Soil methane uptake is tightly linked to carbon dioxide emission in global upland ecosystems

The fluxes of methane (CH4) and carbon dioxide (CO2) between soils and atmosphere play vital roles in regulating climate change and global carbon cycling. There is evidence that CH4 uptake and CO2 emission are correlated on the plot scale. However, it remains unclear whether the fluxes of these two greenhouse gases are tightly linked on temporal and large spatial scales in upland ecosystems. Here, through three independent approaches (in-situ observation, data extracted from ten typical field experiments, and a global meta-analysis), we found that soil CH4 uptake rate was positively associated with soil CO2 emission rate on a temporal scale over two years in our in-situ experiments. Data extracted from other typical field experiments verified that the tight linkage between soil CH4 uptake and CO2 emission on a temporal scale was common, even after the effects of soil temperature and moisture were removed. Moreover, a global meta-analysis further confirmed that the tight linkage between the fluxes of these two greenhouse gases also exists on a large spatial scale. Model selection analysis and structural equation modelling all verified that soil CO2 emission rate was the key predictor for soil CH4 uptake rate after accounting for several important factors, such as climate and soil properties. The estimated annual global forest soil CH4 sink based on our findings is 19.70 +/- 6.37 Tg C yr(-1). The tight linkage between soil CH4 uptake and CO2 emission rate on temporal and spatial scales we found in this study open new insights to easily model soil CH4 uptake based on soil CO2 emission measurement, since it is easier and more cost-efficient to measure than CH4 flux. Overall, our study highlights the role of soil CO2 emissions in predicting soil CH4 uptake, which should be taken into consideration for global CH4 budget quantification.