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 forest in the Netherlands. N2O production, in the presence of high concentrations of acetylene, was detected in the intact needle fraction but was absent in the fragmentation layer of this forest soil. To identify the factors regulating denitrification activity, in the present study the effects of oxygen, pH and organic carbon were investigated in the needle and fragmentation fraction of acid coniferous forest soils. Under natural circumstances denitrification in the Scots pine needles was higher than in Douglas fir needles and absent in fragmentation material. Under anaerobic conditions comparable N2O production in the two soil types was found in needle suspensions of both forest types, indicating that differences in anaerobic microsites were responsible for different N2O production under aerobic circumstances. Denitrifying capacity was absent in the fragmentation layer; under anaerobic circumstances little N2O was produced. Neither an addition of available carbon (glucose and succinate) nor an increase in pH revealed a denitrifying capacity comparable to that observed in needles. The increase in pH, under anaerobic circumstances, was most effective on N2O production in the fragmentation material. The denitrifying capacity in the fragmentation layer remained low during short-term incubation under optimal conditions. This indicates the presence of a low denitrifying population, most likely due to aerobic conditions, low pH and low available organic carbon. Although the significance of N2O production under natural conditions remains speculative, this study seeks to clarify soil-layer specific denitrifying activity in acid coniferous forest soils.