Microbial nitrogen-turnover processes within the soil profile of a nitrogen-saturated spruce forest and their relation to the small-scale pattern of seepage-water nitrate

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)] of a N-saturated spruce stand at the Hoglwald 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 (NO3-) concentration at 40 cm depth. The N-15-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 (NH4+) and NO3-, as well as amounts of microbial biomass C (C-mic) and N (N-mic) 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 M-1, and 25% in M-2 (i.e., a considerable amount of net NO3- 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. NH4+ and NO3- concentrations in the organic layer as well as NO3- concentrations in M-2 were significantly correlated with NO3- concentrations in seepage water at 40 cm depth. However, no significant correlation between NO3- 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.