From particulate to mineral-associated carbon: Inputs drive soil carbon storage in tropical managed pastures

Improved management practices in degraded grasslands, have been promoted as effective strategies to restore soil organic carbon (SOC). However, the mechanisms controlling SOC sequestration and its upper storage limits remain poorly understood. We evaluated a long-term field experiment comparing a degraded pasture, a managed pasture with rotational grazing, liming, and nitrogen fertilization, and an adjacent native vegetation area. We investigated how pasture management modulates SOC stocks partitioned into particulate organic matter (POM) and mineral-associated organic matter (MAOM), microbial community structure (PLFA profiles), and their relationships with clay specific surface area and pedogenic oxides. POM stocks were higher under native vegetation than under pastures, whereas managed pastures exhibited higher MAOM stocks than native vegetation. PLFA profiles indicated that land-use conversion shifted microbial dominance from fungi in native vegetation to bacteria in pastures, reflecting contrasting pathways of POM decomposition and persistence. Degraded pastures exhibited microbial physiological stress, while managed pasture with higher soil pH, fertility, and forage productivity yielded MAOM with more microbial signatures. NanoSIMS analyses showed that SOC occurred as discrete micropatches with Al-, Fe-, and Si-bearing mineral phases, decoupled from clay specific surface area and pedogenic oxide abundance. Instead, improved management increased SOC coverage on clay particles. Overall, enhanced organic inputs and soil fertilization under managed pasture, together with animal trampling that fragments dead forage, created favorable microsites for microbial growth and accelerated the conversion of POM into MAOM. Organic inputs, rather than mineral surface availability alone, fostered MAOM formation, promoting SOC storage in managed pastures.