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Human activities have released a large amount of nitric oxides (NOx) into the atmosphere, leading to elevated regional atmospheric NOx concentration and nitrogen (N) deposition. Several studies have observed a downward NOx or NOy (NOx and its oxidation products) flux to the forest canopy, but attributed it to the photochemical reactions and conversion from gaseous N to particulate N. Here, we focus on the possibility of NOx sink by forest canopy based on an observed downward NOx flux of 0.28 kg N ha−1 yr−1 in a temperate forest of northeastern China, using hourly monitoring of NOx fluxes. In combination with results from previous studies, we find that the downward NOx or NOy fluxes ranged from 0.2 to 1.4 kg N ha−1 yr−1 in temperate forests and from 5.3 to 8.5 kg N ha−1 yr−1 in subtropical forests and they were positively related to atmospheric NOx concentration, mean annual air temperature and annual precipitation across eight forest sites from around the world. If assuming 50% of downward NOx or NOy was directly taken up by the canopy, we estimate a NOx or NOy sink of 0.2 and 1.1 Tg N yr−1 by global respective temperate and subtropical forests, representing an effective scavenger of atmospheric NOx and a considerable N source to forests. We suggest that interception of NOx or NOy by forest canopy should be considered in global NOx budgets and their contribution to the global forest N pool in global Earth System's models. Plain Language Summary: Canopy interception of reactive N has gained wide attention in recent several years, however, its quantification is still unclear. In the present work, we demonstrate the NOx downward movement in a typical temperate forest in northeastern China was partially driven by canopy biological uptake, and then combined results of our in situ observation of canopy NOx flux with other existing 10 monitoring sites worldwide. We estimated an N input of 0.2 and 1.1 Tg N yr−1 in global respective temperate and subtropical forests via downward NOx movement, representing an effective scavenger of atmospheric NOx and a considerable N source to forests. Key Points: The potential importance of canopy NOx uptake by forests for atmospheric NOx concentration and forest N pools is unclearCombining present and previous study results, we estimated a canopy N input of 0.2–1.1 Tg yr−1 in global temperate and subtropical forestsWe suggest that interception of NOx by forest canopy should be considered in global NOx budgets and forest N pools [ABSTRACT FROM AUTHOR]

Forests worldwide store large quantities of carbon (C), particularly in soils as soil organic C (SOC). In northeastern China, two dominant forest types, secondary mixed forest (MF) and larch plantation forest (LF), cover extensive areas. However, we lack an understanding of the patterns and the mechanisms of SOC storage and stabilization in MF and LF, especially in deep soil layers. This research aims to illustrate the vertical distribution and mineral protection of SOC over soil depth; we also used δ13C values of soil fractions to evaluate SOC stability. Samples from the surface litter (Oi), organic layer (Oa+e), and 0–40 cm mineral soils were collected from both MF and LF plots. We used two different methods to separate bulk soils into distinguished fractions: (1) macro- and micro-aggregates and the non-aggregated fraction, and (2) particulate organic matter (POM) and mineral-associated organic matter (MAOM). The C concentrations, C stocks, and δ13C of all soil fractions were determined. Our findings were as follows: (1) SOC was mainly stored in mineral soils and was 13.6% lower in LF (8609 ± 1180 g C m−2) than MF (9969 ± 2084 g C m−2). (2) In both MF and LF, the SOC stock was mainly stored in aggregates (averaged 92.7%); macroaggregates dominated in the surface layers (Oa+e layer and 0–10 cm) but microaggregates dominated in the deep layers (10–20 cm and 20–40 cm). In mineral soils, MAOM was the dominant fraction of the C stock (averaged 81.6%). (3) The proportion of C distributed in microaggregates and MAOM increased from Oa+e to the 20–40 cm layer. (4) The C/N ratios and δ13C values of MAOM were smaller and heavier compared to those of POM. Our study demonstrated that in both forests, aggregate formation and mineral association predominantly contributed to SOC storage, and large stocks of SOC were distributed in the deep soil. The increasing proportion of SOC in microaggregates and MAOM along the soil depth, most likely derived from microbial turnover and microbial necromass, influenced SOC stability in both forest types. [ABSTRACT FROM AUTHOR]

Societal Impact Statement: Increasing atmospheric nitrogen (N) deposition represents a major global change factor, but its long‐term effect on tree growth and carbon (C) sequestration remains uncertain. Our manipulation experiment and meta‐analysis reveal that N deposition in temperate and boreal forests promoted tree growth and the allocation of more C into wood than into leaves in China and worldwide. Thus, N deposition may increase forest C sequestration through enhanced wood production and distribution of C into stable sinks. In the context of achieving "Carbon Neutrality," understanding how N deposition affect long‐term forest C sinks will help us with mitigation strategies under climate change. Summary: Increased nitrogen (N) deposition is driving many temperate and boreal forests in the Northern Hemisphere towards N saturation. However, it is uncertain how long‐term N deposition affects tree growth and carbon (C) allocation in these forests.To investigate this, we treated temperate larch and mixed forests in northeastern China with N additions for 8 years. In addition, we collected data from 25 N‐addition experiments in temperate and boreal forests worldwide to reveal the overall effects of N on tree growth and C allocation.Nitrogen additions significantly promoted total biomass increment by 24% in both study forests, with on average additional 8 kg C per kg N gain into woody biomass over the study period. Nitrogen additions increased the ratio of woody biomass increment to foliage litterfall production in the larch forest (by 34%). Literature data analysis also revealed greater N promotion on wood (24%) over foliage (9%) production. However, the positive effect on foliage diminished over time.These results combined imply that N deposition may promote tree growth in temperate and boreal regions and drive proportionally more photosynthate allocation into wood than leaves, thus may enhance forest C sequestration in the long run. [ABSTRACT FROM AUTHOR]

Emissions of dinitrogen (N2) and nitrous oxide (N2O) from soil are important components of the global nitrogen cycle. Soil N2O emissions from terrestrial ecosystems have been well studied. However, patterns and mechanisms of N2 emissions remain unclear due to the technical difficulty in measuring N2 production. In this study, an in situ 15N labeling method was employed to determine soil N2 and N2O emission rates from the lower, middle, and upper slopes, which correspond to different moisture conditions, in a temperate forest in Northeast China. We found that N2 emissions varied from 85 to 3442 μg N m−2 h−1 across the slopes and were dominated by denitrification. The emissions of bulk N2O (22 to 258 μg N m−2 h−1) and denitrification-derived N2O (14 to 246 μg N m−2 h−1) were significantly lower than N2 emissions from their corresponding slope positions. Both N2 and N2O emissions significantly increased when soils become wetter. The ratios of N2O/(N2O + N2) were significantly higher at the upper and middle slopes (0.22 and 0.20, respectively) compared with those at the lower slope (0.08 ± 0.01). At the catchment scale, N2 accounted for 85% of the total gaseous N losses (N2O + N2). Our study shows that soil moisture drives the patterns of N2 and N2O emissions and field quantification of N2O/(N2O + N2) ratio should further consider the effect of slope position of forest ecosystems to estimate total soil gaseous N losses. [ABSTRACT FROM AUTHOR]

Background: Forest succession is an important ecological process and has been studied for more than a century. However, changes in nitrogen (N) availability during succession remain unclear as they may lead to either N saturation or N limitation. Here, we propose a conceptual model to illustrate changes in N availability during four stages of secondary succession using the natural abundance of 15N in plant leaves (foliar δ15N). We predicted that N availability would decline in the early stages of succession and then increase in late stages, coinciding with the changes in foliar δ15N, with the inflection point varying in different climate zones. Data on foliar δ15N from 16 succession sequences were synthesized to explore changes in N availability during forest succession. Results: The compiled data were consistent with the proposed conceptual model. Foliar δ15N in boreal and temperate forests decreased significantly in the first two stages of succession (estimated to last at least 66 years in temperate forests), at a rate of 0.18‰ and 0.38‰ per decade, respectively, and decreased slightly in tropical forests in the first 23 years. Foliar δ15N is projected to increase in later stages in all forests, which is supported by observations in both temperate and tropical forests. The inflection points of N availability when N limitation peaked during succession were different in different climate zones, implying different ecosystem N turnovers. Conclusions: Our study reconciles the controversies regarding changes in N availability during forest secondary succession. Our findings are also useful for predicting the recovery of N and carbon accumulation during succession. Nonetheless, studies on forest secondary succession using foliar δ15N have thus far been limited, and more research should be conducted to further verify the conceptual model proposed here. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The effects of nitrogen (N) deposition on forests largely depend on its fate after entering the ecosystem. While several studies have addressed the forest fate of N deposition using 15N tracers, the long‐term fate and redistribution of deposited N in tropical forests remains unknown. Here, we applied 15N tracers to examine the fates of deposited ammonium (NH4+) and nitrate (NO3‐) separately over 3 years in a primary and a secondary tropical montane forest in southern China. Three months after 15N tracer addition, over 60% of 15N was retained in the forests studied. Total ecosystem retention did not change over the study period, but between 3 months and 3 years following deposition 15N recovery in plants increased from 10% to 19% and 13% to 22% in the primary and secondary forests, respectively, while 15N recovery in the organic soil declined from 16% to 2% and 9% to 2%. Mineral soil retained 50% and 35% of 15N in the primary and secondary forests, with retention being stable over time. The total ecosystem retention of the two N forms did not differ significantly, but plants retained more 15NO3‐ than 15NH4+ and the organic soil more 15NH4+ than NO3‐. Mineral soil did not differ in 15NH4+ and 15NO3‐ retention. Compared to temperate forests, proportionally more 15N was distributed to mineral soil and plants in these tropical forests. Overall, our results suggest that atmospherically deposited NH4+ and NO3‐ is rapidly lost in the short term (months) but thereafter securely retained within the ecosystem, with retained N becoming redistributed to plants and mineral soil from the organic soil. This long‐term N retention may benefit tropical montane forest growth and enhance ecosystem carbon sequestration. [ABSTRACT FROM AUTHOR]

Summary: 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. [ABSTRACT FROM AUTHOR]

The impacts of anthropogenic nitrogen (N) deposition on forest ecosystems depend in large part on its fate. However, our understanding of the fates of different forms of deposited N as well as the redistribution over time within different ecosystems is limited. In this study, we used the 15N‐tracer method to investigate both the short‐term (1 week to 3 months) and long‐term (1–3 yr) fates of deposited NH4+ or NO3− by following the recovery of the 15N in different ecosystem compartments in a larch plantation forest and a mixed forest located in northeastern China. The results showed similar total ecosystem retention for deposited NH4+ and NO3−, but their distribution within the ecosystems (plants vs. soil) differed distinctly particularly in the short‐term, with higher 15NO3− recoveries in plants (while lower recoveries in organic layer) than found for 15NH4+. The different short‐term fate was likely related to the higher mobility of 15NO3− than 15NH4+ in soils instead of plant uptake preferences for NO3− over NH4+. In the long‐term, differences between N forms became less prevalent but higher recoveries in trees (particularly in the larch forest) of 15NO3− than 15NH4+ tracer persisted, suggesting that incoming NO3− may contribute more to plant biomass increment and forest carbon sequestration than incoming NH4+. Differences between the two forests in recoveries were largely driven by a higher 15N recovery in the organic layer (both N forms) and in trees (for 15NO3−) in the larch forest compared to the mixed forest. This was due to a more abundant organic layer and possibly higher tree N demand in the larch forest than in the mixed forest. Leachate 15N loss was minor (<1% of the added 15N) for both N forms and in both forests. Total 15N recovery averaged 78% in the short‐term and decreased to 55% in the long‐term but with increasing amount of 15N label (re)‐redistributed into slow turn‐over pools (e.g., trees and mineral soil). The different retention dynamics of deposited NH4+ and NO3− may have implications in environmental policy related to the anthropogenic emissions of the two N forms. [ABSTRACT FROM AUTHOR]

The impacts of anthropogenic nitrogen (N) deposition on forest ecosystems have been well recognized. However, our understanding of the fate of deposited N and its retention dynamics in different ecosystems is limited. This study used the 15N-tracer method to investigate both the short-term (1 week - 3 months) and long-term (1 - 3 years) fates of deposited NH4+ or NO3- in two forests in northeastern China, namely a larch plantation forest and a mixed forest. We determined retention dynamics of 15N-labelled NH4+ or NO3- (by following the recovery of the 15N) in different ecosystem compartments over three years. In the first three months, total N retention declined from 97 -100% to 52 - 72% in the larch forest and from 84 - 88 to 53 - 56% in the mixed forest, indicating a rapid initial loss of newly deposited N regardless of forest type. Here after, 15N retention stayed relatively constant in both forests, but retained 15N was redistributed from the organic layer to the mineral soil with no significant change in plants over time. At the end of three years, 15N retention were 48 - 51% and 42 - 47% in the larch and mixed forest, respectively, suggesting that once retained over the first growing season, most deposited N was recycled within ecosystem in the long-term. The N retention efficiency in our studied forests was lower than that observed in many temperate forests in Europe and US. But leachate 15N loss was minor (<1% of the added 15N). Total N retention did not significantly differ between NH4+ and NO3- inputs in either forest. However, the distribution of NH4+ and NO3- inputs in different ecosystem compartments was distinctly different, with higher 15NO3- retained in plants (while lower retention in organic layer) than found for 15NH4+. This suggests that NO3- deposition could be more beneficial to forest growth and CO2 sequestration into plant biomass. The distinctly different retention of deposited NH4+ and NO3- may have long-term consequences, which has important implications in environmental policy development to regulate anthropogenic deposition of NO3- and NH4+ and their impacts on forest ecosystems.Keywords: 15N tracer, temperate forests, N retention and redistribution, N deposition, northeastern China [ABSTRACT FROM AUTHOR]

• First long-term daily soil NO flux measurement with an automated system from a temperate forest in Northeast China. • Soil temperature explained 90–92% of the seasonal variation in NO emission, with a Q 10 of 3.67. • The optimum soil moisture for NO emission was 37% WFPS during the growing season. • Annual soil NO emission was 0.42±0.04 kg N ha−1, being 31% of the global forest average. • Soil NO emission during the non-growing season was only 7.7% of the annual flux. Nitric oxide (NO) is a highly reactive trace gas affecting atmospheric chemistry and air quality. Although forest soils have been recognized as an important source of atmospheric NO, there are large uncertainties in global forest soil NO emission inventories (ranged from 0.03 to 8.00 kg N ha−1, averaged 1.34 ± 0.28 kg N ha−1), partly due to the paucity of high-frequency monitoring from unmanaged forests. In this study, we used an automated sampling system to measure NO fluxes with a high temporal resolution over two years (daily measurements from January 2019 to December 2020) in a mixed forest in Northeast China. We found that the mean annual NO emission was 0.42 ± 0.04 kg N ha−1, being 31% of the global forest average. The contribution of NO emission during the growing season was 92% of the yearly NO flux. Soil temperature was the most important edaphic factor in regulating NO emission, explaining 90–92% of the seasonal variation. The apparent temperature sensitivity (Q 10) of NO flux was 3.67. In the growing season, NO emission was also influenced by soil moisture, with optimum soil moisture of 37% WFPS. By providing a detailed measurement of diurnal, seasonal, and annual dynamics of NO emissions and their environmental controls from forest soils, our data are useful to develop more accurate biogeochemical models that will improve upscaled global NO budgets. [ABSTRACT FROM AUTHOR]