Effect of biochar on the stability and distribution of phosphorus fractions of brown soil aggregates

Soil aggregates play an important role in regulating the storage and turnover of phosphorus in the soil. In this study, we evaluated the regulatory effects of biochar-mediated soil aggregates on the form, transformation, and distribution of soil phosphorus. This study was based on long-term field experiments with biochar. Four treatments were selected: no fertilization (CK), biochar application (BC), NPK fertilizer application (NPK), and NPK fertilizer combined with biochar application (NPK+BC). After the maize harvest in 2021, 0–20 cm soil samples were collected to study the effects of biochar on the composition and stability of soil aggregates, as well as the content of various phosphorus fractions in each particle size aggregates. The distribution of phosphorus fractions in the soil aggregates and the relative contribution rates of soil aggregates of different particle sizes to different forms of phosphorus were also clarified. First, the application of biochar significantly increased the proportion of coarse aggregates (>2 mm) by 8.7% compared to CK treatment and 15.8% compared to NPK treatment, whereas that of silt and clay particles (<0.053 mm) was reduced by 11.5% compared to CK treatment and 21.8% compared to NPK treatment. The application of biochar significantly increased the mean weight diameter and mean geometric diameter of the soil aggregates by 9.6% and 22.2%, respectively. Second, the application of biochar increased the content of total inorganic phosphorus (mainly manifested as the increase in NaHCO₃-Pi and NaOH-Pi contents) in aggregates with particle size >0.25 mm and that of total organic phosphorus (mainly manifested as the increase in NaOH-Po and Residual-P contents) in fine and large aggregates (0.25–2 mm). Third, the application of biochar reduced the relative contribution of silt and clay particles (<0.053 mm) to total phosphorus and different phosphorus fractions and increased that of coarse aggregates (>2 mm) to total phosphorus and different phosphorus fractions. In conclusion, the application of biochar improved the stability and structure of soil aggregates and promoted the distribution of phosphorus fractions in soil aggregates with larger particle sizes. Biochar application is an important agricultural measure for improving soil structure, promoting soil phosphorus retention, and reducing the risk of soil phosphorus loss.