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

PurposeTropical forests exchange large amounts of greenhouse gases (GHGs: carbon dioxide, CO2; methane, CH4; and nitrous oxide, N2O) with the atmosphere. Forest soils and stems can be either sources or sinks for CH4 and N2O, 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.MethodsFluxes 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).ResultsTree stems and soils were always sources of CO2 with higher fluxes in SF compared to TF and SL. Fluxes of CH4 and N2O were more variable, even within one habitat. Results showed that, in TF, soils acted as sinks for N2O whereas, in SF and SL, they acted as sources. In contrast, tree stems which were predominantly sources of N2O in SF and TF, were sinks in SL. In the soil, N2O fluxes were significantly influenced by both temperature and soil water content, whereas CH4 fluxes were only significantly correlated with soil water content.ConclusionSF areas were major sources of the three gases, whereas SL and TF soils and tree stems acted as either sources or sinks for CH4 and N2O. Our results indicate that tree stems represent overlooked sources of CH4 and N2O in tropical forests that need to be further studied to refine GHG budgets.