Tree stem and soil methane and nitrous oxide fluxes, but not carbon dioxide fluxes, switch sign along a topographic gradient in a tropical forest

Purpose Tropical forests exchange large amounts of greenhouse gases (GHGs: carbon dioxide, CO 2 ; methane, CH 4 ; and nitrous oxide, N 2 O) with the atmosphere. Forest soils and stems can be either sources or sinks for CH 4 and N 2 O, but little is known about what determines the sign and magnitude of these fluxes. Here, we aimed to study how stem and soil GHG fluxes vary along a topographic gradient in a tropical forest. Methods Fluxes of GHG from 56 individual tree stems and adjacent soils were measured with manual static chambers. The topographic gradient was characterized by a soil moisture gradient, with one end in a wetland area (“seasonally flooded”; SF), the other end in an upland area (“terra firme”; TF) and in between a transitional area on the slope (SL). Results Tree stems and soils were always sources of CO 2 with higher fluxes in SF compared to TF and SL. Fluxes of CH 4 and N 2 O were more variable, even within one habitat. Results showed that, in TF, soils acted as sinks for N 2 O whereas, in SF and SL, they acted as sources. In contrast, tree stems which were predominantly sources of N 2 O in SF and TF, were sinks in SL. In the soil, N 2 O fluxes were significantly influenced by both temperature and soil water content, whereas CH 4 fluxes were only significantly correlated with soil water content. Conclusion SF areas were major sources of the three gases, whereas SL and TF soils and tree stems acted as either sources or sinks for CH 4 and N 2 O. Our results indicate that tree stems represent overlooked sources of CH 4 and N 2 O in tropical forests that need to be further studied to refine GHG budgets.