A new framework for interpreting ex situ wetland methane production and consumption rates

Ex situ incubations are the primary method used to quantify rates of gross methane production and consumption, which together determine net wetland methane emissions. However, a clear framework for interpreting these data is lacking, and it remains uncertain whether ex situ rates and their environmental drivers conform to expectations from in situ ecological theory. We synthesized published rates of methanogenesis, aerobic methane oxidation, and anaerobic methane oxidation and used boosted regression tree models to identify key drivers. We found median aerobic methane oxidation rates were 2.7 times greater than those of methanogenesis—a paradox given wetlands are net methane sources. This discrepancy arises from methodological artifacts: aerobic methane oxidation is measured as a potential rate, resulting in overestimation, whereas methanogenesis is largely measured as a substrate-limited rate, resulting in underestimation. This conclusion is grounded in our finding that potential methanogenesis rates were an order of magnitude higher than their substrate-limited counterparts (p < 0.001). Our driver analysis indicates that 1. rates of methanogenesis are strongly controlled by labile carbon availability from the soil surface, 2. the balance between methanogenesis and anaerobic methane oxidation is shifting with warming, and 3. species-specific plant effects structure all pathways of methane production and consumption via rhizosphere interactions. This synthesis demonstrates the value of ex situ data for generating mechanistic hypotheses while highlighting the need for future research to prioritize in situ measurements to obtain accurate rate estimates.