Water table management impacts on phosphorus loads in tile drainage

Increased phosphorus (P) concentrations, consistently exceeding Quebec's surface water quality standard of 0.03 mg L−1 total P, caused increased P loads in tile drainage from controlled drainage/subirrigation (CD/SI) plots compared to free drainage (FD) plots in a field study carried out in 2005 in Coteau-du-Lac (southwestern Quebec, Canada). This happened even though the total outflow volumes from CD/SI plots were reduced by 27% compared to FD plots. Of the total P concentration, around 96% was in the form of dissolved P under both treatments. The results also showed that CD/SI had no effect on the soil P concentration and P saturation.During the experiment, the SI water added about 0.84 kg P ha−1, which was negligible compared to the fertilizer input on the field, but represented about 8.5 times the average total P loss from CD/SI plots. However, a laboratory soil column experiment, in which the two drainage treatments were simulated and P-free water was used for SI, also showed increased P concentrations under CD/SI. This confirmed that the increased P loads in tile drainage under CD/SI were most likely caused by an increase in P solubility due to the shallow water table inherent to the water table management system rather than by the addition of P from the SI water.