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

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.