Humid Subtropical Forests Constitute a Net Methane Source: A Catchment‐Scale Study

Commonly, well‐drained forest soils are net methane (CH 4 ) sinks, whereas poorly drained soils in groundwater discharge zones could contribute significant CH 4 emissions. Climate change is projected to bring more summer precipitation extremes to subtropics, which may affect forest CH 4 balance. Chinese forests often have a hilly topography with well‐drained hillslopes and pronounced groundwater discharge zones. Although different landscape elements are likely to have different source and sink functions for CH 4 , the climatic influence on CH 4 budget at the catchment scale remains poorly studied. Here, we measured CH 4 fluxes over three years along a topographic gradient in a subtropical forested catchment in SW China. CH 4 fluxes exhibited a clear spatial pattern indicating moderate CH 4 uptake or emission from the well‐drained soils on the hillslope (−5.8 to +1.0 kg CH 4 ‐C·ha −1 ·year −1 ) and significant CH 4 emission from the wetter soils in the groundwater discharge zone (94.3 to 479.5 kg CH 4 ‐C·ha −1 ·year −1 ). Despite its small area contribution to the catchment (0.6%), the groundwater discharge zone emitted substantial amounts of CH 4 in monsoonal summers, affecting the whole‐catchment budget. Hillslope soils showed weaker CH 4 uptake or even turned into a small CH 4 source under wet climatic conditions. Estimates of catchment‐scale CH 4 budgets indicate that subtropical forest catchments can tip from a net CH 4 sink in drier years to a net CH 4 source in wetter years, highlighting the importance of interannual climate variabilities. Our findings suggest that subtropical forests under projected climate change could contribute a net CH 4 source with consequences for the regional CH 4 balance.