Soil organic matter as a key controlling variable for methane oxidation in forest soils – microbial analysis and global estimation

Methane (CH₄) is over 25 times stronger greenhouse gas than CO₂ on a molar basis, and its concentration increases continuously. Its global budget, however, is yet to be constrained, and the greatest uncertainty is associated with natural sources and sinks. Terrestrial ecosystems are known to uptake atmospheric CH₄ by a group of microbes known as methanotrophs. Previous studies have noted that the controlling variables for methane oxidation in forest soils are moisture content, temperature and nutrient availability. However, previous estimation on CH₄ oxidation in terrestrial soils exhibit great discrepancy between model estimates and field observations. In this study, we measured CH₄ uptake rates monthly in temperate pine forests and seasonally in subtropical forests in Korea for 2 years. In addition, soil chemical properties and microbial composition were monitored to reveal the key controlling variable for the uptake rates. Deciduous forests showed the highest CH₄ uptake rate followed by mixed forests and the lowest was observed in coniferous forests. Air-filled porosity and the abundance of methanotrophs were correlated with CH₄ uptake rate. Our results as well as meta-analysis of 207 measurements from 84 literature showed that soil organic matter (SOM) content significantly correlated with soil CH₄ uptake rate at both regional and global scales, indicating that SOM can be a robust controlling factor for CH₄ oxidation. We speculate that SOM content affects soil CH₄ oxidation via alters air-filled porosity and available carbon source for facultative methanotrophs. The amount of CH₄ oxidation in global forests estimated by a model based on SOM content is 22.22 Tg, which far exceeds previous estimation of 17.46 Tg.