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.