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.