Accumulation and release of organic phosphorus (P) from legacy P-affected soils to adjacent drainage water

Legacy effects of P in agricultural soils have been highlighted in recent literature. However, co-accumulation and release of organic P (Po) have often been ignored in current agro-environmental assessments. The mineralizable Po fraction has a potential to increase the activity of phosphate in pore water, increasing fertility or degrading water quality. In this study, the effects of agricultural management practices (fertilizer applied corn-soybean rotation cropland and dairy manure applied pasture) on the Po/phosphate ratio were investigated in P-rich (290-1232 mg kg^-1) agricultural soils and adjacent ditchwater using experimental soil-water chemistry. The effect of agricultural management was significant on both Po and the Po/phosphate ratio in soil and adjacent ditchwater. The P_o content, dominated by orthophosphate monoesters, in the manure-amended pasture (average ~ 245 mg kg^-1) was significantly greater than that in the fertilizer-applied cropland (average 103 mg kg^-1). The Po/phosphate ratio was also significantly greater in the manure-amended pasture (0.54) than in the fertilizer-applied cropland (0.42). Similarly, water quality data also showed that ditchwater near the pasture had a significantly greater flux of dissolved non-reactive P and a greater Po/phosphate ratio compared to the water near the fertilizer-applied sites. Furthermore, a greater Po/phosphate ratio in ditchwater was often observed during wet periods, and the ratio was positively correlated to the discharge (r = 0.42, p = 0.003). The study showed the agricultural management-specific Po accumulation and release and - Po/phosphate ratio that might affect the fate of P in agroecosystems.