Topography-related controls on N2O emission and CH4 uptake in a tropical rainforest catchment.

Forest soils in the warm-humid tropics significantly contribute to the regional greenhouse gas (GHG) budgets. However, spatial heterogeneity of GHG fluxes is often overlooked. Here, we present a study of N2O and CH4 fluxes over 1.5 years, along a topographic gradient in a rainforest catchment in Xishuangbanna, SW China. From the upper hillslope to the foot of the hillslope, and further to the flat groundwater discharge zone, we observed a decrease of N2O emission associated with an increase of soil water-filled-pore-space (WFPS), which we tentatively attribute to more complete denitrification to N2 at larger WFPS. In the well-drained soils on the hillslope, denitrification at anaerobic microsites or under transient water-saturation was the potential N2O source. Negative CH4 fluxes across the catchment indicated a net soil CH4 sink. As the oxidation of atmospheric CH4 is diffusion-limited, soil CH4 consumption rates were negatively related to WFPS, reflecting the topographic control. Our observations also suggest that during dry seasons N2O emission was significantly dampened (<10 µg N2O-N m-2 h-1) and CH4 uptake was strongly enhanced (83 µg CH4-C m-2 h-1) relative to wet seasons (17 µg N2O-N m-2 h-1 and 56 µg CH4-C m-2 h-1). In a post-drought period, several rain episodes induced exceptionally high N2O emissions (450 µg N2O-N m-2 h-1) in the groundwater discharge zone, likely driven by flushing of labile organic carbon accumulated during drought. Considering the global warming potential associated with both GHGs, we found that N2O emissions largely offset the C sink contributed by CH4 uptake in soils (more significant in the groundwater discharge zone). Our study illustrates important topographic controls on N2O and CH4 fluxes in forest soils. With projected climate change in the tropics, weather extremes may interact with these controls in regulating forest GHG fluxes, which should be accounted for in future studies.