Microbial nitrogen turnover in soils under different types of natural forest

Microbial nitrogen (N) turnover was compared among 12 forest stands with a natural vegetation composition. They comprised six typical forest types in the eastern part of Austria, including oak and beech forests, spruce–fir–beech forests, floodplain and pine forests. The aim was to provide a reference basis for measurements in managed, especially disturbed or damaged forest ecosystems. We measured total C- and N-concentrations in soil, extractable NO3- and NH4+ concentrations, microbial biomass N, nitrogen mineralisation potential and potential N2O production in soil. As indicators of NH4+ availability and NO3- translocation into deeper soil horizons, accumulation of NH4+ and NO3- on resin bags was evaluated. Differences between the forest stands in potential N turnover were related to the size of the total N stores in soil and to vegetation composition. N mineralisation potential and microbial biomass were generally higher in the spruce–fir–beech, floodplain and pine forests with large N pools than in the oak and beech forests with lower N pools. Fastest N turnover was found in soils of a virgin spruce–fir–beech forest situated on nutrient-rich bedrock. Mineralisation potentials in these soils doubled averages in the other soils. N2O production rates measured by an intact soil coreincubation method varied widely among the forest stands and were highest (up to 170 (mu)g N2O-N m^-2 h^-1) in soils of an oak–hornbeam and a woodruff–beech forest situated close to the city of Vienna, probably as a result of high atmospheric N input. Within the study sites, the soil moisture content was the factor mainly responsible for seasonal and microscale variability in N2O production rates.