Soil net nitrogen (N) mineralization and nitrification as well as gross nitrification rates were studied in a forest soil within a 30×18m homogeneous plot located in an N saturated mature spruce stand at the Höglwald Forest (Bavaria, Germany) in order to explain the small-scale variation in nitrate (NO 3 − ) concentration in seepage water. Seepage water was sampled below the main rooting zone in 40cm depth with suction cups over two periods at 20 measuring spots respectively. The sampling spots were uniformly distributed over the plot for both sampling periods, and represented the whole concentration range of seepage water NO 3 − concentrations measured within a close mesh of 121 suction cups. At each measuring spot soil net N mineralization, gross and net nitrification, heterotrophic soil respiration, extractable soil ammonium (NH 4 + ) and NO 3 − , and additional physical and chemical soil parameters were measured in the organic layer and correlated with the NO 3 − concentrations in seepage water. Furthermore, the effects of environmental parameters on N conversion processes were evaluated using multiple linear regression analysis. We found that the small-scaled variations in seepage water NO 3 − concentration were related to similar small-scaled variations in key processes of microbial N turnover rates in the organic layer. Within this study net N mineralization in the organic layer could explain 51-59% of the corresponding small-scale variation of nitrate concentrations in seepage water below the main rooting zone using a multiple linear regression model with stepwise procedure. In addition, we found that small-scale patterns of N turnover in the organic layer were strongly influenced by water content in the organic layer and the dry mass of organic matter.

Microbial N‐turnover processes were investigated in three different forest soil layers [organic (O) layer, 0–10 cm depth (M1), 10–40 cm depth (M2)] of a N‐saturated spruce stand at the Höglwald Forest (Bavaria, SW Germany). The aim of the study was to provide a detailed insight into soil‐layer‐specific microbial production and consumption of inorganic N within the main rooting zone. Furthermore, we intended to clarify the relevance of each investigated soil layer on the observed high spatial variation of seepage‐water nitrate (NO$ _3^- $) concentration at 40 cm depth. The 15N‐pool dilution technique was applied for determination of gross and net N‐turnover rates in the different soil layers. Moreover, soil pH, C : N ratio, pool sizes of soil ammonium (NH$ _4^+ $) and NO$ _3^- $, as well as amounts of microbial biomass C (Cmic) and N (Nmic) were determined. The O layer had the greatest microbial‐biomass density along with the highest gross and net N‐turnover rates. 55% of the net nitrification occurred in the O layer, 20% in M1, and 25% in M2 (i.e., a considerable amount of net NO$ _3^- $ production was located in the mineral soil). Spatial variability of N‐turnover rates even increased with increasing soil depth due to higher spatial variation of microbial biomass and C and N contents in soil. NH$ _4^+ $ and NO$ _3^- $ concentrations in the organic layer as well as NO$ _3^- $ concentrations in M2 were significantly correlated with NO$ _3^- $ concentrations in seepage water at 40 cm depth. However, no significant correlation between NO$ _3^- $ concentrations in seepage water and any N‐turnover process was found. We suggest that in contrast to in situ field measurements the dislocation of the soil samples from their natural environment may have altered the spatial variability of N‐turnover rates.

Microbial N turnover processes were investigated in three different forest soil layers [organic (O) layer, 0-10 cm depth (M 1 ), 10-40 cm depth (M 2 )] after the clear cutting of a nitrogen (N) saturated spruce stand at the Höglwald Forest (Bavaria, Germany). The aim of the study was to provide detailed insight into soil-layer specific microbial production and the consumption of inorganic N within the main rooting zone. Furthermore, we intended to clarify the relevance of each soil layer investigated in respect of the observed high spatial variation of seepage water nitrate (NO 3 − ) concentration at a depth of 40 cm. The buried bag and the 15 N pool dilution techniques were applied to determine the net and gross N turnover rates. In addition, soil pH, C:N ratio, pool sizes of soil ammonium (NH 4 + ) and NO 3 − , as well as quantities of microbial biomass carbon (C mic ) and nitrogen (N mic ) were determined. The 40 cm thick upper mineral soil was found to be the main place of NO 3 − production with a NO 3 − supply or net nitrification three times higher than in the considerably thinner O layer. Nevertheless, O layer nitrification processes determined via in situ field experiments showed significant correlation with seepage water NO 3− . An improved correlation noted several months after the cut may result from a transport-induced time shift of NO 3 − with downstream hydrological pathways. In contrast, the soil laboratory incubation experiments found no indication that mineral soil is relevant for the spatial heterogeneity of seepage water NO 3 − . The results from our study imply that in situ experiments may be better suited to studies investigating N turnover in relation to NO 3 − loss via seepage water in similar ecosystems in order to gain representative data.

& Context In N-saturated forests nitrate concentrations in seepage water (NO 3 À 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 NO 3 À 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 NO 3 À 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 NO 3 À seepage in linear regression models. Net N-mineralization rates in the humus layer and fine root biomass in the upper mineral soil influenced NO 3 À 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 NO 3 À seepage 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 NO 3 À seepage .