Alum reduced phosphorus release from flooded soils under cold spring weather conditions.

The effectiveness of amendments such as alum [Al2 (SO4 )3 ·18H2 O] in reducing phosphorus (P) loss to floodwater has been reported under summer conditions and laboratory-controlled environments, but not under actual spring weather conditions in cold climate regions with high diurnal temperature variations when potential for P losses is high. The effectiveness of alum in reducing P release under Manitoba spring weather conditions was evaluated in a 42-day experiment using 15-cm soil monoliths from eight agricultural soils, which were unamended or alum-amended (5 Mg ha-1 ) and flooded to a 10-cm head. Dissolved reactive P (DRP) concentrations and pH of porewater and floodwater were determined on flooding day and every 7 days after flooding (DAF). Porewater and floodwater DRP concentrations in unamended soils increased 1.4- to 4.5-fold, and 1.8- to 15.3-fold, respectively, from 7 to 42 DAF. In alum-amended soils, DRP concentrations averaged across soils was 43%-73% (1.0-2.0 mg L-1 ) lower in porewater, and 27%-64% (0.1-1.2 mg L-1 ) lower in floodwater than unamended soils during the flooding period. The reduction of DRP by alum was more pronounced under high fluctuating diurnal spring air temperature than with controlled air temperature (4°C) in a previous similar study. Acidic pH in porewater and floodwater due to alum did not persist over 7 days. This study showed that alum application is a viable option in reducing P released to floodwater in agricultural soils of cold regions where flooding-induced P loss is prevalent in the spring.

Phosphorus mobilization from intact soil monoliths flooded under simulated summer versus spring snowmelt with intermittent freeze-thaw conditions.

Enhanced phosphorus (P) release from flooded, anaerobic soils has been extensively studied under summer temperatures but not under cold temperatures with intermittent freeze-thaw events. We investigated the temperature and freeze-thaw effects during flooding on the release of P to floodwater from soil monoliths (15-cm depth) collected from eight agricultural fields in Manitoba. Soil monoliths were flooded with reverse osmosis water and incubated for 56 d under simulated summer flooding (SSF; 22 ± 1 °C) or snowmelt flooding with intermittent freeze-thaw (IFT; 4 ± 1 °C with intermittent freezing) in triplicates. Redox potential (Eh), pore water and floodwater dissolved reactive P (DRP) concentrations, pH, and concentrations of Ca, Mg, Fe, and Mn were determined weekly. In seven soils, Eh decreased rapidly with days after flooding (DAF) under SSF to values <200 mV but not under IFT. Both pore water and floodwater DRP concentrations significantly increased with DAF in all soils under SSF and in seven soils under IFT. Although DRP concentrations were consistently greater under SSF than IFT in four soils, other soils had similar concentrations at certain DAF. Significant relationships between ion concentrations and redox status that fitted both IFT and SSF data in most soils suggest that similar redox-driven mechanisms are responsible for the P release; however, less P was released under IFT than under SSF because soils were not severely reduced under IFT. Substantial P release in a few soils under IFT appeared to be unrelated to redox status, suggesting other P release mechanisms that are not redox driven.