Changes in long-term land use alter deep soil microbial necromass and organic carbon stabilization

Carbon sequestration in grassland ecosystems plays an important role in alleviating global climate changes. However, the conversion of natural grassland to agricultural cropland has a profound implication for soil organic carbon (OC) sequestration, particularly regarding deep soil carbon stability. Here, we addressed the uncertainties surrounding deep soil OC mineralization by investigating the distribution and stabilization of OC pools in topsoil (0-20 cm) in comparison with that in deep soil (80-100 cm) after 11 and 40 years of agricultural cropland conversion from natural grassland at Hulunbuir, China. It was observed that the conversion substantially reduced the bulk OC in the deep soil, from 44.70 g kg^-1 in grassland to 8.76-6.22 g kg^-1 in agricultural cropland. Despite this decline, the contribution of mineral-associated OC (MAOC), conversion of microbial necromass C to bulk soil OC, and potential stability of OC increased, indicating a shift towards stabler soil OC forms in agricultural soils. The dissolved organic carbon of the topsoil in the agricultural cropland became more recalcitrant than that in the grassland, while the aliphatic carbon of the MAOC in the deep soil was increased. Although OC mineralization rates were lower in agricultural soils than in the grassland, the temperature sensitivity of OC decomposition (Q₁₀) increased. These findings underscore the importance of assessing soil OC stability under long-term land use changes, with implications for sustainable agricultural management and deep soil carbon's role in climate regulation.