Temporal variations of fluxes of NO, NO2, N2O, CO2, and CH4 in a tropical rain forest ecosystem

Abstract

Fluxes of N2O, NO, NO2, CO2, and CH4 were measured with high temporal resolution for 3 months at a tropical rain forest site in Queensland, Australia, using automated measuring systems. During this period, representing the transition between dry and wet season, huge pulses of NO emissions from the soil exceeding 500 μg N m−2 h−1 were observed with the onset of the first rainfalls. The magnitude of fluxes was explained by intensive mineralization of accumulated litter from a previous long‐lasting dry period. The mean NO emission rate was 207.1 μg N m−2 h−1 (range: 0.1–773.8 μg N m−2 h−1) and thus ∼8 times higher as compared to N2O emissions (25.6 μg N m−2 h−1, range: 0–101.3 μg N m−2 h−1). NO and N2O emissions showed pronounced temporal variations, which were almost exclusively triggered by changes in soil moisture. Total NO and N2O losses summed to ∼3.5 kg N ha−1. Though a significant amount of the NO emitted from the soil was redeposited to the soil surface after its oxidation to NO2 (0.7 kg N ha−1), the observed magnitude of net NOx release from the soil indicates that NOx emissions from tropical rain forest ecosystems are seriously underestimated. The mean CO2 emission was 159.0 mg C m−2 h−1 (range: 36.3–284.8 mg C m−2 h−1) and thus >2 magnitudes higher than NO emissions. Among the C‐ and N‐trace gases measured, the temporal variability of CH4 uptake was the lowest. The mean uptake rate for CH4 was −23.8 μg C m−2 h−1 (range: −50.0–0 μg C m−2 h−1). The emission ratios of NO:N2O, CO2:N2O and CO2:NO varied substantially with time. During dry periods the NO‐N:N2O‐N ratio was as high as 60:1, whereas for wetter periods it decreased to <7:1. A comparable trend was also observed for the ratio between CO2‐C:N2O‐N emissions. The largest ratio of CO2‐C:NO‐N (>1500:1) was observed at intermediate soil moisture values.