N 2O, NO and CH 4 exchange, and microbial N turnover over a Mediterranean pine forest soil
Trace gas exchange of N 2O, NO/NO 2 and CH 4 between soil and the atmosphere was measured in a typical Mediterranean pine ( Pinus pinaster) forest during two intensive field campaigns in spring and autumn 2003. Furthermore, gross and net turnover rates of N mineralization and nitrification as well as soil profiles of N 2O and CH 4 concentrations were determined. For both seasons a weak but significant N 2O uptake from the atmosphere into the soil was observed. During the unusually dry and hot spring mean N 2O uptake was −4.32 µg N m -2 h -1, whereas during the wet and mild autumn mean N 2O uptake was −7.85 µg N m -2 h -1. The observed N 2O uptake into the soil was linked to the very low availability of inorganic nitrogen at the study site. Organic layer gross N mineralization decreased from 5.06 mg N kg -1 SDW d -1 in springtime to 2.68 mg N kg -1 SDW d -1 in autumn. Mean NO emission rates were significantly higher in springtime (9.94 µg N m -2 h -1) than in autumn (1.43 µg N m -2 h -1). A significant positive correlation between NO emission rates and gross N mineralization as well as nitrification rates was found. The negative correlation between NO emissions and soil moisture was explained with a stimulation of aerobic NO uptake under N limiting conditions. Since NO 2 deposition was continuously higher than NO emission rates the examined forest soil functioned as a net NO x sink. Observed mean net CH 4 uptake rates were in spring significantly higher (−73.34 µg C m -2 h -1) than in autumn (−59.67 µg C m -2 h -1). Changes in CH 4 uptake rates were strongly negatively correlated with changes in soil moisture. The N 2O and CH 4 concentrations in different soil depths revealed the organic layer and the upper 0.1 m of mineral soil as the most important soil horizons for N 2O and CH 4 consumption.