Phosphorus Release From Ditch Mesocosms: The Effects of Pedology and Hydrology.

Phosphorus (P) is a major pollutant of the Chesapeake Bay and a growing number of freshwater and brackish systems around the world. In Mid-Atlantic Coastal Plain landscapes open drainage and intensive animal husbandry are common. Common management practices of primary ditches include backhoe cleanouts and the use of controlled drainage. Our objective was to determine if common means of ditch management enhance P retention or the sorption capacity of ditch soil. Experimental mesocosms of intact soil from a farm on the Eastern Shore of Maryland with a long history of poultry manure application were recovered. These mesocosms received treatments testing the effect of cleanouts and controlled drainage on pedon effluent P, as well as treatments to explore the highly reactive, organic matter rich surficial horizons common in primary ditches. Soil treatments were: cleanout, control, surficial material removal and organic matter addition. Hydrology treatments were drained, saturated and alternating. Periodically, simulated groundwater was upwelled through the mesocosms and effluent P was measured. Saturated mesocosms produced more effluent P than drained. Alternating hydrology mesocosms behaved according to their temporary hydrology. Cleanout treatment mesocosms produced less effluent P than non-cleanout mesocosms. Non-cleanout mesocosms, regardless of hydrology released P to effluent. At the end of the experiment mesocosms were opened and soils were analyzed for oxalate extractable P (Pox), aluminum and iron (Feox). Feox and Pox were highly correlated (P<.01, R>0.98). Columns experiencing continuous saturation over the course of the experiment had less Feox and Pox than drained or alternating-hydology mesocosms. Because no treatment enhanced P sorption capacity or reduced effluent P below input P levels, straightforward ditch hydrologic and soil manipulations in our experiment did not serve the purpose of P mitigation.