Reducing Phosphorus Release from Flooded, Manured Soils to Overlying Floodwater with Recycled Gypsum Amendment.

Phosphorus (P) loss from agricultural soils to water bodies is an environmental concern, since it is the limiting nutrient for freshwater eutrophication.  Prolonged flooding of agricultural soils can trigger a series of redox processes affecting P transformations that can enhance P release to overlying floodwater. Soil amendment with gypsum has been found to be effective in reducing P runoff and leaching losses from soils. We tested the hypothesis that recycled wallboard gypsum (RWG) amendment can reduce P release from flooded, calcareous soils to overlying floodwater, using three liquid swine manure-amended, pre-incubated, calcareous soils from Manitoba; a Pembina loam, a Dencross clay, and a Denham sandy loam. Pre-treated soils were packed into incubation vessels either unamended (control), or with RWG amendment (at 5 t/ha) and kept flooded for a period of eight weeks. Soil redox potential (Eh), pore water pH, dissolved reactive P (DRP) concentrations in pore water and floodwater, were monitored weekly. Amendment with RWG did not have a significant effect on soil Eh, except for Dencross clay where the Eh decrease was less in RWG amended treatment after 21 days. Pore water pH was greater in RWG-amended than unamended treatments in all soils at most incubation times. Both pore water and floodwater DRP concentrations were significantly less in RWG-amended than in unamended soils. The decrease in pore water DRP concentrations with RFG-amendment in relation to unamended soils ranged from 6-32%, 27-43% and <1 to 21%, in Pembina loam, Denham sandy loam and Dencross clay, respectively, while the corresponding reduction in floodwater DRP concentrations were 10-38%, 30-57% and 14-35%, respectively. The results suggest that RWG amendment not only resulted in reducing P release from flooded soils to pore water, but also reduced the P transfer from pore water to floodwater, very likely due to precipitation reactions of P with calcium.