Distinct physical stabilization of organic carbon as it is protected in the silt plus clay fraction of a shallow loamy sand soil in a woody-encroached savanna grassland

Abstract Aims Although there is increasing evidence that the storage of soil organic carbon (SOC) is affected by woody plant encroachment in savannas, the underlying physical stabilization mechanisms controlling SOC partitioning among aggregate fractions remain poorly resolved, particularly in sandy soils. We assessed changes in SOC concentration and SOC stocks (SOCs), soil water‑stable aggregates, and physical protection mechanisms within aggregate size fractions of a woody-encroached savanna grassland in South Africa. Methods We sampled the topsoil layer of open grassland and woody-encroached grassland loamy sand soil. The collected bulk soil samples were separated into four aggregate size fractions: large macroaggregates (LM), small macroaggregates (SM), microaggregates (M) and silt plus clay (SICL) by wet sieving. Afterwards, bulk soil and separated aggregate fractions were analysed for SOC. Results On average, SOC was 45% higher in the woody-encroached grassland compared to open grassland. Correspondingly, SOCs in the woody-encroached grassland were 29% higher relative to open grassland. A larger proportion of the aggregates were dominated by SM and M, with a greater proportion of the SOC stabilized within the SICL of the woody-encroached grassland soil. Conclusions These findings indicate distinct mechanisms of SOC stability, storage and protection in woody-encroached grasslands linked to enhanced retention within the SICL fraction, facilitated by greater surface area and more exchange sites for SOC adsorption. The entrapment of SOC within the SICL fraction is desirable because it is highly stable, has a longer turnover time and contributes to long-term storage of SOCs in savanna grassland soils.