Soil layer-specific variability in net nitrification and denitrification in an acid coniferous forest

High spatial variation in nitrification potentials has been observed in for est soils, but explanations for this variability have remained speculative. In the present study we determined whether sample treatment, sample size, denitrification or small-scale variations in abiotic properties could expla in spatial variation in nitrogen transformations in the organic horizon of a pine forest soil. Net nitrate production in homogenates of the organic ho rizon was extremely variable. Sample size (60-600 cm(2)) had no significant effect on nitrate production. In homogenised samples no increased nitrogen production was observed compared to intact incubated cores. High small-sca le variation in nitrate production was observed in the litter (L) horizon. When this stratified L layer was subdivided, high net nitrate production wa s observed in moss (LM) and fragmented needles, whereas no net nitrate prod uction was found in intact needles. The addition of acetylene, inhibiting n itrous oxide reductase, led to significant nitrous oxide production in the L layer. Low nitrous oxide production was found in the LM layer and none in the fragmentation layer. These results show that denitrification can expla in part of the spatial variation and plays a major role in nitrogen transfo rmations in the L layer. The relatively higher pH and the presence of fungi are suggested as factors responsible for high denitrification rates in the L layer. As a consequence homogenisation of the organic horizon could lead to highly variable nitrate production due to denitrifying activity from th e needles being introduced into other layers.