Restricted colloidal-bound phosphorus release controlled by alternating flooding and drying cycles in an alkaline...

Colloid-facilitated phosphorus (P) migration plays an important role in P loss from farmland to adjacent water bodies. However, the dynamics of colloidal P (P_coll) release as influenced by irrigation in alkaline calcareous soil remains a knowledge gap. The present study, monitored the dynamic change of P_coll under different water management strategies: 1) control, 2) flooding, and 3) alternating flooding and drying cycles. Soil water-dispersible colloids (0.6 nm-1 μm) were extracted by combining filtration and ultrafiltration methods. The contents of P, cation and organic carbon in the water-dispersible colloids were determined and the stability and mineral composition of colloidal fractions were characterized. The results showed that P_coll ranged from 16.5 to 25.5 mg kg^-1 and represented 42.8%-64.9% of the water-extracted P in the control. Flooding significantly decreased the P_coll content by 16.0%-62.1% (mean 32.7%) and it may be attributed to the dissolution of colloidal iron (Fe) bound P. The alternating flooding and drying treatment significantly reduced the P_coll content by 11.6%-88.0% (mean 67.6%). The P_coll content of the flooding event was always greater than the P_coll content of the drying event during flooding and drying cycles. Redundancy analysis and random forest modeling showed that the colloidal calcium (Ca) and ionic strength in soil solutions had negative correlations with the P_coll content, and pH, ionic strength and truly dissolved P were the critical factors affecting P_coll. Drying of the flooded soil led to the decrease of pH and the increase of ionic strength, colloidal Ca content and positive charges of colloid surfaces, which promoted colloid aggregation and enhanced soil P sorption capacity. This restricted the loss potential of P_coll. In summary, controlled flooding and drainage when managed correctly have a role to play in mitigating P_coll loss from P-enriched calcareous soils.