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

Abstract

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.