15N Labeling Research Articles

Nutrient mediation of sink strength in the Orobanche minor – Red clover association

Holoparasites are non-photosynthetic plants which derive all their growth requirements from their host plant and are thought to act as a very strong sink for host resources. Here, we grew red clover plants in split-root boxes to explore the effect of nutrient supply to Orobanche minor parasitized or unparasitized host roots. Where nutrients were supplied to parasitized roots, parasite growth was strongly promoted at the expense of the host. Conversely, host growth did not differ significantly from unparasitized controls where nutrients were supplied to unparasitized roots. While 15N labelling suggested both strong parasitic ammonium abstraction and reduced nitrate uptake in parasitized roots, the total N content of systems where nutrients were fed to parasitized roots was approximately 26 % higher than control plants, suggesting that changes in host and parasite growth rates were due to changes in sink strength, rather than nutrient uptake. Parasitism and nutrient supply had strong effects on leaf carbohydrate metabolism but did not affect photosynthetic rates or leaf N concentration. In the second experiment, we investigated the importance of light level on the host – parasite relationship, concluding that parasitism had a diminished effect on host growth under low light conditions. Total system mass was unaffected by the apparent sink strength of the parasite. Our results suggest a dynamic relationship between host shoot and parasite sink strengths, mediated by changes in nutrient status.

Measuring 15N and 13C Enrichment Levels in Sparsely Labeled Proteins Using High-Resolution and Tandem Mass Spectrometry.

Isotope labeling of both 15N and 13C in selected amino acids in a protein, known as sparse labeling, is an alternative to uniform labeling and is particularly useful for proteins that must be expressed using mammalian cells, including glycoproteins. High levels of enrichment in the selected amino acids enable multidimensional heteronuclear NMR measurements of glycoprotein three-dimensional structure. Mass spectrometry provides a means to quantify the degree of enrichment. Mass spectrometric measurements of tryptic peptides of a selectively labeled glycoprotein expressed in HEK293 cells revealed complicated isotope patterns which consisted of many overlapping isotope patterns from intermediately labeled peptides, which complicates the determination of the label incorporation. Two challenges are uncovered by these measurements. Metabolic scrambling of amino groups can reduce the 15N content of enriched amino acids or increase the 15N in nontarget amino acids. Also, undefined, unlabeled medium components may dilute the enrichment level of labeled amino acids. The impact of this unexpected metabolic scrambling was overcome by simulating isotope patterns for all isotope-labeled peptide states and generating linear combinations to fit to the data. This method has been used to determine the percent incorporation of 15N and 13C labels and has identified several metabolic scrambling effects that were previously undetected in NMR experiments. Ultrahigh mass resolution is also utilized to obtain isotopic fine structure, from which enrichment levels of 15N and 13C can be assigned unequivocally. Finally, tandem mass spectrometry can be used to confirm the location of heavy isotope labels in the peptides.

In situ 15N labeling reveals high soil N2 emission during anaerobic soil disinfestation period in a greenhouse vegetable production system

In situ 15N labeling reveals high soil N2 emission during anaerobic soil disinfestation period in a greenhouse vegetable production system

Measurement of absolute abundance of crystallins in human and αA N101D transgenic mouse lenses using 15N-labeled crystallin standards

Stable isotope labeled standards of all major human lens crystallins were created to measure the abundance of lens endogenous crystallins from birth to adulthood. All major human crystallins (αA, αB, βA2, βA3/A1, βA4, βB1, βB2, βB3, γA, γB, γC, γD, γS) were cloned with N-terminal 6 x His tagged SUMO for ease of purification and the ability to generate natural N-termini by SUMO protease cleavage when producing crystallins for structure/function studies. They were then expressed in 15N-enriched media, quantified by mass spectrometry, and mixed in proportions found in young human lens to act as an artificial lens standard. The absolute quantification method was tested using soluble protein from 5-day, 23-day, 18-month, and 18-year-old human lenses spiked with the 15N artificial lens standard. Proteins were trypsinized, relative ratios of light and heavy labeled peptides determined using high-resolution precursor and data independent MS2 scans, and data analysis performed using Skyline software. Crystallin abundances were measured in both human donor lenses and in transgenic mouse αA N101D cataract lenses. Technical replicates of human crystallin abundance measurements were performed with average coefficients of variation of approximately 2% across all 13 crystallins. αA crystallin comprised 27% of the soluble protein of 5-day-old lens and decreased to 16% by 18-years of age. Over this time period αB increased from 6% to 9% and the αA/αB ratio decreased from 4.5/1 to 2/1. γS-crystallin also increased nearly 2-fold from 7% to 12%, becoming the 3rd most abundant protein in adult lens, while βB1 increased from 14% to 20%, becoming the most abundant crystallin of adult lens. Minor crystallins βA2, βB3, and γA comprised only about 1% each of the newborn lens soluble protein, and their abundance dropped precipitously by adulthood. While 9 of the SUMO tagged crystallins were useful for purification of crystallins for structural studies, γA, γB, γC, and γD were resistant to cleavage by SUMO protease. The abundance of WT and N101D human αA in transgenic mouse lenses was approximately 40-fold lower than endogenous mouse αA, but the deamidation mimic human αA N101D was less soluble than human WT αA. The high content of αA and the transient abundance of βA2, βB3, and γA in young lens suggest these crystallins play a role in early lens development and growth. βB1 becoming the most abundant crystallin may result from its role in promoting higher order β-crystallin oligomerization in mature lens. The full set of human crystallin expression vectors in the Addgene repository should be a useful resource for future crystallin studies. 15N labeling of these crystallins will be useful to accurately quantify crystallins in lens anatomic regions, as well as measure the composition of insoluble light scattering crystallin aggregates. The standards will also be useful to measure the abundance of crystallins expressed in transgenic animal models.

1H, 13C and 15N backbone resonance assignment of the calcium-activated EndoU endoribonuclease.

The catalytic domain of the calcium-dependent endoribonuclease EndoU from Homo sapiens was expressed in E. coli with 13C and 15N labeling. A nearly complete assignment of backbone 1H, 15N, and 13C resonances was obtained, as well as a secondary structure prediction based on the assigned chemical shifts. The predicted secondary structures were almost identical to the published crystal structure of calcium-activated EndoU. This is the first NMR study of an eukaryotic member of the EndoU-like superfamily of ribonucleases.

Parasitic plants regulate C and N distribution among common mycorrhizal networks linking host and neighboring plants.

Common mycorrhizal networks (CMNs) can link multiple plants and distribute nutrients among them. However, how parasitic plants regulate the carbon and nutrient exchange between CMNs and the linked plants is unknown. Thus, we conducted a container experiment with two Trifolium pratense grown in two plastic cores and connected only by CMNs using a 25-μm nylon fabric in each container. Host T. pratense was parasitized or not parasitized by Cuscuta gronovii. CMNs were left intact or broken by rotating the cores with the host or neighboring T. pratense. The dual 15N and 13C labeling method was used to evaluate the N distributed by CMNs to the host and neighboring T. pratense and the recently fixed C from the host and neighboring T. pratense to CMNs. The results showed that CMNs distributed more 15N to unparasitized neighboring T. pratense than the parasitized host T. pratense. Moreover, the unparasitized neighboring T. pratense provides more recently fixed C to CMNs than the parasitized host T. pratense. These results revealed that the parasite regulated C and nutrient exchange between CMNs and the linked plants following the reciprocal rewards rule. Moreover, this study highlights the importance of parasitic plants in the regulation of mutualistic interactions in ecological webs.

An in situ 15N labeling experiment unveils distinct responses to N application approaches in a mountain beech forest.

Atmospheric nitrogen (N) deposition has notably increased since the industrial revolution, doubling N inputs to terrestrial ecosystems. This could mitigate N limitations in forests, potentially enhancing productivity and carbon sequestration. However, excessive N can lead to forest N saturation, causing issues like soil acidification, nutrient imbalances, biodiversity loss, increased tree mortality and a potential net greenhouse gas emission. Traditional experiments often overlook the canopy's role in N fate, focusing instead on direct N addition to the forest floor. In our study, we applied 20kgNha y-1 of labeled 15NH415NO3 solution (δ15N=30‰) both above and below the canopy, maintaining also control plots. We assessed ecosystem components before and after treatment, calculated N stocks, and used mass balance for fertilizer recovery analysis. Findings revealed that the above-canopy N addition intercepted up to 31±4% of added N in foliage, a significant contrast to the negligible recovery in leaves with below-canopy treatment. Overall plant recovery was higher in the above-canopy treatment (43±11%) compared with below (9±24%). Post-vegetative season, about 15±1% of above-canopy added N was transferred to soil via litterfall, indicating substantial N reabsorption or loss through volatilization, stemflow or throughfall. In contrast, the below-canopy approach resulted in just 4.0±0.6% recovery via litterfall. These results highlight a significant difference in N fate based on the application method. Nitrogen applied to the canopy showed distinct recovery in transient compartments like foliage. However, over a few months, there was no noticeable change in N recovery in long-lived tissues across treatments. This implies that N application strategy does not significantly alter the distribution of simulated wet N deposition in high Carbon/N tissues, underscoring the complex dynamics of forest N cycling.

The distribution of particulate organic matter in the heterogeneous soil matrix - Balancing between aerobic respiration and denitrification

Denitrification, a key process in soil nitrogen cycling, occurs predominantly within microbial hotspots, such as those around particulate organic matter (POM), where denitrifiers use nitrate as an alternative electron acceptor. For accurate prediction of dinitrogen (N2) and nitrous oxide (N2O) emissions from denitrification, a precise quantification of these microscale hotspots is required. The distribution of POM is of crucial importance in this context, as the local oxygen (O2) balance is governed not only by its high O2 demand but also by the local O2 availability.Employing a unique combination of X-ray CT imaging, microscale O2 measurements, and 15N labeling, we were able to quantify hotspots of aerobic respiration and denitrification. We analyzed greenhouse gas (GHG) fluxes, soil oxygen supply, and the distribution of POM in intact soil samples from grassland and cropland under different moisture conditions. Our findings reveal that both proximal and distal POM, identified through X-ray CT imaging, contribute to GHG emissions. The distal POM, i.e. POM at distant locations to air-filled pores, emerged as a primary driver of denitrification within structured soils of both land uses. Thus, the higher denitrification rates in the grassland could be attributed to the higher content of distal POM. Conversely, despite possessing compacted areas that could favor denitrification, the cropland had only small amounts of distal POM to stimulate denitrification in it. This underlines the complex interaction between soil structural heterogeneity, organic carbon supply, and microbial hotspot formation and thus contributes to a better understanding of soil-related GHG emissions.In summary, our study provides a holistic understanding of soil-borne greenhouse gas emissions and emphasizes the need to refine predictive models for soil denitrification and N2O emissions by incorporating the microscale distribution of POM.

Grazing regimes alter the fate of 15N‐labeled urea in a temperate steppe

AbstractClarifying the fate of different nitrogen (N) species in different pools of terrestrial ecosystems is a prerequisite for a comprehensive understanding of the influence of human activities on the N cycle. Grazing has always been an important way of grassland management for centuries in the temperate grasslands of North China. However, how grazing regimes affect the N fate of urea derived from the livestock in the plant–soil systems of grazed grasslands remains poorly understood. Therefore, an in situ three‐factor (grazing regime, soil depth, and sampling time) 15N‐labeling experiment in a temperate steppe was conducted to answer this question. After 48 days of 15N labeling, the 15N recovered in shoots under no grazing (7.3% ± 1.8%) was approximately 2.3 times of that under rotational (3.2% ± 0.4%) and 2.5 times of that under continuous overgrazing (2.9% ± 0.5%). More 15N was recovered in roots under rotational than continuous overgrazing (19.8% ± 2.4% vs. 10.4% ± 0.5%, respectively), indicating that rotational overgrazing could promote more N retention in the roots. However, the 15N recovered in the soil was lower under continuous (23.7% ± 2.0%) than that of no grazing (42.0% ± 5.6%). Additionally, overgrazing reduced the magnitude of the soil active N pool in microbial biomass N and soluble N relative to no grazing. The grazing regimes would have significantly influenced both the soil and plants. That is to say, grazing regimes have directly impacted plant growth, and subsequently indirectly affected soil properties. Overgrazing often led to excessive vegetation consumption, resulting in decreased soil water content (SWC) and reduced soil organic carbon (SOC), ultimately caused alterations in plant species composition. The retention of 15N within the plant–soil system under continuous overgrazing was notably lower compared to that of no grazing. Continuous overgrazing has led to a shift in the dominant plant species from Leymus chinensis to Stipa grandis, by decreasing the proportion of perennial grasses by 10%, and increasing the annual and biennial plants by 8%. The fate of 15N was also altered in response to the variations in grazing regimes. Consequently, the recovery of 15N within the plant–soil system under continuous overgrazing was significantly lower compared to that of no grazing. In conclusion, overgrazing reduces the recovery of 15N within the plant–soil system in the temperate steppe, and rotational grazing is more preferable over continuous grazing as it could promote higher N retention in grassland ecosystems.

Oxidative Nitrogen Insertion into Silyl Enol Ether C═C Bonds.

Here, we demonstrate a fundamentally new reactivity of the silyl enol ether functionality utilizing an in situ-generated iodonitrene-like species. The present transformation inserts a nitrogen atom between the silyl enol ether olefinic carbons with the concomitant cleavage of the C═C bond. Overall, this facile transformation converts a C-nucleophilic silyl enol ether to the corresponding C-electrophilic N-acyl-N,O-acetal. This unprecedented access to α-amido alkylating agents enables modular derivatization with carbon and heteroatom nucleophiles and the unique late-stage editing of carbon frameworks. The reaction efficiency of this transformation is well correlated with enol ether nucleophilicity as described by the Mayr N scale. Applications presented herein include late-stage nitrogen insertion into carbon skeletons of natural products with previously unattainable regioselectivity as well as modified conditions for 15N labeling of amides and lactams.

Probing nitro(so) and chloro byproducts and their precursors in natural organic matter during UV/NH2Cl treatment by FT-ICR MS with machine learning insights

The UV/monochloramine (UV/NH2Cl) process, while efficiently eliminating micropollutants, produces toxic byproducts. This study utilized Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to investigate molecular-level changes in natural organic matter (NOM) and to disclose formation pathways of nitro(so) and chloro byproducts in the UV/NH2Cl process. The UV/NH2Cl process significantly increased the saturation and oxidation levels and altered the elemental composition of NOM. Using 15N labeling and a screening workflow, nitro(so) byproducts with nitrogen originating from inorganic sources (i.e., reactive nitrogen species (RNS) and/or NH2Cl) were found to exhibit total intensities comparable to those from NOM. RNS, rather than NH2Cl, played a significant role in incorporating nitrogen into NOM. Through linkage analysis, nitro(so) addition emerged as an important reaction type among the 25 reaction types applied. By using phenol as a representative model compound, the nitro byproducts were confirmed to be mainly generated through the oxidation of nitroso byproducts instead of nitration. Machine learning and SHAP analysis further identified the major molecular indices distinguishing nitro(so) and chloro precursors from non-precursors. This study enhances our fundamental understanding of the mechanisms driving the generation of nitro(so) and chloro byproducts from their precursors in complex NOM during the UV/NH2Cl process.

1H, 13C and 15N backbone resonance assignment of Cel45A from Phanerochaete chrysosporium

A glycoside hydrolase family 45 (GH45) enzyme from the white-rot basidiomycete fungus Phanerochaete chrysosporium (PcCel45A) was expressed in Pichia pastoris with 13C and 15N labelling. A nearly complete assignment of 1H, 13C and 15N backbone resonances was obtained, as well as the secondary structure prediction based on the assigned chemical shifts using the TALOS-N software. The predicted secondary structure was almost identical to previously published crystal structures of the same enzyme, except for differences in the termini of the sequence. This is the first NMR study using an isotopically labelled GH45 enzyme.

Efficient Synthetic Access to Stable Isotope Labelled Pseudouridine Phosphoramidites for RNA NMR Spectroscopy.

Here we report the efficient synthetic access to 13C/15N-labelled pseudouridine phosphoramidites, which were incorporated into a binary H/ACA box guide RNA/product complex comprising 77 nucleotides (nts) in total and into a 75 nt E. coli tRNAGly. The stable isotope (SI) labelled pseudouridines were produced via a highly efficient chemo-enzymatic synthesis. 13C/15N labelled uracils were produced via chemical synthesis and enzymatically converted to pseudouridine 5'-monophosphate (ΨMP) by using YeiN, a Ψ-5'-monophosphate C-glycosidase. Removal of the 5'-phosphate group yielded the desired pseudouridine nucleoside (Ψ), which was transformed into a phosphoramidite building suitable for RNA solid phase synthesis. A Ψ -building block carrying both a 13C and a 15N label was incorporated into a product RNA and the complex formation with a 63 nt H/ACA box RNA could be observed via NMR. Furthermore, the SI labelled pseudouridine building block was used to determine imino proton bulk water exchange rates of a 75 nt E. coli tRNAGly CCmnm5U, identifying the TΨC-loop 5-methyluridine as a modifier of the exchange rates. The efficient synthetic access to SI-labelled Ψ building blocks will allow the solution and solid-state NMR spectroscopic studies of Ψ containing RNAs and will facilitate the mass spectrometric analysis of Ψ-modified nucleic acids.

Single-Scan Heteronuclear 13C-15N J-Coupling NMR Observations Enhanced by Dissolution Dynamic Nuclear Polarization.

Heteronuclear 13C-15N couplings were measured in single-scan nuclear magnetic resonance (NMR) experiments for a variety of nitrogen-containing chemical compounds with varied structural characteristics, by using a one-dimensional (1D) 13C-15N multiple-quantum (MQ)-filtered experiment. Sensitivity limitations of the MQ filtering were overcome by the combined use of 15N labeling and dissolution dynamic nuclear polarization (dDNP), performed at cryogenic conditions and followed by quick and optimized sample melting and transfer procedures. Coupling information could thus be obtained from nucleotide bases, amino acids, urea, and aliphatic and aromatic amides, including the measurement of relatively small J-couplings directly from the 1D filtered spectra. This experiment could pave the way for NMR-based analytical applications that investigate structural and stereochemical insights into nitrogen-containing compounds, including dipeptides and proteins, while relying on heteronuclear couplings and nuclear hyperpolarization.

High sugarcane yield and large reduction in reactive nitrogen loss can be achieved by lowering nitrogen input

Nitrogen (N) fertilisation is critical for sugarcane crop production. Consequently, overuse of N fertiliser in sugarcane is widespread and it is most prevalent in China. Yet, a comprehensive analysis of sugarcane N balance accounting for different reactive N (Nr) loss pathways, crop N use and soil N retention using 15N labelled fertilisers under commercial high-input cropping conditions is lacking. A two-year field experiment with 5 N levels (0–560 kg N ha−1; N0, N210, N300, N390, N560) was conducted to quantify the effect of N rate on sugarcane yield, Nr loss through different loss pathways, crop N use and soil N retention, and to identify an optimal N rate that sustain high yield whilst reducing Nr loss in Southern China. The results showed that optimal N rate (N300) had similar yield compared with farmer practice (N560) for the plant crop and the ratoon crop. Nitrogen leaching and ammonia loss were increased significantly with increase in N rate, with farmer practice recording the highest Nr loss. Compared with farmer practice, under optimal N application, N leaching was reduced from 211 kg ha−1 to 107 kg ha−1 and from 244 kg ha−1 to 165 kg ha−1 for the plant and ratoon crops, respectively. The ammonia volatilization from the plant crop and the ratoon crop in the optimal N treatment was 24.8 kg ha−1 and 27.0 kg ha−1, respectively, which are considerably less compared with the emission from the farmer practice. There was no significant difference in nitrous oxide emission between optimal N rate and farmer practice. 15N labelling studies showed that crop N recovery, soil N residue and fertiliser N lost at the optimal N rate were 20.9%, 24.9% and 54.2%, respectively, as opposed to 12.5%, 14.8% and 72.7%. respectively, under farmer practice. To the best of our knowledge, this study forms the first comprehensive evaluation of sugarcane N balance accounting Nr loss, N use and native N soil stock covering a wide range of N supply simultaneously in the plant and ratoon crops under commercial crop production condition. This study thus provides very valuable new knowledge not only for crop N supply optimisation in high-input sugarcane production systems as in China but also for sugarcane greenhouse gas accounting studies particularly from climate change mitigation and bioenergy production context.

Different nitrogen concentrations affect strawberry seedlings nitrogen form preferences through nitrogen assimilation and metabolic pathways

The preference for different forms of nitrogen (N) is an essential means to improve plant nitrogen utilization efficiency (NUE). Currently, little information is known about the N uptake preferences of strawberries and their interaction with N concentration. In this study, the preference and distribution of nitrate nitrogen (NO3−-N) and ammonium nitrogen (NH4+-N) in strawberry seedlings were investigated in response to different N concentrations. This study used two labeled N sources (15NH4NO3 and NH415NO3) for tracer labeling. Three levels of NH4NO3 were set: low N 5 mM (N1), medium N 15 mM (N2), and high N 25 mM (N3). Each N level included two 15N labeling treatments, totaling six treatments. The results showed that under low N, medium N, and high N conditions, strawberry seedlings exhibited preferences for NH4+-N, NO3−-N, and NH4+-N, respectively. Compared with N1 and N3 treatments, N2 treatment significantly increased the activity of NR, NiR, GS, and GOGAT in strawberry seedlings roots and leaves, and the relative expression levels of FaNRT1.1, FaNRT2.1, and FaAMT1.1 in roots were significantly upregulated; When the N supply concentration increases, the distribution center of NO3−-N in strawberry seedlings shifts from stem to leaf, and under high N conditions, NH4+-N accumulates significantly in the strawberry roots. According our research, the N assimilation and metabolic capacity of strawberries themselves are important factors affecting N form preferences.

Short-term nitrogen addition reduces soil microbial nitrogen fixation rate in subtropical Pinus taiwanensis and Castanopsis faberi forests

Biological nitrogen (N) fixation is an important source of N in terrestrial ecosystems, but the response of soil microbial N fixation rate to N deposition in different forest ecosystems still remains uncertain. We conducted a field N addition experiment to simulate atmosphere N deposition in subtropical Pinus taiwanensis and Castanopsis faberi forests. We set up three levels of nitrogen addition using urea as the N source: 0 (control), 40 (low N), and 80 g N·hm-2·a-1(high N) to examine the chemical properties, microbial biomass C, enzyme activities, and nifH gene copies of top soils (0-10 cm). We also measured the microbial N fixation rate using the 15N labeling method. Results showed that N addition significantly reduced the soil microbial N fixation rate in the P. taiwanensis and C. faberi forests by 29%-33% and 10%-18%, respectively. Nitrogen addition significantly reduced N-acquiring enzyme (i.e., β-1, 4-N-acetylglucosaminidase) activity and nifH gene copies in both forest soils. There was a significant positive correlation between the microbial N fixation rate and soil dissolved organic C content in the P. taiwanensis forest, but a significant negative relationship between the rate of soil microbial nitrogen fixation and NH4+-N content in the C. faberi forest. Overall, soil microbial N fixation function in the P. taiwanensis forest was more sensitive to N addition than that in the C. faberi forest, and the factors affecting microbial N fixation varied between the two forest soils. The study could provide insights into the effects of N addition on biological N fixation in forest ecosystems, and a theoretical basis for forest management.

Species-specific responses of C and N allocation to N addition: evidence from dual 13C and 15N labeling in three tree species

Soil nitrogen (N) availability affects plant carbon (C) utilization. However, it is unclear how various tree functional types respond to N addition in terms of C assimilation, allocation, and storage. Here, a microcosm experiment with dual 13C and 15N labeling was conducted to study the effects of N addition (i.e., control, 0 g N kg−1; moderate N addition, 1.68 g N kg−1; and high N addition, 3.36 g N kg−1 soil) on morphological traits, on changes in nonstructural carbohydrates (NSC) in different organs, as well as on C and N uptake and allocation in three European temperate forest tree species (i.e., Acer pseudoplatanus, Picea abies and Abies alba). Our results demonstrated that root N uptake rates of the three tree species increased by N addition. In A. pseudoplatanus, N uptake by roots, N allocation to aboveground organs, and aboveground biomass allocation significantly improved by moderate and high N addition. In A. alba, only the high N addition treatment considerably raised aboveground N and C allocation. In contrast, biomass as well as C and N allocation between above and belowground tissues were not altered by N addition in P. abies. Meanwhile, NSC content as well as C and N coupling (represented by the ratio of relative 13C and 15N allocation rates in organs) were affected by N addition in A. pseudoplantanus and P. abies but not in A. alba. Overall, A. pseudoplatanus displayed the highest sensitivity to N addition and the highest N requirement among the three species, while P. abies had a lower N demand than A. alba. Our findings highlight that the responses of C and N allocation to soil N availability are species-specific and vary with the amount of N addition.

Climate controls on nitrate dynamics and gross nitrogen cycling response to nitrogen deposition in global forest soils

Understanding the patterns and controls regulating nitrogen (N) transformation and its response to N enrichment is critical to re-evaluating soil N limitation or availability and its environmental consequences. Nevertheless, how climatic conditions affect nitrate dynamics and the response of gross N cycling rates to N enrichment in forest soils is still only rudimentarily known. Through collecting and analyzing 4426-single and 769-paired observations from 231 15N labeling studies, we found that nitrification capacity [the ratio of gross autotrophic nitrification (GAN) to gross N mineralization (GNM)] was significantly lower in tropical/subtropical (19%) than in temperate (68%) forest soils, mainly due to the higher GNM and lower GAN in tropical/subtropical regions resulting from low C/N ratio and high precipitation, respectively. However, nitrate retention capacity [the ratio of dissimilatory nitrate reduction to ammonium (DNRA) plus gross nitrate immobilization (INO3) to gross nitrification] was significantly higher in tropical/subtropical (86%) than in temperate (54%) forest soils, mainly due to the higher precipitation and GNM of tropical/subtropical regions, which stimulated DNRA and INO3. As a result, the ratio of GAN to ammonium immobilization (INH4) was significantly higher in temperate than in tropical/subtropical soils. Climatic rather than edaphic factors control heterotrophic nitrification rate (GHN) in forest soils. GHN significantly increased with increasing temperature in temperate regions and with decreasing precipitation in tropical/subtropical regions. In temperate forest soils, gross N transformation rates were insensitive to N enrichment. In tropical/subtropical forests, however, N enrichment significantly stimulated GNM, GAN and GAN to INH4 ratio, but inhibited INH4 and INO3 due to reduced microbial biomass and pH. We propose that temperate forest soils have higher nitrification capacity and lower nitrate retention capacity, implying a higher potential risk of N losses. However, tropical/subtropical forest systems shift from a conservative to a leaky N-cycling system in response to N enrichment.

Deep N acquisition in cultivated grasslands: Uptake of slow-release 15N-labeled ammonium in hemiboreal monospecific leys

AimsTo develop a methodology to study uptake and redistribution by plants of NH4+ from deep soil, applying it to investigate deep root N uptake by cultivated grassland species.MethodsA slow-release 15NH4+ label adsorbed to clinoptilolite was placed into soil (depth 42 cm) well below the densest root zone in well-established monospecific stands of five grass and two clover species. Species showing a variety of deep rooting patterns, N acquisition strategy, forage qualities, and persistence in hemiboreal conditions were chosen. The label was placed in early spring and tracked throughout one or two growing seasons in two repeated experiments.ResultsAfter two growing seasons ~ 90% of the label was tracked in the soil and harvested herbage of grasses, less in clovers. Deep N uptake was limited in spring, increased during mid-season, and was strongest in autumn in all species, despite lower herbage yield in autumn. Species differed in ability to recover and maintain 15N in the soil–plant system. In one growing season, Lolium perenne L., Phleum pratense L., Schedonorus pratensis (Huds.) P.Beauv. and Schedonorus arundinaceus (Schreb.) Dumort herbage recovered ~ 65% of the label, Poa pratensis L. 54%, and Trifolium pratense L. and Trifolium repens L. 36–48%. Label transport to topsoil was observed, mainly attributable to plant nutrient redistribution rather than physical diffusion.ConclusionsThe innovative slow-release 15N label enabled tracing species differences and seasonal changes in uptake of NH4+ from deep soil. Among the tall-growing grasses, growth vigor appeared as important for deep N uptake as expected root depth.