Abstract. Effects of anthropogenic nitrogen (N) deposition and the ability of terrestrial ecosystems to store carbon (C) depend in part on the amount of N retained in the system and its partitioning among plant and soil pools. We conducted a meta-analysis of studies at 48 sites across four continents that used enriched 15 N isotope tracers in order to synthesize information about total ecosystem N retention (i.e., total ecosystem 15 N recovery in plant and soil pools) across natural systems and N partitioning among ecosystem pools. The greatest recoveries of ecosystem 15 N tracer occurred in shrublands (mean, 89.5%) and wetlands (84.8%) followed by forests (74.9%) and grasslands (51.8%). In the short term (,1 week after 15 N tracer application), total ecosystem 15 N recovery was negatively correlated with fine-root and soil 15 N natural abundance, and organic soil C and N concentration but was positively correlated with mean annual temperature and mineral soil C:N. In the longer term tracer were below ground in forests, shrublands, and grasslands, we conclude that growth enhancement and potential for increased C storage in aboveground biomass from atmospheric N deposition is likely to be modest in these ecosystems. Total ecosystem 15 N recovery decreased with N fertilization, with an apparent threshold fertilization rate of 46 kg NÁha À1 Áyr À1 above which most ecosystems showed net losses of applied 15 N tracer in response to N fertilizer addition.

N saturation induced by atmospheric N deposition can have serious consequences for forest health in many regions. In order to evaluate whether foliar d 15 N may be a robust, regional-scale measure of the onset of N saturation in forest ecosystems, we assembled a large dataset on atmospheric N deposition, foliar and root d 15 N and N concentration, soil C:N, mineralization and nitrification. The dataset included sites in northeastern North America, Colorado, Alaska, southern Chile and Europe. Local drivers of N cycling (net nitrification and mineralization, and forest floor and soil C:N) were more closely coupled with foliar d 15 N than the regional driver of N deposition. Foliar d 15 N increased non-linearly with nitrification:mineralization ratio and decreased with forest floor C:N. Foliar d 15 N was more strongly related to nitrification rates than was foliar N concentration, but concentration was more strongly correlated with N deposition. Root d 15 N was more tightly coupled to forest floor properties than was foliar d 15 N. We observed a pattern of decreasing foliar d 15 N values across the following species: American beech>yellow birch>sugar maple. Other factors that affected foliar d 15 N included species composition and climate. Relationships between foliar d 15 N and soil variables were stronger when analyzed on a species by species basis than when many species were lumped. European sites showed distinct patterns of lower foliar d 15 N, due to the importance of ammonium deposition in this region. Our results suggest that examining d 15 N values of foliage may improve understanding of how forests respond to the cascading effects of N deposition.

In many nitrate (NO(3)(-)) source partitioning studies, the delta(18)O value for NO(3)(-) produced from nitrification is often assumed to reflect the isotopic compositions of environmental water (H(2)O) and molecular oxygen (O(2)) in a 2:1 ratio. Most studies that have measured or observed this microbial endmember have found that the delta(18)O-NO(3)(-) was more positive (up to +15 per thousand higher) than the assumed value. Current understanding of the mechanism(s) responsible for this discrepancy is limited. Incubations of one temperate forest soil (organic) and two agricultural soils (mineral) were conducted with (18)O-labeled H(2)O to apportion the sources of oxygen in NO(3)(-) generated from nitrification. The NO(3)(-) produced in all soils had delta(18)O values that could not be explained by a simple endmember mixing ratio of 2:1. A more comprehensive model describing the formation of microbial NO(3)(-) was developed, which accounts for oxygen exchange between H(2)O and NO(2)(-), and includes terms for kinetic and equilibrium isotope effects. Oxygen isotope exchange (i.e., the fraction of NO(3)(-)-oxygen that originates from the abiotic exchange of H(2)O and NO(2)(-)) varied widely between the temperate forest soil (0.37) and the two agricultural soils (0.52 and 0.88). At present, the microbial endmember for nitrification cannot be successfully predicted.

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[1] Two adjacent catchments with similar temperate forest cover and podzolic soils have annual nitrate (NO 3 -) export that differs by a factor of 10. Monthly rates of mineralization and nitrification measured by the buried bag technique, soil C/N ratios, and the contribution of microbial NO -during NO 3 -loss do not show evidence of denitrification, although denitrification proceeding to completion in isolated pockets followed by mixing with higher NO 3 -groundwaters would yield the same result. Alternatively, active uptake of NO 3 -by vegetation following spring melt will also deplete the groundwater NO 3 -in the shallow soil depths without isotopic fractionation. The low NO 3 -catchment also has lower NO 3 -in shallow soil waters during spring melt. Shallower slopes promote near-surface flow paths in organic-rich soil horizons which may facilitate denitrification during spring melt. Although the catchment with low NO 3 -export has a large wetland near the catchment outlet, the NO 3 -attenuating capacity of this wetland is largely unused except in the late fall because growing season groundwater concentrations of NO 3 -are undetectable and the wetland is frozen during snowmelt. In the high NO 3 -catchment, organic-rich soils and vegetation in the riparian zone cannot completely attenuate high NO 3 -in discharging groundwaters. In our study, factors controlling NO 3 -in groundwater such as slope, stratigraphy, and hydraulic conductivity can play a larger role than riparian zones in controlling differences in annual NO 3 -export observed between catchments.

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