Hump-Shaped Relationship Between Microbial Carbon Use-Efficiency and Soil Organic Carbon in Alpine Grasslands

Microbial carbon use efficiency (CUE) mediates soil organic carbon (SOC) turnover, yet its drivers and role in carbon stabilization in alpine grasslands remain unclear. Using data from 45 sites across the Qinghai-Tibetan Plateau and ¹⁸O-water to analyze substrate-independent CUE, this study reveals a hump-shaped relationship between microbial CUE and SOC. Below an SOC threshold of 54 g C kg^-1, increases in precipitation, plant biomass, soil total nitrogen, and phosphorus contents raise CUE, supporting microbial in vivo turnover. High CUE implies more carbon incorporation into microbial cells, facilitating mineral-associated organic carbon (MAOC) formation via sorption on clays. This process is reflected by rising MAOC/SOC ratios with increasing clay content. Above the SOC threshold of 54 g C kg^-1, phosphorus limitation and high clay content suppress CUE, with the influence of clay amplifying under wetter conditions. In carbon-rich soils, the newly discovered inverse correlation between MAOC and microbial efficiency in alpine systems highlights that CUE only reflects the potential for carbon sequestration. These findings emphasize the need to balance soil carbon accumulation and nutrient availability for microorganisms to maintain the soil carbon storage capacity of climate-sensitive ecosystems.