Differentiating biological and chemical factors of top and deep soil carbon sequestration in semi-arid tropical Inceptisol: an outcome of structural equation modeling

Though soils sequester large amounts of carbon (C), variations in physical and chemical characteristics of top and deep layers necessitate the study of factors governing topsoil and deep soil C sequestration to predict land-use changes to alleviate climate change. Land-use systems involving pasture, trees, trees  pasture and fallow were considered. The upper soil (0–15 cm) had ∼12, 34 and 59% higher microbial biomass C than the 15–30, 30–45 and 45–60 cm layers, respectively. Fluorescein diacetate (FDA) and dehydrogenase activities had similar trends. Across the land uses, topsoil layers had ∼17% lower silt + clay (s + c) content than deep layers. Amorphous iron content significantly increased with soil depth. In the top two soil layers, s + c accounted for ∼19–30% of total soil organic carbon (SOC); in the next two layers s + c could store >30% of total SOC. Stepwise regression analysis revealed FDA to be the most significant biological driver for SOC sequestration. Structural equation modeling showed that biological factors controlled C sequestration in topsoil layers, while s + c and amorphous iron were the major factors of C sequestration in deep layers. Current land uses are largely deficient of SOC and have the potential to store an additional22 Mg CO2e per ha