Biochar addition enhances soil aggregate stability by modulating soil internal forces: The critical role of readily oxidized organic carbon and particulate organic carbon

Abstract. Degradation of soil structure and function is a significant obstacle to sustainable agriculture, yet using biochar is known to be an effective way of increasing soil organic carbon and boosting structure. Aggregate stability is greatly influenced by soil internal forces (SIFs), such as van der Waals, hydration, and electrostatic forces. However, it is still not clear which specific organic carbon components play a significant role in the stability of aggregates and the action of soil internal forces. In this study, a field simulation experiment was conducted using maize straw and wheat straw biochars to investigate how biochar addtion affect soil aggregate stability. The findings showed that as the rate of biochar application increased from 0 % to 10 % over 180 day and 365 days, the contents of soil light fraction organic carbon (LFOC), particulate organic carbon (POC), dissolved organic carbon (DOC), readily oxidized organic carbon (ROC), as well as soil specific surface area and surface charge number, increased. The content of different carbon fractions decreased with prolonged incubation time, while soil electrochemical parameters continued to increase throughout the incubation period. Pearson correlation and redundancy analyses showed that soil active organic carbon fractions were strongly correlated with soil surface electrochemical properties, with ROC and POC being the primary factors. Soil aggregate stability increased with higher biochar application rates, and the beneficial effect of biochar on aggregate stability became increasingly pronounced over time. Based on the quantitative estimates of SIFs, the addition of biochar facilitated soil aggregates stability by increasing van der Waals attractive pressure and decreasing electrostatic repulsive pressure, and finally lowering net repulsive pressure between soil particles. The soil aggregate breaking strength increased quickly at initially and then leveled out as the electrolyte content decreased; 10⁻² mol L⁻¹ marked the tipping point. The study identified the active organic carbon (ROC and POC) fractions which could mediate the soil surface electrochemical properties and soil internal forces, finally enhance soil aggregate stability. Our research provides a theoretical basis for the targeted use and modifying of biochar to efficiently improve soil structure.