Effects of increased precipitation combined with nitrogen addition and increased temperature on methane fluxes in alpine meadows of the Tibetan Plateau.

Climate change and anthropogenic activities have resulted in increased atmospheric methane (CH4) concentration. Increased nitrogen deposition and precipitation accompanies climate warming and can change soil carbon and nitrogen dynamics and microbial processes and alter CH4 fluxes. To quantify the sink of the vast alpine meadows of the Tibetan Plateau and to examine how precipitation addition (P), warming (W), and nitrogen addition (N) affect CH4 fluxes in alpine meadows, we conducted continuous 3-growing season experiments in an alpine meadow using the static chamber and gas chromatograph method. Soil CH4 samples were collected during the early, peak, and late stages of the growing season from 2015 to 2017. Our results suggested that neither P, W, nor N had an interaction effect on soil CH4 uptake. P significantly increased and decreased the copies number of particulate methane monooxygenase alpha subunit (pmoA) and methyl-coenzyme M reductase alpha subunit (mcrA), respectively. However, P significantly decreased CH4 uptake, particularly under the combined treatment of P and N. Compared with the control, CH4 uptake decreased under P, N, PW, and PN by 50.64%, 6.24%, 39.37%, and 75.06%, respectively, whereas under W and WN CH4 uptake increased by 16.19% and 7.56%, respectively. Soil CH4 uptake was positively correlated with soil temperature and pmoA and negatively correlated with soil moisture and NH4+-N content. CH4 uptake was significantly affected by the sampling period. CH4 uptake was significantly lower rates during peak growing season compared with those during the early and late stages of the growing season. Our results suggest that, (1) CH4 fluxes of alpine grassland ecosystems are more sensitive to P than W or N, and (2) precipitation controls CH4 flux response to increasing nitrogen deposition in alpine meadows on the Tibetan Plateau. Therefore, future research should focus on the response and feedback of CH4 uptake to changes in precipitation.