Soil Moisture Threshold of Methane Uptake in Alpine Grassland Ecosystems

ABSTRACT Methane (CH 4 ) uptake in alpine ecosystems is an important component of the global CH 4 sink. However, large uncertainties remain regarding the magnitude and spatial patterns of CH 4 uptake, owing to its extensive spatial variability, diverse controlling factors, and limited regional‐scale observations. Here, we investigated field ecosystem CH 4 uptake along a 3200‐km transect across various alpine grasslands on the Qinghai–Tibetan Plateau (QTP). We found a substantial spatial variation in in situ CH 4 uptake among alpine grasslands, with the highest rates in drier regions of the mid‐western QTP. Soil moisture was the most important factor controlling CH 4 uptake, exhibiting a remarkably low threshold of 6.2 ± 0.1 v/v %. Below this threshold, CH 4 uptake was constrained by soil moisture, moisture‐induced nitrogen limitation, and high temperatures. Above this threshold, CH 4 uptake was mainly limited by gas diffusion and low temperatures. By integrating grid predictors with a random forest model trained on 1851 field measurements encompassing both our observations and a regional synthesis across the QTP, we estimated a regional CH 4 uptake of 0.88 ± 0.020 Tg CH 4 year −1 from all alpine grasslands on the QTP. This higher estimate, primarily driven by alpine steppes, was significantly greater than current regional estimates from global CH 4 models. Our findings highlight the importance of CH 4 sink in dry alpine ecosystems characterized by low soil moisture, suggesting that the contribution of CH 4 sink in drylands may have been substantially underestimated in the current global CH 4 budget.