N-induced soil acidification triggers metal stimulation of soil methane oxidation in a temperate steppe ecosystem

Abstract

Methane (CH4) is a potent greenhouse gas, and it is well established that low nitrogen (N) stimulates- and high N suppresses CH4 oxidation in grassland ecosystems. In this study, we examined the response of CH4 uptake to long-term (>10 years) multi-level N additions in a temperate steppe of northern China. The N impacts on CH4 uptake transitioned from positive to negative at the N addition rate of 4 g N m−2 y−1. However, the high-N suppression on CH4 uptake was partially relieved when the continuous (>10 years) high N inputs (16 g N m−2 y−1 to 64 g N m−2 y−1) caused soil pH drop to < 5.7. Further experiments revealed that continuous high-N inputs acidified soil to pH < 5.7 that released metal ions to stimulate monooxygenase enzyme activity; therefore, the CH4 oxidation suppression was partially relieved under continuous N loading. Structural equation model results confirmed that metal ions, such as Iron (Fe3+), Manganese (Mn4+), and Copper (Cu) desorbed by soil particles, were negatively correlated with soil pH while positively correlated with CH4 oxidation under high N inputs. These analyses suggested that continuous high N inputs-induced soil acidification can release previously bounded metal elements that partially alleviated the N suppression on CH4 uptake. The N-induced soil acidification impacts on CH4 uptake might play a critical role in global CH4 cycling and deserve further investigation as external N inputs continue climbing.