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

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.