Denitrification and gaseous nitrogen losses from an acid spruce forest soil

To quantify gaseous N-emissions in an approximately 80-y-old spruce forest, in-situ measurements were carried out every 2 wk over one year. Using the acetylene inhibition technique, soil cores (0–10 cm) were incubated for 24 h under 10 kPa acetylene to detect (N2 + N2O)N from denitrification. Additional soil cores were incubated without C2H2 to measure the overall N2ON emission (derived from denitrification and nitrification). The site receives an N-input of approximately 11.5 kg N ha−1 y−1 through wet deposition. On an annual basis, N2O emissions from this forest soil were calculated to be 81 g N ha−1 y−1, whereas N-losses from both denitrification and nitrification were lower than 176 g ha−1 y−1. Low net nitrification may be the main reason for the low N2O and N2 emission. Gaseous N-losses showed a slight seasonal trend as emissions were lowest during the winter season (December-March), whereas highest values were measured in spring (under 10 kPa C2H2) or early summer (without C2H2). The highest denitrification rate occurred when the NO3−-content was high in the OfOh-horizon, indicating that the denitrifying activity may rise significantly if a critical NO3−-concentration is reached. NO3 was also shown to limit denitrification under laboratory conditions using intact soil cores.