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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]

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]

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]

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]

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]

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]

Nitrogen (N) deposition affects forest biogeochemical cycles worldwide, often contributing to N saturation. Using long-term (>30-year) records of stream nitrate (NO) concentrations at Fernow Experimental Forest (West Virginia, USA), we classified four watersheds into N saturation stages ranging from Stage 0 (N-limited) to Stage 3 (N-saturated). We quantified NO contributions from atmospheric and microbial sources using δN, δO, and ΔO of NO and characterized the concentrations and isotopes of NO in precipitation. Despite receiving identical atmospheric inputs, the proportions of atmospheric NO in streams averaged from 7 to 10% in the hardwood watersheds (stages 1, 2, and 3) and 54% in the conifer watershed (Stage 0). This suggests that the hardwood watersheds may be less responsive to future reductions in N deposition than the conifer watershed, at least in the short term. As shown in other studies, atmospheric NO proportions were higher during stormflow. Despite large proportions of atmospheric NO in the Stage 0 stream, total atmospheric NO-N flux from this watershed (2.9 g ha) was lower than fluxes in the other watersheds (range = 117.8-338.5 g ha). Seasonal patterns of δN-NO in the hardwood watersheds suggest enrichment of the soil NO pool during the growing season due to plant uptake. In all watersheds, δO-based mixing models over-estimated atmospheric NO contributions to streams by up to 12% compared to ΔO-based estimates. Our results highlight the importance of atmospheric deposition as a NO source in low-concentration streams and demonstrate the advantage of using ΔO-NO over δO-NO for NO source apportionment. [ABSTRACT FROM AUTHOR]

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), 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. [ABSTRACT FROM AUTHOR]

Nitrogen (N) is the nutrient that most frequently limits the productivity of forest ecosystems. Understanding N cycling and forest response to altered N inputs and climate change is an ongoing research challenge. In several intensively studied forests in northeastern North America, well-characterized N inputs are not balanced by measured N losses, suggesting that an unmeasured N loss pathway such as denitrification may be important. We studied soil denitrification gas fluxes in northern hardwood forests at the Hubbard Brook long-term ecological research site in New Hampshire, USA, and found that denitrification in apparently oxic soils could account for N losses greater than half of annual atmospheric N inputs. Denitrification rates were strongly affected by elevation and season, with higher rates occurring at high elevation plots and during snowmelt. These results suggest that denitrification accounts for a major portion of the increasing amounts of 'missing N' reported for this site, and that a significant amount of the anthropogenic N that enters terrestrial ecosystems in northeastern North America is returned to the atmosphere as N. These dynamics are highly vulnerable to change, however, as soil moisture levels and conditions during snowmelt are changing rapidly along with climate. [ABSTRACT FROM AUTHOR]

Elevated levels of inorganic nitrogen (N) deposition and earthworm invasion have the potential to alter N dynamics in eastern North American temperate forests. A regional comparison was conducted across 21 sugar maple ( Acer saccharum Marsh) stands in southern Ontario, where forest floor C:N ratios ranged from 17 to 38 showed that, similar to many other studies, rates of potential net mineralization and nitrification increased below a forest floor C:N ratio threshold of approximately 25 and that nitrification rates are positively correlated with foliar N concentration. However, detailed measurements at four representative stands, receiving between 9.8 and 19 kg N ha y in throughfall, showed that foliar N levels were highest at the site with the lowest N deposition. The primary difference amongst these sites was the presence of invasive earthworms. Specifically, sites without earthworms had significantly higher forest floor N with a lower C:N ratio than the sites with earthworms. There was no significant difference in the rate of sugar maple litter decomposition or chemistry amongst the sites assessed after 540 days using fine (2-mm mesh) litter bags, suggesting that differences in forest floor N levels were most likely due to consumption of litter by large earthworm species and that the lower C:N ratio of the forest floor in sites without earthworms is brought about primarily by a much longer residence time. This work supports the conclusions that forest floor N concentration (or C:N ratio) has a very strong control on N dynamics in forests, but shows that the presence of earthworms can have an impact on forest floor C:N ratio and hence N dynamics that is greater than current levels of atmospheric inorganic N deposition in temperate forests of Ontario. [ABSTRACT FROM AUTHOR]

The long-term capacity of riparian zones in regulating groundwater nitrate fluxes is not well understood. This study analyses patterns of nitrate removal for the period 1994-2012 at two sites in a river floodplain that have received high groundwater nitrate loading from a large upland aquifer for over 32 years. During the study, mean NO-N concentrations entering the riparian zone varied between 20-30 and 30-42 mg/L at the upstream and downstream sites respectively, but did not show any clear inter-annual trend. A permeable sand layer in the riparian zone is underlain by a regional aquitard at a depth of 5-6 m and 4 m at the upstream site and downstream site respectively. Denitrification resulted in a decline in nitrate concentrations as lateral groundwater flow in the sand layer interacted with buried peat and channel bar deposits that range up to 3 m in depth at both riparian sites. This interaction was greater at the downstream site where the organic deposits extend down to <1 m from the aquitard in some locations. At the upstream site nitrate removal efficiency in the sand layer, at depths of 3-4 m ~20 m from the river bank, declined from 68 % in 1996-1998 to 42 % in 2009-2012. A smaller decline from 92 to 82 % occurred in the sand layer 10 m from the river bank during the study. In contrast, no clear pattern of change was evident at the downstream site where a nitrate removal efficiency of 98-100 % occurred at the river bank in most years between 1994 and 2012. These data suggest that the long-term nitrate removal performance of some riparian zones may decline if carbon availability for denitrification becomes limited as a result of variations in the quantity, quality and location of subsurface organic deposits that interact with deeper groundwater flowpaths. [ABSTRACT FROM AUTHOR]

Urbanization alters nitrogen (N) cycling, but the spatiotemporal distribution and impact of these alterations on ecosystems are not well-quantified. We measured atmospheric inorganic N inputs and soil leaching losses along an urbanization gradient from Boston, MA to Harvard Forest in Petersham, MA. Atmospheric N inputs at urban sites (12.3 ± 1.5 kg N ha year) were significantly greater than non-urban (5.7 ± 0.5 kg N ha year) sites with NH (median value of 77 ± 4 %) contributing thrice as much as NO. Proximity to urban core correlated positively with NH (R = 0.57, p = 0.02) and total inorganic N inputs (R = 0.61, p = 0.01); on-road CO emissions correlated positively with NO inputs (R = 0.74, p = 0.003). Inorganic N leaching rates correlated positively with atmospheric N input rates (R = 0.61, p = 0.01), but did not differ significantly between urban and non-urban sites ( p > 0.05). Our empirical measurements of atmospheric N inputs are greater for urban areas and less for rural areas compared to modeled regional estimates of N deposition. Five of the nine sites had NO leached that came almost entirely from nitrification, indicating that the NO in leachate came from biological processes rather than directly passing through the soil. A significant proportion (17-100 %) of NO leached from the other four sites came directly from the atmosphere. Surprisingly, the four sites where atmospheric sources made up the largest proportion of leachate NO also had relatively low N leaching rates, suggesting that atmospheric N inputs added to terrestrial ecosystems can move to multiple sinks and losses simultaneously, rather than being lost via leaching only after abiotic and biotic sinks have become saturated. This study improves our understanding of atmospheric N deposition and leaching in urban ecosystems, and highlights the need to incorporate urbanization effects in N deposition models. [ABSTRACT FROM AUTHOR]

Heathlands and grasslands occur in montane regions, naturally or due to anthropogenic land-use. These are typically nutrient-poor but exposure to elevated nitrogen deposition and intensive livestock grazing causes large-scale ecological change. We studied the long-term implications of grazing removal on soil and drainage water biogeochemistry and the implications for nitrogen cycling in 50-year replicated grazing exclosures on a montane grassland exposed to high rates of ambient nitrogen deposition. Evidence of 'ecosystem recovery' represented by successional change from graminoid to shrub-dominance after cessation of grazing was not reflected in the soil biogeochemistry. Cessation of grazing had a negative impact, with increased soil extractable and soil solution nitrate concentrations; an apparent shift towards a more nitrogen-rich, bacterially dominated microbial community; and the acidification of soils and leachate. The increase in nitrate leaching appears to have been counterbalanced by a decrease in dissolved organic nitrogen leaching, approximately maintaining the overall nitrogen balance of the system, whilst apparently altering ecosystem functioning. High rates of organic matter cycling and inorganic nitrogen uptake in grazed grassland may have sustained ecosystem N limitation under elevated nitrogen deposition. Grazing removal caused long-term over-supply of nitrogen from mineralisation of enriched organic matter, exacerbated by continued high nitrogen deposition, exceeding the uptake demand of heath vegetation and resulting in nitrification and nitrate leaching. This disequilibrium between vegetation and soil following grazing removal has implications for restoration after periods of intensive grazing. Grazing may not simply leave a legacy of nutrient enrichment but its cessation may trigger nitrogen saturation and soil and freshwater eutrophication and acidification which counteract the immediate benefits of natural vegetation recovery. Long term, nitrogen saturation of abandoned grasslands is likely to reduce ecosystem resilience to invasion by nitrophilous species, pathogen attack and vulnerability to environmental pressures such as climate change. We conclude that partial and/or phased reduction in grazing levels may permit the more synchronised recovery of soils and vegetation, thereby avoiding imbalances between nitrogen supply and nitrogen demand and detrimental ecological effects. [ABSTRACT FROM AUTHOR]

Assessment of temporal trends and rates of change in hydrochemical parameters and forest cover has been conducted to elucidate key drivers of surface water acidification in glacial lakes in the Czech Republic. Since 1984, the key driver in acidification reversal was sulphate (SO 4) concentration (median decrease of −3.58 μeq L −1 yr −1) which fell in line with reductions in sulphur (S) deposition. Reduction of nitrogen (N) deposition was followed by proportional reduction in nitrate (NO 3) leaching although decline in NO 3 concentrations was more pronounced at two sites, the Čertovo Lake (CT) and Prášilské Lake (PR) until 2006; only Žďárské pond showed effective catchment N immobilization. Coherent decline of chloride concentration was detected across all sites. The decrease of strong mineral acids was partly compensated by decrease of inorganic aluminium (Al in), especially at sites most acidified in the beginning of observations (ANC 1984–1986 between −160 and −90 μeq L −1 at CT, Černé Lake—CN and Plešné lake—PL) and by reductions of base cations and increases of pH. All lakes (CN, CT, PL, PR and LK) moved to the ANC range between −29 and 30 μeq L −1 (2010–2012) where sensitivity of pH to further reductions in acid anions may be expected. Concurrently, charge of weak organic acids (OAs) increased and partly balanced the strong mineral acid decrease as a consequence of (i) significant DOC (dissolved organic carbon) increase (median change of 0.13 mgC L −1 yr −1 since 1993) and (ii) deprotonation of weak OAs caused by pH rise. Since 2000s, bark beetle induced forest decline accelerated NO 3 leaching at most of the catchments (by 200 % at LK, PL and PR). However, elevated N leaching was effectively neutralized by base cations (K, Mg, Ca) originating from decaying fresh litter, thus acidification recovery was not reversed, but slowed down. After cessation of NO 3 leaching we hypothesise that collapsed tree canopy across catchments (from 12 to 87 % compared to 1984) will cause lower total acid input in precipitation (S + N) and regrowth of vegetation may stimulate higher N immobilization (in biomass and soil); processes which could lead to further increase of ANC and pH, key indicators for biological recovery. [ABSTRACT FROM AUTHOR]

Nitrogen immobilization in fresh litter represents a significant N flux in forest ecosystems, and changes in this process resulting from atmospheric N deposition could have important implications for ecosystem responses. We conducted two leaf decay experiments, using N-labeled sugar maple leaf litter, to quantify N transport from old litter and soil to fresh litter during early stages of decomposition, and we examined the influence of litter N concentration and soil N availability on upward N transfer in a northern hardwood forest. After one year of decay, the average N transfer from soil to fresh litter (2.63 mg N g litter) was much higher than the N transfer from older litter (1- to 2-years-old) to fresh litter (0.37 mg N g litter). We calculated the ratio of annual N transfer per unit of excess N pool for these two N sources. The ratio was not significantly different between old litter and soil, suggesting that fungi utilize N in the old litter and mineral soil pools for transport to decaying fresh litter with similar efficiency. Initial litter N concentration had a significant effect on upward N flux into decaying leaf litter, whereas no effect of soil N fertilization was observed. Reduction in the flux from soil to fresh litter owing to anthropogenic N inputs probably contributes significantly to changing soil N dynamics. Future work is needed on fungal N acquisition and transport as well as the fungal taxa involved in this process and their responses to changing environments. [ABSTRACT FROM AUTHOR]

Belowground dynamics of terrestrial ecosystems are responding to global increases in anthropogenic N deposition with important consequences for productivity and ecosystem health. We compared root characteristics across five root orders in Pinus tabuliformis plantations treated for 3 years to a gradient of N addition (0-15 g m year). In reference plots, the roots of P. tabuliformis were finer and with higher specific root length than reported for other pine species, suggesting severe N limitation. Addition of N resulted in slightly reduced fine root biomass and significant changes in root morphology, responses that were associated primarily with first and second order roots. In particular, root number, cumulative root length, individual root length, and specific root length all declined with increasing N addition for first and second order roots, with most of the responses elicited at <9 g m year N addition. These responses (1) support the concept of ephemeral root modules consisting of first and second orders and (2) are consistent with a change in functional demand from uptake to transport with increasing soil resource availability. Traditionally, fine roots have been identified by a somewhat arbitrary diameter cut-off (e.g., 1 or 2 mm); as an index of fine root function, diameter would fail to reveal most of the functional response. [ABSTRACT FROM AUTHOR]

Flux of dissolved inorganic nitrogen (DIN-primarily nitrate) from terrestrial ecosystems has been considered an important contributor to acidification of linked aquatic systems. The basis of this concern is the nitrogen (N) saturation hypothesis, positing that additions of N to terrestrial ecosystems in excess of biological requirements will result in DIN leaching. There is a consensus (implicit hypothesis) in the literature that atmospheric deposition of DIN in excess of a threshold of approximately 10 kg ha year leads to significant flux. Diverse data from USA indicate that DIN flux is highly variable both in space and time; the spatial uncertainty as measured by the pooled coefficient of variation is about 0.95, and the temporal (inter-year) uncertainty is about 0.75. The relationship between atmospheric deposition of DIN and annual flux is near-linear within the range of current deposition for US sites (≤8 kg ha year wet deposition). If wet and dry depositions are approximately equal, over 85 % of total DIN deposition is retained. This is nearly equal to the retention reported by the US Geological Survey National Water-Quality Assessment Program, which considered all nonpoint sources of N as inputs and both DIN and organic N as fluxes. Although input-output data have high uncertainty, the 85 % retention of atmospheric DIN by terrestrial watersheds casts doubt on its importance as a contributor to aquatic acidification. There is no obvious threshold of deposition leading to DIN leaching. The nitrogen saturation hypothesis may not fully explain N behavior in terrestrial ecosystems. [ABSTRACT FROM AUTHOR]

The effect of chronic high groundwater nitrate loading on riparian forests is poorly understood. The growth patterns of northern white cedar ( Thuja occidentalis) and related plant-soil processes were examined at four riparian sites in southern Ontario, Canada which have similar vegetation, soils, and hydrology but have differed in adjacent land use for >60 years. Fertilized cropland at two riparian sites produced groundwater-fed surface flows with high mean NO-N concentrations of 9 and 31 mg l, whereas mean concentrations were <0.5 mg l at two control sites down slope from forest. Tree-ring analysis at the two nitrate-rich sites indicated a positive growth trend in 1980-2004 and an absence of a positive growth trend in the 1945-1970 period that preceded high rates of synthetic nitrogen fertilizer use on cropland. However, a significant increase in growth also occurred in 1980-2004 at the two control riparian sites suggesting that high groundwater nitrate inputs did not influence tree growth. Cedar foliar and litter N content did not differ significantly between the high nitrate and control sites. Litter decomposition rates measured by the litterbag technique at a nitrate-enriched and control site were similar. Litter from a high nitrate and a control site produced a similar rate of potential denitrification in lab incubations of riparian surface peat. This study indicates that prolonged nitrate inputs in groundwater did not increase nitrogen uptake and growth of white cedar or stimulate decomposition and denitrification as a result of changes in the quality of plant material. In the absence of anthropogenic nitrate inputs, riparian wetland soils are typically high in ammonium and low in nitrate, and as a consequence, white cedar may have a limited ability to utilize nitrate. [ABSTRACT FROM AUTHOR]

Elevated emissions of nitrogen oxides (NO) in the Athabasca Oil Sands Region, Alberta and higher foliar nitrogen (N) concentrations in jack pine ( Pinus banksiana) needles close to major emission sources has led to concerns that the surrounding boreal forest may become N-saturated. Despite these concerns, N deposition and impacts on upland forests in the region is poorly quantified. The objective of this study was to characterize N cycling in five plots representing the two dominant upland forest types (jack pine and trembling aspen, Populus tremuloides) close (<30 km) to the largest mining operations in the region, during a 2-year period. Despite the high level of NO emissions, bulk throughfall and deposition measured at both study sites were surprisingly very low (<2 kg N ha year). Internal N cycling was much greater in aspen stands; annual N input in litterfall was ten times greater, and net N mineralization rates were two to five times greater than in jack pine stands. Nitrogen use efficiency (NUE) was much greater in jack pine when calculated based on N litterfall indices, but not when N pools in biomass were considered. Despite differences in internal cycling among forest types, nitrate leaching from mineral soil in both forest types was negligible (<0.1 kg N ha year) and patterns of N in roots, foliage, and mineral soil were typical of N-limited ecosystems, and both sites show no evidence of N saturation. [ABSTRACT FROM AUTHOR]

Various studies over the last 15 years have attempted to describe the processes of N retention, saturation and NO leaching in semi-natural ecosystems based on stable isotope studies. Forest ecologists and terrestrial biogeochemists have used N labelled NO and NH tracers to determine the fate of atmospheric deposition inputs of N to terrestrial ecosystems, with NO leaching to surface waters being a key output flux. Separate studies by aquatic ecologists have used similar isotope tracer methods to determine the fate and impacts of inorganic N species, leached from terrestrial ecosystems, on aquatic ecosystems, usually without reference to comparable terrestrial studies. A third group of isotopic studies has employed natural abundances of N and O in precipitation and surface water NO to determine the relative contributions of atmospheric and microbial sources. These three sets of results often appear to conflict with one another. Here we attempt to synthesize and reconcile the results of these differing approaches to identifying both the source and the fate of inorganic N in natural or semi-natural ecosystems, and identify future research priorities. We conclude that the results of different studies conform to a consistent conceptual model comprising: (1) rapid microbial turnover of atmospherically deposited NO at multiple biologically active locations within both terrestrial and aquatic ecosystems; (2) maximum retention and accumulation of N in carbon-rich ecosystems and (3) maximum leaching of NO, most of which has been microbially cycled, from carbon-poor ecosystems exposed to elevated atmospheric N inputs. [ABSTRACT FROM AUTHOR]

Karst watersheds are a major source of drinking water in the European Alps. These watersheds exhibit quick response times and low residence times, which might make karst aquifers more vulnerable to elevated nitrogen (N) deposition than non-karst watersheds. We summarize 13 years of monitoring NO, NH, and total N in two forest ecosystems, a Norway spruce ( Picea abies (L.) Karst.) forest on Cambisols/Stagnosols (IP I) and a mixed beech ( Fagus sylvatica L.) spruce forest on Leptosols (IP II). N fluxes are calculated by multiplying concentrations, measured in biweekly intervals, with hydrological fluxes predicted from a hydrological model. The total N deposition in the throughfall amounts to 26.8 and 21.1 kg/ha/year in IP I and IP II, respectively, which is high compared to depositions found in other European forest ecosystems. While the shallow Leptosols at IP II accumulated on average 9.2 kg/ha/year of N between 1999 and 2006, the N budgets of the Cambisols/Stagnosols at IP I were equaled over the study period but show high inter-annual variation. Between 1999 and 2006, on average, 9 kg/ha/year of DON and 20 kg/ha/year of DIN were output with seepage water of IP I but only 4.5 kg/ha/year of DON and 7.7 kg/ha/year of DIN at IP II. Despite high DIN leaching, neither IP I nor IP II showed further signs of N saturation in their organic layer C/N ratios, N mineralization, or leaf N content. The N budget over all years was dominated by a few extreme output events. Nitrate leaching rates at both forest ecosystems correlated the most with years of above average snow accumulation (but only for IP I this correlation is statistically significant). Both snow melt and total annual precipitation were most important drivers of DON leaching. IP I and IP II showed comparable temporal patterns of both concentrations and flux rates but exhibited differences in magnitudes: DON, NO, and NH inputs peak in spring, NH showed an additional peak in autumn; the bulk of the annual NO and DON output occurred in spring; DON, NO, and NH output rates during winter months were low. The high DIN leaching at IP I was related to snow cover effects on N mineralization and soil hydrology. From the year 2004 onwards, disproportional NO leaching occurred at both plots. This was possibly caused by the exceptionally dry year 2003 and a small-scale bark beetle infestation (at IP I), in addition to snow cover effects. This study shows that both forest ecosystems at Zöbelboden are still N limited. N leaching pulses, particularly during spring, dictate not only annual but also the long-term N budgets. The overall magnitude of N leaching to the karst aquifer differs substantially between forest and soil types, which are found in close proximity in the karstified areas of the Northern Limestone Alps in Austria. [ABSTRACT FROM AUTHOR]

Information about the impact of nitrogen (N) deposition on the fate of deposited N in peatland ecosystems is lacking. Thus we investigated the fate of experimentally added N in long-term N-fertilized treatments in a Sphagnum-dominated ombrotrophic bog. Fertilization significantly stimulated vascular plant and suppressed Sphagnum and Polytrichum moss growth. N content in peat, mosses, and vascular plants was raised by the fertilizer addition and reached a maximum at 3.2 g m N input level with phosphorus (P) and potassium (K) addition. Most of N was retained in the vegetation and upper 10 cm of the peat. When N deposition equalled 1.6 g m and less, or 3.2 g m N with P and K addition, no inorganic N leaching was observed on the plots. This result indicates that co-fertilization with P and K raised the N retention capacity and that critical N loads with respect to N saturation depend on P and K availability. Most of the deposited N was recovered in the bulk peat, which may be related to a rapid immobilization of inorganic N by microorganisms and mycorrhizal assimilation. Increase of N, P, and K fertilization increased the contribution of vascular plants to N retention significantly and reduced those of mosses. The increase was mainly related to enhanced productivity, vascular biomass and N content in tissues; the reduced retention by mosses resulted from both reduced moss biomass and assimilation. The study shows that the N filter function of ombrotrophic bogs will be influenced by interactions with other nutrients and shifts in plant community structure. [ABSTRACT FROM AUTHOR]

Ecosystem acidification and eutrophication resulting from increased deposition of dissolved inorganic nitrogen (DIN) are issues of increasing global concern. Consequently, costly policy decisions are being implemented to decrease nitrogen oxide (NO) emissions. Although declining DIN deposition along with rapid declines of DIN in surface waters have been reported in parts of Europe, the same observation is just emerging in North America. Here we find a significant decline in bulk deposition NO during the later part of a 28-year record in southcentral Ontario, Canada. Despite high N retention and substantial inter-annual variability in the long-term record due to periods of drought, we find significant declines in annual NO concentrations and export at six out of 11 streams that drain upland-dominated catchments. In contrast, five streams draining primarily wetland-dominated catchments with lower levels of NO show no decreasing trend in NO concentration or export. The rapid response in stream NO to declining atmospheric inputs was observed at sites with historically moderate inputs of DIN (~870 mg m y) in bulk deposition. Topographic features such as slope, and related catchment features including wetland cover, appear to influence which catchments will respond positively to declining DIN deposition. These findings force us to revise our original conceptualization of the N saturation status of these catchments. [ABSTRACT FROM AUTHOR]

Nitrogen compounds generated by anthropogenic combustion deposits in forest watersheds and induce nitrogen saturation of the area. Because excess nitrogen is derived from atmospheric deposition, this action is expected to uniformly affect a wide area of forest soils. Geographically, heterogeneous nitrate concentration of stream water within a small area has been attributed to the tree type, geological setting and tree cut. In this article, we hypothesized that the effect of the atmospheric nitrogen deposition in the forest watershed may vary within a small area, and that such variation is induced by the degree of air mass containing a high concentration of nitrogen deposition of combustion origin. We measured major ion concentrations, including nitrate, nitrite oxygen and nitrogen stable isotope of nitrate sampled at 24 water streams in the Chichibu region, which is 50-100 km from the Tokyo metropolitan area. The nitrate concentration showed a wide range (25.6-237 μmol L) within 300 km, which was explained sufficiently by the air mass advection path and its contact with the mountain's surface. The nitrate concentration showed a significant positive correlation with chloride ( r = 0.73; p < 0.001). As chloride originates outside of the Chichibu region, the positive correlation between two ions showed that the nitrate concentration of the stream water was affected by the nitrogen compound from the Tokyo Metropolitan area as a form of atmospheric deposition. Between the nitrate concentration and the stable isotope ratio of oxygen of nitrate, there was a positive correlation until nitrate concentration of 100 μmol L. When the nitrate is over 100 μmol L, δO shows a stable value of ca. 5.7‰. This indicates that the nitrification proceeds when the nitrate concentration was low to middle, but the reaction slowed when the nitrate concentration became high. Oxygen stable isotope of nitrate along with a set of nitrate concentrations can be used as a good indicator of nitrogen saturation. [ABSTRACT FROM AUTHOR]

The objectives of this study were to quantify rates of nitrogen inputs to the forest floor, determine rates of nitrogen losses via leaching and to partition the sources of NO from healthy, declining, and salvage or preemptively cut eastern hemlock ( Tsuga canadensis) stands in both an urban forest at the Arnold Arboretum in Boston, MA and a rural forest at Harvard Forest in Petersham, MA. Rates of nitrogen inputs (NH and NO) to the forest floor were 4-5 times greater, and rates of nitrogen losses via leachate were more than ten times greater, at the Arnold Arboretum compared to Harvard Forest. Nitrate that was lost via leachate at Harvard Forest came predominantly from atmospheric deposition inputs, whereas NO losses at the Arnold Arboretum came predominantly from nitrification. Although our study was limited to one urban and one rural site, our results suggest that current management regimes used to control the hemlock woolly adelgid ( Adelges tsugae), such as salvage cutting, may be reducing nitrogen losses in urban areas due to rapid regrowth of vegetation and uptake of nitrogen by those plants. In contrast, preemptive cutting of trees in rural areas may be leading to proportionately greater losses of nitrogen in those sites, though the total magnitude of nitrogen lost is still smaller than in urban sites. Results of our study suggest that the combination of the hemlock woolly adelgid, nitrogen inputs, and management practices lead to changes in the movement and source of NO losses from eastern hemlock forest ecosystems. [ABSTRACT FROM AUTHOR]

Ion concentrations and fluxes in seepage water (below the main rooting zone) were compared before and after clear cutting at two similar long-term experimental Norway spruce forest plots. While Ballyhooly (Ireland) was influenced by sea salt deposition, Höglwald (Germany) received high nitrogen (N) deposition. These differences were reflected in seepage water concentrations with sodium (Na) and chloride (Cl) dominating at Ballyhooly and high nitrate (NO) and aluminium concentrations at Höglwald. Following clear cutting of the forest plots, NO concentrations peaked (Ballyhooly: 2018 μmol L, Höglwald: 2595 μmol L). Moreover, at Ballyhooly, NO concentrations and fluxes were continuously elevated for ~1.5 years. At Höglwald, the clear cut plot, which was replanted with spruce and beech saplings, exhibited periodically elevated NO concentrations with two distinct peaks. However, low concentrations, compared to the control (uncut) plot, were also observed. Further, at Höglwald a plot with a pre-existing dense natural regeneration of Norway spruce exhibited much lower NO concentrations before and after clear cutting. Nonetheless, NO concentrations following clear cut at both sites were elevated at least periodically above European drinking water standards (50 mg L). An important prerequisite for NO leaching is that forests are N saturated or at least not N-limited; consequently chronic elevated N deposition may lead to increased deterioration of seepage water quality across Europe following forest disturbances (harvesting, windthrow, insect attacks). Clear cutting at Ballyhooly was responsible for significant element loss, especially of potassium, N and calcium, while magnesium loss was compensated by high sea salt inputs. At Höglwald the contamination of seepage water with NO has been the main problem for more than 20 years at the mature stand. A pre-existing regeneration can help to reduce NO and cation leaching after cutting. [ABSTRACT FROM AUTHOR]

Southern Ontario receives the highest levels of atmospheric nitrogen (N) deposition in Canada and there are concerns that forests in the region may be approaching a state of ‘N saturation’. In order to evaluate whether potential chemical indices provide evidence of N saturation, 23 hardwood plots were sampled along a modeled N-deposition gradient ranging from 9.3 to 12.8 kg/ha/year. All plots were dominated by sugar maple ( Acer saccharum Marsh.) and foliar N and foliar δ15N were positively correlated with modeled N deposition. However, forest floor N content and the C:N ratio were unrelated to N deposition, but were instead related to soil pH and annual temperature; lower C:N ratios and higher N content in the forest floor were found at the most acidic sites in the cooler, northern part of the study region despite lower N deposition. Likewise, δ15N values in surface mineral soil and the 15N enrichment factor of foliage (δ15N foliage − δ15N soil) are correlated to soil pH and temperature and not N deposition. Further, potential N mineralization, ammonification, and nitrification in Ontario maple stands were highest in the northern part of the region with the lowest modeled N deposition. Nitrogen cycling in soil appears to be primarily influenced by the N status of the forest floor and other soil properties rather than N deposition, indicating that chemical indices in soil in these hardwood plots may not provide an early indicator of N saturation. [ABSTRACT FROM AUTHOR]

There is growing evidence from different sources that prolonged high N deposition causes a shift from nitrogen (N) limitation to nitrogen and phosphorus (P) co-limitation or even P limitation in many terrestrial ecosystems. However, the number of ecosystems where the type of limitation has been directly tested by longer-term full-factorial field experiments is very limited. We conducted a 5-year fertilization experiment with N and P in the Lüneburger Heide (NW Germany) to test the hypothesis that, following decades of elevated atmospheric N inputs, plant growth in dry lowland heaths may have shifted from N to N–P co-limitation or P limitation. We also tested whether the plant tissue N:P ratio reflects the type of nutrient limitation in a continental lowland heathland. Experimental plots dominated by Calluna vulgaris received regular additions of N (50 kg N ha−1 y−1), P (20 kg P ha−1 y−1), a combination of both, or water only (control) from 2004 to 2008. Over the whole study period, a highly significant positive N effect on shoot length was found, thus indicating N limitation. We conclude that a clear shift from N limitation to N–P co-limitation or P limitation has not yet occurred. Tissue N:P ratios showed a high temporal variability and no relationship between tissue N:P ratio and the shoot length response of Calluna to nutrient addition was found. The N:P tool is thus of limited use at the local scale and within the range of N:P ratio observed in this study, and should only be used as a rough indicator for the prediction of the type of nutrient limitation in lowland heathland on a larger geographical scale with a broader interval of N:P ratio. [ABSTRACT FROM AUTHOR]

Rapid industrialization in East Asia is causing adverse effects due to atmospheric deposition in terrestrial and freshwater ecosystems. Decreasing stream pH and alkalinity and increasing NO3 − concentrations were observed throughout the 1990s in the forested Lake Ijira catchment in central Japan. We investigated these changes using data on atmospheric deposition, soil chemistry, stream water chemistry, and forest growth. Average atmospheric depositions (wet + dry) of 0.83, 0.57, and 1.37 kmol ha−1 year−1 for hydrogen, sulfur, and nitrogen, respectively, were among the highest levels in Japan. Atmospheric deposition generally decreased before 1994 and increased thereafter. The catchment was acid-sensitive; stream alkalinity was low (134 μmolc l−1) and pH in surface mineral soils decreased from 4.5 in 1990 to 3.9 in 2003. Stream NO3 − concentration nearly doubled (from 22 to 42 μmolc l−1) from the late 1980s to the early 2000s. Stream NO3 − concentration was controlled primarily by water temperature before 1996/1997 and by stream discharge thereafter. Stream NO3 − concentrations decreased during the growing season before 1996/1997, but this seasonality was lost thereafter. The catchment became nitrogen-saturated (changing from stage 1 to 2) in 1996/1997, possibly because of declining forest growth rates due to the 1994 summer drought, defoliation of Japanese red pine by pine wilt disease, maturation of Japanese cedar stands, and stimulation of nitrogen mineralization and nitrification due to alkalinization of soils (increased exchangeable Ca2+ and soil pH) after the summer drought. Stream pH and alkalinity began decreasing in 1996/1997. The enhanced growing-season NO3 − discharge since 1996/1997 appeared to be the major cause of stream acidification. Increased atmospheric deposition since 1994 may have contributed to this change. [ABSTRACT FROM AUTHOR]

The article investigates the spatial and temporal variation of pollutant and nutrient loads in various environmental media in permanent forest and grassland plots in Baden-Württemberg, Germany. Study shows that lead and sulphur concentrations are the most marked downward trends in three foliage and grassland samples. It indicates that the reduced levels of these elements in the biomonitors reflect the successful execution of clean air policies.

Nitrogen additions (NH4NO3) at rates of three- and ten-fold ambient atmospheric deposition (8.5 kg ha − 1 year − 1) were realised in an acid- and base-poor northern hardwood forest of Québec, Canada. Soil solution chemistry, foliar chemistry, crown dieback and basal area growth of sugar maple ( Acer saccharum Marsh.) were measured. Except for a transitory increase of NO3 and NH4 concentrations, there was no persistent increase in their level in soil solution 3 years after N treatments, with the exception of one plot out of three, that received the highest N addition, beginning to show persistent and high NO3 concentrations after 2 years of N additions. Three years of N additions have significantly increased the N DRIS index of sugar maple but not N foliar concentration. Potassium, Ca and Mn foliar concentrations, as well as P and Ca DRIS indices, decreased in treated plots after 3 years. No treatment effect was observed for basal area growth and dieback rate. One unexpected result was the significant decrease in foliar Ca even in the treated plots that received low N rates, despite the absence of significant NO3-induced leaching of Ca. The mechanism responsible for the decrease in foliar Ca is not known. Our results, however, clearly demonstrate that increased N deposition at sites with low base saturation may affect Ca nutrition even when clear signs of N saturation are not observed. [ABSTRACT FROM AUTHOR]

Dissolved organic nitrogen (DON) has recently been recognized as an important component of terrestrial N cycling, especially under N-limited conditions; however, the effect of increased atmospheric N deposition on DON production and loss from forest soils remains controversial. Here we report DON and dissolved organic carbon (DOC) losses from forest soils receiving very high long-term ambient atmospheric N deposition with or without additional experimental N inputs, to investigate DON biogeochemistry under N-saturated conditions. We studied an old-growth forest, a young pine forest, and a young mixed pine/broadleaf forest in subtropical southern China. All three forests have previously been shown to have high nitrate (NO) leaching losses, with the highest loss found in the old-growth forest. We hypothesized that DON leaching loss would be forest specific and that the strongest response to experimental N input would be in the N-saturated old-growth forest. Our results showed that under ambient deposition (35–50 kg N ha−1 y−1 as throughfall input), DON leaching below the major rooting zone in all three forests was high (6.5–16.9 kg N ha−1 y−1). DON leaching increased 35–162% following 2.5 years of experimental input of 50–150 kg N ha−1 y−1. The fertilizer-driven increase of DON leaching comprised 4–17% of the added N. A concurrent increase in DOC loss was observed only in the pine forest, even though DOC:DON ratios declined in all three forests. Our data showed that DON accounted for 23–38% of total dissolved N in leaching, highlighting that DON could be a significant pathway of N loss from forests moving toward N saturation. The most pronounced N treatment effect on DON fluxes was not found in the old-growth forest that had the highest DON loss under ambient conditions. DON leaching was highly correlated with NO leaching in all three forests. We hypothesize that abiotic incorporation of excess NO (through chemically reactive NO) into soil organic matter and the consequent production of N-enriched dissolved organic matter is a major mechanism for the consistent and large DON loss in the N-saturated subtropical forests of southern China. [ABSTRACT FROM AUTHOR]

Fluxes of major ions in rainfall (RF), throughfall plus stemflow (TF + SF), and stream water (SW) were measured for five water years in a small catchment of a Japanese cedar forest near the Sea of Japan. The fluxes of most ions in RF and in TF + SF, including the non-sea-salt constituents, increased from late autumn to midwinter owing to the seasonal westerly wind. The concentrations of most ions in SW showed no obvious seasonal trend during the study period, whereas $${\text{NO}}_3 ^ - $$ concentrations were lowest in summer, with a small seasonality. The Ca2+ and Mg2+ outputs in SW were approximately 3.7 and 1.8 times the TF + SF inputs of these cations, respectively. The large net outputs of base cations in the catchment may indicate a decrease in the soil's acid-neutralizing capacity. Annual dissolved inorganic nitrogen inputs in RF and in TF + SF were 17.7 and 17.9 kg N ha−1 year−1, respectively, which exceeded previously published thresholds in Europe and the U.S. (i.e., the values at which these inputs increased $${\text{NO}}_3 ^ - $$ levels in SW) and equaled the highest level of nitrogen deposition previously reported in Japan. The $${\text{NO}}_{\text{3}} ^{\text{ - }} $$ concentrations in SW were relatively high even in summer. During high-precipitation events, $${\text{NO}}_{\text{3}} ^{\text{ - }} $$ concentrations in SW increased with increasing water discharge, and the pH decreased simultaneously during several events. Nitrogen deposition may contribute to the high $${\text{NO}}_{\text{3}} ^{\text{ - }} $$ concentrations in SW and the temporary acidification that occurred during the rain events. [ABSTRACT FROM AUTHOR]

Stream nitrogen (N) export and nitrate $$(\text {NO}^-_3) $$ concentration were measured at 14 forested watersheds (GEOMON network) in the Czech Republic between 1994 and 2005. In the last several decades, emissions of sulfur (S) and N compounds have decreased throughout much of Europe. In the Czech Republic, atmospheric deposition of S has decreased substantially since the beginning of 1990s, whereas N deposition remains largely unchanged at most sites. The mean dissolved inorganic nitrogen (DIN) streamwater export ranged from 0.2 to 12.2 kg ha−1 y−1 at the GEOMON sites. Despite decades of elevated N deposition, 44–98% of DIN inputs to these watersheds were retained or denitrified, and many watersheds showed seasonal variation in nitrate concentrations. Dissolved organic N export was quantified in 1 year only and ranged from 0.05 to 3.5 kg ha−1 y−1. Spatial variability in DIN export among watersheds was best explained by spatial variability in average acidic deposition, particularly S deposition ( R 2 = 0.81, P < 0.001); DIN input and forest floor carbon:nitrogen (C/N) also provided significant explanatory power. DIN export was strongly influenced by the forest floor C/N ratio and depth of the forest floor soils ( R 2 = 0.72, P < 0.001). The only variable that predicted variations in forest floor C/N ( R 2 = 0.32, P < 0.05) among watersheds was S deposition. Forest floor depth was also related to deposition variables, with S deposition providing the most explanatory power ( R 2 = 0.50, P < 0.01). Variation in forest floor depth was also associated with climatic factors (precipitation and temperature). Temporal variability in DIN export was primarily associated with changes in acidic deposition over time; S deposition explained 41% of variability in DIN exports among all watersheds and years. Extensive acidification of forested watersheds was associated with the extraordinarily high S inputs to much of the Czech Republic during earlier decades. We hypothesize that recovery from acidification has led to improved tree health as well as enhanced microbial activity in the forest floor. As these watersheds move into a new regime with dramatically lower sulfur inputs, we expect continued declines in nitrate output. [ABSTRACT FROM AUTHOR]

Persistently high Nitrogen (N) deposition may have caused widespread N saturation in Central Europe’s forests. Simple and inexpensive methods are required for estimating the N status. This study suggests that the current N status of forest ecosystems can be estimated by measuring CaCl2-extractable nitrate concentrations in the soil below the main rooting zone. We tested this possibility using a large number of samples (135 in total) in a nested sampling design in two homogeneous Norway spruce forests in southern Bavaria. This approach was accompanied by a small scale survey with suction cups ( N = 54) in one forest. Nitrate concentrations determined by soil extracts varied widely (coefficients of variance 95 and 125%) and were well comparable with those of the simultaneous investigation of seepage water. Site and stand conditions explained only a small portion (<10%) of the total variation. Mineral soil nitrate concentrations were not spatially dependent at the medium and large scales (about 10 m to several km) in both forests. Therefore the reliability of estimates at these scales depends mainly on the sample size. At the small scale (

Soil processes often exhibit spatial heterogeneity that may influence plant community structure. This study was conducted to determine fine-scale spatial patterns and degrees of variability of soil nutrients and plant communities within different vegetation types in a stream floodplain in southwestern West Virginia. One 5 m x 5 m site was established in each of two vegetation/drainage types: pasture (PA) and seasonal wetland (SW). Sites were located ∼25 m apart on flat bottomland. A 10 m x 1 m transect was also established perpendicular to the visible boundary between PA and SW drainage types, but separate from the PA and SW sites. Each site was divided into 1-m² plots (n = 25) and the transect was divided into 0.25-m² plots (n = 40). Mineral soil was taken to a 5 cm depth. Soil organic matter was measured as loss-on-ignition. Extractable NH4+ and NO3- were determined before and after laboratory incubation (28 days at 27°C) to determine net N mineralization and nitrification. Cations were analyzed using inductively coupled plasma emission spectrometry. Vegetation was assessed using estimated percent cover (sites) and aboveground harvested biomass (transect). Mean organic matter was significantly higher (P < 0.05) in SW than in PA (10.6% and 8.3%. respectively). Nitrification was nearly 100% of mineralization in all soils, and was significantly lower (P < 0.05) in PA than in SW (0.7 and 1.8 µg NO3-N/g soil/d, respectively). Aluminum was significantly higher (P < 0.05) in SW than in PA (202.1 and 0.5 µg Al/g soil, respectively). Calcium and pH were significantly higher (P < 0.05) in PA than in SW (768.5 µg/g soil and 4.4, respectively). Magnesium was significantly higher (P < 0.05) in SW than in PA 1174.2 and 121.9 µg/g soil, respectively). Transect results were similar to PA and SW sites and an abrupt transition was found between PA and SW site types. Vegetation analysis revealed two distinct communities with SW dominated by wetland species and PA dominated by a mixture with slightly more upland species. Transect vegetation also consisted of largely wetland species within wetland and a mixture in pasture, however Arthraxon hispidus (Thunb.) Makino dominated for ∼1 m at the boundary. Spatial variability of organic matter was much lower than spatial variability of nitrification. Thus, availability of organic substrates to N-processing microbes is less variable than N processing itself, underlining the complexity of biotic factors responsible for regulating soil N processes. [ABSTRACT FROM AUTHOR]

Long term trends (1978–2005) of N–NO3 concentrations in river water were investigated for 10 rivers draining forested catchments in Piedmont, North-Western Italy, and Canton Ticino, Switzerland. All the river catchments come into the category of the medium-high stage of N saturation (levels 2–3 of the Stoddard’s classification). The seasonal signal in N–NO3 concentrations and its changes in time over the course of the study period was also evaluated. Single trends were analysed for significance and magnitude; statistical techniques for the detection of common trends were then applied to identify a common pattern in the N–NO3 time series. Both the increasing NO3 levels and the limited seasonal pattern in recent years indicate an aggrading level of N saturation in time. Synchronous trends of N–NO3 export were found for 8 rivers. The main common trend was used to test relationships with: (i) temperature, (ii) precipitation, and (iii) N deposition. Step-changes in the data series were also assessed, and the main points of change are discussed in relation to meteorological factors and response to the N saturation status. Temperature proved to be the main factor affecting the temporal pattern of N–NO3 concentrations: warm periods were usually followed by an N–NO3 increase in river water due to enhanced mineralisation and nitrification in soil. [ABSTRACT FROM AUTHOR]

The objective of this study was to evaluate the nitrogen (N) biogeochemistry of an 18–22 year old forested watershed in western Maryland. We hypothesized that this watershed should not exhibit symptoms of N saturation. This watershed was a strong source of nitrate (NO3 −) to the stream in all years, with a mean annual export of 9.5 kg N ha−1 year−1 and a range of 4.4–18.4 kg N ha−1 year−1. During the 2001 and 2002 water years, wet deposition of inorganic N was 9.0 kg N ha−1 year−1 and 6.3 kg N ha−1 year−1, respectively. Watershed N export rates in 2001 and 2002 water years were 4.2 kg N ha−1 year−1 and 5.3 kg N ha−1 year−1, respectively. During the wetter water years of 2003 and 2004, the watershed exported 15.0 kg N ha−1 year−1 and 18.4 kg N ha−1 year−1, rates that exceeded annual wet deposition of N by a factor of two (7.5 kg N ha−1 year−1 in 2003) and three (5.5 kg N ha−1 year−1 in 2004). Consistent with the high rates of N export, were high concentrations (2.1–3.3%) of N in foliage, wood (0.3%) and fine roots, low C:N ratios in the forest floor (17–24) and mineral soil (14), high percentages (83–96%) of the amount of mineralized N that was nitrified and elevated N concentrations (up to 3 mg N l−1) in soil solution. Although this watershed contained a young aggrading forest, it exhibited several symptoms of N saturation commonly observed in more mature forests. [ABSTRACT FROM AUTHOR]

The consequences of nitrogen (N) enrichment for terrestrial and freshwater ecosystems are of increasing concern in many areas due to continued or increasing high emission rates of reactive N. Within terrestrial ecosystems various conceptual frameworks and modelling approaches have been developed which have enhanced our understanding of the sequence of changes associated with increased N availability and help us predict their future impacts. Here, some recent findings are described and their implications for these conceptual frameworks and modelling approaches discussed. They are: (a) an early loss of plant species that are characteristic of low N conditions as N availability increases and a loss of species with high N retention efficiencies (so called N ‘filters’), (b) suppression of microbial immobilisation of deposited $${\text{NO}}^{ - }_{3} $$ due to increased $${\text{NH}}^{ + }_{4} $$ availability in the early stages of N saturation, (c) the early onset of $${\text{NO}}^{ - }_{3} $$ leaching due to these changes (a and b above) in both plant and microbial functioning, (d) reduced sensitivity of vegetation to N additions in areas with high historical N deposition, (e) delayed changes in soil C:N changes due to increased net primary productivity and reduced decomposition of soil organic matter. Some suggestions of early indicators of N saturation are suggested (occurrence of mosses; $${\text{NH}}^{ + }_{4} :{\text{NO}}^{ - }_{3} $$ ratio in surface soils) which indicate either a shift in ecosystem function and/or structure. [ABSTRACT FROM AUTHOR]

In 1989, a watershed acidification experiment was begun on the Fernow Experimental Forest in West Virginia, USA. Ammonium sulfate fertilizer (35.5 kg N ha−1 yr−1and 40.5 kg S ha−1 yr−1) was applied to a forested watershed (WS3) that supported a 20-year-old stand of eastern deciduous hardwoods. Additions of N and S are approximately twice the ambient deposition of nitrogen and sulfur in the adjacent mature forested watershed (WS4), that serves as the reference watershed for this study. Acidification of stream water and soil solution was documented, although the response was delayed, and acidification processes appeared to be driven by nitrate rather than sulfate. As a result of the acidification treatment, nitrate solution concentrations increased below all soil layers, whereas sulfate was retained by all soil layers after only a few years of the fertilization treatments, perhaps due to adsorption induced from decreasing sulfate deposition. Based on soil solution monitoring, depletion of calcium and magnesium was observed, first from the upper soil horizons and later from the lower soil horizons. Increased base cation concentrations in stream water also were documented and linked closely with high solution levels of nitrate. Significant changes in soil chemical properties were not detected after 12 years of treatment, however. [ABSTRACT FROM AUTHOR]

Nitrogen (N) budget was estimated with dissolved inorganic N (DIN) and dissolved organic N (DON) in a forested mountainous watershed in Tsukui, Kanagawa Prefecture, about 50 km west of Tokyo in Central Japan. The forest vegetation in the watershed was dominant by Konara oak ( Quercus serrata) and Korean hornbeam ( Carpinus tschonoskii), and Japanese cedar ( Cryptomeria japonica). Nitrate (NO3 −) concentration in the watershed streamwater was averagely high (98. 0 ±± 19 (±± SD, n = 36) μmol L−1) during 2001–2003. There was no seasonal and annual changes in the stream NO− 3 concentration even though the highest N uptake rate presumably occurred during the spring of plant growing season, a fact indicating that N availability was in excess of biotic demands. The DON deposition rates (DON input rates) in open area and forest area were estimated as one of the main N sources, accounting for about 32% of total dissolved N (TDN). It was estimated that a part of the DON input rate contributed to the excessive stream NO− 3 output rate under the condition of the rapid mineralization and nitrification rates, which annual DON deposition rates were positively correlated with the stream NO3 − output rates. The DON retention rate in the DON budget had a potential capacity, which contributed to the excessive stream NO− 3 output rate without other N contributions ( e. G. Forest floor N or soil N). [ABSTRACT FROM AUTHOR]

Compared to upland forests, riparian forest soils have greater potential to remove nitrate (NO3) from agricultural runoff through denitrification. It is unclear, however, whether prolonged exposure of riparian soils to nitrogen (N) loading will affect the rate of denitrification and its end products. This research assesses the rate of denitrification and nitrous oxide (N2O) emissions from riparian forest soils exposed to prolonged nutrient runoff from plant nurseries and compares these to similar forest soils not exposed to nutrient runoff. Nursery runoff also contains high levels of phosphate (PO4). Since there are conflicting reports on the impact of PO4 on the activity of denitrifying microbes, the impact of PO4 on such activity was also investigated. Bulk and intact soil cores were collected from N-exposed and non-exposed forests to determine denitrification and N2O emission rates, whereas denitrification potential was determined using soil slurries. Compared to the non-amended treatment, denitrification rate increased 2.7- and 3.4-fold when soil cores collected from both N-exposed and non-exposed sites were amended with 30 and 60 µg NO3-N g−1 soil, respectively. Net N2O emissions were 1.5 and 1.7 times higher from the N-exposed sites compared to the non-exposed sites at 30 and 60 µg NO3-N g−1 soil amendment rates, respectively. Similarly, denitrification potential increased 17 times in response to addition of 15 µg NO3-N g−1 in soil slurries. The addition of PO4 (5 µg PO4-P g−1) to soil slurries and intact cores did not affect denitrification rates. These observations suggest that prolonged N loading did not affect the denitrification potential of the riparian forest soils; however, it did result in higher N2O emissions compared to emission rates from non-exposed forest soils. [ABSTRACT FROM AUTHOR]

Although the large variations in nitrate export from forested watersheds have been attributed to a variety of natural and disturbance-related factors, baseflow nitrate concentrations in 49 mid-Appalachian forested watersheds were most strongly related to differences in bedrock geology. Within the mid-Appalachian region of Pennsylvania, Maryland and West Virginia, watersheds dominated by Pottsville and Allegheny sandstone (PVA), Catskill, Chemung, and Pocono shale and sandstone (CCP), and Mauch Chunk shale and Greenbrier limestone (MCG), respectively, exhibited significantly different low, intermediate, and high mean stream nitrate concentrations. Soil pH, soil percent N concentration (%N), soil C:N mass ratio, soil exchangeable Ca, watershed slope, and the occurrence of white ash (Fraxinus americanaL.), sugar maple (Acer saccharumMarsh.), and eastern hemlock (Tsuga canadensisL.) were related significantly to bedrock geology type as well as stream nitrate levels. Other factors such as past land disturbances (fire and agriculture) and stand age (old-growth) typically were associated with only one bedrock geology type. However, within a bedrock geology type, past agriculture and the presence of old-growth forest may be important in explaining stream nitrate concentrations on an individual watershed basis. The basal area of black locust (Robinia pseudoacaciaL.), a species that enhances soil nitrogen levelsvianitrogen fixation, showed a moderate positive correlation with stream nitrate concentrations. Bedrock geology explained the most variation in winter (49%) and summer (32%) stream nitrate concentrations. Bedrock geology may have been a better predictor of stream nitrate concentrations than soil chemistry, because the geologic variation was better assessed at the regional scale of this study compared to soil chemistry, which varies at the micro-scale due to topographic, vegetation, microbial, and climatic influences. Results of this study suggest that bedrock geology is an important factor to consider when assessing forest nitrogen dynamics at a broad landscape scale. [ABSTRACT FROM AUTHOR]

Soil- and stream-water data from the Plynlimon research area, mid-Wales, have been used to develop a conceptual model of spatial variations in nitrogen (N) leaching within moorland catchments. Extensive peats, in both hilltop and valley locations, are considered near-complete sinks for inorganic N, but leach the most dissolved organic nitrogen (DON). Peaty mineral soils on hillslopes also retain inorganic N within upper organic horizons, but a proportion percolates into mineral horizons as nitrate (NO), either through incomplete immobilisation in the organic layer, or in water bypassing the organic soil matrix via macropores. This NOreaches the stream where mineral soilwaters discharge (via matrix throughflow or pipeflow) directly to the drainage network, or via small N-enriched flush wetlands. NOin hillslope waters discharging into larger valley wetlands will be removed before reaching the stream. A concept of catchment ‘nitrate leaching zones’ is proposed, whereby most stream NOderives from localised areas of mineral soil hillslope draining directly to the stream; the extent of these zones within a catchment may thus determine its overall susceptibility to elevated surface water NOconcentrations. [ABSTRACT FROM AUTHOR]

A long-term field experiment was initiated to simulate chronic atmospheric N deposition, a widespread phenomenon in industrial regions of the world. Eight years of experimental nitrate ($NO3-$) additions (3 g $NO3-$-N m-2 per year) to four different northern hardwood forests located along a 500 km geographic gradient dramatically increased leaching losses of $NO3-$-N, dissolved organic carbon (DOC), and dissolved organic nitrogen (DON). During the last two water years, the average increase in solution $NO3-$-N and DON leaching from the $NO3-$-amended plots was 2.2 g N m-2, equivalent to 72% of the annual experimental N addition. Results indicate that atmospheric N deposition may rapidly saturate some northern hardwood ecosystems across an entire biome in the upper Great Lakes Region of the USA. Changes in soil C and N cycling induced by chronic N deposition have the potential in this landscape to significantly alter the flux of DOC and DON from upland to aquatic ecosystems. Michigan Gradient study site characteristics are similar to those of European forests most susceptible to N saturation. [ABSTRACT FROM AUTHOR]

The impact on an ecosystem of an environmental stress, such as climate change or air pollution, can be studied through experimentation, through comparisons of sites across a gradient of the stress, through long-term studies at a single site, or through theoretical or modelling approaches. Although the former three techniques often are used to develop and test models, it is much rarer to explicitly link experimental, comparative or long-term studies together. Here we present a concept for combining experimental and comparative research to assess the direction and rate of change, the expected long-term state, and the rate at which the long-term state is achieved after an ecosystem is exposed to an environmental stress. We do this by comparing the response of a forest in Denmark to experimentally increased N deposition with the expected long-term response based on a European database of forests exposed to different levels of N deposition over long time periods. The analysis suggests that if N deposition were to increase by 3-fold to about 50 kg N ha-1 a-1 at the Danish site, and remain at this level, the N concentration in needles would respond within 2–4 yr after the onset of the enhanced N deposition, and would rapidly plateau to an expected mean value of 18.0 mg N g-1 dry mass (95% confidence interval ± 2.5 mg g-1). The N concentration of new litter also would respond rapidly (1–2 yr) to reach an expected value of 16.6 mg N kg-1 dry mass (± 3). The N concentration of the organic layer in the soil would increase much more slowly, but a significant increase would be expected within 5–10 yr. Mineral soil pH would take more than 7 yr to change. Finally, the flux of dissolved inorganic N in leachate would begin to increase immediately, but would take many years to reach the expected level of 22.4 kg N ha-1 a-1 (± 4). [ABSTRACT FROM AUTHOR]

Links between forest floor carbon:nitrogen (C:N) ratios, atmospheric N deposition and nitrate leaching into surface waters have been reported for forest ecosystems, but similar studies have not been reported previously for the equivalent compartments of moorland ecosystems in Great Britain, despite the importance of nitrate in contributing to the acidification of moorland streams and lakes in British uplands. In this paper, the relationships between the C:N ratio of moorland soil surface organic matter, N deposition, and nitrate leaching are explored for 13 soils in four moorland catchments. Although there is spatial variability in the C:N ratio of soils, major differences are apparent between soils and especially between catchments. The C:N ratio appears to be inversely related to modelled inorganic N deposition and, to a lesser degree, measured nitrate leaching, for three of the four catchments studied (Allt a'Mharcaidh, Afon Gwy, and Scoat Tarn). Nitrification may make an important contribution to nitrate leaching at the two higher deposition sites. At the fourth site, the heavily acidified River Etherow catchment, extremely high rates of nitrate leaching are not accompanied by low C:N ratios or high nitrification potentials in the upper soil horizons. Hence the C:N ratio of surface soil organic matter may have potential as an indicator of nitrogen saturation and leaching in some systems, but it is not universally applicable. [ABSTRACT FROM AUTHOR]