Anaerobic conditions developed in soils with flooding can enhance the release of soil phosphorus (P) to overlying water, but little information is available for soils with a long history of manure application. We examined the P release from manure-amended soils under simulated snowmelt flooding. Intact monoliths from manured (solid-swine, SSM or liquid swine, LSM) and unamended (control) field plots were collected from Carman, Manitoba. Monoliths were frozen for seven days, then thawed, flooded and incubated at +4±1 °C. Redox potential, pH, and concentrations of dissolved reactive P (DRP), Ca, Mg, Fe and Mn in pore water and floodwater were determined weekly up to 56 days after flooding (DAF), and then at 84 DAF. Redox potential decreased with DAF with a greater, and more rapid, decrease in SSM (from initial value of ~300 mV to < 0 mV by 84 DAF) compared to LSM and control (~100 mV by 84 DAF). Pore water and floodwater DRP concentrations were significantly greater in manured treatments than the control at all DAFs, and in SSM than LSM for most DAFs. While floodwater DRP concentrations remained relatively stable in the control treatment, concentrations in manured treatments increased substantially from the onset of flooding to 35-42 DAF (3-4-fold increase) and remained relatively stable thereafter. Significantly greater P release from SSM- than LSM-treated monoliths was due to greater input of P, as well as the higher organic matter content in SSM treated soils. These favored the rapid development of anaerobic conditions that further induced P release.

Soil monoliths were collected from replicated (4) field plots of three selected treatments (control, liquid swine manure and solid swine manure) from a long term manure field trial. Mololiths were flooded under simulated snowmelt condition. Porewater was extracted at 5 cm depth and floodwater from the overlying floodwater at weekly intervals upto 8 weeks and then at 12 weeks. pH and dissolved reactive P in pore water and floodwater were analyzed. The dissolved reactive P concentrations (mg/L) and pH are given for pore water and floodwater seprately for each replicate soil monolith for the three treatments. 

Details of the experiment and the analysis are provided in the manuscript.