A global inventory of the soil CH4 sink

Methane uptake by soils is a small but important flux in the global budget of atmospheric methane, and could be susceptible to changes in land use and climate. Estimates of this sink vary between 20 and 45 Tg yr −1 . We propose to develop a better constrained estimate using a mechanistic understanding of the biogeochemical controls of soil CH 4 uptake. We reviewed over 120 published papers reporting field measurements of CH 4 uptake and made over 318 annual estimates of CH 4 uptake for various types of ecosystems. We collected data from these papers for a number of parameters that are known to influence the magnitude of the sink including climatic zone, ecosystem, latitude, annual mean rainfall, annual mean temperature, and the soil texture. Regression analyses with the continuous variables (latitude, rainfall, and temperature) yielded results with poor predictive ability and no significant relationship. Stratification by class variables such as climatic zone, ecosystem type and soil texture provided better predictive ability (R 2 = 0.29, P < 0.0001). The mean largest uptake rates were observed in temperate forests with coarse soil texture, but the variance within this stratum was also large. Without any stratification, we estimate that the global soil CH 4 sink is 36 ± 23 Tg yr −1 . With stratification, the best current estimate of the global soil uptake of CH 4 is 22 ± 12 Tg yr −1 . The ecosystem type accounted for the largest part of the variation in the global data set. This inventory showed that ecosystem type, geographic zone, and soil texture strongly control CH 4 uptake. Inventory methods that take into account underlying factors that control the process provide better estimates of sink strength.