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.