Soil organic matter enhances aboveground biomass in alpine grassland under drought

Increasing the resilience of plant productivity to drought is crucial for provisioning ecosystem services. Although soil organic matter (SOM) promotes plant growth, whether SOM mitigates aboveground biomass (AGB) loss due to drought in alpine grasslands remains unclear. Here, we evaluated the link between AGB and SOM across 209 alpine grassland sites along a 3,500-km aridity gradient on the Tibetan Plateau. We observed that AGB decreased in response to increasing aridity severity only when the aridity level was above a threshold of 0.37. We further found a stronger positive relationship between AGB and SOM in more arid conditions than in less arid ones, with an abrupt increase beyond an aridity threshold of 0.64. Our results confirm that SOM alleviates drought stress on AGB, mainly by reducing bulk density, increasing fine fraction aggregates, and increasing cation exchange capacity. The identified aridity thresholds and mechanisms emphasise the importance of soil carbon sequestration strategies for biomass production and climate change mitigation in arid areas.