Mechanisms of the Effects of Carbon Source Addition on Carbon Transport and Sequestration in Purple Soil Sloping Farmland under Natural Rainfall

[Objective] Degradation of purple soil and water loss is serious, and it is difficult to increase and sequester carbon in the soil. The aim of the study was to explore the impact of carbon source addition on carbon sequestration of purple soil sloping farmland. [Methods] Purple soil sloping farmland in Sichuan Province was selected as the research object. Six treatments were set up on 10° purple soil slopes, including no fertilization (CK), fertilizer+straw returning (T1), fertilizer+straw biochar (T2), fertilizer+cow dung (T3), fertilizer+organic soil matrix (T4), fertilizer+biochar cow dung and mushroom slag compost (T5), and each treatment was repeated three times. The change characteristics of runoff, sediment and carbon content under rainfall and different carbon sources were analyzed. [Results] 1) Rainfall >30 mm and short-term heavy rainfall (16.1 mm/h) could lead to surface runoff. The addition of carbon sources notably decreased surface runoff. In comparison with CK, the runoff reduction benefits of T2, T3, and T5 reached 38.64%, 38.61%, and 62.95%, respectively. The sediment erosion in different treatments was observed to follow the order of T1>T4>CK>T2>T3>T5, with the highest reduction being 58.99% compared to CK. 2) The loss of organic carbon through surface runoff was primarily attributed to sediment organic carbon, which accounted for 67.47%—90.43% of the total organic carbon loss. T2, T3, and T5 were effective in reducing the transport of organic carbon, resulting in a total organic carbon loss reduction of 21.74%, 19.40%, and 49.28%, respectively, compared to CK. Furthermore, the carbon fractions of T5, including dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), dissolved total carbon (DTC), and sediment organic carbon, exhibited a reduction in transport fluxes by 70.88%, 70.92%, 37.52% and 60.07%, respectively, compared to CK. 3) PLS-PM showed that the carbon sequestration of soil organic carbon was influenced to varying degrees by rainfall characteristics, soil physicochemical properties, and the addition of different carbon sources. [Conclusion] Carbon source addition (T2, T3, T5) can reduce the total amount of organic carbon loss. T5 can increase organic carbon sequestration; T2 and T3 can reduce the loss of each carbon component of surface runoff, but also reduce organic carbon sequestration. Addition of carbon source can increase carbon content and reduce carbon transport, but reduce carbon sequestration. The results need to be verified by long-term experiments. This study is of significant importance for enhancing the fertility of purple soil, maintaining soil carbon balance, and reducing soil carbon loss.