Dual role of silt and clay in the formation and accrual of stabilized soil organic carbon

Soil organic carbon (SOC) is the largest terrestrial carbon (C) pool and is vulnerable to climate and land-use changes. Promoting the stabilization of SOC will reduce climate change-induced C losses. Mineral-associated organic carbon (MAOC), formed by the association of organic carbon with silt- and clay-sized minerals, is the major stabilized SOC fraction and key to sustaining soil health and mitigating climate change. However, the role of silt and clay in MAOC formation remains unclear in dryland ecosystems where microbes and plants are frequently under water stress. The current paradigm assumes that the main role of silt and clay is to adsorb and aggregate organic compounds. In a semi-arid environment, using a naturally occurring gradient of increasing soil silt and clay content partially due to aeolian dust inputs, we show that silt and clay also enhanced microbial decomposition of plant C inputs and microbial turnover, increasing microbial C inclusion into the MAOC pool. Finer-textured soils had higher soil water availability and higher volume of habitat available to microorganisms. The enhanced microbial processing interacted with changes in plant C inputs to further control the relative contribution of MAOC to SOC. Our results suggest SOC models should include soil textural effects on microbial activities and microbial C production in addition to physical protection of SOC. Our study also suggests that aeolian dust inputs can increase soil silt and clay contents and improve nutrient availability in dryland ecosystems, potentially mitigating the SOC loss under climate change and increasing the resilience of ecosystems to drought.