Topography-related controls on NO emission and CH 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 NO and CH 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 NO emission associated with an increase of soil water-filled-pore-space (WFPS), which we tentatively attribute to more complete denitrification to N at larger WFPS. In the well-drained soils on the hillslope, denitrification at anaerobic microsites or under transient water-saturation was the potential NO source. Negative CH fluxes across the catchment indicated a net soil CH sink. As the oxidation of atmospheric CH is diffusion-limited, soil CH consumption rates were negatively related to WFPS, reflecting the topographic control. Our observations also suggest that during dry seasons NO emission was significantly dampened (<10 μg NO-N m h) and CH uptake was strongly enhanced (83 μg CH-C m h) relative to wet seasons (17 μg NO-N m h and 56 μg CH-C m h). In a post-drought period, several rain episodes induced exceptionally high NO emissions (450 μg NO-N m h) 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 NO emissions largely offset the C sink contributed by CH uptake in soils (more significant in the groundwater discharge zone). Our study illustrates important topographic controls on NO and CH 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.