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Raised emissions of biologically reactive nitrogen (N) have intensified N deposition, enhancing tree productivity globally. Nonetheless, the drivers of forest sensitivity to N deposition remain unknown. We used stem growth data from 62,000 trees across Europe combined with N deposition data to track the effects of air temperature and precipitation on tree growth's sensitivity to N deposition and how it varied depending on leaf form over the past 30 years. Overall, N deposition enhanced conifer growth (until 30 kg N ha -1  yr -1 ) while decreasing growth for broadleaved angiosperms. Lower temperatures led to higher growth sensitivity to N deposition in conifers potentially exacerbated by N limitation. In contrast, higher temperatures stimulated growth sensitivity to N deposition for broadleaves. Higher precipitation equally increased N deposition sensitivity in all leaf forms. We conclude that air temperature and leaf form are decisive in disentangling the effect of N deposition in European forests, which provides crucial information to better predict the contribution of N deposition to land carbon sink enhancement., Competing Interests: Declaration of competing interest We declare no competing interests., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Northern temperate and boreal forests are large biomes playing crucial ecological and environmental roles, such as carbon sequestration. Despite being generally remote, these forests were exposed to anthropogenic nitrogen (N) deposition over the last two centuries and may still experience elevated N deposition as human activities expand towards high latitudes. However, the impacts of long-term high N deposition on these N-limited forest ecosystems remain unclear. For 18 years, we simulated N deposition by chronically adding ammonium nitrate at rates of 3 (LN treatment) and 10 (HN treatment) times the ambient N deposition estimated at the beginning of the experiment at a temperate sugar maple and a boreal balsam fir forest site, both located in northeastern America. LN and HN treatments corresponded respectively to addition of 26 kgN·ha -1 ·yr -1 and 85 kgN·ha -1 ·yr -1 at the temperate site and 17 kgN·ha -1 ·yr -1 and 57 kgN·ha -1 ·yr -1 at the boreal site. Between 2002 and 2018, soil solution was collected weekly during summer and concentrations of NO 3 - , NH 4 + , Ca 2+ and pH were measured, totalling ~12,700-13,500 observations per variable on the study period. N treatments caused soil solution NO 3 - , NH 4 + and Ca 2+ concentrations to increase while reducing its pH. However, ion responses manifested through punctual high concentration events (predominantly on the HN plots) that were very rare and leached N quantity was extremely low at both sites. Therefore, N addition corresponding to 54 years (LN treatment) and 180 years (HN treatment) of accelerated ambient N deposition had overall small impacts on soil solution chemistry. Our results indicate an important N retention of northeastern American forests and an unexpected strong resilience of their soil solution chemistry to long-term simulated N deposition, potentially explained by the widespread N-limitation in high latitude ecosystems. This finding can help predict the future productivity of N-limited forests and improve forest management strategies in northeastern America., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Nitrogen (N) deposition is a significant threat to the functioning of forests and negatively impacts the delivery of forest goods and services. Contemporary management approaches seek to adapt forests to such N-deposition stressors, but to date how plant populations in natural forests respond to N deposition and what factors determine the contrasting responses among populations are still unclear. Here, we investigated the impact of N-addition (control: 0 kg ha -1 yr -1; low: 25 kg ha -1 yr -1 ; medium: 50 kg ha -1 yr -1 ; high: 75 kg ha -1 ha yr -1 ) on tree population temporal stability and how initial tree size, mycorrhizal type, and leaf N content (LNC; as a surrogate for functional trait composition) mediate tree population responses to N-addition in a Korean pine and mixed broadleaved dominated temperate forest in northern China. We quantified tree species population temporal stability as the ratio of mean to standard deviation of the year-by-year stem increments recorded in individual trees from 2015 to 2022 experimental period. The results showed different temporal stabilities of tree species among four N-addition levels, with the highest population stability observed within the high N-addition plots. Furthermore, initial tree size had significantly (p < 0.001) positive effects on population temporal stability. The effect of LNC and initial tree size were also contingent on the level of N applied. Specifically, increase in tree population LNC reduced population temporal stability in all plots where N was added. Our results imply that retention of large-sized trees and species with resource-conservative strategies (e.g., low LNC) could enhance forest stability under N deposition., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Forest water reclamation is a decades-old practice of repurposing municipal reclaimed water using land application on forests to filter nutrients and increase wood production. However, long-term application may lead to nutrient saturation, leaching, and potential impairment of ground and surface water quality. We studied long-term effects of reclaimed water application on nutrient leaching potential in a four-decade time series of forest water reclamation facilities in northern Idaho. Our approach compared reclaimed water treated plots with untreated control plots at each of the forest water reclamation facilities. We measured soil nitrifier abundance and net nitrification rates and used tension lysimeters to sample soil matrix water and drain gauges to sample from a combination of matrix and preferential flow paths. We determined nutrient leaching as the product of soil water nutrient concentrations and model-estimated drainage flux. There was more than 450-fold increase in nitrifier abundance and a 1000-fold increase in net nitrification rates in treated plots compared with control plots at long-established facilities, indicating greater nitrate production with increased cumulative inputs. There were no differences in soil water ammonium, phosphate, and dissolved organic nitrogen concentrations between control and effluent treatments in tension lysimeter samples. However, concurrent with increased nitrifier abundance and net nitrification, nitrate concentration below the rooting zone was 2 to 4-fold higher and nitrate leaching was 4 to 10-fold higher in effluent treated plots, particularly at facilities that have been in operation for over two decades. Thus, net nitrification and nitrifier abundance assays are likely indicators of nitrate leaching potential. Inorganic nutrient concentrations in drain gauge samples were 2 to 11-fold higher than lysimeter samples, suggesting nutrient losses occurred predominantly through preferential flow paths. Nitrate was vulnerable to leaching during the wet season under saturated flow conditions. Although nitrogen saturation is a concern that should be mitigated at long-established facilities, these forest water reclamation facilities were able to maintain average soil water nitrate concentrations to less than 2 mg L -1 , so that nitrogen and phosphorous are effectively filtered to below safe water standards., Competing Interests: Declaration of competing interest The authors declare no conflict of interest., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Increased atmospheric nitrogen deposition generally increases plant biomass production until reaching soil N saturation, which could increase the uncertainty of changes in ecosystem temporal stability and their mechanisms. Yet, the response of ecosystem stability to N enrichment and their underlying mechanisms are uncertain, especially when N saturation reached. Here, we conducted a multi-level N addition (0, 2, 5, 10, 15, 25, and 50 g N m -2  year -1 ; high added rates reached N saturation) experiment from 2018 to 2022 to estimate the effect of simulated N deposition on ecosystem biomass stability in a subalpine grassland located on the Qilian mountain of north-eastern Tibetan Plateau. Our results show that community biomass production increased with an increase in N addition in the first N addition experiment year, but decreased with increase in N addition rates after N saturation in subsequent years. We first find a negative quadratic relationship between biomass temporal stability and added N rate, whereby above N saturation threshold (5 g N m -2  year -1 at this site), increases in N addition reduces biomass temporal stability. The changes in biomass temporal stability are largely determined by dominant species stability, species asynchronous, and species richness. These results provide a better understanding of N-induced effect on ecosystem stability and their underlying mechanisms, which is important to evaluate functioning and services of ecological systems under global change scenarios., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Nitrogen deposition has serious consequences to global change. Excessive nitrogen deposition leads to nitrogen saturation in forests, resulting in soil acidification, nitrate leaching, an increase in nitrous oxide emissions, and a decrease in plant species diversity and vegetation productivity. Under the reduction of atmospheric nitrogen deposition in Europe, North America, and China, summarizing the response of forests to decreasing nitrogen deposition can not only improve the knowledge framework of the impact of nitrogen deposition on forests, but also evaluate the effects of emission abatement actions, as well as provide scientific basis for future air pollution control. This study reviewed the response of soil, surface water, nitrogen cycle, and vegetation of temperate forests in Europe and North America and subtropical forests in southwest China to the reduction in atmospheric nitrogen pollution gases and thus nitrogen deposition. The soil water nitrogen concentration responded rapidly to the nitrogen deposition reduction, although the trend was inconsistent. The soil acidification and nitrogen cycles showed a delayed response of recovery from high nitrogen deposition. The nitrogen mineralization and immobilization, soil carbon retention, and net primary production might take decades to respond to the nitrogen deposition reduction. However, the soil inorganic nitrogen pool and nitrogen leaching decreased with the decline in nitrogen deposition, although a one-or two-year lag existed. The surface water nitrogen concentration was closely related to nitrogen status in forests. After the nitrogen deposition decreased, the nitrogen leaching and thus the surface water nitrogen concentration decreased in the areas with historically high nitrogen deposition. However, the low surface water nitrogen concentration in the nitrogen-limited forests was not significantly affected by the nitrogen deposition changes. The recovery of surface water acidification was affected by soil sulfur desorption/mineralization and nitrification/denitrification. The foliar nitrogen concentration decreased with the decline in nitrogen deposition. The nitrogen-saturated forests and regional surface water in southwest China showed a recovery trend from high nitrogen deposition, as a consequence of the implementation of the Total Emissions Control of Air Pollutants and later the Action Plan of Air Pollution Prevention and Control.

Although tropical and subtropical regions have replaced temperate regions as the global-change hotspots for increased atmosphere nitrogen (N) deposition, whether the regional forests reach N saturation is still unclear. Understory or floor N addition has been commonly used in N-deposition studies, but the results of such studies have recently been challenged because they fail to account for canopy interception, assimilation, and leaching processes. Here, we conducted a field experiment to quantify the effects of over-canopy and understory N addition on N budgets in a lower subtropical monsoon evergreen broadleaved (LSMEB) forest. We found that the LSMEB forest was not N saturated after receiving additional N at 25 and 50 kg ha -1 yr -1 for 6 years. Plants were able to absorb the added N by increasing the N concentrations in their organs, with 120-412 % increasing trend of plant N pools under N-addition treatments. Canopy absorption of N resulting from over-canopy N addition led to increases in N concentrations in tree organs but not to increases in tree biomass. Understory N addition could underestimate the effects of N deposition in forests due to neglecting canopy N interception and canopy effects on N redistribution. Additional experiments using over-canopy N addition are needed to assess the true effects of N deposition on different forest ecosystems in different climate zones., Competing Interests: Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other person interest of any nature of kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Nitrogen budgets of a lower subtropical forest as affected by 6 years of over-canopy and understory nitrogen additions” which was submitted to publication as a research paper in Science of the Total Environment., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Increasing atmospheric nitrogen (N) deposition greatly affects species diversity, productivity, and stability of ecosystems. It is thus of the great importance to understand how grassland N pools respond to the increased atmospheric N deposition. This study was conducted in a meadow steppe in Erguna, Inner Mongolia, China. There were six levels of N addition ( i.e ., 0, 2, 5, 10, 20 and 50 g·m -2 ·a -1 ) and two levels of mowing ( i.e ., mowing and unmown). Samples of aboveground tissues of dominant plant, root, aboveground litter, and soil to the depth of 100 cm were collected in the seventh year after treatments. The N content was measured and the N pool was calculated. The results showed that N addition significantly increased the N content of aboveground plant tissues and litter, as well as N pools of Leymus chinensis , plant community, litter and ecosystem. Mowing significantly increased the N content of L. chinensis leaf and litter, but reduced N pools of L. chinensis , plant community and litter, and did not affect their responses to N addition. There was a significant interactive effect between mowing and N addition on plant community N pool. High levels of N addition in the unmown treatment led to more N stored in the litter pool, with the saturation threshold for the plant community N pool occurred at 10 g·m -2 ·a -1 . Under mowing treatment, the plant community N pool increased with the increasing N addition, and more N stored in plant community N pool after mowing. Mowing could alleviate the negative impacts of increasing N deposition on biodiversity and ecosystem stability, and extended postponing the occurrence of ecosystem N saturation induced by increasing N deposition.

With climate change the occurrence of summer droughts is expected to increase in Central Europe. This could lead to increased nitrate (NO 3 - ) leaching when water scarcity affects the N-uptake capacity of trees and increases soil N availability due to early leaf senescence and higher litter input. In the present study, we used 16 years of ecological monitoring data from the LTER research site "Zöbelboden" in Austria. The monitoring site is a mixed Spruce-Sycamore-Ash-Beech forest on karst, which is representative for many watersheds that supply drinking water in Austria. We found that in the year after a summer drought, NO 3 - leaching via soil water seepage was significantly elevated compared to the long-term mean. While in normal years, NO 3 - leaching was primarily affected by soil water seepage volume, after a summer drought these controls changed and NO 3 - leaching was controlled by NO 3 - input via precipitation, tree N uptake, and vapor-pressure deficit. Furthermore, higher aboveground litter input during dry years was correlated with increased NO 3 - leaching in the following year. Our findings show that NO 3 - leaching from temperate mountain forests on karst is susceptible to summer drought, which could affect drinking water quality in the Central European Alps in the future, especially in combination with forest disturbances like bark beetle outbreaks, which are often a direct consequence of drought damage to trees., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

The emission of nitrogen oxides (NO x ) in China decreased by 15% from 2010 to 2015 (without a significant decrease in NH 3 emission), resulting in the decline of nitrogen (N) deposition in East Asia. Empirical N critical load exceedance was used to assess the benefit of the NO x emission reduction in China to natural ecosystems in East Asia. Empirical N critical loads for major forest and grassland types in East Asia were assigned based on field manipulation experiments for N effects. The critical load map based on the minimum of the critical load range of each vegetation type showed that empirical critical loads were generally lower in the Tibetan Plateau and some parts of northeastern China (≤5 kgN·ha -1 ·a -1 ), and higher in northern and southern China (≥20 kgN·ha -1 ·a -1 ). Empirical critical loads were also low in some parts of central and northern Japan (≤5 kgN·ha -1 ·a -1 ) and the south Korean Peninsula (5-10 kgN·ha -1 ·a -1 ). As a benefit of NO x emission reduction in China, N deposition in East Asia decreased significantly from 2010 to 2015. The total area and total amount of critical load exceedance in East Asia declined 4.6% and 14.3% respectively, suggesting great benefits to natural ecosystems., (Copyright © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Deposition of reactive nitrogen (N) is a major threat to terrestrial ecosystems associated with impacts on ecosystem properties and functions including carbon (C) and nutrient stocks, soil water quality and nutrient retention. In the oceanic-alpine Racomitrium heath habitat, N deposition is associated with moss mat degradation and a shift from bryophyte to graminoid dominance. To investigate the effects of moss mat decline on C and N stocks and fluxes, we collected Racomitrium heath vegetation/soil cores from sites along a gradient of N deposition in the UK. Cores were maintained under controlled conditions and exposed to scenarios of current (8-40 kg N ha -1 y -1 ), reduced (8 kg N ha -1 y -1 ) and elevated (50 kg N ha -1 y -1 ) N deposition. Cores from high N deposition sites had smaller aboveground C and N stocks and, under current conditions, leached large amounts of inorganic N and had low soil water pH compared with low N deposition sites. With reduced N deposition there was evidence for rapid recovery of soil water quality in terms of reduced N leaching and small increases in pH. Under high N deposition, cores from low N deposition sites retained much of the applied N while those with a history of high N deposition leached large amounts of inorganic N. Carbon fluxes in soil water and net CO 2 fluxes varied according to core source site but were not affected by the N deposition scenarios. We conclude that C and N stocks and cycling in Racomitrium heath are strongly affected by long-term exposure to N deposition but that soil water quality may improve rapidly, if N deposition rates are reduced. The legacy of N deposition impacts on moss mat cover and vegetation composition however, mean that the ecosystem remains sensitive to future pulses in N input., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The recent deposition rates of atmospheric nitrate derived from east Asia to the Japanese forested watershed facing the Sea of Japan are of serious concern. However, export ratios and the seasonality of atmospheric nitrate versus microbial nitrate from forest soils to upstreams have not yet been quantified. Furthermore, the influence of local nitrogen sources and internal biogeochemical processes are still unclear. To determine the influence of watershed properties and atmospheric nitrogen deposition on nitrate dynamics in two adjacent basins (the Kita and Minami Rivers) located in central Japan, we conducted seasonal synoptic surveys using the dual isotopes of nitrate. It was found that nitrate regenerated through nitrification in the forest soil was likely the dominant nitrogen source in both basins from the upstream to downstream waters. However, nitrate concentrations and the direct leaching ratio of atmospheric nitrate were considerably higher in the Kita River Basin than in the Minami River Basin, possibly due to the difference in forest environments. In the Kita River Basin, geographic trait such as altitude may be one factor regulating the sensitivity of forest ecosystem to nitrogen deposition. Quantitative assessments of nitrate outflows from the sub-basins revealed that nitrogen leached from the forest soil was a major source (61-81%) of nitrate loading to the coastal sea., (Copyright © 2019. Published by Elsevier B.V.)

Nitrogen (N) inputs from atmospheric deposition can increase soil organic carbon (SOC) storage in temperate and boreal forests, thereby mitigating the adverse effects of anthropogenic CO 2 emissions on global climate. However, direct evidence of N-induced SOC sequestration from low-dose, long-term N addition experiments (that is, addition of < 50 kg N ha -1  y -1 for > 10 years) is scarce worldwide and virtually absent for European temperate forests. Here, we examine how tree growth, fine roots, physicochemical soil properties as well as pools of SOC and soil total N responded to 20 years of regular, low-dose N addition in two European coniferous forests in Switzerland and Denmark. At the Swiss site, the addition of 22 kg N ha -1  y -1 (or 1.3 times throughfall deposition) stimulated tree growth, but decreased soil pH and exchangeable calcium. At the Danish site, the addition of 35 kg N ha -1  y -1 (1.5 times throughfall deposition) impaired tree growth, increased fine root biomass and led to an accumulation of N in several belowground pools. At both sites, elevated N inputs increased SOC pools in the moderately decomposed organic horizons, but decreased them in the mineral topsoil. Hence, long-term N addition led to a vertical redistribution of SOC pools, whereas overall SOC storage within 30 cm depth was unaffected. Our results imply that an N-induced shift of SOC from older, mineral-associated pools to younger, unprotected pools might foster the vulnerability of SOC in temperate coniferous forest soils., Competing Interests: Compliance with Ethical StandardsThe authors declare that they have no conflict of interest.

Chronic high deposition of nitrogen (N) to forest ecosystems can lead to increased leaching of inorganic N to surface waters, enhancing acidification and eutrophication. For 26 years nitrogen has been added as ammonium nitrate (NH 4 NO 3 ) at 40 kg N ha -1 yr -1 to a whole forested catchment ecosystem at Gårdsjön, Sweden, to experimentally simulate the transition from a N-limited to N-rich state. Over the first 10 years of treatment there was an increasing amount of nitrate (NO 3 - ) and to a lesser extent ammonium (NH 4 + ) lost in runoff, but then N leaching stabilised, and for the subsequent 16 years the fraction of N added lost in runoff remained at 9%. NO 3 - concentrations in runoff were low in the summer during the first years of treatment, but now are high throughout the year. High frequency sampling showed that peaks in NO 3 - concentrations generally occurred with high discharge, and were enhanced if high discharge coincided with occasions of N addition. Approximately 50% of the added N has gone to the soil. The added N is equivalent to 140 years of ambient N deposition. At current ambient levels of N deposition there thus appears to be no immediate risk of N saturation at this coniferous forest ecosystem, and by inference to other such N-limited forests in Scandinavia., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Elevated atmospheric nitrogen (N) deposition is a major driver of change, altering the structure/functioning of nutrient-poor Calluna vulgaris-heathlands over Europe. These effects amply proven for north-western/central heathlands may, however, vary across the ecosystem's distribution, especially at the range limits, as heathlands are highly vulnerable to land-use changes combined with present climate change. This is an often overlooked and greatly understudied aspect of the ecology of heathlands facing global change. We investigated the effects of five N-fertilisation treatments simulating a range of N deposition rates (0, 10, 20, and 50 kg N ha -1  yr -1 for 1 year; and 56 kg N ha -1  yr -1 for 9 years) on the Calluna-plants, the plant functional groups, species composition and richness of two life-cycle stages (building/young- and mature-phase) of Calluna-heathlands at their rear-edge limit. Our findings revealed a dose-related response of the shoot length and number of flowers of young and mature Calluna-plants to the addition of N, adhering to the findings from other heathland locations. However, cumulative high-N loading reduced the annual growth and flowering of young plants, showing early signs of N saturation. The different plant functional groups showed contrasting responses to the cumulative addition of N: annual/perennial forbs and annual graminoids increased with quite low values; perennial graminoids were rather abundant in young heathlands but only slightly augmented in mature ones; while bryophytes and lichens strongly declined at the two heathland life-cycle stages. Meanwhile there were no significant N-driven changes in plant species composition and richness. Our results demonstrated that Calluna-heathlands at their low-latitude distribution limit are moderately resistant to cumulative high-N loading. As north-western/central European heathlands under high-N inputs broadly experienced the loss of plant diversity and pronounced changes in plant species dominance, rear-edge locations may be of critical importance to unravel the mechanisms of heathland resilience to future global change., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Chronic elevated nitrogen (N) deposition has altered the N status of temperate forests, with significant implications for ecosystem function. The Bear Brook Watershed in Maine (BBWM) is a whole paired watershed manipulation experiment established to study the effects of N and sulfur (S) deposition on ecosystem function. N was added bimonthly as (NH 4 ) 2 SO 4 to one watershed from 1989 to 2016, and research at the site has studied the evolution of ecosystem response to the treatment through time. Here, we synthesize results from 27 years of research at the site and describe the temporal trend of N availability and N mineralization at BBWM in response to chronic N deposition. Our findings suggest that there was a delayed response in soil N dynamics, since labile soil N concentrations did not show increases in the treated watershed (West Bear, WB) compared to the reference watershed (East Bear, EB) until after the first 4 years of treatment. Labile N became increasingly available in WB through time, and after 25 years of manipulations, treated soils had 10× more extractable ammonium than EB soils. The WB soils had 200× more extractable nitrate than EB soils, driven by both, high nitrate concentrations in WB and low nitrate concentrations in EB. Nitrification rates increased in WB soils and accounted for ~ 50% of net N mineralization, compared to ~ 5% in EB soils. The study provides evidence of the decadal evolution in soil function at BBWM and illustrates the importance of long-term data to capture ecosystem response to chronic disturbance.

In alpine ecosystems, nitrogen (N) deposition has been linked to plant community composition change, including loss of bryophytes and increase of graminoids. Since bryophyte growth is stimulated by increased N availability, it has been hypothesized that loss of bryophyte cover is driven by enhanced decomposition. As bryophyte mats are a significant carbon (C) store, their loss may impact C storage in these ecosystems. We used an N deposition gradient across 15 sites in the UK to examine effects of N deposition on bryophyte litter quality, decomposition and C and N stocks in Racomitrium moss-sedge heath. Increasing N deposition reduced C : N in bryophyte litter, which in turn enhanced decomposition. Soil N stocks increased significantly in response to increased N deposition, and soil C : N declined. However, depletion of the bryophyte mat and its replacement by graminoids under high N deposition was not associated with a change in total ecosystem C stocks. We conclude that decomposition processes in Racomitrium heath are very sensitive to N deposition and provide a mechanism by which N deposition drives depletion of the bryophyte mat. Nitrogen deposition did not measurably alter C stocks, but changes in soil N stocks and C : N suggest the ecosystem is becoming N saturated., (© 2018 The Authors. New Phytologist © 2018 New Phytologist Trust.)

A field experiment was established in a high elevation red spruce (Picea rubens Sarg.) - balsam fir (Abies balsamea) forest on Mount Ascutney Vermont, USA in 1988 to test the nitrogen (N) saturation hypothesis, and to better understand the mechanisms causing forest decline at the time. The study established replicate control, low and high dose nitrogen addition plots (i.e., 0, 15.7 and 31.4kgNH 4 Cl-Nha -1 yr -1 ). The treatments began in 1988 and continued annually until 2010, but monitoring has continued to present. During the fertilization period, forest floor C:N, net in situ N mineralization, spruce foliar Ca%, and live spruce basal area decreased with increasing N addition, while foliar spruce N% and forest floor net nitrification increased with increasing N addition. The control plots aggraded forest floor N at a rate equal to the sum of the net in situ N mineralization plus average ambient deposition. Conversely, N addition plots lost forest floor N. Following the termination of N additions in 2010, the measured tree components returned to pre-treatment levels, but forest floor processes were slower to respond. During the 30year study, site surface air temperature has increased by 0.5°C per decade, and total N deposition has decreased 5.5 to 4.0kgNha -1 yr -1 . There have also been three significant drought years and at least one freeze injury year after which much of the forest mortality on the N addition plots occurred. Given that there was no control for the air temperature increase, discussion of the interactive impacts of climate and change and N addition is only subjective. Predicted changes in climate, N deposition and other stressors suggest that even in the absence of N saturation, regeneration of the spruce-fir ecosystem into the next century seems unlikely despite recent region-wide growth increases., (Published by Elsevier B.V.)

The effects of atmospheric nitrogen (N) deposition on ecosystem functioning largely depend on the retention of N in different ecosystem compartments, but accumulation and partitioning processes have rarely been quantified in long-term field experiments. In the present study we analysed for the first time decadal-scale flows and allocation patterns of N in a heathland ecosystem that has been subject to airborne N inputs over decades. Using a long-term 15 N tracer experiment, we quantified N retention and flows to and between ecosystem compartments (above-ground/below-ground vascular biomass, moss layer, soil horizons, leachate). After 9 years, about 60% of the added 15 N-tracer remained in the N cycle of the ecosystem. The moss layer proved to be a crucial link between incoming N and its allocation to different ecosystem compartments (in terms of a short-term capture, but long-term release function). However, about 50% of the 15 N captured and released by the moss layer was not compensated for by a corresponding increase in recovery rates in any other compartment, probably due to denitrification losses from the moss layer in the case of water saturation after rain events. The O-horizon proved to be the most important long-term sink for added 15 N, as reflected by an increase in recovery rates from 18 to 40% within 8 years. Less than 2.1% of 15 N were recovered in the podzol-B-horizon, suggesting that only negligible amounts of N were withdrawn from the N cycle of the ecosystem. Moreover, 15 N recovery was low in the dwarf shrub above-ground biomass (<3.9% after 9 years) and in the leachate (about 0.03% within 1 year), indicating still conservative N cycles of the ecosystem, even after decades of N inputs beyond critical load thresholds. The continuous accumulation of reactive forms of airborne N suggests that critical load-estimates need to account for cumulative effects of N additions into ecosystems.

Increasing atmospheric reactive nitrogen (N) deposition due to human activities could change N cycling in terrestrial ecosystems. However, the differences between the fates of deposited NH4+ and NO3- are still not fully understood. Here, we investigated the fates of deposited NH4+ and NO3-, respectively, via the application of 15 NH 4 NO 3 and NH 4 15 NO 3 in a temperate forest ecosystem. Results showed that at 410 days after tracer application, most 15NH4+ was immobilized in litter layer (50 ± 2%), while a considerable amount of 15NO3- penetrated into 0-5 cm mineral soil (42 ± 2%), indicating that litter layer and 0-5 cm mineral soil were the major N sinks of NH4+ and NO3-, respectively. Broad-leaved trees assimilated more 15 N under NH 4 15 NO 3 treatment compared to under 15 NH 4 NO 3 treatment, indicating their preference for NO3--N. At 410 days after tracer application, 16 ± 4% added 15 N was found in aboveground biomass under 15NO3- treatment, which was twice more than that under 15NH4+ treatment (6 ± 1%). At the same time, approximately 80% added 15 N was recovered in soil and plants under both treatments, which suggested that this forest had high potential for retention of deposited N. These results provided evidence that there were great differences between the fates of deposited NH4+ and NO3-, which could help us better understand the mechanisms and capability of forest ecosystems as a sink of reactive nitrogen., (© 2016 John Wiley & Sons Ltd.)

871 I. 871 II. 874 III. 875 IV. 878 V. 882 884 References 884 SUMMARY: Humans have long utilized resources from all forest biomes, but the most indelible anthropogenic signature has been the expanse of human populations in temperate forests. The purpose of this review is to bring into focus the diverse forests of the temperate region of the biosphere, including those of hardwood, conifer and mixed dominance, with a particular emphasis on crucial challenges for the future of these forested areas. Implicit in the term 'temperate' is that the predominant climate of these forest regions has distinct cyclic, seasonal changes involving periods of growth and dormancy. The specific temporal patterns of seasonal change, however, display an impressive variability among temperate forest regions. In addition to the more apparent current anthropogenic disturbances of temperate forests, such as forest management and conversion to agriculture, human alteration of temperate forests is actually an ancient phenomenon, going as far back as 7000 yr before present (bp). As deep-seated as these past legacies are for temperate forests, all current and future perturbations, including timber harvesting, excess nitrogen deposition, altered species' phenologies, and increasing frequency of drought and fire, must be viewed through the lens of climate change., (© 2016 The Author. New Phytologist © 2016 New Phytologist Trust.)

Elevated anthropogenic nitrogen (N) deposition has greatly altered terrestrial ecosystem functioning, threatening ecosystem health via acidification and eutrophication in temperate and boreal forests across the northern hemisphere. However, response of forest soil acidification to N deposition has been less studied in humid tropics compared to other forest types. This study was designed to explore impacts of long-term N deposition on soil acidification processes in tropical forests. We have established a long-term N-deposition experiment in an N-rich lowland tropical forest of Southern China since 2002 with N addition as NH4 NO3 of 0, 50, 100 and 150 kg N ha(-1)  yr(-1) . We measured soil acidification status and element leaching in soil drainage solution after 6-year N addition. Results showed that our study site has been experiencing serious soil acidification and was quite acid-sensitive showing high acidification (pH(H2O) <4.0), negative water-extracted acid neutralizing capacity (ANC) and low base saturation (BS,< 8%) throughout soil profiles. Long-term N addition significantly accelerated soil acidification, leading to depleted base cations and decreased BS, and further lowered ANC. However, N addition did not alter exchangeable Al(3+) , but increased cation exchange capacity (CEC). Nitrogen addition-induced increase in SOC is suggested to contribute to both higher CEC and lower pH. We further found that increased N addition greatly decreased soil solution pH at 20 cm depth, but not at 40 cm. Furthermore, there was no evidence that Al(3+) was leaching out from the deeper soils. These unique responses in tropical climate likely resulted from: exchangeable H(+) dominating changes of soil cation pool, an exhausted base cation pool, N-addition stimulating SOC production, and N saturation. Our results suggest that long-term N addition can contribute measurably to soil acidification, and that shortage of Ca and Mg should receive more attention than soil exchangeable Al in tropical forests with elevated N deposition in the future., (© 2014 John Wiley & Sons Ltd.)