Quantification and uncertainty of global upland soil methane sinks: Processes, controls, model limitations, and improvements

Upland soils constitute the second largest and the only manageable methane (CH₄) sink, yet current estimations remain substantially uncertain. This review identifies the primary sources of model uncertainties and emphasize the need for improved model accuracy and necessary comprehensiveness to better estimate upland soil CH₄uptake under global change. We highlight that the limitations of diffusion-reaction models include oversimplified assumptions of upland soils as constant CH₄sinks and insufficient parameterization of the microbial CH₄ oxidation constants. In process-based biogeochemical models, uncertainties stem from the omission of soil O₂ status and oversimplified Michaelis–Menten kinetics parameterization for upland soils. We also provide three suggestions for better addressing the spatiotemporal variations in soil CH₄ uptake globally. 1) Accounting for the balance between methanotrophy and methanogenesis is the key to accurately assessing CH₄ fluxes at fine to large scales. 2) Improved response curves of methanotrophy to soil moisture, temperature, and mineral nitrogen, as the most important regulators, are needed to correct the underestimated spatial variations in the size of the soil CH₄ sink globally. 3) Improving parameterizations based on the relationships between environmental factors and methanotrophic communities is necessary. Our synthesized model estimations and field observations reveal that inconsistent estimations of the spatial variations in forest soil CH₄ sinks, and the neglect of the drylands (arid and semiarid ecosystems) CH₄ sink are the major sources of uncertainty for global upland soil CH₄ sinks. Given the great potential of soil CH₄ uptake in mitigating the imbalanced global CH₄ budget, we emphasize the necessity of addressing the soil CH₄ exchanges in these key ecosystems, particularly under the impacts of global changes, by integrating continuous in-situ observations with improved models to fully account for the dynamics of the terrestrial CH₄ sink. This review contributes to a more accurate estimation, management, and optimization of global upland soil CH₄ sinks, aiding in the development of effective climate change mitigation strategies.