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The impacts of enhanced nitrogen (N) deposition on the global forest carbon (C) sink and other ecosystem services may depend on whether N is deposited in reduced (mainly as ammonium) or oxidized forms (mainly as nitrate) and the subsequent fate of each. However, the fates of the two key reactive N forms and their contributions to forest C sinks are unclear. Here, we analyze results from 13 ecosystem-scale paired 15 N-labelling experiments in temperate, subtropical, and tropical forests. Results show that total ecosystem N retention is similar for ammonium and nitrate, but plants take up more labelled nitrate ( 20 15 25 %) ( mean minimum maximum ) than ammonium ( 12 8 16 %) while soils retain more ammonium ( 57 49 65 %) than nitrate ( 46 32 59 %). We estimate that the N deposition-induced C sink in forests in the 2010s  is 0.72 0.49 0.96  Pg C yr −1 , higher than previous estimates because of a larger role for oxidized N and greater rates of global N deposition. A study using paired 15 N tracers shows atmospheric N deposited in oxidized form is more likely retained by trees, while the reduced form is retained in soil. The authors argue that this is a greater contribution of deposited N to the global forest C sink than previously reported.

Denitrification is an important process in the global nitrogen cycle. The genes encoding NirK and NirS ( nirK and nirS ), which catalyze the reduction of nitrite to nitric oxide, have been used as marker genes to study the ecological behavior of denitrifiers in environments. However, conventional polymerase chain reaction (PCR) primers can only detect a limited range of the phylogenetically diverse nirK and nirS . Thus, we developed new PCR primers covering the diverse nirK and nirS . Clone library and qPCR analysis using the primers showed that nirK and nirS in terrestrial environments are more phylogenetically diverse and 2–6 times more abundant than those revealed with the conventional primers. RNA- and culture-based analyses using a cropland soil also suggested that microorganisms with previously unconsidered nirK or nirS are responsible for denitrification in the soil. PCR techniques still have a greater capacity for the deep analysis of target genes than PCR-independent methods including metagenome analysis, although efforts are needed to minimize the PCR biases. The methodology and the insights obtained here should allow us to achieve a more precise understanding of the ecological behavior of denitrifiers and facilitate more precise estimate of denitrification in environments.

Hadal oceans at water depths below 6,000 m are the least-explored aquatic biosphere. The Challenger Deep, located in the western equatorial Pacific, with a water depth of ∼11 km, is the deepest ocean on Earth. Microbial communities associated with waters from the sea surface to the trench bottom (0 ∼10,257 m) in the Challenger Deep were analyzed, and unprecedented trench microbial communities were identified in the hadal waters (6,000 ∼10,257 m) that were distinct from the abyssal microbial communities. The potentially chemolithotrophic populations were less abundant in the hadal water than those in the upper abyssal waters. The emerging members of chemolithotrophic nitrifiers in the hadal water that likely adapt to the higher flux of electron donors were also different from those in the abyssal waters that adapt to the lower flux of electron donors. Species-level niche separation in most of the dominant taxa was also found between the hadal and abyssal microbial communities. Considering the geomorphology and the isolated hydrotopographical nature of the Mariana Trench, we hypothesized that the distinct hadal microbial ecosystem was driven by the endogenous recycling of organic matter in the hadal waters associated with the trench geomorphology.

• Conifers are considered to prefer to take up ammonium (NH₄⁺) over nitrate (NO₃⁻). However, this conclusion is mainly based on hydroponic experiments that separate roots from soils. It remains unclear to what extent mature conifers can use nitrate compared to ammonium under field conditions where both roots and soil microbes compete for nitrogen (N). • We conducted an in situ whole mature tree nitrogen-15 (15N) labeling experiment (15NH₄⁺ vs 15NO₃⁻) over 15 d to quantify ammonium and nitrate uptake and assimilation rates in four 40-yr-old monoculture coniferous plantations (Pinus koraiensis, Pinus sylvestris, Picea koraiensis and Larix olgensis, respectively). • For the whole tree, 15NO₃⁻ contributed 39% to 90% to total 15N tracer uptake among four plantations during the study period. At day 3, the 15NO₃⁻ accounted for 77%, 64%, 62% and 59% by Larix olgensis, Pinus koraiensis, Pinus sylvestris and Picea koraiensis, respectively. • Our study indicates that mature coniferous trees assimilated nitrate as efficiently as ammonium from soils even at low soil nitrate concentration, in contrast to the results from hydroponic experiments showing that ammonium uptake dominated over nitrate. This implies that mature conifers can adapt to increasing availability of nitrate in soil, for example, under the context of globalization of N deposition and global warming.

The Argentine ant, Linepithema humile Mayr, has spread to almost all continents. In each introduced region, L. humile often forms a single large colony (supercolony), the members of which share the haplotype “LH1”, despite the presence of other supercolonies with different genetic structures. However, the mechanisms underlying the successful invasion of LH1 ants are unclear. Here, we examined whether diet breadth differs between more successful (LH1) and less successful (LH2, LH3, LH4) L. humile supercolonies in Japan to better understand the processes responsible for invasion success. The standard ellipse areas (SEAs) of δ 13 C and δ 15 N and their ranges (CR and NR) were used as diet breadth indices. The SEAs of LH1 were much larger than those of the less successful supercolonies despite no differences in the baseline SEAs of arthropods within the supercolony habitats, indicating that the invasion success of a supercolony is associated with its diet breadth. Furthermore, LH1 had a broader CR than the other supercolonies, suggesting that which might be derived from superior resource exploitation ability. Our study highlights the importance of focusing on intraspecific differences in diet breadth among supercolonies when assessing organisms that can potentially invade and become dominant in new habitats.

Significance Nitrogen (N) losses from terrestrial ecosystems can occur as inert forms or heat-trapping greenhouse gases, and via nitrate (NO ₃⁻) leaching to drainage waters, which can contribute to eutrophication and anoxia in downstream ecosystems. Here, we use natural isotopes to demonstrate that microbial gaseous N production via denitrification is the dominant pathway of NO ₃⁻ removal from forest ecosystems, with gaseous N losses that are up to ∼60-fold higher than those based on traditional techniques. Denitrification becomes less efficient compared with NO ₃⁻ leaching in more N-polluted ecosystems, which has important implications for assessing the connections between terrestrial soils and downstream ecosystems under rising anthropogenic N deposition. Denitrification removes fixed nitrogen (N) from the biosphere, thereby restricting the availability of this key limiting nutrient for terrestrial plant productivity. This microbially driven process has been exceedingly difficult to measure, however, given the large background of nitrogen gas (N ₂) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO ₃⁻) to examine dentrification rates across six forest sites in southern China and central Japan, which span temperate to tropical climates, as well as various stand ages and N deposition regimes. Our multiple stable isotope approach across soil to watershed scales shows that traditional techniques underestimate terrestrial denitrification fluxes by up to 98%, with annual losses of 5.6–30.1 kg of N per hectare via this gaseous pathway. These N export fluxes are up to sixfold higher than NO ₃⁻ leaching, pointing to widespread dominance of denitrification in removing NO ₃⁻ from forest ecosystems across a range of conditions. Further, we report that the loss of NO ₃⁻ to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.

Dissimilatory nitrate reduction to ammonium (DNRA), driven by nitrate-ammonifying bacteria, is an increasingly appreciated nitrogen-cycling pathway in terrestrial ecosystems. This process reportedly generates nitrous oxide (N2O), a strong greenhouse gas with ozone-depleting effects. However, it remains poorly understood how N2O is produced by environmental nitrate-ammonifiers and how to identify DNRA-derived N2O. In this study, we characterize two novel enzymatic pathways responsible for N2O production in Geobacteraceae strains, which are predominant nitrate-ammonifying bacteria in paddy soils. The first pathway involves a membrane-bound nitrate reductase (Nar) and a hybrid cluster protein complex (Hcp–Hcr) that catalyzes the conversion of NO2− to NO and subsequently to N2O. The second pathway is observed in Nar-deficient bacteria, where the nitrite reductase (NrfA) generates NO, which is then reduced to N2O by Hcp–Hcr. These enzyme combinations are prevalent across the domain Bacteria. Moreover, we observe distinctive isotopocule signatures of DNRA-derived N2O from other established N2O production pathways, especially through the highest 15N-site preference (SP) values (43.0‰–49.9‰) reported so far, indicating a robust means for N2O source partitioning. Our findings demonstrate two novel N2O production pathways in DNRA that can be isotopically distinguished from other pathways.IMPORTANCEStimulation of DNRA is a promising strategy to improve fertilizer efficiency and reduce N2O emission in agriculture soils. This process converts water-leachable NO3− and NO2− into soil-adsorbable NH4+, thereby alleviating nitrogen loss and N2O emission resulting from denitrification. However, several studies have noted that DNRA can also be a source of N2O, contributing to global warming. This contribution is often masked by other N2O generation processes, leading to a limited understanding of DNRA as an N2O source. Our study reveals two widespread yet overlooked N2O production pathways in Geobacteraceae, the predominant DNRA bacteria in paddy soils, along with their distinctive isotopocule signatures. These findings offer novel insights into the role of the DNRA bacteria in N2O production and underscore the significance of N2O isotopocule signatures in microbial N2O source tracking.

Background and aims Plant carbon (C), nitrogen (N), phosphorus (P) levels and their stoichiometry and N uptake strategies are important aspects influencing vegetation composition and C dynamics in boreal peatlands. However, C, N and P levels and N sources of different plants remain poorly understood, which prevents a better assessment of plant responses to projected increasing N availability in boreal ecosystems with climate warming and increasing N deposition. Methods We investigated differences of leaf C, N and P concentrations and C and N isotopes (δ 13 C and δ 15 N) between graminoids and shrubs in 18 peatlands in northeastern China. Results Ericoid mycorrhizal (ERM) and ectomycorrhizal (ECM) shrubs have higher C and P while lower C/N and C/P than nonmycorrhizal (NM) graminoids. Shrubs and graminoids have similar leaf N/P, mainly exhibiting N limitation or N and P co-limitation. ECM shrubs show higher N and lower δ 15 N than NM graminoids despite having similar rooting depths, indicating higher N availability and more uptake of 15  N-depleted organic N of ECM shrubs. However, deep-rooted ECM shrubs show slightly higher N than shallow-rooted ERM shrubs, and their δ 15 N differences are insignificant. Shallow-rooted ERM shrubs have higher N and lower δ 15 N than deep-rooted NM graminoids. Conclusions Our results imply lower N and P use efficiencies of shrubs than graminoids, and the important role of mycorrhizal association in differentiating N availabilities and sources between shrubs and graminoids. These findings are useful for understanding peatland plant responses to environmental changes.

Fundamental questions of how plant species within secondary forests and plantations in northeast China partition limited nitrogen (N) resource remain unclear. Here we conducted a N tracer greenhouse study to determine glycine, ammonium, and nitrate uptake by the seedlings of two coniferous species, ( ) and ( ), and two broadleaf species, ( ) and ( ), that are common in natural secondary forests in northeast China. Glycine contributed 43% to total N uptake of , but only 20, 11, and 21% to N uptake by , and , respectively (whole plant), whereas nitrate uptake was 24, 74, 88, and 68% of total uptake for these four species, respectively. Retention of glycine carbon versus nitrogen in roots indicated that 36% of glycine uptake was retained intact. Nitrate was preferentially used by , and , with nitrate uptake constituting 68∼88% of total N use by these three species. These results demonstrated that these dominant tree species in secondary forests in northeast China partitioned limited N resource by varying uptake of glycine, ammonium and nitrate, with all species, except , using nitrate that are most abundant within these soils. Such N use pattern may also provide potential underlying mechanisms for the higher retention of deposited nitrate than ammonium into aboveground biomass in these secondary forests.

Assessment of nitrogen (N) saturation of forests is critical to the evaluation of the manner in which ecosystems will respond to current and future global changes such as N deposition. However, quantifying N saturation remains a challenge. We developed a conceptual model of N saturation stages in forest ecosystems based on (1) a hypothetical relative rate of ammonification, nitrification, and denitrification, (2) concentrations of ammonium and nitrate in the soil, and (3) 15N enrichment pattern of bulk soil N, ammonium, and nitrate in the soil profile. We tested the hypotheses using data from forests located at five sites across eastern Asia. The fraction of nitrate in total inorganic N indicated that the sites represent an N saturation gradient with one boreal forest at stage 1 (least saturated), three temperate forests at stage 2, and one tropical forest at stage 3 (most saturated). The δ15N of bulk soil N increased from topsoil to subsoil more sharply at N‐limited sites than at the N‐rich sites along the N deposition gradient. We also found distinct 15N enrichment patterns of bulk soil N, ammonium, and nitrate in the soil profile across the study sites. At stage 1, nitrate was more 15N‐depleted than ammonium only in the organic soil horizon, indicating limited nitrification, while the 15N depletion of nitrate to ammonium was observed in the deeper mineral soil at stages 2 and 3. Furthermore, ammonium was more 15N‐depleted than bulk soil N at stages 1 and 2 but more 15N‐enriched than bulk soil N at stage 3. Our study suggests that soil profile patterns of δ15N of bulk soil N, ammonium, and nitrate provide information about the relative rates of mineralization, nitrification, and denitrification and thus can be an additional measure of N saturation of forest ecosystems across broad environmental gradients.