Sensitivity of Methane Emissions to Later Soil Freezing in Arctic Tundra Ecosystems Journal of Geophysical Research: Biogeosciences

The atmospheric methane (CH4) concentration, a potent greenhouse gas, is on the rise once again, making it critical to understand the controls on CH4 emissions. In Arctic tundra ecosystems, a substantial part of the CH4 budget originates from the cold season, particularly during the “zero curtain” (ZC), when soil remains unfrozen around 0 °C. Due to the sparse data available at this time, the controls on cold season CH4 emissions are poorly understood. This study investigates the relationship between the fall ZC and CH4 emissions using long-term soil temperature measurements and CH4 fluxes from four eddy covariance (EC) towers in northern Alaska. To identify the large-scale implication of the EC results, we investigated the temporal change of terrestrial CH4 enhancements from the National Oceanic and Atmospheric Administration monitoring station in Utqiaġvik, AK, from 2001 to 2017 and their association with the ZC. We found that the ZC is extending later into winter (2.6 ± 0.5 days/year from 2001 to 2017) and that terrestrial fall CH4 enhancements are correlated with later soil freezing (0.79 ± 0.18-ppb CH4 day−1 unfrozen soil). ZC conditions were associated with consistently higher CH4 fluxes than after soil freezing across all EC towers during the measuring period (2013–2017). Unfrozen soil persisted after air temperature was well below 0 °C suggesting that air temperature has poor predictive power on CH4 fluxes relative to soil temperature. These results imply that later soil freezing can increase CH4 loss and that soil temperature should be used to model CH4 emissions during the fall. Grant no. NA16SEC4810008

大气甲烷（CH4）作为一种强效温室气体，其浓度正再度攀升，因此明确甲烷排放的调控机制至关重要。在北极苔原生态系统中，甲烷收支的相当一部分源自寒冷季节，尤其是“零幕期（zero curtain, ZC）”——此时土壤在0℃左右维持未冻结状态。由于当前该时段的观测数据较为匮乏，寒冷季节甲烷排放的调控机制仍未得到充分阐明。本研究借助阿拉斯加北部4座涡度协方差（eddy covariance, EC）塔的长期土壤温度观测与甲烷通量数据，探究了秋季零幕期与甲烷排放之间的关联。为明确该涡度协方差研究结果的大尺度意义，我们分析了美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration, NOAA）位于阿拉斯加州乌特恰维克（Utqiagvik）的监测站2001至2017年的陆地甲烷浓度增量的时间变化特征，及其与零幕期的关联。研究结果显示：2001至2017年间，零幕期的结束时间正逐年延后（速率为2.6±0.5天/年）；陆地秋季甲烷浓度增量与土壤冻结延后呈显著相关（未冻结土壤条件下，相关系数为0.79±0.18 ppb CH4·day⁻¹）。在2013至2017年的观测期内，所有涡度协方差塔的监测数据均表明，零幕期的甲烷通量显著高于土壤完全冻结后。当空气温度远低于0℃时，土壤仍维持未冻结状态，这说明相较于土壤温度，空气温度对甲烷通量的预测能力较弱。本研究结果表明，土壤冻结延后会加剧甲烷排放损失，因此在秋季甲烷排放模拟中应采用土壤温度作为关键参数。资助编号：NA16SEC4810008