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 (CH 4 ) 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 CH 4 uptake under global change. We highlight that the limitations of diffusion-reaction models include oversimplified assumptions of upland soils as constant CH 4 sinks and insufficient parameterization of the microbial CH 4 oxidation constants. In process-based biogeochemical models, uncertainties stem from the omission of soil O 2 status and oversimplified Michaelis - Menten kinetics parameterization for upland soils. We also provide three suggestions for better addressing the spatiotemporal variations in soil CH 4 uptake globally. 1) Accounting for the balance between methanotrophy and methanogenesis is the key to accurately assessing CH 4 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 CH 4 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 CH 4 sinks, and the neglect of the drylands (arid and semiarid ecosystems) CH 4 sink are the major sources of uncertainty for global upland soil CH 4 sinks. Given the great potential of soil CH 4 uptake in mitigating the imbalanced global CH 4 budget, we emphasize the necessity of addressing the soil CH 4 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 CH 4 sink. This review contributes to a more accurate estimation, management, and optimization of global upland soil CH 4 sinks, aiding in the development of effective climate change mitigation strategies.