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 core-incubation method varied widely among the forest stands and were highest (up to 170 mug 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. (C) 2003 Elsevier B.V. All rights reserved.