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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]

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]

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]

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]

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]

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]

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]

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]

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]

Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species importance is the influence of N2- fixing red alder (Alnus rubra) on nutrient cycling in the forests of the Pacific Northwest. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder: 94% of basal area), particularly when near-coastal sites were excluded (r² = 0.65 and 0.68 for nitrate-N and DON, respectively). Nitrate and DON concentrations were more strongly related to broadleaf cover within entire watersheds than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands plays an important role in watershed nitrogen (N) export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4- 30.8 kg N ha-1 y-1), described by a multiple linear regression combining broadleaf and mixed broad- leaf-conifer cover (r² = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single plant species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds. [ABSTRACT FROM AUTHOR]

Despite the widely recognized importance of disturbance in accelerating the loss of elements from land, there have been few empirical studies of the effects of natural disturbances on nitrogen (N) dynamics in forest ecosystems. We were provided the unusual opportunity for such study, partly because the intensively monitored watersheds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, experienced severe canopy damage following an ice storm. Here we report the effects of this disturbance on internal N cycling and loss for watershed 1 (W1) and watershed 6 (W6) at the HBEF and patterns of N loss from nine other severely damaged watersheds across the southern White Mountains. This approach allowed us to test one component of N limitation theory, which suggests that N losses accompanying natural disturbances can lead to the maintenance of N limitation in temperate zone forest ecosystems. Prior to the ice storm, fluxes of nitrate (NO3-) at the base of W1 and W6 were similar and were much lower than N inputs in atmospheric deposition. Following the ice storm, drainage water NO3- concentrations increased to levels that were seven to ten times greater than predisturbance values. We observed no significant differences in N mineralization, nitrification, or denitrification between damaged and undamaged areas in the HBEF watersheds, however. This result suggests that elevated NO3- concentrations were not necessarily due to accelerated rates of N cycling by soil microbes but likely resulted from decreased plant uptake of NO3-. At the regional scale, we observed high variability in the magnitude of NO3- losses: while six of the surveyed watersheds showed accelerated rates of NO3- loss, three did not. Moreover, in contrast to the strong linear relationship between NO3- loss and crown damage within HBEF watersheds [r2: (W1 = 0.91, W6 = 0.85)], stream water NO3- concentrations were weakly related to crown damage (r2 = 0.17) across our regional sites. The efflux of NO3- associated with the ice storm was slightly higher than values reported for soil freezing and insect defoliation episodes, but was approximately two to ten times lower than NO3- fluxes associated with forest harvesting. Because over one half of the entire year’s worth of N deposition was lost following the ice storm, we conclude that catastrophic disturbances contribute synergistically to the maintenance of N limitation and widely observed delays of N saturation in northern, temperate zone forest ecosystems. [ABSTRACT FROM AUTHOR]

Theories of forest nitrogen (N) cycling suggest that stream N losses should increase in response to chronic elevated N deposition and as forest nutrient requirements decline with age. The latter theory was supported initially by measurements of stream NO3- concentration in old-growth and successional stands on Mount Moosilauke, New Hampshire (Vitousek and Reiners 1975; Bioscience 25:376–381). We resampled 28 of these and related streams to evaluate their response to 23 years of forest aggradation and chronic N deposition. Between 1973–74 and 1996–97, mean NO3- concentration in quarterly samples from Mount Moosilauke decreased by 71% (25 μmol/L), Ca2+ decreased by 24% (8 μmol/L), and Mg2+ decreased by 22% (5 μmol/L). Nitrate concentrations decreased in every stream in every season, but spatial patterns among streams persisted: Streams draining old-growth stands maintained higher NO3- concentrations than those draining successional stands. The cause of the NO3- decline is not evident. Nitrogen deposition has changed little, and although mechanisms such as insect defoliation and soil frost may contribute to the temporal patterns of nitrate loss, they do not appear to fully explain the NO3- decline across the region. Although the role of climate remains uncertain, interannual climate variation and its effects on biotic N retention may be responsible for the synchronous decrease in NO3- across all streams, overriding expected increases due to chronic N deposition and forest aging. Corresponding author's current address: Woods Hole Research Center, P.O. Box 296, Woods Hole, Massachusetts 02543, USA; e-mail: cgoodale@whrc.org. --> [ABSTRACT FROM AUTHOR]

Nitrate produced by bacterially mediated nitrification in soils is isotopically distinct from atmospheric nitrate in precipitation. 15N/14N and 18O/16O isotopic ratios of nitrate can therefore be used to distinguish between these two sources of nitrate in surface waters and groundwaters. Two forested catchments in the Turkey Lakes Watershed (TLW) near Sault Ste. Marie, Ontario, Canada were studied to determine the relative contributions of atmospheric and microbial nitrate to nitrate export. The TLW is reasonably undisturbed and receives a moderate amount of inorganic nitrogen bulk deposition (8.7 kg N · ha−1· yr−1) yet it exhibits unusually low inorganic nitrogen retention (average = 65% of deposition). The measured isotopic ratios for nitrate in precipitation ranged from +35 to +59‰ (VSMOW) for δ18O and −4 to +0.8‰ (AIR) for δ15N. Nitrate produced from nitrification at the TLW is expected to have an average isotope value of approximately −1.0‰ for δ18O and a value of about 0 to +6‰ for δ15N, thus, the isotopic separation between atmospheric and soil sources of nitrate is substantial. Nitrate produced by nitrification of ammonium appears to be the dominant source of the nitrate exported in both catchments, even during the snowmelt period. These whole catchment results are consistent with the results of small but intensive plot scale studies that have shown that the majority of the nitrate leached from these catchments is microbial in origin. The isotopic composition of stream nitrate provides information about N-cycling in the forested upland and riparian zones on a whole catchment basis. [ABSTRACT FROM AUTHOR]

Traditional biogeochemical theories suggest that ecosystem nitrogen retention is controlled by biotic N limitation, that stream N losses should increase with successional age, and that increasing N deposition will accelerate this process. These theories ignore the role of dissolved organic nitrogen (DON) as a mechanism of N loss. We examined patterns of organic and inorganic N export from sets of old-growth and historically (80–110 years ago) logged and burned watersheds in the northeastern US, a region of moderate, elevated N deposition. Stream nitrate concentrations were strongly seasonal, and mean (± SD) nitrate export from old-growth watersheds (1.4 ± 0.6 kg N ha−1 y−1) was four times greater than from disturbed watersheds (0.3 ± 0.3 kg N ha−1 y−1), suggesting that biotic control over nitrate loss can persist for a century. DON loss averaged 0.7 (± 0.2) kg N ha−1 y−1 and accounted for 28–87% of total dissolved N (TDN) export. DON concentrations did not vary seasonally or with successional status, but correlated with dissolved organic carbon (DOC), which varied inversely with hardwood forest cover. The patterns of DON loss did not follow expected differences in biotic N demand but instead were consistent with expected differences in DOC production and sorption. Despite decades of moderate N deposition, TDN export was low, and even old-growth forests retained at least 65% of N inputs. The reasons for this high N retention are unclear: if due to a large capacity for N storage or biological removal, N saturation may require several decades to occur; if due to interannual climate variability, large losses of nitrate may occur much sooner. [ABSTRACT FROM AUTHOR]

This article reports responses of two different forest ecosystems to 9 years (1988–96) of chronic nitrogen (N) additions at the Harvard Forest, Petersham, Massachusetts. Ammonium nitrate (NH4NO3) was applied to a pine plantation and a native deciduous broad-leaved (hardwood) forest in six equal monthly doses (May–September) at four rates: control (no fertilizer addition), low N (5 g N m-2 y-1), high N (15 g N m-2 y-1), and low N + sulfur (5 g N m-2 y-1 plus 7.4 g S m-2 y-1). Measurements were made of net N mineralization, net nitrification, N retention, wood production, foliar N content and litter production, soil C and N content, and concentrations of dissolved organic carbon (DOC) and nitrogen (DON) in soil water. In the pine stand, nitrate losses were measured after the first year of additions (1989) in the high N plot and increased again in 1995 and 1996. The hardwood stand showed no significant increases in nitrate leaching until 1995 (high N only), with further increases in 1996. Overall N retention efficiency (percentage of added N retained) over the 9-year period was 97–100% in the control and low N plots of both stands, 96% in the hardwood high N plot, and 85% in the pine high N plot. Storage in aboveground biomass, fine roots, and soil extractable pools accounted for only 16–32% of the added N retained in the amended plots, suggesting that the one major unmeasured pool, soil organic matter, contains the remaining 68–84%. Short-term redistribution of 15N tracer at natural abundance levels showed similar division between plant and soil pools. Direct measurements of changes in total soil C and N pools were inconclusive due to high variation in both stands. Woody biomass production increased in the hardwood high N plot but was significantly reduced in the pine high N plot, relative to controls. A drought-induced increase in foliar litterfall in the pine stand in 1995 is one possible factor leading to a measured increase in N mineralization, nitrification, and nitrate loss in the pine high N plot in 1996. [ABSTRACT FROM AUTHOR]

Boreal Shield rocky ridges at the Experimental Lakes Area, northwestern Ontario, contain two plant/soil communities with contrasting N cycles. Picea mariana–Pinus banksiana”forest islands” are N limited whereas the lichen, moss, and grass community (or “lichen patches”) on the surrounding bedrock outcrops appear intrinsically N saturated. The potential for this landscape to retain a N input of eightfold ambient levels was tested with a 2-y addition of 40 kg N ha-1 y-1 as NaNO3 to one small catchment (0.40 ha). The elevated N input was poorly retained by the whole catchment during snowmelt. However, during the growing season, N retention in the treated catchment remained as efficient as in references. Forest islands and bedrock surfaces responded in opposite fashions to the elevated N input. By the second year of N addition, bedrock surfaces no longer retained additional N inputs. In contrast, N-amended and reference forest islands retained a similar proportion of N inputs, indicating that forest islands did not become N saturated. The response of the whole catchment to N addition was more similar to forest islands than bedrock surfaces. Even if forest islands only cover a small proportion of catchment area, they can have a strong impact on whole catchment element export because most of the water must move through at least one island before leaving the system. Because the different components of the boreal shield landscape are hydrologically connected, N saturation may occur as a cascading effect in this ecosystem. Monitoring boreal shield landscapes by using outlets at the lower end of the hydrological cascade can fail to detect the impacts of perturbations such as increased N deposition on upper components. [ABSTRACT FROM AUTHOR]

We compared simulated time series of nitrogen-15 (15N) redistribution following a large-scale labeling experiment against field recoveries of 15NH4 + and 15NO3 − in vegetation tissues. We sought to gain insight into the altered modes of N cycling under long-term, experimentally elevated N inputs. The study took place in two contrasting forests: a red pine stand and a mixed deciduous stand (predominantly oak) at the Harvard Forest, Massachusetts, USA. We used TRACE, a dynamic simulation model of ecosystem biogeochemistry that includes 15N/14N ratios in N pools and fluxes. We simulated input–output and internal fluxes of N, tracing the labeled cohorts of N inputs through ecosystem pools for one decade. TRACE simulated the peaks and timing of 15N recovery in foliage well, providing a key link between modeling and field studies. Recovery of tracers in fine roots was captured less well. The model was structured to provide rapid, initial sinks for 15NO3 − and 15NH4 + in both forests, as indicated by field data. In simulations, N in litter turned over rapidly, even as humus provided a long-term sink for rapidly cycling N. This sink was greater in the oak forest. Plant uptake fluxes of N in these fertilized plots were on the same order of magnitude as net assimilation fluxes in forest-floor humus. A striking result was the small rate of incorporation of N in humus resulting from the transfer of litter material to humus, compared with large fluxes of N into humus and its associated microorganisms through direct transfers from pools of inorganic N in soils. [ABSTRACT FROM AUTHOR]