Biochar effects on soil aggregation, phosphorus distribution, and colloidal phosphorus content in paddy soils: a comparative study

Soil colloids significantly influence the transfer of phosphorus (P) from agricultural lands to aquatic ecosystems. Nevertheless, the impact of soil amendments, such as biochar, on colloidal P release and mobilization is not fully understood. This study aimed to explore how biochar amendments affect the dynamics of soil colloidal P in paddy soils, particularly focusing on the interaction between soil aggregation and colloidal P mobilization. We conducted a soil incubation experiment to investigate the effects of rape straw-derived biochar (SBC) and chicken manure-derived biochar (MBC) on soil aggregate stability, P distribution, and colloidal P content in paddy soils with different fertility levels. Soil aggregates were fractionated using wet sieving and sedimentation methods, whereas colloidal P was determined through ultracentrifugation. Our findings demonstrated that both MBC and SBC addition increased soil aggregation by promoting the formation of larger aggregates (> 250 μm and 53–250 μm), while reducing the proportion of smaller soil aggregates (< 2 μm, 2–20 μm, and 20–53 μm). Biochar addition led to an increased allocation of P in soil aggregates of > 250 μm and 53–250 μm, with the observed rise in total P content primarily attributed to the inherent P within the added biochar. MBC exhibited a more pronounced impact on soil P dynamics than SBC. Despite enhanced soil aggregation, MBC addition substantially increased soil colloidal P and dissolved P, suggesting potential environmental risks associated with P loss. The elevated degree of P saturation resulting from MBC addition appeared to play a more critical role in regulating soil colloidal P release and mobilization. Our study highlights the importance of carefully considering the long-term application of P-enriched biochar in paddy soils, as this practice may lead to P loss even in the presence of increased soil aggregate stability.