Microbial N turnover processes in three forest soil layers following clear cutting of an N saturated mature spruce stand

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)] after the clear cutting of a nitrogen (N) saturated spruce stand at the Hoglwald 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 15N 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 (Cmic) and nitrogen (Nmic) 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.