Incorporating Soybean Crop and Fallow Practice Measures Increases Soil Organic Carbon Storage by Improving Soil Aggregate‐Associated Characteristics: A 12‐Year Field Experiment in China

ABSTRACT Soil aggregates' physical protection is an important mechanism for soil organic carbon (SOC) storage, and this process is sensitive to various agricultural production measures. However, knowledge gaps regarding the aggregates' physical protection mechanism when long‐term incorporating soybean crop and fallow practice measures that affect SOC storage remain unclear. Therefore, a 12‐year experiment clarifies the effects of soybean and fallow practices (winter wheat‐summer soybean, WS; winter wheat‐summer fallow, WF; and winter wheat‐summer maize, as a control, CK) on SOC storage (the parameters included: SOC stock, soil particulate organic carbon (POC), soil mineral associated organic carbon (MOC), soil microbial biomass organic carbon (MBC), and accumulative CO 2 emissions), soil aggregate‐associated characteristics (the parameters included: aggregate size class, mean weight diameter (MWD), SOC content, and aromatic‐C and aliphatic‐C distributions), as well as their relationships on the soil profile (0–20 and 20–40 cm). Both WS and WF significantly increased the SOC stock (0–20 cm: 16.9% and 15.3%; 20–40 cm: 38.4% and 20.8%) and MBC (0–20 cm: 26.7% and 22.3%; 20–40 cm: 61.4% and 35.5%) compared to CK. WS and WF also significantly reduced accumulative CO 2 emissions by 27.3% and 49.3%, respectively, compared to CK. Moreover, WS and WF both significantly increased MWD (0–20 cm: 25.8% and 21.6%; 20–40 cm: 22.4% and 12.3%) compared to CK. The aromatic‐C and SOC content in aggregates significantly increased with WS and WF at both 0 and 20 cm and 20 and 40 cm, compared to CK. Additionally, the MBC and MWD are the key influencing factors of SOC storage, as revealed by a structural equation model. Thus, WS and WF could enhance the physical protection of aggregates, promote SOC stock, and enhance agricultural sustainability.