Alterations of soil aggregates and intra-aggregate organic carbon fractions after soil conversion from paddy soils to upland soils: Distribution, mineralization and driving mechanism

Investigating the impacts of soil conversion on soil organic carbon (OC) content and its fractions within soil aggregates is essential for defining better strategies to improve soil structure and OC sequestration in terrestrial ecosystems. However, the consequences of soil conversion from paddy soil to upland soil for soil aggregates and intra-aggregate OC pools are poorly understood. Therefore, the objective of this study was to quantify the effects of soil conversion on soil aggregate and intra-aggregate OC pool distributions. Four typical rice-producing areas were chosen in North and South China, paired soil samples (upland soil converted from paddy soil more than ten years ago vs. adjacent paddy soil) were collected (0–20 cm) with three replicates in each area. A set of core parameters (OC preservation capacity, aggregate carbon (C) turnover, and biological activity index) were evaluated to assess the responses of intra-aggregate OC turnover to soil conversion. Results showed that soil conversion from paddy soil to upland soil significantly improved the formation of macro-aggregates and increased aggregate stability. It also notably decreased soil intra-aggregate OC pools, including easily oxidized OCa (EOCa), particulate OCa (POCa), and mineral-bound (MOCa) OC, and the sensitivity of aggregate-associated OC pools to soil conversion followed the order: EOCa (average reduction of 21.1%) > MOCa (average reduction of 15.4%) > POCa (average reduction of 14.8%). The potentially mineralizable C (C₀) was significantly higher in upland soil than in paddy soil, but the corresponding decay constant (k) was lower in upland soil than in paddy soil. Random forest model and partial correlation analysis showed that EOCa and pH were the important nutrient and physicochemical factors impacting k of C mineralization in paddy soil, while MOCa and C-related enzyme (β-D-cellobiohydrolase) were identified as the key factors in upland soil. In conclusion, this study evidenced that soil conversion from paddy soil to upland soil increased the percentage of macro-aggregates and aggregate stability, while decreased soil aggregate-associated C stock and k of soil C mineralization on a scale of ten years. Our findings provided some new insights into the alterations of soil aggregates and potential C sequestration under soil conversion system in rice-producing areas.