Dynamics and environmental drivers of methane and nitrous oxide fluxes at the soil and ecosystem levels in a wet tropical forest

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Tropical forests are critical for maintaining the global carbon balance and mitigating climate change, yet their exchange of greenhouse gases with the atmosphere remains understudied, particularly for methane (CH4) and nitrous oxide (N2O). This study reports on continuous measurements of CH4 and N2O fluxes at the ecosystem and soil levels, respectively through eddy covariance and an automated chamber technique, in a wet tropical forest in French Guiana over a period of 26 months. We studied the magnitude of CH4 and N2O fluxes and their drivers (climatic variables) during the driest and wettest seasons. Seasonal ecosystem fluxes showed near-zero net CH4 uptake during the driest season and emissions during the wettest season that were larger in magnitude than the uptake. Meanwhile, N2O emissions were of similar magnitudes in both seasons. Upland soils within the footprint of the eddy covariance tower emitted N2O in both seasons, although these fluxes were particularly small. In contrast, upland soils were characterised by CH4 uptake. Overall, seasonal ecosystem and soil CH4 and N2O fluxes were partially explained by seasonal variations in soil water content, soil temperature and global radiation. The magnitude and sign of the net ecosystem fluxes of CH4 and N2O were likely due to outgassing from aboveground biomass and the presence of seasonally flooded areas within the footprint of the eddy covariance system. Further studies of other ecosystem compartments in different forest habitats are needed to better understand the temporal variations in CH4 and N2O fluxes in wet tropical forests.