Balanced fertilization modulates colloidal phosphorus dynamics in paddy fields

The role of organic amendments in soil phosphorus (P) dynamics in paddy fields has been widely studied, yet the impact of balanced fertilization (achieved through organic substitution at equivalent nutrient levels) on the transport and distribution of colloidal P remains poorly understood. This study investigates the influence of balanced fertilization on colloidal P dynamics in paddy field soils and surface water by categorizing colloidal P into nano colloidal (1–100 nm), medium colloidal (100–450 nm), and coarse colloidal (450–1000 nm) fractions. We assessed these colloidal forms in surface water and soil leachates at different depths under various fertilization treatments, including partial organic substitution for chemical nitrogen (N) over 25 years, and balanced N and P substitution over 5 years. Results revealed that equal N substitution over 25 years significantly increased both truly dissolved P and colloidal P concentrations compared to chemical fertilizer treatment, respectively by averaged 3.8 and 1.4 times of colloidal P in surface water and leachates (0–80 cm). In contrast, balanced N and P substitution over 5 years had no significant effect on total colloidal P concentrations, but did increase nano colloidal P by 10 times. Colloidal P size distribution and elemental composition varied, with Fe and Al predominantly found in smaller colloids and Ca in coarser colloids. These findings highlight the critical role of colloidal P in nutrient transport and its potential environmental risks, particularly eutrophication . Optimizing the use of organic and inorganic fertilizers is crucial for balancing agricultural productivity and environmental sustainability in P management strategies for rice–wheat rotation systems.