Runoff and inorganic nitrogen export from Boreal Plain watersheds six years after wildfire and one year after harvest

Abstract

This investigation in the Swan Hills, Alberta, located on the Canadian Boreal Plain, examined May through October runoff before, during and six years after wildfire in a 4th order watershed, compared to a 3rd order reference watershed. It also examined runoff and inorganic nitrogen flow-weighted mean concentration and areal export for 1 year after winter harvest in four 1st order watersheds compared to five reference 1st and 2nd order watersheds. Runoff and areal exports were normalized to precipitation at each site. Runoff impact ratios (year 1 post-disturbance value divided by the pre-disturbance value) for burned and harvested watersheds were 60 and 70% higher, respectively, than reference watersheds (P = 0.06). Runoff from the burned watershed remained elevated 6 years after fire. A trend for higher nitrate and ammonium concentrations, combined with higher runoff yielded impact ratios for areal ammonium and nitrate exports that were 130 and 170% higher, respectively, in harvested than reference watersheds (P = 0.08 for both). The proportion of the watershed harvested was positively related to runoff (r2 = 0.94, P = 0.03) and ammonium impact ratios (r2 = 0.96, P = 0.02), but not nitrate impact ratios (P = 0.30). Areal nitrate export in snowmelt was low in harvested watersheds compared to their pre-harvest condition and to reference watersheds.