Predicting Changes in Dissolved Reactive Phosphorus in Surface and Pore Water of Flooded Alkaline to Calcareous Soils.

Phosphorus (P) released from agricultural soils to surface flood water and soil pore water during flooding contributes to P loading in watersheds and eutrophication in lakes. Identifying soils with a high risk of P losses to flood water is important in selecting proper management practices to minimize P losses. This study examined the effectiveness of various soil P extraction methods in predicting the potential P release to surface flood and soil pore water. Twelve alkaline to calcareous soils from across Manitoba were subjected to two treatments: an unamended control and solid dairy cattle manure amended at the rate of 100 kg P ha^-1, and were incubated aerobically at room temperature for four weeks prior to flooding. After incubation and prior to flooding, soil subsamples were analyzed for extractable P using the Olsen (Ols-P), Mehlich-3 (extractable P measured as molybdate reactive P (M3P_MRP) and by ICP-AES (M3P_ICP)), water (WEP) and modified Hedley fractionation methods. Soils were flooded for 56 days at room temperature.  During the flooding period, dissolved reactive P (DRP) was measured in surface flood water and soil pore water on a weekly basis. Release of P into surface and pore water was highly variable among soils depending on the soil properties. According to regression analysis, M3P_ICP (r²=0.67 to 0.89) followed by WEP (r²=0.41 to 0.85) were the most reliable P extraction methods for predicting the DRP concentration in surface and pore water three weeks after flooding. The M3P_MRP and Ols-P extraction methods were also good predictors but did not predict DRP concentrations as reliably as the other two methods, especially for manured soils. For the fractionation analysis, Ca-bound P which was extracted by 0.1 M HCl was most consistently related with DRP concentration in surface and pore water (r²=0.26 to 0.67).