Pronounced non-growing season warming and changes in soil freeze-thaw (F-T) cycles can dramatically alter net methane (CH₄) exchange rates between soils and the atmosphere. However, the magnitudes and drivers of warming impacts on CH₄ uptake in different stages of the F-T cycle are poorly understood in cold alpine ecosystems, which have been found to be a net sink of atmospheric CH₄. Here, we reported a year-round ecosystem daily CH₄ uptake in an alpine meadow on the Qinghai-Tibetan Plateau after a five-year warming experiment that included a control, a low-level warming treatment (+2.4℃ at 5 cm soil depth), and a high-level warming treatment (+4.5℃ at 5 cm soil depth). We found that warming shortened the F-T cycle under the low-level warming and soils did not freeze under the high-level warming. Although both warming treatments increased the mean CH₄ uptake rate, only the high-level warming significantly increased annual CH₄ uptake compared to the control. The warming-induced stimulation of CH₄ uptake mainly occurred in the cold season, which was mostly during spring thaw under low-level warming and during the frozen winter under high-level warming due to a longer period with thawed soil. We also found that warming significantly stimulated daily CH₄ uptake mainly by reducing near-surface soil water content in the warm season, whereas both soil water content and temperature controlled daily CH₄ uptake in different ways during the autumn freeze, frozen winter, and spring thaw periods of the control. Our study revealed a strong warming effect on CH₄ uptake during the entire F-T cycle in the alpine meadow, especially the unfrozen winter. Our results also suggested the important roles of soil pH, available phosphorus, and methanotroph abundance in regulating annual CH₄ uptake in response to warming, which should be incorporated into biogeochemical models for accurately forecasting CH₄ fluxes under future climate scenarios.

Ecosystem methane flux, and soil moisture and soil temperature at 5 cm, 15 cm, 30 cm soil depths were observed for an entire F-T cycle from June 2019 to June 2020 in a typical alpine meadow on the Qinghai- Tibetan Plateau (QTP), where a long-term warming experiment was established in 2014. Soil temperature and VWC at all three depths from June 2019 to June 2020 were analysed and used in this study, and soil temperature at 5 cm depth from June 2016 to June 2021 was included in the supplemental materials to support the results. Precipitation was recorded at 30-min intervals by the Hongyuan county weather station. The warming experiment included three treatments: ambient temperature (CK), low-level warming (LW, +2.4℃ at 5 cm soil depth), and high-level warming (HW, +4.5℃ at 5 cm soil depth). The measurement intervals between treatments were 5−20 mins and all data were daily averaged for data analysis in the paper. Methane uptake was gap-filled by using a random forest algorithm in R. 

Soil F-T cycle for each plot of all treatments were divided into the following stages: thawed, freezing, frozen, and thawing (Figure 1d) according to the definitions used in previous studies (Zona et al., 2016; Taylor et al., 2018; Arndt et al., 2019, Bao et al., 2021). Specifically, the thawed stage was when daily mean soil temperature at 5 cm depth (daily mean T_soil at 5 cm) was > 0.75℃ for at least five consecutive days. The frozen stage was when daily mean T_soil at 5 cm was < −0.75℃ for at least five consecutive days. The freezing and thawing stages were defined as freezing-frozen and frozen-thawing transition periods, respectively, when daily mean T_soil at 5 cm was between −0.75℃ and 0.75℃ (Figures 1b, d and Table S1). According to these F-T stages, all of which occurred in the control plots, we divided our measurements into four seasons: autumn freeze (Saf), frozen winter (Sfw), spring thaw (Sst), and warm season (WS). The cold season (CS) included autumn freeze, frozen winter, and spring thaw.

Mineral soil samples were obtained from depths of 0−10 cm, 10−20 cm, and 20−40 cm from each plot in August 2020 for soil chemical and microbial analyses including soil pH, available phosphorus (AP), soil total carbon (TC) and nitrogen (TN) contents, soil microbial biomass carbon (MBC) and nitrogen (MBN) and methanotrophs abundance.