Changes in Particulate and Mineral Associated Organic Carbon with Land Use in Contrasting Soils

Abstract. Soil organic carbon (OC) is the largest terrestrial C stock and soils' capacity to preserve OC varies with many factors including land use, soil type and depth. We investigated the effect of land use change on particulate organic matter (POM) and mineral-associated organic matter (MOM) in soils. Surface (0–10 cm) and sub-surface (60–70 cm) soil samples were collected from paired-sites (native and cropped lands) of four contrasting soils. Bulk soils were isolated into POM and MOM fractions, which were analysed for mineralogy, OC and nitrogen, isotopic signatures and ¹⁴C content. POMs of surface soils were relatively unaffected by land use change, possibly because of continuous input of crop residues, while corresponding POM in sub-surface lost more OC. In surface soils, oxides-dominated MOM lost more OC than phyllosilicates- and quartz -dominated MOM, which is attributed to diverse OM input and the extent of OC saturation limit of soils. In contrast, oxides-associated fractions were less affected in the sub-surface soils than the other two MOM fractions, possibly due to OC protection via organo–mineral associations. Changed isotopic signature (linked with vegetation) across the fractions suggested that fresh crop residues constituted the bulk of OM in surface soils (supported by greater ¹⁴C). Increased isotopic signatures and lower ¹⁴C in sub-surface MOM fractions suggested the association of more microbially processed, aged OC in oxides-rich fractions than other MOMs. Results reveal that quantity and quality of OC after land use change was influenced by the nature of C input in surface soils and by mineral-organic association in sub-surface soils.