Nitrate leaching in forest soils: an analysis of long-term monitoring sites in Germany

Elevated atmospheric inputs of NH4+ and NO3- have caused N saturation of many forest ecosystems in Central Europe, but the fate of deposited N that is not bounded by trees remains largely unknown. It is expected that an increase of NO3 leaching from forest soils may harm the quality of groundwater in many regions. The objective of this study was to analyze the input and output of NH4+ and NO 3 - at 57 sites with mature forest stands in Germany. These long-term study sites are part of the European Level 11 program and comprise 17 beech, 14 spruce, 17 pine, and 9 oak stands. The chloride balance method was used to calculate seepage fluxes and inorganic N leaching below the rooting zone for the period from 1996 to 2001. Nitrogen input by throughfall was significantly different among most forest types, and was in the order: spruce > beech/oak > pine. These differences can be largely explained by the amount of precipitation and, thus, it mirrors the regional and climatic distribution of these forest types in Germany. Mean long-term N output with seepage was log-normal distributed, and ranged between 0 and 26.5 kg N ha(-1) yr(-1), whereby 29 % of the sites released more than 5 kg N ha(-1) yr(-1). Leaching of inorganic N was only significantly lower in the pine stands (P < 0.05) compared with leaching rates of the spruce stands. Median N output : input ratio ranged between 0.04 and 0.11 for the beech, oak, and pine stands, while the input : output ratio of the spruce stands was 0.24, suggesting a higher risk of NO3 leaching in spruce forests. Following log-transformation of the data, N input explained 38 % of the variance in N output. The stratification of the data by the C : N ratio of the 0 horizon or the top mineral soil revealed that forests soils with a C : N ratio < 25 released significantly more NO3 (median of 4.6 kg N ha(-1) yr(-1)) than forests with a C : N ratio > 25 (median of 0.8 kg N ha(-1) yr(-1)). The stratification improved the correlation between N input and N output for sites with C : N ratios < 25 (r2 = 0.47) while the correlation for sites with C : N ratios > 25 was weaker (r(2) = 0.21) compared with the complete data set. Our results suggest that NO3- leaching may increase in soils with wide C: N ratios when IN deposition remains on a high level and that the potential to store inorganic N decreases with C : N ratios in the O horizons becoming more narrow.