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Background and aims: Growing evidence has shown that nitrogen (N) deposition can lead to soil acidification and tree nutrient imbalance. Tree species-specific differences in plant-soil interactions may render different forest types exhibiting contrasting responses to N deposition, yet this remains largely untested. Methods: We conducted N addition experiments (0 and 50 kg N ha−1 yr−1) separately in a larch (Larix kaempferi) forest and an adjacent larch-broadleaf mixed forest, and examined whether soil N availability, soil acid–base chemistry, leaf nutrients and stoichiometry of these two forests responded differently to four-year N addition. Results: We found that N addition increased soil nitrate concentrations at four soil layers (i.e. Oa + e, 0–10, 10–20, and 20–40 cm), and resulted in soil acidification at Oa + e and 0–10 cm layers characterized by decreased pH and exchangeable base cations and increased hydrolyzing cations in the larch forest. In contrast to soil chemical properties, larch leaf nutrient stoichiometry except the C:N ratio showed no significant responses to N addition in the larch forest. Moreover, N addition did not significantly affect soil inorganic N concentration, soil acid–base chemistry, and tree leaf nutrients in the larch-broadleaf mixed forest. Conclusions: Our results suggest the higher resistance of larch-broadleaf mixed forests than larch forests against soil acidification under N addition, and highlight the establishment of conifer-broadleaf mixed forests is an important silvicultural practice to alleviate soil acidification induced by N deposition. [ABSTRACT FROM AUTHOR]

Excessive anthropogenic nitrogen fixation alters the nitrogen cycle and increases nitrogen deposition, leading to nitrogen saturation, which in turns leads to forest decline and nitrate leaching into stream waters. Kureha Hill in Toyama Prefecture, Japan, is considered to be in nitrogen saturation, since many streams have contained high concentrations of nitrate for more than 20 years. In this study, the latest status of nitrogen saturation was verified by comparing most recent data with 20 years observational data, focusing on various indicators of nitrogen saturation, such as stream water quality, nitrogen budget of the watershed, and soil nitrification and mineralization potential. Both the nitrogen deposition on the Hyakumakidani watershed on Kureha Hill and the amount of nitrate leaching to the stream tended to decrease. The reduction in nitrate leaching could be attributed to the reduction in nitrogen deposition and other factors, such as nitrogen pools in the soil. Despite the recent decline in nitrate concentration of the Hyakumakidani stream, the pH level has remained unchanged. This may be due to decreased concentrations of sulfate and acid neutralizing capacity (ANC). The C/N ratio of the soil ranged from 12 to 18 from 2000 to 2023, which was lower than the nitrogen saturation threshold of 25. The net nitrification rate and net mineralization rate showed no significant changes between 2002 and 2023, indicating that the potential to produce nitrate has been unchanged. Similar values in net nitrification and net nitrogen mineralization rates reflected that the Hyakumakidani watershed remained in Stage‐3 nitrogen saturation. [ABSTRACT FROM AUTHOR]

The effects of nitrogen (N) deposition on forests largely depend on the ecosystem N status and the fates of deposited N. Boreal forests are typically N-limited ecosystems and are considered to be more efficient in retaining deposited N relative to temperate and tropical forests. As a primary disturbance in boreal forests, wildfires may alleviate N limitation in the burned ecosystem and increase mineralization, resulting in the altered outcomes of the N deposition. In order to explore the effects of a severe wildfire on the retention of deposited N, we investigated the fates of newly deposited N in burned and unburned boreal larch forests by applying 15NH4NO3 tracers to the forest floors. Results showed that total ecosystem retention for the deposited N was 60% in the forest recovering from a severe wildfire burned five years ago, significantly lower than in the unburned mature forest (89%). The difference was mainly attributed to the substantially lower retention in vegetation (8.3%) in the burned site than in the unburned forest (32.4%), as tracer recoveries in soil were similar (51.2 and 56.6%, respectively). Although most 15N tracer was immobilized in organic soil in both burned and unburned forests (33 and 47%, respectively), a noticeably higher amount of 15N was found in mineral soil in the burned forest (19%) than in the unburned forest (10%), suggesting mineral soil as a significant sink for N deposition in the burned forest. A higher total 15N retention in the unburned forest implies that more new N input may stimulate C sequestration and promote the productivity of the Eurasian boreal forest under the background of atmospheric N deposition. However, a considerable amount of deposited N may be lost from the disturbed boreal larch forest ecosystem after a severe wildfire. [ABSTRACT FROM AUTHOR]

Atmospheric nitrogen (N) deposition and climate change are transforming the way N moves through dryland watersheds. For example, N deposition is increasing N export to streams, which may be exacerbated by changes in the magnitude, timing, and intensity of precipitation (i.e., the precipitation regime). While deposition can control the amount of N entering a watershed, the precipitation regime influences rates of internal cycling; when and where soil N, plant roots, and microbes are hydrologically coupled via diffusion; how quickly plants and microbes assimilate N; and rates of denitrification, runoff, and leaching. We used the ecohydrological model RHESSys to investigate (a) how N dynamics differ between N‐limited and N‐saturated conditions in a dryland watershed, and (b) how total precipitation and its intra‐annual intermittency (i.e., the time between storms in a year), interannual intermittency (i.e., the duration of dry months across multiple years), and interannual variability (i.e., variance in the amount of precipitation among years) modify N dynamics and export. Streamflow nitrate (NO3−) export was more sensitive to increasing rainfall intermittency (both intra‐annual and interannual) and variability in N‐limited than in N‐saturated model scenarios, particularly when total precipitation was lower—the opposite was true for denitrification which is more sensitive in N‐saturated than N‐limited scenarios. N export and denitrification increased or decreased more with increasing interannual intermittency than with other changes in precipitation amount. This suggests that under future climate change, prolonged droughts that are followed by more intense storms may pose a major threat to water quality in dryland watersheds. Plain Language Summary: Fossil fuel combustion and industrial agriculture have increased atmospheric nitrogen (N) pollution. Atmospheric N can travel with prevailing winds and be deposited on soil surfaces in terrestrial ecosystems. This can in turn increase N cycling and export to aquatic ecosystems, where excess N acts as a pollutant. The timing and amount of rainfall can influence how much N is transported from terrestrial to aquatic ecosystems, but it remains unclear how future N deposition and precipitation patterns will interact to influence stream water quality and drinking water security. To address this, we used a simulation model to investigate (a) how changes in the timing and/or amount of precipitation influence N export from watersheds, and (b) how the effects of precipitation change with increased atmospheric N deposition. We found that N export to streams increases with precipitation intermittency and variability, particularly when N deposition is low. Under high N deposition, export to streams is substantially elevated, regardless of precipitation timing. Our findings suggest that under future climate change, prolonged droughts that are followed by more intense storms may increase hydrologic N export and pose a major threat to water quality in dryland watersheds. Key Points: Simulating how N deposition interacts with precipitation seasonality can enable us to better predict when dryland watersheds become N‐saturatedAs rainfall regimes become more intermittent and/or variable, streamflow nitrate export is likely to increase, particularly when a watershed is N‐limitedUnder future climate change in drylands, prolonged droughts that are followed by more intense storms may pose a major threat to water quality [ABSTRACT FROM AUTHOR]

Abstract Atmospheric nitrogen (N) deposition and climate change are transforming the way N moves through dryland watersheds. For example, N deposition is increasing N export to streams, which may be exacerbated by changes in the magnitude, timing, and intensity of precipitation (i.e., the precipitation regime). While deposition can control the amount of N entering a watershed, the precipitation regime influences rates of internal cycling; when and where soil N, plant roots, and microbes are hydrologically coupled via diffusion; how quickly plants and microbes assimilate N; and rates of denitrification, runoff, and leaching. We used the ecohydrological model RHESSys to investigate (a) how N dynamics differ between N‐limited and N‐saturated conditions in a dryland watershed, and (b) how total precipitation and its intra‐annual intermittency (i.e., the time between storms in a year), interannual intermittency (i.e., the duration of dry months across multiple years), and interannual variability (i.e., variance in the amount of precipitation among years) modify N dynamics and export. Streamflow nitrate (NO3−) export was more sensitive to increasing rainfall intermittency (both intra‐annual and interannual) and variability in N‐limited than in N‐saturated model scenarios, particularly when total precipitation was lower—the opposite was true for denitrification which is more sensitive in N‐saturated than N‐limited scenarios. N export and denitrification increased or decreased more with increasing interannual intermittency than with other changes in precipitation amount. This suggests that under future climate change, prolonged droughts that are followed by more intense storms may pose a major threat to water quality in dryland watersheds.

Nitrogen (N) deposition usually increases plant tissue N concentrations and thus phosphorus (P) demand in young and/or N‐limited forests, but the N deposition effect on plant P demand has rarely been assessed in N‐saturated forests.Impacts of 18‐year external N additions (Control: 0, Low N: 50, Moderate N:100 and High N: 150 kg N ha−1 year−1) on leaf P of four plant life‐forms (tree, shrub, herb and liana), P fractions of bulk and rhizosphere soils were examined in a N‐saturated mature tropical forest in southern China.Leaf N, P and N: P ratios of all plant life‐forms remained stable under three N additions. Among soil P fractions, moderate labile organic P increased by 25%–33% across three N additions; and soil total P was increased by 11.76% under Low N, and 8.87% under High N, compared with the control. The PLS‐PM results showed that path coefficient of microbial community to available P significantly increased and of inorganic P to available P significantly decreased under N additions than control. N additions improved soil P availability through microbe‐mediated P transformation: Low N significantly increased soil microbial taxonomic diversity, and a higher microbial diversity could enlarge the sources of nutrient acquisition and stimulate decomposition of recalcitrant organic matters; while High N significantly decreased soil microbial taxonomic diversity, the remaining microorganisms that were screened by N‐rich environments had the characteristics of resisting the N addition effects and maintained efficient P acquisition.Synthesis. Our findings provide a novel line of evidence that long‐term N deposition did not increase plant P demand in a N‐saturated mature tropical forest. The underlying mechanism is that plants did not increase N uptakes therefore nor increase P uptakes (a stable leaf N: P stoichiometry) in an already N‐saturated ecosystem. Different N addition rates regulated soil P transformation via microbial community transition. These findings help improve the understanding of plant P acquisition and modelling of biogeochemical N–P cycling and vegetation productivity in N‐rich forest ecosystems, particularly considering the fact that chronic N deposition may likely lead to soil N richness and even saturation of many forests in the future. [ABSTRACT FROM AUTHOR]

Unfettered agricultural activities have severely degraded vast areas of grasslands over the last decade. To rehabilitate and restore the productivity in affected grasslands, rangeland management practices still institute vast nitrogen-based fertilization regimes. However, excessive fertilization can often have damaging environmental effects. Over-fertilization can lead to nitrogen saturation. Although early indicators of nitrogen saturation have been documented, research detailing the near-real-time nitrogen saturation status of grasslands is required to better facilitate management protocols and optimize biomass production within degraded grasslands. Hence, the aim of this study was to discriminate nitrogen-saturated tropical grasses grown under a diverse fertilization treatment trial, using Worldview-3 satellite imagery and decision tree techniques. To accomplish this, nitrogen-saturated plots were first identified through specific physiological-based criteria. Thereafter, Worldview-3 satellite imagery (400–1040 nm) and decision tree techniques were applied to discriminate between nitrogen-saturated and -unsaturated grassland plots. The results showed net nitrate (NO3−-N) concentrations and net pH levels to be significantly different (α = 0.05) between saturated and non-saturated plots. Moreover, the random forest model (overall accuracy of 91%) demonstrated a greater ability to classify saturated plots as opposed to the classification and regression tree method (overall accuracy of 79%). The most important variables for classifying saturated plots were identified as: the Red-Edge (705–745 nm), Coastal (400–450 nm), Near-Infrared 3 (838–950 nm), Soil-Adjusted Vegetation Index (SAVI) and the Normalized Difference Vegetation Index 3 (NDVI3). These results provide a framework to assist rangeland managers in identifying grasslands within the initial stages of nitrogen saturation. This will enable fertilization treatments to be adjusted in near-real-time according to ecosystem demand and thereby maintain the health and longevity of Southern African grasslands. [ABSTRACT FROM AUTHOR]

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. • Investigated N-addition effects on tree population temporal stability. • Temporal stabilities of tree species differed among N-addition levels. • Initial tree size and functional traits influenced temporal stability. • Mycorrhizal type did not determine temporal stability. [ABSTRACT FROM AUTHOR]

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. • Forest water reclamation (FWR) is a cost-effective approach to manage wastewater. • Annual osccilation in nutrient leaching depended on seasonal rate of drainage. • Long-term FWR increased nitrifier abundance, net nitrification, and leaching. • Nitrifier abundance and net nitrification are precursors to N saturation. • Phosphate leaching was minimal; occurring mainly through preferential flow. [ABSTRACT FROM AUTHOR]

Unfettered agricultural activities have severely degraded vast areas of grasslands over the last decade. To rehabilitate and restore the productivity in affected grasslands, rangeland management practices still institute vast nitrogen-based fertilization regimes. However, excessive fertilization can often have damaging environmental effects. Over-fertilization can lead to nitrogen saturation. Although early indicators of nitrogen saturation have been documented, research detailing the near-real-time nitrogen saturation status of grasslands is required to better facilitate management protocols and optimize biomass production within degraded grasslands. Hence, the aim of this study was to discriminate nitrogen-saturated tropical grasses grown under a diverse fertilization treatment trial, using Worldview-3 satellite imagery and decision tree techniques. To accomplish this, nitrogen-saturated plots were first identified through specific physiological-based criteria. Thereafter, Worldview-3 satellite imagery (400–1040 nm) and decision tree techniques were applied to discriminate between nitrogen-saturated and -unsaturated grassland plots. The results showed net nitrate (NO3−-N) concentrations and net pH levels to be significantly different (α = 0.05) between saturated and non-saturated plots. Moreover, the random forest model (overall accuracy of 91%) demonstrated a greater ability to classify saturated plots as opposed to the classification and regression tree method (overall accuracy of 79%). The most important variables for classifying saturated plots were identified as: the Red-Edge (705–745 nm), Coastal (400–450 nm), Near-Infrared 3 (838–950 nm), Soil-Adjusted Vegetation Index (SAVI) and the Normalized Difference Vegetation Index 3 (NDVI3). These results provide a framework to assist rangeland managers in identifying grasslands within the initial stages of nitrogen saturation. This will enable fertilization treatments to be adjusted in near-real-time according to ecosystem demand and thereby maintain the health and longevity of Southern African grasslands.

Legacy nitrogen (N) sources originating from anthropogenic N inputs (NANI) may be a major cause of increasing riverine N exports in many regions, despite a significant decline in NANI. However, little quantitative knowledge exists concerning the lag effect of NANI on riverine N export. As a result, the N leaching lag effect is not well represented in most current watershed models. This study developed a lagged variable model (LVM) to address temporally dynamic export of watershed NANI to rivers. Employing a Koyck transformation approach used in economic analyses, the LVM expresses the indefinite number of lag terms from previous years' NANI with a lag term that incorporates the previous year's riverine N flux, enabling us to inversely calibrate model parameters from measurable variables using Bayesian statistics. Applying the LVM to the upper Jiaojiang watershed in eastern China for 1980-2010 indicated that ~97% of riverine export of annual NANI occurred in the current year and succeeding 10 years (~11 years lag time) and ~72% of annual riverine N flux was derived from previous years' NANI. Existing NANI over the 1993-2010 period would have required a 22% reduction to attain the target TN level (1.0 mg N L(-1)), guiding watershed N source controls considering the lag effect. The LVM was developed with parsimony of model structure and parameters (only four parameters in this study); thus, it is easy to develop and apply in other watersheds. The LVM provides a simple and effective tool for quantifying the lag effect of anthropogenic N input on riverine export in support of efficient development and evaluation of watershed N control strategies.

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The effects of deposited nitrogen (N) on forest ecosystems largely depend on the amount of N retained in the ecosystems and its partitioning among ecosystem pools. However, our understanding of the capacity of tropical plantations to retain deposited N is limited. To evaluate the retention of deposited N in a human-disturbed pine plantation in southern China and compare the result with previous findings in an adjacent old-growth forest, we added 15N-tracer monthly to the forest floor for one year and determined its recovery in ecosystem compartments four months after the last addition. We monitored 15N recoveries in soil solution monthly to quantify leaching losses. The pine forest retained about 58 ± 5% of the 15N-labeled deposited N, which is lower than that reported in the adjacent old-growth forest (72 ± 6%). Both forests experience chronic N deposition (recently measured at 51 kg N ha−1 yr−1) and we attribute the difference in retention to effects of previous disturbance mainly understory and litter harvesting in the pine plantation. Only 3 kg N ha−1 yr−1 (5% of the 15N-labeled deposited N) out of the measured total leaching (54 kg N ha−1 yr−1) originated from deposited (and labeled) N from the measurement year, suggesting that N leaching is dominated by unlabeled N sources. Furthermore, results from our study and other similar 15N labeling experiments together show similar patterns of total ecosystem retention of deposited N in tropical and temperate forests, but here we demonstrate a decreasing retention of N with increased N deposition in these forests. Our findings indicate that plantation forests that experience human-disturbance and chronic N deposition have lower N retention compared to old-growth forests, and thus elevated N inputs in such ecosystems can cause risk of hydrological N losses, soil acidification, and freshwater pollution. [ABSTRACT FROM AUTHOR]

Background and aims: Through agriculture and industry, humans are increasing the deposition and availability of nitrogen (N) in ecosystems worldwide. Carbon (C) isotope tracers provide useful insights into soil C dynamics, as they allow to study soil C pools of different ages. We evaluated to what extent N enrichment affects soil C dynamics in experiments that applied C isotope tracers. Methods: Using meta-analysis, we synthesized data from 35 published papers. We made a distinction between "new C" and "old C" stocks, i.e., soil C derived from plant C input since the start of the isotopic enrichment, or unlabeled, pre-existing soil C. Results: Averaged across studies, N addition increased new soil C stocks (+30.3%), total soil C stocks (+6.1%) and soil C input proxies (+30.7%). Although N addition had no overall, average, effect on old soil C stocks and old soil C respiration, old soil C stocks increased with the amount of N added and respiration of old soil C declined. Nitrogen-induced effects on new soil C and soil C input both decreased with the amount of extraneous N added in control treatments. Conclusion: Although our findings require additional confirmation from long-term field experiments, our analysis provides isotopic evidence that N addition stimulates soil C storage both by increasing soil C input and (at high N rates) by decreasing decomposition of old soil C. Furthermore, we demonstrate that the widely reported saturating response of plant growth to N enrichment also applies to new soil C storage. [ABSTRACT FROM AUTHOR]

Ten research plots were selected on the Eagle Mts. (Orlické hory, Czech Republic) ridge, within complexes of young Norway spruce stands. Since 2002 the defoliation of tree crowns and height increment have been assessed every year after the vegetation season. Sampling of needles for nutrient analyses has been carried out since 2004 every autumn together with the crown condition assessment. Soil samples were taken in 2006, 2010, 2014 and 2018. Samples of the upper organic layer and of the mineral soil in 0–25 cm depth were taken separately. Samples of assimilation organs, humus and mineral soil were prepared and analysed according to the standard ICP Forests methods. Long term results have confirmed that the crown defoliation is decreasing. Health status significantly improved, and crown defoliation values got comparable to those of other forest regions in the Czech Republic. Foliar concentrations of stress elements (sulfur, fluorine) decreased, and foliar analysis demonstrated a significant decrease in air pollution load. There is a problem with insufficient uptake of phosphorus. Soil analyses confirmed the fact that the soil environment is acidic, and the amounts of accessible phosphorus, exchangeable magnesium and calcium are significantly deficient in the mineral soil. In this region there is still high deposition load – total N deposition varies usually between 30–40 kg.ha-1.y-1 and total S deposition between 15–30 kg.ha-1.y-1, i.e. 2–3 times higher than in other regions of the country. [ABSTRACT FROM AUTHOR]

Chronic elevated nitrogen deposition has increased nitrogen availability in many forest ecosystems globally, and this phenomenon has been suggested to increase soil nitrification. Although it is believed that increased nitrogen availability would also increase nitrous oxide (N2O) emissions from forest ecosystems, its impact on N2O flux is poorly known. In this study, 3-years monitoring of N2O emissions was performed in a forested watershed receiving elevated nitrogen deposition and located in the suburbs of Tokyo, Japan. In addition, a comparative field survey was carried out in nine temperate forest sites with varying nitrogen availabilities. In the intensively studied forest site showing typical nitrogen saturation, the average annual N2O emissions from the whole watershed were estimated to be 0.88 kg N ha−1 year−1, comparable to the highest observed levels for temperate forests except for some very high emission sites in Europe. Although no correlation was found for humid spots with WFPS > 60%, a clear positive correlation was noted between N2O flux and net nitrification rate in situ for plots with water-filled pore space (WFPS) < 60%. The N2O flux varied across nine forest sites almost in proportional to the stream water NO3− concentration in the watershed that ranged from 0.14 to 1.64 mg N/L. We conclude that N2O emissions are related to nitrification in moist temperate forest, which may be associated with the magnitude of nitrogen saturation. [ABSTRACT FROM AUTHOR]

Although several studies have assessed the effects of thinning on water quality, including nitrogen (N) exports, few have examined N-saturated plantations. This study assessed the short-term effect of thinning on N exports from a N-saturated plantation forest in northern Kyushu, western Japan, that was thinned (43% of basal area) during January–March 2012. Water levels at the gauging station were continually measured, and stream-water baseflow and stormflow samples were collected before (2011) and after (2013) forest thinning. Annual precipitation before (2469 mm) and after (2418 mm) forest thinning differed little, and annual water discharge after thinning (1641 mm) was similar to that before thinning (1609 mm). However, direct flow during stormflow periods was higher after thinning (260 mm) than before (153 mm). The concentrations of major ions in stream water did not differ before and after thinning. As a result, the high annual dissolved inorganic N (DIN) exports differed little before (35.8 kg N ha−1 a−1) and after (36.5 kg N ha−1 a−1) forest thinning. DIN exports during stormflow periods were slightly higher after (12.2 kg N ha−1 a−1) than before thinning (10.5 kg N ha−1 a−1) and were proportional to the increased direct flow after thinning (561 mm after vs. 470 mm before thinning). We concluded that thinning does not affect annual N exports from the N-saturated plantation forest, but it can increase the proportion of N exported during stormflow periods in proportion to increased water volume of direct flow after thinning. [ABSTRACT FROM AUTHOR]

Study region: Kita and Minami River basins in Japan. Study focus: The coastal watershed in central Japan along the Sea of Japan has suffered large amounts of atmospheric nitrogen (N) deposition from northeastern Asia. However, the quantitative influences of atmospheric N deposition onto forested watersheds in the two basins and riverine N export into coasts remain unclear. To evaluate the current contribution of atmospheric N deposition, N deposition rates from the atmosphere to both basins, and N export rates from the rivers to the sea were quantified. New hydrological insight for the region: Deposition rates of bulk N in each basin exceeded 1000 mg m−2 year−1, more than 60% of which was supplied from winter to early spring by westerly winds. Annual deposition rates in the two basins did not differ, but annual export rates of inorganic N from the Kita River were significantly higher than those from the Minami River. These results suggest that symptoms of N saturation in the Kita River forested watershed are more serious. Furthermore, recent increasing trends of riverine N concentrations may have caused shifts in the limiting nutrient for coastal primary production from N to phosphorous. We suggest reductions in nitrate exports from forests as a strategy to improve nitrate pollution to both downstream waters and coastal ecosystems; however such efforts would involve intercontinental-scale actions in reducing N emissions.

Forest soils harbor diverse microbial communities responsible for the cycling of elements including carbon (C), nitrogen (N), and phosphorus (P). Conversely, anthropogenic N deposition can negatively feed back on soil microbes and reduce soil organic matter (SOM) decomposition. Mechanistically, this can include reductions of decomposer biomass, especially fungi, and decreases in activities of lignin-modifying enzyme (LMEs). Moreover, N inputs can lower resource C:N and thus decrease the C:N imbalance between microbial decomposers and their resources. As a result, microbially-mediated decomposition might slow down, resulting in larger SOM pools with consequences for ecosystem nutrition and climate regulation. Here, we studied the long-term impact of experimental N addition on soil microbes and microbially-mediated decomposition in two coniferous forests in Switzerland and Denmark. We measured microbial biomass C and N, phospholipid fatty acid (PLFA) biomarkers and potential enzyme activities related to C, N, and P acquisition along the topsoil profile (0–30 cm). In particular, we investigated shifts in microbial C:N homeostasis and relative C:N:P limitation. Contrary to prevailing theory, microbial biomass and community composition were remarkably resistant against two decades of 750 and 1,280 kg ha−1 of cumulative N inputs at the Swiss and Danish site, respectively. While N reduced fungal-specific PLFAs and lowered fungi-to-bacteria (F:B) ratios in some (mainly organic) horizons where soil organic carbon (SOC) has accumulated, it increased F:B ratios in other (mainly mineral) horizons where SOC has declined. We did not find a consistent reduction of LME activities in response to N. Rather, relationships between LME activities and SOC concentrations were largely unaffected by N addition. This questions prevalent theories of lignin decomposition and SOC storage under elevated N inputs. By using C:N stoichiometry, we further show that microbial communities responded in part non-homeostatically to decreasing resource C:N, in addition to a likely increase in their carbon use efficiency and a decrease in nitrogen use efficiency. While the expected increased allocation to C- and decreased allocation to N-acquiring enzymes was not found, microbial investment in P acquisition (acid phosphatase activity) increased in the nutrient-poor Podzol (but not in the nutrient-rich Gleysol). Enzyme vector analysis showed decreasing C but increasing P limitation of soil microbial communities at both sites. We conclude that simulated N deposition led to physiological adaptations of soil microbial communities across the topsoil profile in two independent experiments, with long-term implications for tree nutrition and SOC sequestration. However, we expect that microbial adaptations are not endless and may reach a tipping point when ecosystems experience nitrogen saturation.

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. Unlabelled Image • A large amount of nitrogen deposited onto the basins from winter to spring. • Atmospheric nitrate concentrations in upstream waters showed the elevational gradient. • The major source of nitrate was nitrification in forest soil through the basins. • Nitrate leached from the forest soil was the main source for coastal ecosystem. [ABSTRACT FROM AUTHOR]

The Athabasca Oil Sands in Alberta, Canada, is one of the largest point sources of nitrogen oxides in Canada. There are concerns that elevated nitrogen (N) deposition will adversely impact forest ecosystems located downwind of emission sources. The role of the forest floor in regulating these potential eutrophication effects was investigated following a 5-year enrichment study in which N was applied as NH4NO3 above the canopy of a jack pine (Pinus banksiana Lamb) stand in northern Alberta close to Fort McMurray at rates ranging from 5 to 25 kg N ha−1 y−1 in addition to background deposition of approximately 2 kg N ha−1 y−1. Chemical analysis of lichen mats revealed that the N concentration in the apical (upper) lichen tissue and necrotic tissue increased with treatment. When expressed as a N pool, the fibric–humic material held the largest quantity of N across all treatments due to its relatively large mass (172–214 kg N ha−1), but there was no significant treatment effect. Soil net N mineralization and net nitrification rates did not differ among N treatments after five years of application. A 15N tracer applied to the forest floor showed that N is initially absorbed by the apical lichen (16.6% recovery), FH material (29.4% recovery), and the foliage of the vascular plant Vaccinium myrtilloides (31.7% recovery) in particular. After 2 years, the FH 15N pool size was elevated and all other measured pools were depleted, indicating a slow transfer of N to the FH material. Applied 15N was not detectable in mineral soil. The microbial functional gene ammonia monooxygenase (amoA) responsible for catalyzing the first step in nitrification was undetectable using PCR screening of mineral soil microbial communities in all treatments, and broad fungal/bacterial qPCR assays revealed a weak treatment effect on fungal: bacterial ratios in mineral soil with decreasing relative fungal abundance under higher N deposition. This work suggests that terricolous lichen mats, which form the majority of ground cover in upland jack pine systems, have a large capacity to effectively retain elevated N deposition in soil humus. [ABSTRACT FROM AUTHOR]

Highlights • Boreal forests are known to be strongly nitrogen limited. • Chronic ammonium-nitrate additions were made for 8 years at two boreal forest sites in Canada. • The two sites showed a strong resilience to N addition in terms of soil response. • Only the site receiving the lowest natural N deposition showed a response to N addition. • The sites receiving low N deposition are prone to respond in terms of nutrient status and growth. Abstract The boreal forest is recognised as one of the world's most N-limited ecosystems. Increased N deposition is thought to stimulate boreal forest growth and carbon sequestration. However, evidence is lacking about the impact of increasing N supply over long time scales. Moreover, N deposition in forest ecosystems may cause N saturation, which leads to soil cation depletion and, ultimately, affects tree health. Ammonium nitrate (NH 4 NO 3) was applied during an 8-year period in two stands in the boreal forest in Québec (Canada): one composed of balsam fir (Abies balsamea [Linné] Miller), and the other, of black spruce (Picea mariana [Mill.] B.S.P.). Application rates were 3 and 10 times the atmospheric nitrogen (N) deposition rate measured at each site, which was 6 kg ha−1 year−1 at the balsam fir site and 3 kg ha−1 year−1 at the black spruce site. Soil solution (at depths of 30 and 60 cm), mosses, soil and foliar chemistry as well as tree growth were analyzed. After 8 years of N additions, no signs of N saturation were observed in the soil solution of either site, and no treatment response was observed at the balsam fir site. By contrast, in the treated plots at the black spruce site, N concentration increased in mosses and tree needles. Tree basal areal increment also increased by 21% compared with the control. The different responses between the two forest types could be ascribed to the fact that the black spruce site is located further north and receives less N deposition than the balsam fir site. The tree growth increase at the black spruce site (concomitant with foliar N increase) suggests that C sequestration in tree biomass is more likely to increase in areas having soils with high C:N ratio and receiving low N deposition. [ABSTRACT FROM AUTHOR]

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 CO2 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. [ABSTRACT FROM AUTHOR]

The structure and function of terrestrial ecosystems are maintained by processes that vary with temporal and spatial scale. This study examined temporal and spatial patterns of net nitrogen (N) mineralization and nitrification in mineral soil of three watersheds at the Fernow Experimental Forest, WV: 2 untreated watersheds and 1 watershed receiving aerial applications of N over a 25-year period. Soil was sampled to 5 cm from each of seven plots per watershed and placed in two polyethylene bags—one bag brought to the laboratory for extraction/analysis, and the other bag incubated in situ at a 5 cm depth monthly during growing seasons of 1993-1995, 2002, 2005, 2007-2014. Spatial patterns of net N mineralization and nitrification changed in all watersheds, but were especially evident in the treated watershed, with spatial variability changing non-monotonically, increasing then decreasing markedly. These results support a prediction of the N homogeneity hypothesis that increasing N loads will increase spatial homogeneity in N processing. Temporal patterns for net N mineralization and nitrification were similar for all watersheds, with rates increasing about 25-30% from 1993 to 1995, decreasing by more than 50% by 2005, and then increasing significantly to 2014. The best predictor of these synchronous temporal patterns across all watersheds was number of degree days below 19°C, a value similar to published temperature maxima for net rates of N mineralization and nitrification for these soils. The lack of persistent, detectable differences in net nitrification between watersheds is surprising because fertilization has maintained higher stream-water nitrate concentrations than in the reference watersheds. Lack of differences in net nitrification among watersheds suggests that N-enhanced stream-water nitrate following N fertilization may be the result of a reduced biotic demand for nitrate following fertilization with ammonium sulfate. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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Although the effects of adding nitrogen (N) to grasslands have been studied worldwide, little is known about how those effects are, in turn, influenced by climate, soil, and vegetation. Data from such experiments conducted in two main types of grassland in China, alpine and semi-arid, which differed widely in terms of climate, soil, and vegetation, were compiled and analysed to examine the response of above-ground biomass to N. Adding N increased the above-ground biomass by 51% on average. Although the response of biomass showed no obvious trend with the dose of N in the alpine grasslands, the response in the semi-arid grassland peaked at 15–20 g m −2 of N applied annually. Nitrogen addition efficiency in the alpine grasslands was higher than that in the semi-arid grasslands. The difference was mainly due to the differences in total soil N and its interactions with the above-ground biomass, which were particularly influenced by the soil's capacity to provide available N. These results suggest that soil N and features of local vegetation should be taken into account in arriving at the appropriate dose of N for a given site. [ABSTRACT FROM AUTHOR]

Abstract: Over the past few decades, nitrate‐nitrogen (NO3‐N) concentrations have increased within streams of the central Rockies, a pattern linked to regional N deposition trends. As NO3‐N concentrations increase, in‐stream biological demand may become saturated and stream N export may increase. In mountain landscapes, streams generally flow through steep, narrow valleys with limited riparian area and strong stream‐hillslope connectivity. Interspersed between the narrow valleys are wide segments where substantial floodplain riparian areas can develop. Here, we coupled measures of stream reach NO3‐N flux balances with nutrient enrichment experiments along two stream reaches of contrasting valley morphology in Rocky Mountain National Park. The stream reaches were (1) a narrow valley segment with limited floodplain riparian area and (2) a longitudinally adjacent (directly downstream) wide valley segment with extensive floodplain riparian area. We found that in‐stream biological uptake of added NO3‐N was limited in both segments, presumably as a consequence of saturating conditions. Assessment of mass flux indicated that the narrow valley segment was a consistent source of water and NO3‐N across flow states, while the wide segment was a sink at high flow and a source at low flow. Due to low in‐stream biological retention, gross gains and losses of water and NO3‐N to and from the stream exerted primary constraint on segment mass balances. Our results suggest that the exchange of water and nutrients between the stream and adjacent landscape can exert strong control on reach‐scale nutrient export, particularly in streams experiencing or approaching N saturation. [ABSTRACT FROM AUTHOR]

Summary: 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. [ABSTRACT FROM AUTHOR]

The capacity of stream biofilms to transform and assimilate N in highly N-loaded streams is essential to guarantee the water quality of freshwater resources in urbanized areas. However, the degree of N saturation experienced by urban streams and their response to acute increases in N concentration are largely unknown. We measured changes in the rates of NH4+ uptake (UNH4) and oxidation (UAO) resulting from experimental increases in NH4+-N concentration in mature biofilms growing downstream of a wastewater treatment plant (WWTP) and, thus, naturally exposed to high N concentration. We investigated the responses of UNH4 and UAO to NH4+-N increases and the abundance of NH4+ oxidizing bacteria and archaea (AOB and AOA) in epilithic and epipsammic biofilms. UNH4 and UAO increased with increasing NH4+-N concentration for the 2 biofilm types, suggesting no N saturation under ambient levels of NH4+-N. Thus, these biofilms can contribute to mitigating N excesses and the variability of NH4+-N concentrations from WWTP effluent inputs. The 2 biofilm types exhibited different Michaelis–Menten kinetics, indicating different capacity to respond to acute increases in NH4+-N concentration. Mean UNH4 and UAO were 5× higher in epilithic than epipsammic biofilms, coinciding with a higher abundance of AOA+AOB in the former than in the later (76 × 104 vs 14 × 104 copies/cm2). AOB derived from active sludge dominated in epilithic biofilms, so our results suggest that WWTP effluents can strongly influence in-stream NH4+ processing rates by increasing N inputs and by supplying AOA+AOB that are able to colonize some stream habitats. [ABSTRACT FROM AUTHOR]

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 15N 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 15N-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 15N 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 15N, as reflected by an increase in recovery rates from 18 to 40% within 8 years. Less than 2.1% of 15N were recovered in the podzol-B-horizon, suggesting that only negligible amounts of N were withdrawn from the N cycle of the ecosystem. Moreover, 15N recovery was low in the dwarf shrub above-ground biomass (