Plant species richness mediates the responses of microbial necromass carbon accumulation to climate aridity in alpine meadows

Abstract Plant diversity loss caused by climate change decreases soil organic carbon (SOC) sequestration, but the mechanism involved remains unclear. Investigating the changes in soil microbial necromass carbon (MNC) accumulation along a climate–plant species diversity gradient can help clarify this mechanism, as it is crucial for the stability of SOC. We conducted large‐scale sampling across a 2500‐km transect through grasslands on the Tibetan Plateau to investigate the MNC content and its contribution to SOC at depths of 0–20 and 20–40 cm in response to environmental and plant species diversity gradients. Plant species richness, plant biomass, and the proportion of above‐ground biomass accounted for by Cyperaceae (sedges) increased with decreasing aridity along the gradient of rising altitude. Meanwhile, the MNC content, its contribution to SOC, and the ratio of fungal to bacterial necromass carbon in both soil layers also increased with decreasing aridity. These results indicate that, in addition to changing climate factors along the altitudinal gradient, plant species richness plays a pivotal role in facilitating soil MNC accumulation and thus the accrual of SOC. Structural equation modelling revealed that plant diversity increases the MNC content by enhancing the abundance of Cyperaceae in the grassland. A higher abundance of Cyperaceae significantly increased the root biomass and level of rhizodeposition, thereby increasing the diversity and activity of soil microbes, and ultimately the accumulation of MNC. Synthesis . We conclude that changes in plant species richness in response to aridity dominate the MNC content and its proportion in SOC on the Tibetan Plateau. These findings demonstrate the importance of incorporating plant species diversity into conservation efforts in grasslands to mitigate the negative impacts of climate change and human activities on SOC.