Transforming slope cropland into terraced cropland to enhance the retention of labile SOC fractions by aggregates in southwestern China

Improving agricultural carbon sinks is an important approach to mitigating climate change and ensuring grain security. The converting of slope cropland into terraced cropland is an effective measure to promote the development of agriculture in mountainous areas. Studying the potential and mechanism of carbon sequestration of terraces assists land management strategies to address climate change. Therefore, we analyzed variations in labile SOC fractions accumulation and distribution in 132 soil aggregate samples from 3, 6, 9, 13, and 20 years of terrace development (rainfed cropland (CL) and orchard (OR)) and slope cropland in the Sichuan Basin. We observed that the contribution rate of SOC fraction to SOC was the highest in aggregates with >2 mm (0.6 %–28.2 %). Over 20 years, the SOC and labile SOC fractions of terraces initially decreased, then increased, and finally gradually declined. Compared with slope cropland, the dissolved organic carbon (DOC), particulate organic carbon (POC), and microbial biomass carbon (MBC) in the aggregates of terraces increased by 1.5 %, 2.3 % and 28. 8 %. Compared with OR, CL promotes the content of SOC, EOC, DOC and MBC in aggregates, increased by 6.1 %, 3.6 %, 3.6 %, and 28.6 %, respectively. Age, aggregate stability, and the interaction between age and land use patterns were the main factors for SOC accumulation. Path analysis indicated that both soil physical properties and aggregate stability have significant influences on SOC accumulation, with total effects of 0.6 and 0.7, respectively, whereas temperature and precipitation reduced sequestration (total effect = −0.2). Therefore, terracing promotes the physical protection of labile SOC fractions by aggregates, and contributes to the improvement of the agricultural carbon sink.