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

摘要

Microbial N-turnover processes were investigated in three different forest soil layers [organic (O) layer, 0-10cm depth (M_1), 10-40cm depth (M_2)] of a N-saturated spruce stand at the H?glwald 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 40cm 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 (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 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 M_2 were significantly correlated with NO_3~- concentrations in see-page water at 40cm 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.