15N Natural Abundance Research Articles

Black Seedcoat Pigmentation Is a Marker for Enhanced Nodulation and N2 Fixation in Bambara Groundnut (Vigna Subterranea L. Verdc.) Landraces

Field studies on the effect of differing seedcoat colour on plant growth and symbiotic performance of three Bambara groundnut landraces using the 15N natural abundance technique and ureide analysis in the xylem stream and petiole extracts revealed a consistent pattern in nodule function between ureide levels and shoot δ15N values, as well as between ureide levels and percent N derived from fixation at Gbalahi, Kpalisogu, and Manga. At those three sites, where shoot δ15N was low and percent N derived from fixation high, ureide concentrations in the xylem sap and petiole extracts were also high, indicating that the two techniques were comparatively quite robust in estimating N2 fixation at a higher precision in the test Bambara groundnut landraces. A comparison of the effect of differing seedcoat pigmentation on the concentration of phenolics released by imbibed seeds showed that the Black landrace contained higher levels of flavonoids and anthocyanins, followed by the Red, and lowest in the Cream landrace. The Black landrace also recorded much higher nodule number and nodule fresh weight per plant, greater shoot biomass, lower shoot δ15N values, greater N derived from fixation, higher symbiotic N contribution, and increased water-use efficiency across all the five study sites, followed by the Red landrace, which produced the highest grain yield. Black seedcoat pigmentation in Bambara groundnut is therefore a marker for increased nodulation, N2 fixation, and plant growth that can be tapped in breeding programs for developing high N2-fixing, water-use efficient grain legumes for use by farmers in Africa.

Strategy for the Sampling of Sugarcane Plants for the Reliable Quantification of N2 Fixation Using 15N Natural Abundance

Strategy for the Sampling of Sugarcane Plants for the Reliable Quantification of N2 Fixation Using 15N Natural Abundance

Plasma proteins δ15N vs plasma urea as candidate biomarkers of between-animal variations of feed efficiency in beef cattle: Phenotypic and genetic evaluation

Identifying animals that are superior in terms of feed efficiency may improve the profitability and sustainability of the beef cattle sector. However, measuring feed efficiency is costly and time-consuming. Biomarkers should thus be explored and validated to predict between-animal variation of feed efficiency for both genetic selection and precision feeding. In this work, we aimed to assess and validate two previously identified biomarkers of nitrogen (N) use efficiency in ruminants, plasma urea concentrations and the 15N natural abundance in plasma proteins (plasma δ15N), to predict the between-animal variation in feed efficiency when animals were fed two contrasted diets (high-starch vs high-fibre diets). We used an experimental network design with a total of 588 young bulls tested for feed efficiency through two different traits (feed conversion efficiency [FCE] and residual feed intake [RFI]) during at least 6 months in 12 cohorts (farm × period combination). Animals reared in the same cohort, receiving the same diet and housed in the same pen, were considered as a contemporary group (CG). To analyse between-animal variations and explore relationships between biomarkers and feed efficiency, two statistical approaches, based either on mixed-effect models or regressions from residuals, were conducted to remove the between-CG variability. Between-animal variation of plasma δ15N was significantly correlated with feed efficiency measured through the two criteria traits and regardless of the statistical approach. Conversely, plasma urea was not correlated to FCE and showed only a weak, although significant, correlation with RFI. The response of plasma δ15N to FCE variations was higher when animals were fed a high-starch compared to a high-fibre diet. In addition, we identified two dietary factors, the metabolisable protein to net energy ratio and the rumen protein balance that influenced the relation between plasma δ15N and FCE variations. Concerning the genetic evaluation, and despite the moderate heritability of the two biomarkers (0.28), the size of our experimental setup was insufficient to detect significant genetic correlations between feed efficiency and the biomarkers. However, we validated the potential of plasma δ15N to phenotypically discriminate two animals reared in identical conditions in terms of feed efficiency as long as they differ by at least 0.049 g/g for FCE and 1.67 kg/d for RFI. Altogether, the study showed phenotypic, but non-genetic, relationships between plasma proteins δ15N and feed efficiency that varied according to the efficiency index and the diet utilised.

Biological N Fixation and N Transfer in an Intercropping System between Legumes and Organic Cherry Tomatoes in Succession to Green Corn

The aim of this study was to investigate the transfer of N from different legumes to cherry tomatoes in the intercropping system under residual straw of the previous green corn crop using the 15N natural abundance method. We also investigated the temporal variation in nitrogen transfer to a cherry tomato, the biological nitrogen fixation (BNF) of legumes, and the N concentration of green corn cultivated in the intercrop succession. The experimental design was a complete randomized block with eight treatments and five replications, described as follows: two controls consisting of a monocrop of cherry tomato with or without residual straw, cherry tomato and jack bean, sun hemp, dwarf velvet bean, mung bean, and white lupine or cowpea bean in intercropping system. The BNF was responsible for more than half of the N accumulated in the legumes. The N of legumes was transferred to cherry tomato in similar quantities, and the leaves and fruits of cherry tomato received more N transfer than shoots. It was shown that N transfer increases with the growth/development of cherry tomatoes. The intercropping system with legumes did not affect the 15N natural abundance of leaves and the aboveground biomass of green corn cultivated in succession.

Why is phloem sap nitrate kept low?

Since the first description of phloem sap composition nearly 60 years ago, it is generally assumed that phloem sap does not contain nitrate and that there is little or no backflow of nitrate from shoots to roots. While it is true that nitrate can occasionally be absent from phloem sap, there is now substantial evidence that phloem can carry nitrate and furthermore, transporters involved in nitrate redistribution to shoot sink organs and roots have been found. This raises the question of why nitrate may or may not be present in phloem sap, why its concentration is generally kept low, and whether plant shoot-root nutrient cycling also involves nitrate. We propose here that phloem sap nitrate is not only an essential component of plant nutritional signaling but also contributes to physical properties of phloem sap and as such, its concentration must be controlled to ensure proper coordination of plant development and nutrient transport. This article is protected by copyright. All rights reserved.

15N Natural Abundance of Soil Microbial Biomass in Alpine and Tundra Ecosystems

Isotopic composition of nitrogen in soil microbial biomass (δ15Nmicr) is connected with the transformation of nitrogen compounds and with the balance of carbon and nitrogen availability for microorganisms. We have studied the dependence of δ15Nmicr on nitrogen isotopic composition in the substrate (δ15N of total and extractable nitrogen), as well as the dependence of δ15Nmicr and 15N-enrichment of microbial biomass (Δ15Nmicr = δ15Nmicr – δ15Nsubstr) on nitrogen availability parameters (the C/N ratio in soil, the N-mineralization activity, the content of extractable nitrogen, and the nitrogen use efficiency) in soils of four alpine ecosystems in the North Caucasus and four tundra ecosystems in the Khibiny Mountains. It has been shown that δ15Nmiсr varies from –0.2 to +8.4‰ and may be characterized by both 15N-enrichment and depletion (negative Δ15Nmiсr values) relative to the total and extractable soil nitrogen. As a rule, Δ15Nmicr is 1.5–3.1‰ relative to 15Ntotal and 0.6–4.8‰ relative to 15Nextr. However, under the most N-deficiency conditions in soils of mountain tundra lichen and shrub heaths, Nmicr does not accumulate an increased amount of 15N. We have not revealed a close correlation of δ15Nmicr and Δ15Nmicr with the C/N ratio. The accumulation of 15N in microbial biomass is much stronger related to N-mineralization (positively) and the nitrogen use efficiency (negatively). This testifies to the important role of microbial nitrogen dissimilation in controlling the isotopic composition of soil microbial biomass nitrogen.

Nitrogen uptake strategies of mature conifers in Northeastern China, illustrated by the 15N natural abundance method

BackgroundConifers partition different N forms from soil, including ammonium, nitrate, and dissolved organic N (DON), to sustain plant growth. Previous studies focused on inorganic N sources and specific amino acid forms using 15N labelling, but knowledge of the contribution of DON to mature conifers’ N uptake is still scarce. Here, we quantified the contribution of different N forms (DON vs. NH4+ vs. NO3−) to total N uptake, based on 15N natural abundance of plant and soil available N, in four mature conifers (Pinus koraiensis, Pinus sylvestris, Picea koraiensis, and Larix olgensis).ResultsDON contributed 31%, 29%, 28%, and 24% to total N uptake by Larix olgensis, Picea koraiensis, Pinus koraiensis, and Pinus sylvestris, respectively, whereas nitrate contributed 42 to 52% and ammonium contributed 19 to 29% of total N uptake for these four coniferous species.ConclusionsOur results suggested that all four conifers could take up a relatively large proportion of nitrate, while DON was also an important N source for the four conifers. Given that DON was the dominant N form in study soil, such uptake pattern of conifers could be an adaptive strategy for plants to compete for the limited available N sources from soil so as to promote conifer growth and maintain species coexistence.

Experimental Design and Interpretation of Terrestrial Ecosystem Studies Using 15N Tracers: Practical and Statistical Considerations

The stable isotope 15N is an extremely useful tool for studying the nitrogen (N) cycle of terrestrial ecosystems. The affordability of isotope-ratio mass spectrometry has increased in the last decades and routine measurements of δ15N with an accuracy better than 1‰ are now easily achieved. Except perhaps for wood, which has a very high C/N ratio, isotope analysis of samples is, thus, no longer the main challenge in measuring the partitioning of 15N used as tracer in ecosystem studies. The central aim of such experiments is to quantitatively determine the fate of N after it enters an ecosystem, mainly as fertilizer, as atmospheric deposition or as plant litter. By measuring how much of this incoming N goes into different ecosystem pools, inferences can be made about the entire N cycle. Sample collection and preparation can be tedious work. Optimizing sampling schemes is thus an important aspect in the application of 15N in ecosystem research and can be helpful for obtaining a high precision of the results with the available manpower and budget. In this contribution, we combine statistical and practical considerations and give recommendations for the design of labeling experiments and also for assessments of natural 15N abundance. In particular, we discuss soil, vegetation and water sampling. We additionally address the most common questions arising during the calculation of tracer partitioning, and we provide some examples of the interpretation of experimental results.

Dynamics of new- and old- soil organic carbon and nitrogen following afforestation of abandoned cropland along soil clay gradient

Disentangling the changes of newly input and original organic carbon (OC) and nitrogen (N) after agricultural land-use transition is crucial for understanding the mechanisms behind soil OC and N dynamics in terrestrial ecosystems. However, these dynamics have received little attention so far due to the lack of an approach separating them. In this study, we determined the natural abundance of 13C and 15N in soils from afforested forestlands and adjacent croplands, the forestlands were established at different times (10, 20 and 30 years) at four sites with different soil textures across the Loess Plateau of China. The objective was to explore the changes of new- and old- OC and N along with soil clay gradient, and to examine the proportions of these changes in total OC and N along with the clay content. We showed that the conversion of cropland to forestland increased total- and new- OC and N stocks by 0.69 kg m−2 and 0.10 kg m−2, but decreased old OC and N stocks by −0.20 kg m−2 and −0.04 kg m−2, respectively. When averaged across afforestation age, the proportions of new OC and N increased, but the proportions of old OC and N decreased with the clay content. In the soils that have a higher clay content, the accumulation of N rates were greater than that of OC in the new stock, and the loss rates of N were higher than that of OC in the old stock after afforestation. In contrast, in the soils that have a lower clay content, the accumulation rates of N were lower than that of OC in the new stock, and the loss rates of N were smaller than the loss of OC in the old stock. These findings indicated that soil texture regulates the changes of OC and N in new and old soil organic matter pools following cropland afforestation.

Improving the Fertigation of Soilless Urban Vertical Agriculture Through the Combination of Struvite and Rhizobia Inoculation in Phaseolus vulgaris.

Soilless crop production is a viable way to promote vertical agriculture in urban areas, but it relies extensively on the use of mineral fertilizer. Thus, the benefits of fresher, local food and avoiding the transportation and packaging associated with food import could be counteracted by an increase in nutrient-rich wastewater, which could contribute to freshwater and marine eutrophication. The present study aimed to explore the use of mineral fertilizer substitutes in soilless agriculture. Phaseolus vulgaris (common bean) was fertilized with a combination of slow-releasing fertilizer struvite (a source of N, P, and Mg), which is a byproduct of wastewater treatment plants, and inoculation with Rhizobium (a N2-fixing soil bacteria). The experiment included three bean-production lines: (A) 2 g/plant of struvite and rhizobial inoculation; (B) 5 g/plant of struvite and rhizobial inoculation, both irrigated with a Mg-, P-, and N-free nutrient solution; and (C) a control treatment that consisted of irrigation with a full nutrient solution and no inoculation. Plant growth, development, yields, and nutrient contents were determined at 35, 62, and 84 days after transplanting as well as biological N2 fixation, which was determined using the 15N natural abundance method. Treatments A and B resulted in lower total yields per plant than the control C treatment (e.g., 59.35 ± 26.4 g plant–1 for A, 74.2 ± 23.0 g plant–1 for B, and 147.71 ± 45.3 g plant–1 for C). For A and B, the nodulation and N2 fixation capacities appeared to increase with the amount of initially available struvite, but, over time, deficient levels of Mg were reached as well as nearly deficient levels of P, which could explain the lower yields. Nevertheless, we conclude that the combination of struvite and N2-fixing bacteria covered the N needs of plants throughout the growth cycle. However, further studies are needed to determine the optimal struvite quantities for vertical agriculture systems that can meet the P and Mg requirements throughout the lifetime of the plants.

Trees enhance abundance of arbuscular mycorrhizal fungi, soil structure, and nutrient retention in low-input maize cropping systems

Retaining trees in low-input agroecosystems could be key to maintain mycelia of arbuscular mycorrhizal fungi (AMF) and hence, improve soil fertility and crop performance. We assessed the impact of faidherbia (Faidherbia albida, Fabaceae) and mango (Mangifera indica, Anacardiaceae) trees on AMF and soil fertility in smallholder farmers’ maize fields. Along distance-from-tree gradients (1, 4, 10, 15 m), we collected soil to assess AMF hyphal density, soil aggregation, and aggregate-associated carbon (C), nitrogen (N), and phosphorus (P) at the end of the non-cropping season. Further, we determined maize biomass and yield. The impact of faidherbia on maize N nutrition was assessed using the 15N natural abundance methodology. Our results show that hyphal density was largest at 4 and 10 m from trees and greater around faidherbia than mango. Soil aggregation decreased with distance from mango and was greater around faidherbia than mango. Macroaggregate-associated C, N, and P decreased with distance-from-tree, due to differences in aggregate distribution. Maize biomass was smallest at 1 m from trees and did not differ when under faidherbia versus mango. On average 69 ± 14, 24 ± 9, 20 ± 6, and 12 ± 5% of total foliar N of maize grown at 1, 4, 10, and 15 m from faidherbia trees was tree-derived. Our results suggest that faidherbia and mango trees can maintain AMF mycelia and combat declining soil fertility. Faidherbia is particularly suited to enhance measured soil parameters commonly associated with soil fertility and alleviate soil mining for N via improved internal N cycling. As such, agroforestry trees can contribute to a more sustainable agriculture positively affecting the environment via mitigating soil degradation.

Aggregation, carbon, nitrogen, and natural abundance of 13C and 15N in soils under no-tillage system fertilized with injection and surface application of pig slurry for five years

Injection of pig slurry (PS) into soils under no-tillage system (NTS) is more efficient for improving soil chemical and physical attributes, and reducing C and N emissions, than surface applications. This study evaluated the effect of using injection and surface application of PS, compared to NPK and control treatments, on the soil aggregate, C and N contents, and isotopes 13C and 15N. The NTS consisted of rotations of summer (maize) and winter (black oat and wheat) grasses from 2011 to 2015. The treatments were PS injected into the soil (PSI), PS applied on the soil surface (PSS), chemical fertilization (NPK), and control (CTRL). The following soil properties were evaluated in the 0–5, 5–10, and 10–20 cm layers: aggregate stability (geometric mean diameter – GMD; aggregate mass distribution); total organic carbon (TOC) and total nitrogen (TN) in macroaggregates (8.0–0.25 mm), microaggregates (<0.25 mm), and bulk soil (<2.0 mm); and isotopes 13C and 15N in macro and microaggregates. The application of PSI improved the soil physical attributes, presenting higher GMD (0–5 cm) than the PSS, NPK, and CTRL treatments. In the 5–10 cm layer, the PSI treatments were more efficient in increasing the GMD and macroaggregate mass than the NPK. PSI also was more efficient in increasing TOC and TN when compared to PSS, and generated a higher GMD, which are protectors of these elements in the soil. The natural abundance of 15N denoted the lower soil organic matter decomposition in the PSI treatment when compared to the PSS. The natural abundance of 13C showed less-negative values in macroaggregates than in microaggregates, denoting that the soil management practices and crops used (grasses) affected positively the abundance of 13C. After seven applications of PS in maize-oat-wheat rotation in NTS, the application of PSI was more efficient in improving the soil physical and chemical attributes than the application of PSS.

Tripartite symbioses regulate plant–soil feedback in alder

Abstract Plant–soil feedbacks regulate plant productivity and diversity, but potential mechanisms underpinning such feedbacks, such as the allocation of recent plant assimilate, remain largely untested especially for plants forming tripartite symbioses. We tested how soils from under alder Alnus glutinosa and beneath other species of the same and different families affected alder growth and nutrition, and colonization of roots by nitrogen‐fixing Frankia bacteria and ectomycorrhizal fungi. We also measured how the soil environment affected carbon capture and allocation by pulse labelling seedlings with 13CO2. We then tested for linkages between foliar nutrient stoichiometry and carbon capture and allocation and soil origin using statistical modelling approaches. Performance of alder and nitrogen nutrition were best on home and birch Betula pendula soils (both Betulaceae), whereas performance on Douglas fir Pseudotsuga menziesii (Pinaceae) soil was poor. Plants growing in P. menziesii soil were virtually devoid of Frankia and ectomycorrhizas, and the natural abundance 15N signatures of leaves were more enriched indicating distinct nitrogen acquisition pathways. Seedlings in these soils also had smaller 13C fixation and root allocation rates, leading to smaller 13C respiration rates by microbes. Statistical models showed that the best predictors of foliar N concentration were 13C allocation rates to fine roots and net CO2 exchange from the mesocosms. The best predictors for foliar phosphorus concentration were net CO2 exchange from the mesocosms and soil origin; seedlings in home soils tended to have greater foliar phosphorus compared to birch soils while seedlings from Douglas fir soils were no different from the other treatments. Foliar phosphorus concentration was not correlated with plant available or total soil phosphorus for any of the soils. Home soils also resulted in distinct ectomycorrhizal communities on seedlings roots, which could be responsible for greater foliar phosphorus concentration. Our findings show how the association of alder with nitrogen‐fixing Frankia relieved nitrogen limitation in the seedling triggering a performance feedback loop. We propose that relief of nitrogen limitation likely increases plant phosphorus demand, which may promote the formation of ectomycorrhizas in nutrient‐deficient soils. The formation of tripartite symbioses therefore generates positive plant–soil feedbacks, which enables plants to acquire mineral nutrients otherwise inaccessible in trade for carbon. A free Plain Language Summary can be found within the Supporting Information of this article.

15N natural abundance of vehicular exhaust ammonia, quantified by active sampling techniques

Vehicular exhaust has been identified as an important ammonia (NH3) emission source in urban areas using the stable isotope tracer technique (δ15N); however, its δ15N signature remains poorly constrained due to difficulties in concentrating and speciating NHx (NH3 + particulate NH4+). In this study, we deployed two active sampling techniques, MCIs (Multi-nozzle Cascade Impactor sampler; “particle-gas” order) and CCSCs (ChemComb Speciation Cartridge sampler; “gas-particle” order), to collect vehicular exhaust NHx in a long urban tunnel in Shenyang, China, and quantified the 15N from the collected NH3 and NH4+. While no significant difference in tunnel NH3 concentrations between the two samplers was observed (n = 21), the δ15N-NH3 was slightly higher using the MCIs (6.3 ± 1.6‰) compared to the CCSCs (4.8 ± 2.3‰) during the tunnel operation period. This minor absolute difference of 1.5 ± 2.8‰ may derive from denuder-saturation used within the CCSCs that lead to NHx speciation ambiguity, which also caused significant δ15N-pNH4+ differences between the two sampling approaches. Our results suggest that both active samplers can well characterize the δ15N-NHx in the ambient environment, but the “particle - gas” sampling approach performs better in elevated NH3 concentration environments. We recommend that the δ15N value of 6.3‰ is used as the isotope end member for vehicle NH3 when performing NH3 source apportionment.

Nitrogen accumulated and biologically fixed by uninoculated Anadenanthera peregrina (L.) Speg trees under monospecific stands in the Atlantic Forest biome

Nitrogen-fixing legumes are widely used to improve ecosystems. However, N accumulation in biomass, N2 amounts biologically fixed, and the effects on soil N status are poorly understood for plantations with slow-growing N2-fixing legume trees in seasonally dry environments. In this study, we assessed these questions in monospecific stands of uninoculated Anadenanthera peregrina (L.) var. peregrina (angico) trees established in the Atlantic Forest biome. Nine experimental plots with a stand density of 1111 tree ha−1 were examined about five to six years after tree planting. N content in the leaves, branches, bark, and wood was estimated using allometric equations and N concentration in the tissues. To assess the N content, the topsoil layer was sampled in each plot and in one pasture area. The annual N2 fixation rate at the stand scale was estimated by the natural abundance of 15N, using leaves of angico and a mix of leaves of herbs and shrubs growing in the understory in each plot. N accumulation in aboveground biomass was estimated at 260 kg ha−1 and N allocated in the leaves and branches was four-fold that in the tree stem. N derived from biological N2 fixation was estimated at 50% on average, with an annual rate of N fixed in the aboveground biomass of 22 kg ha−1 year−1. Soil N storage was similar between A. peregrina stands and the pasture area at six years after planting. Long-term gains of N2 fixation may be greatly improved by the adoption of breeding programs for this slow-growing species and the use of an adequate rhizobium strain.

Variable root:shoot ratios and plant nitrogen concentrations discourage using just aboveground biomass to select legume service crops

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

Multiyear measurements on 15N natural abundance of precipitation nitrate at a rural forested site

Large anthropogenic NOxemissions increase atmospheric nitrate (NO3−) deposition. The China's Clean Air Action has been implemented to reduce anthropogenic NOxemissions since 2013. To examine if atmospheric NO3− deposition would decrease and its sources had changed as a result of this regulation, we collected the precipitation samples (n = 181) on a daily basis at a rural forested site in Northeast China from April 2014 to December 2017 and measured the concentrations and δ15N values of NO3−. We found annual NO3− deposition ranged from 6.5 to 7.3 kg N ha−1 yr−1 and gradually declined since 2015. The δ15N–NO3- spanned a large range from −17.4 to 13.1‰ with a mass-weighted mean of −3.5‰ during the study period. The δ15N–NO3- was higher by 10‰ in winter (3.3 ± 4.2‰) than in summer (−7.4 ± 3.9‰), which was driven by the changes of both coal combustion and soil emission. Over the study period, NOx from coal combustion, vehicle exhaust, biomass burning and soil emission contributed 28.2 ± 11.9%, 28.8 ± 16.3%, 26.9 ± 14.7%, and 16.1 ± 7.2% to precipitation NO3−, respectively. We found that relative contribution of fossil fuel combustion did not decline with the China's Clean Air Action and considerable NOxreduction at the regional scale.

Influence of PRE-emergence herbicides on soybean development, root nodulation and symbiotic nitrogen fixation

Widespread occurrence of herbicide-resistant weeds has further limited effective POST-emergence herbicide options in soybean (Glycine max (L.) Merr.) leading to an increased adoption of PRE-emergence herbicides. The objective of this study was to investigate the influence of 11 commonly used PRE herbicides on soybean development, root nodulation, and symbiotic N fixation. Soybean plants were grown under greenhouse conditions in pots (10 L; 4 plants per pot) filled with silt loam soil and treated one day after planting with a labeled field rate of imazethapyr, chlorimuron-ethyl, cloransulam-methyl, metribuzin, sulfentrazone, flumioxazin, saflufenacil, acetochlor, S-metolachlor, dimethenamid-P, pyroxasulfone and no herbicide (nontreated control). Sulfentrazone reduced soybean canopy at the VC growth stage but no canopy reduction was observed at the V2 growth stage from any of the herbicide treatments. At the R2 growth stage, herbicides had no effect on soybean development (root and shoot biomass), root nodulation (# nodule per plant, nodule diameter, and nodule biomass) and symbiotic N fixation (acetylene reduction assay and 15N natural abundance). According to our findings, although PRE herbicides may slightly affect early-season soybean development, the impacts on plant growth, root nodulation, and symbiotic N fixation were negligible. Thus, when sprayed according to the label, the benefits of PRE herbicides for weed control likely outweigh any potential concern regarding soybean development, root nodulation, and N fixation.

Graphitic nitrogen in carbon catalysts is important for the reduction of nitrite as revealed by naturally abundant 15N NMR spectroscopy.

Metal-free nitrogen-doped carbon is considered as a green functional material, but the structural determination of the atomic positions of nitrogen remains challenging. We recently demonstrated that directly-excited solid state 15N NMR (ssNMR) spectroscopy is a powerful tool for the determination of such positions in N-doped carbon at natural 15N isotope abundance. Here we report a green chemistry approach for the synthesis of N-doped carbon using cellulose as a precursor, and a study of the catalytic properties and atomic structures of the related catalyst. N-doped carbon (NH3) was obtained by the oxidation of cellulose with HNO3 followed by ammonolysis at 800 °C. It had a N content of 6.5 wt% and a surface area of 557 m2 g-1, and 15N ssNMR spectroscopy provided evidence for graphitic nitrogen besides regular pyrrolic and pyridinic nitrogen. This structural determination allowed probing the role of graphitic nitrogen in electrocatalytic reactions, such as the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nitrite reduction reaction. The N-doped carbon catalyst (NH3) showed higher electrocatalytic activities in the OER and HER under alkaline conditions and higher activity for nitrite reduction, as compared with a catalyst prepared by the carbonization of HNO3-treated cellulose in N2. The electrocatalytic selectivity for nitrite reduction of the N-doped carbon catalyst (NH3) was directly related to the graphitic nitrogen functions. Complementary structural analyses by means of 13C and 1H ssNMR, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and low-temperature N2 adsorption were performed and provided support to the findings. The results show that directly-excited 15N ssNMR spectroscopy at natural 15N abundance is generally capable of providing information on N-doped carbon materials if relaxation properties are favorable. It is expected that this approach can be applied to a wide range of solids with an intermediate concentration of N atoms.

Symbiotic Nitrogen Fixation by Legumes in Alpine Ecosystems: a Vegetation Experiment

The natural nitrogen-15 abundance method does not always make it possible to calculate the rate of symbiotic nitrogen fixation by legumes and needs to be improved. Five legume species typical for the alpine belt of the Teberda Nature Reserve (Anthyllis vulneraria, Astragalus levieri, Hedysarum caucasicum, Oxytropis kubanensis, and Trifolium polyphyllum) have been grown from seeds under conditions of laboratory vegetation experiment. The results show that nodules on the roots of these plants are formed at early stages of their development; Trifolium polyphyllum does not form nodules either under high-mountain conditions or during growth in the laboratory. The natural 15N abundance in the leaves of legume plants in alpine ecosystems makes it possible to calculate the contribution of atmospheric N2 to nitrogen nutrition as early as the first year of their development, while the isotopic nitrogen composition of the roots does not allow this parameter to be determined. The calculation of atmospheric nitrogen fixation rate should take into account isotope fractionation between symbiotic bacteria (nodules) and the host plant; otherwise, the proportion of fixed nitrogen in plant nutrition may be underestimated.