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.