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 sink of atmospheric methane (CH₄). Quantifying spatiotemporal patterns of the net CH₄ exchange between upland soils and atmosphere is critical for refining global CH₄ budget estimates and developing climate mitigation strategies. However, the global CH₄ budget in upland soils remains highly uncertain due to incomplete understanding of the shifts in the source or sink role of upland soils for CH₄ under 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 presented a shift from a weak sink to a source, and grassland soils changed from a sink to a strong source of CH₄. The sink capacity of forest soils for CH₄ sharply decreased by 68.8% over the past three decades, while tundra upland soils acted as a consistent source of CH₄, with a decrease of 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 provide a new perspective on the spatiotemporal patterns of CH₄ fluxes in global upland soils, which update the role of upland soils in the global CH₄ budget, particularly as a potential CH₄ sink.