Seasonal variation in ecosystem and soil methane and nitrous oxide fluxes in a tropical rainforest

Abstract

Tropical forests play a key role in the global carbon balance and in natural climate change mitigation, as they account for 68% of global forest carbon stocks and represent up to 30% of global soil carbon stocks. However, major uncertainties remain regarding the long-term sustainability of their carbon sink capacity when considering the full greenhouse gas exchange, including methane (CH4) and nitrous oxide (N2O) fluxes, and accurately identifying and quantifying all sources and sinks. In this line, we present here original continuous high-frequency ecosystem (eddy covariance) and soil (automated chamber) CH4 and N2O flux data from a 2.5-year study in a seasonally wet tropical forest at the Guyaflux experimental site, French Guiana. The main objective of our study was to assess the seasonal patterns of CH4 and N2O exchange at the ecosystem and soil levels, and to identify the environmental drivers. Seasonal variations in ecosystem and soil CH4 and N2O fluxes were tremendous, with generally higher CH4 and N2O emissions in the wettest than in the driest season. Global radiation, soil water content and soil temperature were the main drivers of seasonal variation in ecosystem and soil CH4 and N2O fluxes. Furthermore, based on eddy covariance measurements of all greenhouse gases, i.e. CH4, N2O and CO2, the forest was overall a significant carbon sink (-1,875 ± 813 kgC ha-1 y-1, i.e. cumulative net ecosystem exchange), although the ecosystem shifted from a small sink to a small source of CH4 during the wettest season, and remained a more or less small but constant source of N2O. In contrast, soil fluxes in the upper part of the forest within the tower footprint were consistently a CH4 sink, while soil N2O fluxes shifted depending on the season, from a small N2O sink in the driest season to a small source in the wettest season. Our study shows that the carbon sink potential of the Guyaflux forest is not yet compromised by CH4 and N2O emissions. However, under the more frequent extreme conditions of contrasting soil water content and global radiation expected in the future, CH4 and N2O emissions may increase and thus reduce the forest carbon sink.