Physicochemical fractionation reveals increased soil organic carbon storage in a wolfberry orchard under cover cropping

Background and aims: Characterizing the responses of soil organic carbon (SOC) fractions to agricultural management practices is essential for understanding SOC stability in agroecosystems. To establish a rational soil management regime for wolfberry (Lycium barbarum L.) production, this study investigated the long-term effects of cover cropping with manure on the storage of SOC and its fractions in a wolfberry orchard in Ningxia, Northwest China. Methods: A field experiment was conducted using wolfberry grown as a monocrop or cover cropped with forage radish under zero, moderate, and high rates of animal manure. Results: After seven years of cover cropping, SOC concentrations in the topsoil (0–20 cm) were higher than those under monocropping, and the difference was most pronounced under moderate manure application. The annual SOC accumulation rates reached ~ 1.00 t haa under cover cropping with moderate and high manure application, and the SOC storage efficiency of exogenous organic carbon input was 33.0%. Cover cropping also increased the concentrations of unprotected coarse particulate organic carbon fraction, as well as physically protected particulate organic carbon, chemically protected clay-sized, and biochemically protected silt-sized fractions in the topsoil. Unprotected SOC was the predominant form of organic carbon accumulated. A positive linear association emerged between SOC stock and exogenous organic carbon input in the topsoil. Despite weak responses of SOC and its fractions in the subsoil (20–40 cm), their trends were basically consistent with those observed in the topsoil. Conclusions: Organic carbon inputs from multiple sources boosted SOC storage in the wolfberry orchard. Cover cropping with moderate manure application effectively improved SOC concentrations in the coarse and intra-aggregate particulate fractions, showing great potential for enhancing SOC storage. Future studies should delve deeper into the response mechanisms of SOC fractions from a microbiological perspective to decipher the role of cover crops and manure in the accumulation and transformation of SOC fractions.