Evaluating the potential for snowmelt phosphorus losses from perennial forage crops.

In cold regions, there is concern that losses of P with snowmelt runoff following freeze and thaw of vegetation may be greater from perennial forages relative to annual crops. We evaluate the drivers of P losses with snowmelt runoff over a network of field-scale small watersheds in Manitoba, Canada, following annual crops (59 site-years), perennial forage (19 site-years), or tillage to terminate a forage (4 site-years). Vegetation type was not significantly related to concentrations of P lost in snowmelt or load (p > 0.05), and 0-5 cm Olsen-P in soil was the best predictor of flow-weighted mean concentrations of total dissolved P (r² = 0.46, p < 0.001) and total P (r² = 0.45, p < 0.001) across the 82 site-years of data. Sites having a recent (10-year) land use history without tillage had greater P stratification in the top 5 cm of soil than those with tillage, irrespective of vegetation type (p < 0.001). Residual variation in snowmelt P concentration and loads were negatively related to water yield and positively related to proportion of soil surface area covered by crop residue (independent of type of residue). Loads of P exported with snowmelt were primarily a function of water yield, and at a similar level of snow water equivalent, perennial forages exhibit lower water yield than annual crop sites. These results suggest that with careful management of soil P, adding perennial plants to crop rotations will not increase losses of P with snowmelt and through impacts on hydrology, reductions in overall loading may occur.