Mechanisms controlling the stability and sequestration of mineral associated organic carbon upon erosion and deposition

Mineral associated organic carbon (MAOC) accounts for a substantial portion of soil organic carbon (SOC) due to its tendency for physicochemical bonding with mineral surfaces. However, the mechanisms controlling MAOC stability and sequestration upon erosion and deposition remain unclear. We collected samples from topsoil (0-20 cm) and subsoil (60-80 cm) in an eroding loess landscape in Northwest China. SOC was fractionated into particulate organic carbon (POC) and MAOC, and OC retained by reactive mineral phases (calcium, iron, and aluminum) were studied. Adsorption-desorption experiments were performed to quantify the OC capture capacity, and thermogravimetric techniques were used to determine the thermal stability of MAOC. Although POC dominated in the soil mass distribution (>60 %) in both sites, the MAOC in the topsoil and subsoil in the depositional sites was characterized by 1.38-3.88 times higher OC contents than the eroding sites. Furthermore, depositional sites featured 1.19-1.45, 1.43-1.72, and 1.28-1.96 times higher maximum adsorption capacity, specific mineral surface area, and silt + clay contents, respectively, than these of the eroding sites, indicating high OC sequestration capacity in the soils of depositional sites. Erosion directly transports soil materials (e.g., OC and fine particles) to the depositional sites, when coupled with the high OC capture capacity, this leads to an increase in the OC content from 0.50 to 1.37 at the eroding sites to 2.19-3.28 mg g(-1) at the depositional site. The content and chemical structure of organic-metal complexes had minor differences, however, MAOC in the depositional sites was observed to have excellent thermal stability. Calcium-, and silt + clay-associated OC (CaOC and CS-OC) were positively correlated with the MAOC stability and content. Overall, our findings highlight the important role of Ca-OC and CS-OC in determining the stability and sequestration of MAOC in eroding loess landscapes.