Soil Moisture Threshold of Methane Uptake in Alpine Grassland Ecosystems

Methane (CH₄) uptake in alpine ecosystems is an important component of the global CH₄ sink. However, large uncertainties remain regarding the magnitude and spatial patterns of CH₄ uptake, owing to its extensive spatial variability, diverse controlling factors, and limited regional-scale observations. Here, we investigated field ecosystem CH₄ 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 CH₄ uptake among alpine grasslands, with the highest rates in drier regions of the mid-western QTP. Soil moisture was the most important factor controlling CH₄ uptake, exhibiting a remarkably low threshold of 6.2 ± 0.1 v/v %. Below this threshold, CH₄ uptake was constrained by soil moisture, moisture-induced nitrogen limitation, and high temperatures. Above this threshold, CH₄ 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 CH₄ uptake of 0.88 ± 0.020 Tg CH₄ 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 CH₄ models. Our findings highlight the importance of CH₄ sink in dry alpine ecosystems characterized by low soil moisture, suggesting that the contribution of CH₄ sink in drylands may have been substantially underestimated in the current global CH₄ budget.