Quantification of Ecosystem-Scale Methane Sinks Observed in a Tropical Rainforest in Hainan, China

Tropical rainforest ecosystems are important when considering the global methane (CH 4 ) budget and in climate change mitigation. However, there is a lack of direct and year-round observations of ecosystem-scale CH 4 fluxes from tropical rainforest ecosystems. In this study, we examined the temporal variations in CH 4 flux at the ecosystem scale and its annual budget and environmental controlling factors in a tropical rainforest of Hainan Island, China, using 3 years of continuous eddy covariance measurements from 2016 to 2018. Our results show that CH 4 uptake generally occurred in this tropical rainforest, where strong CH 4 uptake occurred in the daytime, and a weak CH 4 uptake occurred at night with a mean daily CH 4 flux of −4.5 nmol m −2 s −1 . In this rainforest, the mean annual budget of CH 4 for the 3 years was −1260 mg CH 4 m −2 year −1 . Furthermore, the daily averaged CH 4 flux was not distinctly different between the dry season and wet season. Sixty-nine percent of the total variance in the daily CH 4 flux could be explained by the artificial neural network (ANN) model, with a combination of air temperature (T air ), latent heat flux (LE), soil volumetric water content (VWC), atmospheric pressure (Pa), and soil temperature at −10 cm (T soil ), although the linear correlation between the daily CH 4 flux and any of these individual variables was relatively low. This indicates that CH 4 uptake in tropical rainforests is controlled by multiple environmental factors and that their relationships are nonlinear. Our findings also suggest that tropical rainforests in China acted as a CH 4 sink during 2016–2018, helping to counteract global warming.