The effect of oxygen, pH and organic carbon on soil-layer specific denitrifying capacity in acid coniferous forest

Emissions of N2O from acid coniferous forest soils are found to be low and considered to be due to nitrification rather than denitrification. Recently we have demonstrated soil-layer specific denitrification in a Scots pine f orest in the Netherlands. N2O production, in the presence of high concentra tions of acetylene, was detected in the intact needle fraction but was abse nt in the fragmentation layer of this forest soil. To identify the factors regulating denitrification activity, in the present study the effects of ox ygen, pH and organic carbon were investigated in the needle and fragmentati on fraction of acid coniferous forest soils. Under natural circumstances de nitrification in the Scots pine needles was higher than in Douglas fir need les and absent in fragmentation material. Under anaerobic conditions compar able N2O production in the two soil types was found in needle suspensions o f both forest types, indicating that differences in anaerobic microsites we re responsible for different N2O production under aerobic circumstances. De nitrifying capacity was absent in the fragmentation layer; under anaerobic circumstances little N2O was produced. Neither an addition of available car bon (glucose and succinate) nor an increase in pH revealed a denitrifying c apacity comparable to that observed in needles. The increase in pH, under a naerobic circumstances, was most effective on N2O production in the fragmen tation material. The denitrifying capacity in the fragmentation layer remai ned low during short-term incubation under optimal conditions. This indicat es the presence of a low denitrifying population, most likely due to aerobi c conditions, low pH and low available organic carbon. Although the signifi cance of N2O production under natural conditions remains speculative, this study seeks to clarify soil-layer specific denitrifying activity in acid co niferous forest soils. (C) 2001 Elsevier Science Ltd. All rights reserved.