) losses are poorly constrained in Earth System Models (ESMs). Here, we produce a global map of natural soil 15N abundance and quantify soil denitrification N loss for global natural ecosystems using an isotope-benchmarking method. We show an overestimation of denitrification by almost two times in the 13 ESMs of the Sixth Phase Coupled Model Intercomparison Project (CMIP6, 73 ± 31 Tg N yr−1), compared with our estimate of 38 ± 11 Tg N yr−1, which is rooted in isotope mass balance. Moreover, we find a negative correlation between the sensitivity of plant production to rising carbon dioxide (CO2) concentration and denitrification in boreal regions, revealing that overestimated denitrification in ESMs would translate to an exaggeration of N limitation on the responses of plant growth to elevated CO2. Our study highlights the need of improving the representation of the denitrification in ESMs and better assessing the effects of terrestrial ecosystems on CO2 mitigation.

Effects of groundwater and seasonal streamflow on the symbiotic nitrogen fixation of deep-rooted legumes in a dryland floodplain

Biological nitrogen fixation of deep-rooted legumes is a major pathway of nitrogen (N) inputs to drylands. Water sources including groundwater and streamflow are vital to the growth of deep-rooted legumes in drylands, but it is unclear how their different availability and biochemical properties may influence plant N fixation. We established two sampling transects at the southern fringe of the Taklimakan Desert (Xinjiang province, north-west China) to study the effects of both the groundwater depth and distance to a seasonally active river on the N fixation rate of Alhagi sparsifolia, a dominant deep-rooted legume species. We quantified the N fixation rate of A. sparsifolia using the 15N natural abundance method, and related variation in N fixation to water source use and soil biochemical properties along the two transects. The N fixation rate of A. sparsifolia decreased with groundwater depth and increased with distance from the river channel. Instead of variation in the water source or nutrient concentrations, we found that the gradient in salinity formed by groundwater flow and streamflow was most responsible for the spatial pattern in N fixation of A. sparsifolia. In many dryland regions, water availability controls the spatial variability of symbiotic N fixation along hydrological gradients. Our study system provides a contrasting condition where salinity most strongly regulates N fixation. In addition to understanding the important role of variation in water source and nutrient availability, our study suggests that the impact of soil salinity in mediating N fixation in dryland ecosystems is deserving of attention.