Throughfall Nitrogen Research Articles

Nitrogen deposition causes eutrophication in bryophyte communities in central and northern European forests

Key messageOur results indicate that nitrogen deposition is likely to adversely affect forest bryophyte communities, having negative impacts in terms of increased dominance of nitrophilic species at the expense of N-sensitive species and a decrease in evenness.ContextElevated atmospheric deposition of nitrogen (N) has long been recognised as a threat to biodiversity and, despite declines in European emission levels, will remain a threat in the future.AimsIt has proven difficult to show clear large-scale impacts of N deposition on vascular forest understorey species, and few studies have looked at impacts on forest bryophytes. Here, we assess the impact of nitrogen deposition on forest bryophyte communities.MethodsWe used data from 187 plots included in European monitoring schemes to analyse the relationship between levels of throughfall nitrogen deposition and bryophyte taxonomic and functional diversity and community nitrogen preference.ResultsWe found that nitrogen deposition is significantly associated with increased bryophyte community nitrogen preference and decreases in species evenness.ConclusionOur results indicate that nitrogen deposition is likely to adversely affect forest bryophyte communities, having negative impacts in terms of increased dominance of nitrophilic species at the expense of N-sensitive species and a decrease in species evenness.

Local drivers of heterogeneity in a tropical forest: epiphytic tank bromeliads affect the availability of soil resources and conditions and indirectly affect the structure of seedling communities.

Environmental heterogeneity is a key component in explaining the megadiversity of tropical forests. Despite its importance, knowledge about local drivers of environmental heterogeneity remains a challenge for ecologists. In Neotropical forests, epiphytic tank bromeliads store large amounts of water and nutrients in the tree canopy, and their tank overflow may create nutrient-rich patches in the soil. However, the effects of this nutrient flux on environmental heterogeneity and plant community structure in the understory remain unexplored. In a Brazilian coastal sandy forest, we investigated the effects of the presence of epiphytic tank bromeliads on throughfall chemistry, soil chemistry, soil litter biomass, light, and seedling community structure. In the presence of epiphytic tank bromeliads, the throughfall nitrogen concentration increased twofold, the throughfall phosphorus concentration increased threefold, and the soil patches had a 3.96% higher pH, a 50% higher calcium concentration, and 11.88% less light. By altering the availability of soil resources and conditions, the presence of bromeliads partially shifted the available niche spaces for plant species and indirectly affected the structure of the seedling communities, decreasing their diversity, density, and biomass. For the first time, we showed that the presence of tank bromeliads in the canopy can create characteristic soil patches in the understory, affecting the structure of seedling communities via fertilization. Our results reveal a novel local driver of environmental heterogeneity, reinforcing and expanding the key role of tank bromeliads both in nutrient cycling and plant community structuring of Neotropical coastal sandy forests.

Event Scale Relationships of DOC and TDN Fluxes in Throughfall and Stemflow Diverge From Stream Exports in a Forested Catchment

AbstractAquatic fluxes of carbon and nutrients link terrestrial and aquatic ecosystems. Within forests, storm events drive both the delivery of carbon and nitrogen to the forest floor and the export of these solutes from the land via streams. To increase understanding of the relationships between hydrologic event character and the relative fluxes of carbon and nitrogen in throughfall, stemflow and streams, we measured dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations in each flow path for 23 events in a forested watershed in Vermont, USA. DOC and TDN concentrations increased with streamflow, indicating their export was limited by water transport of catchment stores. DOC and TDN concentrations in throughfall and stemflow decreased exponentially with increasing precipitation, suggesting that precipitation removed a portion of available sources from tree surfaces during the events. DOC and TDN fluxes were estimated for 76 events across a 2‐year period. For most events, throughfall and stemflow fluxes greatly exceeded stream fluxes, but the imbalance narrowed for larger storms (>30 mm). The largest 10 stream events exported 40% of all stream event DOC whereas those same 10 events contributed 14% of all throughfall export. Approximately 2–5 times more DOC and TDN was exported from trees during rain events than left the catchment via streams annually. The diverging influence of event size on tree versus stream fluxes has important implications for forested ecosystems as hydrological events increase in intensity and frequency due to climate change.

Desiccation and rehydration of mosses greatly increases resource fluxes that alter soil carbon and nitrogen cycling

Abstract Mosses often have positive effects on soil carbon and nitrogen cycling, but we know little about how environmentally determined cycles of desiccation and rehydration in mosses influence these processes. In this context, we compared carbon and nitrogen in throughfall after precipitation passed through eight moss species that were either hydrated continuously or desiccated and rehydrated. Also, the throughfall of four moss species was added to soil and used to determine the net effect of carbon and nitrogen added in moss throughfall on soil CO2 and N2O efflux. Depending on the species, desiccated‐rehydrated (rehydrated) mosses lost 2–31 times more carbon in throughfall than mosses that were continuously hydrated (hydrated). Hydrated mosses lost little to no detectable nitrogen, whereas most rehydrated mosses lost some nitrogen in throughfall. Throughfall from both hydrated and rehydrated mosses generated higher CO2 and N2O efflux than water treated soils, but rehydrated moss throughfall promoted larger N2O efflux than hydrated moss throughfall. Throughfall from hydrated mosses caused net negative changes in soil carbon and had very little effect on soil nitrogen, whereas throughfall from rehydrated mosses generated positive changes in soil carbon and nitrogen. Synthesis. Our results indicate that resources lost from desiccated mosses during rehydration influence soil carbon and nitrogen transformations and may be important drivers of carbon and nitrogen cycling and storage in ecosystems.

Fog as a source of nitrogen for redwood trees: evidence from fluxes and stable isotopes

Summary A defining feature of the redwood forest in coastal California is the presence of fog in the summer months, a time when there is typically little rainfall. Our goal was to determine the role of summer fog in canopy transformation of nitrogen, nitrogen uptake by trees and photosynthesis within a coastal redwood forest ecosystem. We measured horizontal and vertical inputs of nitrogen, the isotopic composition of nitrogen in a variety of atmospheric sources (summer fog, winter rain and throughfall throughout the year), nitrogen pools (soil solution) and plant tissue (roots and foliage), as well as rates of photosynthesis and nitrogen uptake by trees. Throughfall nitrogen fluxes were greater at the forest edge compared to the interior both within the canopy (sampled 10 m above‐ground) and onto the forest floor (sampled 1 m above‐ground; P < 0.05). Similarly, soil solution and total inorganic nitrogen were greater at the forest edge compared to the interior (P = 0.0014 and 0.009, respectively). Whereas natural abundance δ15NO3 values were not significantly different between winter rain (measured as bulk precipitation) and summer fog water (average δ15N = −1.2 ± 0.680/00), δ15NH4 values were significantly greater in fog water (11.4 ± 2.70/00) compared to rain (1.2 ± 0.90/00). We found no difference in δ15N in roots from forest edge trees compared to interior trees. In contrast, nitrogen concentrations and δ15N in foliage from forest edge trees were significantly greater compared to interior trees (P < 0.0001), suggesting that the leaves of forest edge trees may be obtaining a greater proportion of their nitrogen from fog compared to those of the interior trees. Natural abundance 13C of leaf sugars and rates of photosynthesis were significantly higher at the forest edge compared to the interior during the fog season (P < 0.05), but not different between locations in the rain season (P > 0.05). Nitrification in the forest floor, rather than the canopy, is the primary source of in these soils throughout the year. Synthesis. Summer fog provides nitrogen directly and indirectly to redwood trees, especially those at the forest edge, and affects the physiologic function of redwood trees.

Nitrogen deposition along differently exposed slopes in the Bavarian Alps

The Alps are affected by high nitrogen deposition, particularly in the fringe of the Northern and Southern Alps. In the framework of a two-year monitoring study performed in 2010 and 2011, we investigated the ammonia and nitrogen dioxide air concentration and ammonium and nitrate deposition at different altitudes between 700 and 1600ma.s.l. in the Garmisch-Partenkirchen district in the Upper Bavaria region (Germany). Four-weekly measurements of deposition collected with bulk open field samplers and under-crown were performed in a profile perpendicular to the axis of the Loisach valley; measurements were conducted at eight sites. Whereas open field deposition ranged from 5 to 11kgha−1a−1, nitrogen throughfall has reached up to 21kgha−1a−1. Data from the valley and the slopes were compared with measurements performed on the platform of the Environmental Research Station Schneefernerhaus (Zugspitze) at an altitude of 2650ma.s.l. For the rough estimation of the total yearly deposition rate of nitrogen, the canopy uptake model was applied. By regarding nitrogen uptake by the trees, total deposition can exceed the throughfall in all sites by up to 50%. Additionally, we estimated the total deposition from the sum of wet and dry deposition. On the one side, the wet deposition could be extrapolated from the open field deposition. On the other side, we used the inferential method to calculate the dry deposition on the basis of NH3 and NO2 air concentrations and their literature based deposition velocities. Since fixed deposition velocities are inappropriate particularly in complex orography, we tried to find correction factors based upon terrain characteristics and meteorological considerations. Temperature monitoring at the eight sites and wind measurements at two sites provided some evidence for the semi-empirical parameterization. Due to numerous imponderabilities, the results of the two methods were not consistent for all sites.

A simple tool for estimating throughfall nitrogen deposition in forests of western North America using lichens

Anthropogenic nitrogen (N) deposition has had substantial impacts on forests of North America. Managers seek to monitor deposition to identify areas of concern and establish critical loads, which define the amount of deposition that can be tolerated by ecosystems without causing substantial harm. We present a new monitoring approach that estimates throughfall inorganic N deposition from N concentration in lichens collected on site. Across 84 study sites in western North America with measured throughfall, a single regression model effectively estimated N deposition from lichen N concentration with an R2 of 0.58 and could be improved with the addition of climate covariates including precipitation seasonality and temperature in the wettest quarter to an R2 of 0.74. By restricting the model to the more intensively sampled region including Oregon, Washington, and California, the R2 increased to 0.77. Because lichens are readily available, analysis is cost-effective, and accuracy is unaffected by mountainous terrain, this method allows development of deposition estimates at sites across broad spatial and topographic scales. Our approach can allow land managers to identify areas at risk of N critical load exceedance, which can be used for planning and management of air pollution impacts.

Detecting the nitrogen critical loads on European forests by means of epiphytic lichens. A signal-to-noise evaluation

Lichens are considered to be among the most sensitive organisms for several types of pollutants. In this work, we analyzed a dataset of 286 epiphytic lichen species observed on 1155 trees at 83 ForestBIOTA plots, which is a subsample of approx. 500 plots of the European ICP Forests Level II network. We aimed at examining the amount of nitrogen deposition for which a significant variation of the relative diversity of morpho-functional groups of epiphytic lichens in the sampled plots is expected. Moreover, the study aimed at determining how much variance of these diversity variables could be explained by nitrogen depositions only. We used correlation and multiple regression models as well as hierarchical partitioning to evaluate the relative importance of environmental predictors in explaining variation in lichen diversity descriptors. The analysis splits the variation explained by each variable into a joint effect together with the other explanatory variables, and into an independent effect not shared with any other variable. The percentage of macrolichens in the plots was shown to be the most important indicator, since 56.7% of its variation could be explained by deposition, particularly by nitrogen compounds. It was shown that approx. 75% of the ForestBIOTA plots are affected by an unsustainably high throughfall nitrogen deposition. Based on these outcomes, it was possible to determine a nitrogen critical load of 2.4kgha−1yr−1.

Sulphur and nitrogen concentrations and fluxes in bulk precipitation and throughfall in the mountain and highland spruce stands in the Czech Republic

Concentrations and fluxes of sulphur and nitrogen compounds in bulk precipitation and in throughfall were evaluated and compared for two experimental sites in the Czech Republic: one situated at Rájec (Drahanská upland, 610 m a.s.l.) and the second one at Bílý Kříž (Moravian-Silesian Beskids, 908 m a.s.l.) both with similar stands of young Norway spruce. The three-year study performed during 2006–2008, revealed statistically significant differences in nitrate nitrogen concentrations in bulk precipitations and in ammonium nitrogen concentrations both in bulk precipitation and in throughfall between the two sites. Higher nitrogen compounds concentrations in bulk precipitation were found at Rájec. Differences between the two sites in sulphur concentrations were not found out neither in bulk precipitation nor in throughfall waters. Total sulphur deposition amounted to 8.1, 8.3 and 6.7 kg S·ha<sup>–1</sup> at Rájec and to 14.8, 16.9 and 15.4 kg S·ha<sup>–1</sup> at Beskids for the three years studied, respectively. Total inorganic nitrogen throughfall flux amounted to 12.1, 11.6 and 11.6 at Rájec and 13.8, 18.9 and 15.0 kg·ha<sup>–1</sup> at Bílý Kříž for the years 2006, 2007 and 2008, respectively.  

Dissolved Organic Nitrogen Concentrations and Ratios of Dissolved Organic Carbon to Dissolved Organic Nitrogen in Throughfall and Soil Waters in Norway Spruce and Scots Pine Forest Stands Throughout Norway

Dissolved organic nitrogen (DON) plays an important ecological role in forest ecosystems, and its concentration is related to that of dissolved organic carbon (DOC). We investigated DON concentrations and ratios of DOC to DON in throughfall and soil waters in 16 Norway spruce and two Scots pine forest stands sampled at weekly intervals between 1996 and 2006. The stands are all included in the ICP Forests Level II monitoring program and are located throughout Norway. DON concentrations were significantly and positively related to DOC concentrations in throughfall (r2 = 0.72, p < 0.0001) and soil water at 5, 15, and 40 cm (r2 = 0.86, 0.32, and 0.84 and p < 0.0001, 0.04, and <0.0001, respectively). At most sites, the annual median DOC/DON ratio in throughfall ranged from 20.3 to 55.5, which is lower than values in soil water, which ranged from 24.5 to 81.3, gradually decreasing with soil depth. DON concentrations varied seasonally in throughfall at many plots and in soil water at 5-cm depth at one plot only, with higher values in the growing season, but there was no noticeable seasonality at greater depth. The ratios of DOC/DON in soil water were significantly positively related to the C/N ratio in soil at the same depth. Above-ground litter input was the main factor having a significant, negative relationship to DOC/DON in soil water at all depths studied. This might reflect the effect of site conditions on both DOC/DON ratios and litter quantity.

Fluxes of oxidised and reduced nitrogen above a mixed coniferous forest exposed to various nitrogen emission sources

Concentrations of nitrogen gases (NH 3, NO 2, NO, HONO and HNO 3) and particles (pNH 4 and pNO 3) were measured over a mixed coniferous forest impacted by high nitrogen loads. Nitrogen dioxide (NO 2) represented the main nitrogen form, followed by nitric oxide (NO) and ammonia (NH 3). A combination of gradient method (NH 3 and NO x ) and resistance modelling techniques (HNO 3, HONO, pNH 4 and pNO 3) was used to calculate dry deposition of nitrogen compounds. Net flux of NH 3 amounted to −64 ng N m −2 s −1 over the measuring period. Net fluxes of NO x were upward (8.5 ng N m −2 s −1) with highest emission in the morning. Fluxes of other gases or aerosols substantially contributed to dry deposition. Total nitrogen deposition was estimated at −48 kg N ha −1 yr −1 and consisted for almost 80% of NH x . Comparison of throughfall nitrogen with total deposition suggested substantial uptake of reduced N (±15 kg N ha −1 yr −1) within the canopy.

Trends in impact of acidification on groundwater bodies in the Czech Republic; an estimation of atmospheric deposition at the horizon 2015

The acidification of surface and groundwaters is a long standing issue in the Czech Republic which culminated in the second half of the last century by mass extinction of forest cover in many mountainous regions. The total deposition of sulfur at that time was reaching as much as about 108 kg/ha/year. This resulted in a decrease of alkalinity in groundwaters, decrease of pH and on the other hand an increase in concentration of nitrates, sulfates, berylium and aluminium. Desulfurization of power plants and attenuation of heavy industry leads to a decrease in sulfur deposition down to the present mean values around 16.5 kg/ha/year. It is expected that around the year 2010 the sulfur deposition should not have a substantial impact on the environment providing the similar trend would continue. However, a reverse trend can be observed in deposition of nitrogen. The growth of road traffic and more frequent use of earth gas for heating lead to a dramatic rise in nitrogen throughfall of which mean value increased from 15.4 kg/ha/year in 1990 up to 25.7 kg/ha/year in 2001. If such trend continues in future, then the mean values of nitrogen throughfall would attain 37.8 kg/ha/year in 2015, so that the nitrogen would take over the role of sulfur in acidification. Therefore, the problem of acidification in Central Europe is by no means resolved, only the structure of acidification input has been changed.

Elevated NH3 and NO2 air concentrations and nitrogen deposition rates in the vicinity of a highway in Southern Bavaria

Abstract A transect study consisting of air concentration and deposition measurements of nitrogen compounds was performed to estimate the potential influence of car emissions on the nitrogen input to ecosystems. Therefore, two transects each consisting of 4 plots, the first in a coniferous forest and the second one in an extensively farmed grassland, were installed perpendicular to a highway south of Munich (Bavaria). Both profiles were influenced mainly by car emissions and showed only small local influences caused by agricultural activities. In the framework of a pilot study based upon denuder measurements we found a strong temporal dependency of both nitrogen dioxide (NO2) and ammonia (NH3) concentrations on traffic density. In the main study air concentrations of NO2 and NH3 were measured by passive samplers; they used as the basis for the estimation of dry deposition. These estimations have been compared with the results of analyses from simultaneously conducted canopy throughfall deposition and open air bulk measurements of nitrate (NO3−) and ammonium (NH4+). Additionally, within the forest transect the variety of different soil vegetation species was recorded and quantified. We obtained a strong gradient of gas concentrations along both profiles. Whereas the bulk deposition remained quite constant along the non-forested transect, the nitrogen throughfall deposition rate diminished substantially with the distance from the highway. The deposition rate at the forest edge was twice of that inside. The nitrogen load estimated for the examined forest in the vicinity of the highway was comparable to other forest ecosystems situated near diffuse emission sources from agriculture. It could be shown that changes in soil composition and soil vegetation along the forest transect are caused by decreasing nitrogen deposition with distance from the highway. The application of road salt in winter leads to further impacts.

Comparison of throughfall and soil solution chemistry between a high-density Corsican pine stand and a naturally regenerated silver birch stand

In Flanders, critical loads for acidification and eutrophication are exceeded in the majority of the forest stands, and many previously nitrogen limited forest ecosystems have become nitrogen saturated. The present study investigates whether a naturally regenerated stand of silver birch ( Betula pendula Roth) contributes less to the acidification and eutrophication of the forest soil than a high-density plantation of Corsican pine ( Pinus nigra ssp. laricio Maire). Throughfall deposition of inorganic nitrogen was about 3.5 times higher in the Corsican pine stand than in the birch stand. Potassium throughfall deposition was significantly higher under birch due to higher canopy leaching. Magnesium throughfall deposition was significantly higher under the pine canopy due to higher dry deposition. The lower nitrogen throughfall deposition in the birch stand was reflected in a 60% lower nitrate percolation at 1 m depth compared with pine. Nitrate soil percolation is linked to losses of aluminium and base cations.

Throughfall Nitrogen Deposition Has Different Impacts on Soil Solution Nitrate Concentration in European Coniferous and Deciduous Forests

Increases in the deposition of atmospheric nitrogen (N) influence N cycling in forest ecosystems and can result in negative consequences due to the leaching of nitrate into groundwaters. From December 1995 to February 1998, the Pan-European Programme for the Intensive and Continuous Monitoring of Forest Ecosystems measured forest conditions at a plot scale for conifer and broadleaf forests, including the performance of time series of soil solution chemistry. The influence of various ecosystem conditions on soil solution nitrate concentrations at these forest plots (n = 104) was then analyzed with a statistical model. Soil solution nitrate concentrations varied by season, and summer concentrations were approximately 25% higher than winter ones. Soil solution nitrate concentrations increased dramatically with throughfall (and bulk precipitation) N input for both broadleaf and conifer forests. However, at elevated levels of throughfall N input (more than 10 kg N ha−1 y−1), nitrate concentrations were higher in broadleaf than coniferous stands. This tree-specific difference was not observed in response to increased bulk precipitation N input. In coniferous stands, throughfall N input, foliage N concentration, organic layer carbon–nitrogen (C:N) ratio, and nitrate concentrations covaried. Soil solution nitrate concentrations in conifer plots were best explained by a model with throughfall N and organic layer C:N as main factors, where C:N ratio could be replaced by foliage N. The organic layer C:N ratio classes of more than 30, 25–30, and less than 25, as well as the foliage N (mg N g−1) classes of less than 13, 13–17, and more than 17, indicated low, intermediate, and high risks of nitrate leaching, respectively. In broadleaf forests, correlations between N characteristics were less pronounced, and soil solution nitrate concentrations were best explained by throughfall N and soil pH (0–10-cm depth). These results indicate that the responses of soil solution nitrate concentration to changes in N input are more pronounced in broadleaf than in coniferous forests, because in European forests broadleaf species grow on the more fertile soils.

Spatial variability of throughfall water and chemistry and forest floor water content in a Douglas fir forest stand

Abstract. This study focuses on spatial variability of throughfall water and chemistry and forest floor water content within a Douglas fir (Pseudotsuga menziesii, Franco L.) forest plot. Spatial patterns of water and chemistry (NH4+, NO3‾, SO42-, Cl‾, Mg2+, Ca2+, Na+ and K+) were compared and tested for stability over time. The spatial coefficient of variation (CV) was between 18 and 26% for amounts of throughfall water and ions, and 17% for forest floor water content. Concentrations and amounts of all ions were correlated significantly. Ion concentrations were negatively correlated with throughfall water amounts, but, except for NH4+, there was no such relation between throughfall water and ion amounts. Spatial patterns of throughfall water fluxes and forest floor water contents were consistent over time; patterns of ion fluxes were somewhat less stable. Because of the spatial variability of forest floor thickness and drainage, it was not possible to relate patterns in throughfall water directly to patterns in water content. The spatial variability of throughfall nitrogen and forest floor water contents can cause significant variability in NO3‾ production within the plot studied. Keywords: nutrient throughfall, forest floor water, spatial variability, time-stability, nitrogen

Dynamic nitrogen deposition thresholds during forest stand development in a Douglas fir forest analysed with two nitrogen models SMART2 and MERLIN

Abstract. This study focuses on spatial variability of throughfall water and chemistry and forest floor water content within a Douglas fir (Pseudotsuga menziesii, Franco L.) forest plot. Spatial patterns of water and chemistry (NH4+, NO3‾, SO42-, Cl‾, Mg2+, Ca2+, Na+ and K+) were compared and tested for stability over time. The spatial coefficient of variation (CV) was between 18 and 26% for amounts of throughfall water and ions, and 17% for forest floor water content. Concentrations and amounts of all ions were correlated significantly. Ion concentrations were negatively correlated with throughfall water amounts, but, except for NH4+, there was no such relation between throughfall water and ion amounts. Spatial patterns of throughfall water fluxes and forest floor water contents were consistent over time; patterns of ion fluxes were somewhat less stable. Because of the spatial variability of forest floor thickness and drainage, it was not possible to relate patterns in throughfall water directly to patterns in water content. The spatial variability of throughfall nitrogen and forest floor water contents can cause significant variability in NO3‾ production within the plot studied. Keywords: nutrient throughfall, forest floor water, spatial variability, time-stability, nitrogen

Atmospheric deposition and canopy exchange processes in alpine forest ecosystems (northern Italy)

Throughfall and bulk precipitation chemistry were studied for five years (June 1994–May 1999) at two high elevation forest sites (Val Gerola and Val Masino) which were known to differ in terms of tree health, as assessed by live crown condition. The ion concentration of bulk precipitation samples did not differ significantly between sites, except for Mg 2+, while the throughfall concentrations differed in the measured values of H +, N-NO 3 −, Cl −, Na +, K +, DOC and weak organic acids. The results of the application of the canopy exchange model indicated a higher contribution from the dry deposition of N-NO 3 −, N-NH 4 + and H + at Val Gerola, where the damage symptoms were more evident. In addition, the canopy leaching of Ca 2+, K + and weak organic acids were 47%, 21% and 27% higher at Val Gerola than at Val Masino. Annual SO 4 2− deposition fluxes (21.3 kg ha −1 yr −1 at Val Masino and 23.6 kg ha −1 yr −1 at Val Gerola) were similar to those reported for moderately polluted European and U.S. sites. Annual N loads were 13.6 and 13.1 kg ha −1 yr −1 in the bulk input, and 15.0 and 18.0 kg ha −1 yr −1 in throughfall inputs, at Val Masino and Val Gerola, respectively. The contribution of the organic fraction to the total N atmospheric deposition load is significant, constituting 17% of the bulk flux and 40% of the throughfall flux. Measured nitrogen loads exceed the critical nutrient loads by several kg N ha −1 at both stations. In particular the nitrogen throughfall load at Val Gerola was about 3 times higher than the critical values.

Evaluation of organic horizon C:N ratio as an indicator of nitrate leaching in conifer forests across Europe

We evaluate the relationship between the carbon-to-nitrogen ratio (C:N) of the soil organic horizon and nitrate leaching in runoff or seepage water from 33 conifer forests across Europe. The sites span a geographical range covering 11 countries from Ireland to western Russia and Finland to the southern Alps, and encompass a wide range in throughfall nitrogen deposition. The aim of the study is to evaluate the hypothesis that the C:N ratio of the organic (OH) horizon can be used to estimate the level of leaching of nitrate from a forest ecosystem. The analysis suggests that C:N ratio can be an indicator of nitrate leaching for conifer forests across Europe if these ecosystems are grouped into broad categories of throughfall nitrogen deposition. At low levels of N deposition (<10 kg N ha −1 year −1), nitrate leaching is low regardless of the OH C:N ratio. At intermediate (10–20 kg N ha −1 year −1) and high (>20 kg N ha −1 year −1) N deposition, nitrate leaching increases with decreasing C:N ratio. In addition, for any given value of C:N, the level of nitrate leaching is higher at high N-deposition sites than at intermediate N-deposition sites. From the current data, OH horizon C:N ratio can give a reasonable estimate of the annual export flux of nitrate (95% confidence interval ca ±5 kg N ha −1 year −1) for sites receiving throughfall-N up to about 30 kg N ha −1 year −1. Above this level, the variability in the data increases, suggesting other factors may need consideration to refine estimates of nitrate leaching.

Natural 15N abundance in two nitrogen saturated forest ecosystems.

Natural 15N abundance values were measured in needles, twigs, wood, soil, bulk precipitation, throughfall and soil water in a Douglas fir (Pseudotsuga menziesii (Mirb.) and a Scots pine (Pinus sylvestris L.) stand receiving high loads of nitrogen in throughfall (>50 kg N ha-1 year-1). In the Douglas fir stand δ15N values of the vegetation ranged between -5.7 and -4.2‰ with little variation between different compartments. The vegetation of the Scots pine stand was less depleted in 15N and varied from -3.3 to -1.2‰δ15N. At both sites δ15N values increased with soil depth, from -5.7‰ and -1.2‰ in the organic layer to +4.1‰ and +4.7‰ at 70 cm soil depth in the Douglas fir and Scots pine stand, respectively. The δ15N values of inorganic nitrogen in bulk precipitation showed a seasonal variation with a mean in NH4+-N of -0.6‰ at the Douglas fir stand and +10.8‰ at the Scots pine stand. In soil water below the organic layer NH4+-N was enriched and NO3--N depleted in 15N, which was interpreted as being caused by isotope fractionation accompanying high nitrification rates in the organic layers. Mean δ15N values of NH4+ and NO3- were very similar in the drainage water at 90 cm soil depth at both sites (-7.1 to -3.8‰). A dynamic N cycling model was used to test the sensitivity of the natural abundance values for the amount of N deposition, the 15N ratio of atmospheric N deposited and for the intrinsic isotope discrimination factors associated with N transformation processes. Simulated δ15N values for the N saturated ecosystems appeared particularly sensitive to the 15N ratio of atmospheric N inputs and discrimination factors during nitrification and mineralization. The N-saturated coniferous forest ecosystems studied were not characterized by elevated natural 15N abundance values. The results indicated that the natural 15N abundance values can only be used as indicators for the stage of nitrogen saturation of an ecosystem if the δ15N values of the deposited N and isotope fractionation factors are taken into consideration. Combining dynamic isotope models and natural 15N abundance values seems a promising technique for interpreting natural 15N abundance values found in these forest ecosystems.