Effects of Substituting Synthetic Fertilizer with Organic Materials on Soil Organic Carbon Sequestration and Aggregate Size Distribution in Red Soil in South China

Substitute synthetic fertilizer by organic amendments practice was considered as an effective practice to improve soil organic carbon (SOC) pool. The comparison among different organic substitution (OS) practices on SOC sequestration and aggregate size distribution in cropland has still been limited at present. We aimed to investigate the influences of different OS practices on the SOC storage, fraction, and aggregate size distribution in red soil in South China. In this study, therefore, the OS practice by using maize straw, biochar, biogas residue, and cow dung were compared with the conventional fertilization (CK) in sweet maize cropping system to substitute 20% and 50% synthetic nitrogen fertilizer in the Pearl River Delta plain. We compared the total soil organic carbon (TOC) content and fractions, soil carbon pool management index (CMI), carbon sequestration index (RI), aggregate particle size distribution, mean weight diameter (MWD), and aggregate-associated SOC content. Results showed that the OS practices increased the TOC contents by 11.51-299.77% and the CMI by 10.14-190.13%, respectively, compared to CK. Therein, the biochar-based OS practices significantly increased the TOC contents by 138.53% and 299.77% under 20% and 50% substitution ratio (P < 0.05), respectively, due to the higher C/N ratio. Also, the biochar-based OS practice promoted the accumulation of SOC in inert organic carbon pool and significantly increased the RI by 76.02% compared to CK (P < 0.05), while the other treatments primarily increased the SOC contents in active organic carbon pool. Moreover, the OS practices promoted to the formation of small macroaggregate and the migration of SOC into large macroaggregate and small macroaggregate in red soil. From the perspective of aggregate protection, the SOC sequestration capacity under the OS practices based on maize straw, biogas residue and cow dung were lower than the CK, because the more SOC were stored in large macroaggregate and small macroaggregate. In summary, we indicated that the biochar-based OS practice at the 50% substitution ratio would be the suitable OS practice for improving the SOC storage and sequestration in red soil in South China. This study provided valuable information for the development of clean agricultural production in typical subtropical areas in East Asia.