Long-term conservation tillage enhances soil organic carbon accumulation in rice-wheat rotation systems by improving aggregate stability and reducing carbon flow from macroaggregates to microaggregates

Although previous studies have extensively investigated the relationship between aggregate stability and soil organic carbon (SOC) dynamics under long-term conservation tillage, the intensity and direction of carbon (C) flow among aggregates remain poorly quantified using the ¹³C natural abundance technique. To address this gap, a rice-wheat rotation study (initiated in 2011) was performed to investigate changes in cation exchange capacity (CEC), pH, SOC content, δ¹³C, and glomalin-related soil protein (GRSP) levels in soil aggregates of >2 mm, 0.25-2 mm, 0.053-0.25 mm, and <0.053 mm aggregate fractions in size. The soil samples used in this study were collected in 2020. The experiment involved four treatments: no-tillage with straw returning (NTS), no-tillage without straw returning (NTNS), conventional tillage with straw returning (CTS), and conventional tillage without straw returning (CTNS). After 9 years of rice-wheat rotations, conservation tillage treatments (NTS and NTNS) significantly improved the aforementioned soil properties compared with conventional tillage treatments (CTS and CTNS). Specifically, NTS and NTNS significantly enhanced CEC and reduced pH in >2 mm, 0.25-2 mm, and 0.053-0.25 mm aggregates. These changes promoted the generation of GRSP. NTS and NTNS increased the proportion of >2 mm and 0.25-2 mm aggregates by 7.6 %-26.2 % and 2.7 %-12.5 %, respectively, and significantly increased the mean weight diameter and geometric mean diameter by 23.5 %-56.9 % and 54.8 %-80.9 %. In addition, NTS and NTNS significantly increased SOC content in >2 mm, 0.25-2 mm, and 0.053-0.25 mm aggregates. Based on the assessment of ¹³C values, NTS and NTNS significantly reduced soil δ¹³C by 5.9 %-11.7 %, 3.8 %-9.2 %, and 5.3 %-8.2 % in >2 mm, 0.25-2 mm, and 0.053-0.25 mm aggregates, respectively. This reduction indicates that long-term conservation tillage significantly reduces C flow from macroaggregates to microaggregates. Furthermore, partial least squares path modeling validated that after 9 years of rice-wheat rotations, conservation tillage treatments increased CEC and decreased pH in >2 mm, 0.25-2 mm, and 0.053-0.25 mm aggregates, thereby promoting GRSP production, increasing aggregate stability, and enhancing SOC accumulation across the three fractions. In conclusion, long-term conservation tillage is an effective strategy for promoting SOC sequestration in rice-wheat rotation systems.