Effects of One-year and Two-year Continuous Application of Biochar on the Stability and Availability of Phosphorus in Paddy Soil

This study examined the influence of biochar application on soil phosphorus dynamics in a typical red paddy soil through a rice pot experiment. The experiment compared three treatments: no biochar application (CK), one-year biochar application (B1), and two-year consecutive biochar application (B2). The concentrations and proportions of total phosphorus (TP), active P (P_SOL, P_M3), moderately stable P (Pi_OH, Po_OH, P_HCl), and highly stable P (P_CL, P_OCL) were analyzed at different rice growth stages. Additionally, the responses of soil legacy P index (LGC), P saturation degree (DPS), and P adsorption capacity (SOR) to biochar addition were investigated. The results reveal that compared to the CK treatment, B1 application significantly reduced active P during the tillering stage but increased it during the maturity stage. Moderately stable P increased significantly during the tillering stage in the B1 treatment. No significant changes were observed in highly stable P for all treatments(P < 0.05). In the B2 treatment, active P significantly increased during the heading and maturity stages, while moderately stable P increased during the tillering and maturity stages. The content of highly stable P remained unchanged, but its proportion decreased significantly by 18.8% and 27.8% during the heading and maturity stages, respectively. Both B1 and B2 treatments led to a significant increase in LGC and DPS, and the B1 treatment showed a significant increase in SOR during the heading stage. Overall, the addition of biochar significantly increased the content and proportion of active P and moderately stable P in paddy soil, decreased the proportion of highly stable P with low plant utilization, and enhanced soil legacy P and bioavailable P. These findings provide valuable insights for optimizing P management strategies in paddy fields.