Generally reduced sink capacity of upland soils for atmospheric methane over the past three decades (1993-2022)

Upland soils act as the second largest and the only manageable methane (CH4) sink. Quantifying spatiotemporal patterns of the net CH4 exchange in upland soils is critical for refining global CH4 budget estimates and developing climate mitigation strategies. However, global CH4 budget in upland soils remains highly uncertain due to incomplete understanding of CH4-related biogeochemical processes and their responses to climate change. In this study, we generated high-resolution global maps of CH4 fluxes from upland soils by integrating field CH4 flux measurement data spanning over the period of 1993-2022 using machine learning models. Collectively, upland soils exhibited a generally reduced sink capacity for atmospheric methane over the past three decades. Cropping uplands underwent a shift from a weak sink to a source over the past three decades. In contrast, a sharp decrease by 68.8% was observed in sink capacity of forest soils for CH4. Grassland soils changed from a sink to a strong source of CH4, while tundra upland soils acted as a consistent source of CH4, although accompanied by a decrease by 55.3% since 1993. The combined effects of changes in precipitation and temperature can explain more than 70% of spatiotemporal variations of CH4 fluxes in upland soils. Our findings provided a new perspective on the spatiotemporal patterns of CH4 fluxes in global upland soils, which updated the role of upland soils in global CH4 budget, particularly as a potential CH4 sink.