Enhanced soil aggregation and reduced colloidal phosphorus loss by straw biochars in tea and medicinal herb cropping systems

Phosphorus (P) loss in agricultural runoff significantly contributes to water eutrophication, yet effective mitigation strategies remain limited. This study aimed to assess the impact of biochar amendments on P loss and soil properties in two cropping systems in Zhejiang Province, China: tea and medicinal herb cropping systems. Field experiments compared four fertilization treatments: no fertilization, conventional fertilization (CF), and CF with the partial substitution of chemical fertilizer by 1.5 t ha⁻¹ rice straw biochar (CFRB) or corn straw biochar (CFCB). Over the monitoring period, the study assessed the impact of straw biochars on P loss in different forms and soil properties (soil organic carbon, pH, and soil aggregation). Results showed colloidal P accounted for 41.4 % and 29.7 % of total P losses in the tea and medicinal herb cropping systems, respectively. Biochar amendments (CFRB and CFCB) significantly reduced total P and colloidal P losses across both cropping systems, decreasing colloidal P losses by up to 30.4 % compared with CF. This was primarily due to biochar amendments enhancing soil organic carbon and elevating soil pH, which improved soil aggregation by increasing the proportion of small and micro-aggregates by 3.7 %–9.0 %, effectively limiting the transport of P-bound colloids. These findings highlight the role of biochar as an effective, sustainable solution for mitigating P loss in tea and medicinal herb cropping systems. • Colloidal P accounted for 41.4 % of total P loss in the tea cropping system. • Colloidal P contributed 29.7 % to total P loss in the medicinal herb cropping system. • Straw biochars reduced total and colloidal P concentrations and loss loads in runoff. • Straw biochars increased the proportion of small and micro-aggregates in the soil.