Impacts of Abandoned Sloping Farmland on Soil Aggregates and Aggregate-Associated Organic Carbon in Rocky Desertification Areas of SW China

Abstract

Abstract Vegetation restoration after sloping farmland abandonment effectively sequesters soil organic carbon (SOC), with soil aggregates playing a pivotal role. However, the relationship between soil aggregates, aggregate-associated organic carbon (AAOC), and total SOC content in abandoned farmland with varying degrees of rocky desertification in karst regions remains unclear. In this study, we investigated the dynamic characteristics of the particle size distribution of soil aggregates, total SOC, and AAOC in abandoned sloping farmland (5 years, 10 years, and 15 years) with different levels of rocky desertification (no rocky desertification, potential rocky desertification, slight rocky desertification, and moderate rocky desertification) in a typical karst area. The results indicated that total SOC content in the 0-20 cm soil layer increased after abandonment in all levels of rocky desertification, peaking after 15 years. The abandoned sloping farmland with moderate desertification showed the best recovery effect. Vegetation restoration post-abandonment increased 5-10 mm aggregates while decreasing 1-2 mm and < 0.25 mm sizes. AAOC's contribution to total SOC significantly rose over time for 5-10 mm aggregates. Among sizes, > 1 mm aggregates strongly correlated with total SOC (p < 0.05). The growth of AAOC in 5-10 mm aggregates was the primary driver of total SOC increase. In summary, vegetation restoration following the abandonment of sloping farmland is an effective approach for enhancing total SOC content in slope farmland of varying degrees of rocky desertification. The observed changes are mainly attributed to the increase in 5-10 mm size aggregates and their associated AAOC.