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Temporal variations of methane flux (FCH4) and its underlying mechanisms still remain poorly understood. To quantify diurnal and seasonal patterns of FCH4 and investigate its determinants, we monitored FCH4 using eddy covariance in an alpine meadow on the Qinghai-Tibetan Plateau, China, from June 2015 to December 2016. As a strong CH4 sink, the alpine meadow on the Qinghai-Tibetan Plateau consumed 0.41 0.04 Tg CH4/year. There was an obvious diurnal pattern with more CH4 uptakes during the nighttime than the daytime for both growing and nongrowing season. The diurnal FCH4 during the growing and nongrowing season were positively correlated with air temperature (Ta), volumetric water content, friction velocity (u(*)), and vapor pressure deficit. The growing season FCH4 showed a significant quadratic polynomial relationship with the canopy conductance (G(w)) and gross primary production). FCH4 was significantly higher in the growing season than in the nongrowing season. The seasonal FCH4 was negatively correlated with soil temperature and net radiation (Rn) but not with volumetric water content and gross primary production. Ridge regression models indicated that Ta and u(*) explained 83% of the variation in the diel dynamics of FCH4 during the growing season and explained 72% of the variation during the nongrowing season. Rn accounted for 49% of variations of FCH4 at the seasonal scale. The temporal patterns and the environmental controlling factors revealed in this study may improve model parameterization for biosphere-atmosphere CH4 exchange simulation as well as the methane budget estimation.

Plain Language Summary Methane (CH4) is a very important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since 1750, and its impact is second only after carbon dioxide (CO2). However, we know much less about CH4 compared to CO2. Many basic but important questions about CH4 still remain unanswered, such as the diurnal and seasonal patterns of CH4 flux as well as the controlling factors of these temporal dynamics of CH4 flux. To answer these questions, we monitored the CH4 flux continuously in an alpine meadow ecosystem located on the eastern Qinghai-Tibetan Plateau, China, from June 2015 to December 2016. At the diurnal scale, we found that the CH4 uptake at nighttime was higher than at daytime. At the seasonal scale, we found higher CH4 uptake during summer than winter. We also found that CH4 fluxes were determined by air temperature and net radiation at the diurnal scale and seasonal scale, respectively. The temporal patterns and the controlling factors revealed in this study may help to improve the prediction of CH4 exchange processes and the estimation of methane budget from the regional to the global scale.