Objective A 38-year location experiment was conducted to investigate the primary mechanisms for enhancing carbon sequestration in red paddy soils, aiming to establish a theoretical foundation and offer technical guidance for improving the agricultural and ecological functions of these soils.

Methods The long-term field experiment, initiated in 1981 at the Science and Technology Park of Jiangxi Agricultural University, encompassed five treatments: a control with no fertilization (CK) and four fertilization treatments under the same annual total NPK input. These fertilization treatments included chemical NPK fertilizers (F), total rice straw return combined with NPK fertilizers (SF), milk vetch return with NPK fertilizers (MF), and a combination of all these practices (MSF). Following the late rice harvest in 2019, soil samples were collected from the 0−20 cm plow layer to determine total organic carbon content (SOC). Particle-size and density fraction methods were utilized to analyze organic carbon components within aggregates of varying sizes. Structural equation modeling (PLS-PM) was employed to explore the pathways influencing SOC sequestration.

Results Long-term fertilization significantly (P<0.05) influenced soil aggregate distribution and the content of SOC. Compared to F, all the SF, MF and MSF treatments increased the proportion of macroaggregates by 13.3%−26.6%, mean weight diameter by 6.2%−31.7%, and SOC by 19.2%−37.1%, MSF treatment accumulated the highest SOC (23.53 mg/kg) among the treatments. Long-term application of organic fertilizers led to significant alterations (P<0.05), relative to CK and F, in components of each organic carbon pool. MSF treatment exhibited the highest oxidizable organic carbon (2.5 g/kg) and microbial biomass carbon contents (1.5 g/kg), while MF treatment. had the highest dissolved organic carbon (0.2 g/kg). Regardless of fertilization or not, the increased SOC was primarily sequestered as the fine-grained organic carbon fraction within macroaggregates, accounting for an average of 38.1% of the total organic carbon content. The sequestered SOC values were positively (P<0.001) correlated with the total SOC in macroaggregates but negatively (P<0.001) correlated with the total SOC in microaggregates. PLS-PM revealed that the organic carbon content of each carbon fraction within macroaggregates was significantly (P<0.001) affected by the average mass diameter of the aggregates and microbial biomass carbon, while microbial biomass carbon also promoted the mineral binding state in microaggregates.

Conclusions The long-term combined application of organic and chemical fertilizers promotes the formation and stability of soil macroaggregates, and significantly increases total SOC concentration. Improved aggregate stability and microbial biomass carbon substantially boost the storage of various carbon fractions within macroaggregates, thereby greatly enhancing SOC sequestration in red paddy soils. These factors are likely the key pathways influencing SOC sequestration in red paddy soils. The combination of straw and milk vetch with chemical fertilizers represents a relatively effective fertilization strategy for enhancing carbon sequestration in red paddy soils.