N₂O, NO and CH₄ exchange, and microbial N turnover over a Mediterranean pine forest soil

Abstract. Trace gas exchange of N₂O, NO/NO₂ and CH₄ 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 N₂O and CH₄ concentrations were determined. For both seasons a weak but significant N₂O uptake from the atmosphere into the soil was observed. During the unusually dry and hot spring mean N₂O uptake was −4.32 µg N m^-2 h^-1, whereas during the wet and mild autumn mean N₂O uptake was −7.85 µg N m^-2 h^-1. The observed N₂O 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 NO₂ deposition was continuously higher than NO emission rates the examined forest soil functioned as a net NO_x sink. Observed mean net CH₄ 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 CH₄ uptake rates were strongly negatively correlated with changes in soil moisture. The N₂O and CH₄ 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 N₂O and CH₄ consumption.