Biochar application driven change in soil internal forces improves aggregate stability: Based on a two-year field study

Feinan Hu, Chenyang Xu, Rentian Ma, Kun Tu, Jiayan Yang, Shiwei Zhao, Mingyi Yang, Fengbao Zhang

Biochar amendments are effective for stabilizing soil aggregates and improving the quality and fertility of soils. Soil internal forces (SIFs), including electrostatic, hydration, and van der Waals forces, can substantially affect aggregate stability; however, there has been relatively little focus on the effects of biochar addition on SIFs and their relation to aggregate stability. In this work, in order to quantitatively investigate the influence of biochar on aggregate stability, we collected soil samples with different biochar application rates (0, 2.5%, 5.5%, and 7.0%; w/w) after a two-year field experiment, and then used Na+-saturated soil aggregates (1–5 mm) to conduct our experiments. Our results demonstrated that after 2-year, specific surface area (SSA), soil organic carbon (SOM), cation exchange capacity (CEC), and surface charge density (σ0) increased while soil pH slightly decreased with biochar application from 0% to 7%. Soil aggregate stability increased with the biochar application rate. Theoretical calculations indicated that forces of electrostatic repulsion and van der Waals attraction both increased in response to biochar incorporation, however the net pressure of SIFs decreased. Soil aggregate stability was well explained by the theoretical calculations. Biochar addition reduced the net pressure of SIFs and stabilized soil aggregates. Overall, the investigation by using the mono-cationic model system showed that SIFs played important roles in the aggregate stability and explained well the aggregate stability of amended soils. The findings of this work initiate the process of elucidating and quantifying the complex biochar-mineral interaction mechanism in soil systems.