Effects of long-term deep tillage, straw returning and nutrient management on organic carbon mineralization of fluvo-aquic soil

Soil organic carbon (SOC) mineralization is closely related to the terrestrial ecosystem carbon cycle and global climate change. Reasonable tillage and nutrient management can be adopted to improve the carbon accumulation and sequestration potential in soil. It is urgent to explore the relationship between tillage-nutrient management and SOC stability in the process of soil biochemistry, within the context of carbon sequestration and emission reduction. Therefore, this study aims to clarify the effects of tillage and nutrient management on SOC mineralization in fluvo-aquic soil of North China. A 15-year long-term positioning experiment was carried out on the tillage and nutrient management (2007-2022). Two main treatments were set: rotate plow without straw return (RP-S), and deep plow with straw return (DP+S). Three secondary treatments were set: controlled fertilization (CK), Conventional fertilization (CON), and Optimized fertilization (OPT), with a total of six treatments. Soil samples were collected at 0-20 cm depth in October 2022. Some parameters were measured, including the contents of SOC and activated carbon components, as well as the carbon pool management index. SOC mineralization rate was determined by the incubation method. A first-order kinetic model was used to calculate the potential mineralization and turnover rates. The structural equation model was fitted to reveal the turnover and sequestration of SOC under different tillage and nutrient management. The experimental results show that the C/N (SOC and TN ratio) generally shared a decreasing trend under the condition of RP-S, while the C/N generally shared an increasing first and then decreased trend under the condition of DP+S. Compared with the RP-S, DP+S treatments increased the contents of activated carbon components in soil, and the carbon pool management index (CPMI) increased significantly. Compared with CON, OPT significantly increased the content of SOC, with incremental rates of 12.35%. OPT significantly increased the readily oxidizable carbon (ROC) and CPMI with the condition of DP+S. SOC mineralization rates were the highest in the 1 d and then decreased rapidly. After 10 d incubation, SOC mineralization rates decreased slowdown until it stabilized. The SOC mineralization rate was in agreement with the logarithmic function. Nutrient management enhanced the mineralization rates of SOC in the fluvo-aquic soils in the following order: OPT, CON, CK. SOC mineralization rates in all treatments were consistent with the first-order kinetic model. DP+S significantly increased the cumulative mineralization rate (value of C₀/SOC) by 23.59% (C₀ is potential mineralizable organic carbon content in soil), while the OPT significantly reduced the value of C₀/SOC by 22.12%. The accumulative mineralization of SOC (C_t is accumulation of soil organic carbon mineralization during cultivation time t) was significantly and positively correlated with the SOC, activated carbon components, and soil carbon pool management index (P<0.01), both of which were significantly and positively correlated with Potential mineralizable of SOC (C₀) in the fluvo-aquic soils (P<0.05). Tillage management, microbial biomass carbon (MBC), and ROC were the direct factors of SOC mineralization and sequestration potential in soil. Tillage and fertilization management dominated the SOC mineralization by the contents of SOC and activated carbon components, then impacting soil carbon sequestration potential. According to the SOC accumulation content, the direct positive impact of long-term deep plow with straw return treatment on soil carbon sequestration potential can be fully counteracted by the indirect negative effect of the increase in SOC and activated carbon components on soil carbon sequestration potential. In conclusion, Long-term DP+S with OPT significantly improved the stability of soil structure and the SOC sequestration potential. Optimal fertilization reduced the cumulative SOC mineralization rate of soil and then enhanced the SOC accumulation and sustainable utilization of farmland resources. The finding can also provide ideal farmland management to optimize the combination of tillage and fertilization in fluvo-aquic soil of North China.