Seasonal dynamic of gross nitrification and N2O emission at two tropical rainforest sites in Queensland, Australia

Combined measurements of nitrification activity and N2O emissions were performed in a lowland and a montane tropical rainforest ecosystem in NE-Australia over a 18 months period from October 2001 until May 2003. At both sites gross nitrification rates, measured by the BaPS technique, showed a strong seasonal pattern with significantly higher rates of gross nitrification during wet season conditions. Nitrification rates at the montane site (1.48 +/- 0.24-18.75 +/- 2.38 mg N kg(-1) day(-1)) were found to be significantly higher than at the lowland site (1.65 +/- 0.21-4.54 +/- 0.27 mg N kg(-1) day(-1)). The relationship between soil moisture and gross nitrification rates could be described best by O'Neill functions having a soil moisture optimum of nitrification at app. 65% WFPS. At the lowland site, for which continuous measurements of N2O emissions were available, nitrification was positively correlated with N2O emission. Nitrification contributed significantly to N2O formation during dry season (app.85%) but less (app. 30%) during wet season conditions. In average 0.19 parts per thousand of the N metabolized by nitrification was released as N2O. The N2O fraction loss for nitrification was positively correlated with changes in soil moisture and varied slightly between 0.15 and 0.22 parts per thousand. Our results demonstrate that combined N2O emission and microbial N turnover studies covering prolonged observation periods are needed to clarify and quantify the role of the microbial processes nitrification and denitrification for annual N2O emissions from soils of terrestrial ecosystems.