DOI: 10.5061/dryad.gtht76hp4
title: Data from: Increased annual methane uptake driven by warmer winters in an alpine meadow

description: 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.
description: 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 &gt; 0.75℃ for at least five
 consecutive days. The frozen stage was when daily mean T_soil at 5 cm was
 &lt; −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.