Causes for the small scale variability of nitrate concentration in seepage water of an N saturated mature spruce stand

Context In N-saturated forests nitrate concentrations in seepage water (NO3-seepage ) regularly show high spatial variability even within homogeneous stands. Up to now the reasons of this variability are not fully understood. & Aims The main objective was to identify the crucial parameters that control spatial variability of NO3-seepage at the Höglwald site. & Methods We investigated a multitude of parameters (e.g. N turnover, root biomass, soil chemistry, soil physics, stand parameters) and related them to NO3-seepage , measured in 40 cm depth with suction cups. & Results A small number of biological parameters (net N mineralization, root distribution, and stand density) explained up to 93 % of the variability of NO3-seepage in linear regression models. Net N-mineralization rates in the humus layer and fine root biomass in the upper mineral soil influenced NO3-seepage positively. Fine root biomass in deeper soil layers (30-40 cm depth) and stand density had a negative influence. & Conclusion The rate of net N mineralization in the organic layer is decisive for the nitrate production in the soil. Roots in the upper mineral soil increase NO3seepage by intensive water uptake but excluding nitrate at the same time. The variation of these two parameters is responsible for most of the small-scale variability of NO3seepage.