Effects of Multiple Fires on Nutrient Yields from Streams Draining Boreal Forest and Fen Watersheds: Nitrogen and Phosphorus

Abstract

Wildfire in the boreal forests at the Experimental Lakes Area in Ontario caused significant losses of nitrogen and phosphorus in streams. Both watershed type and fire intensity appear to determine the extent of losses. The Northeast wetland basin lost more N and P, especially TDN, TDP, TN, and TP, than did terrestrial basins, although nitrate losses were higher from terrestrial basins. Losses of nutrients after the second fire generally were not as high as after the first fire. In the East basin, which burned twice with a high fire intensity, stream concentrations of NO₃⁻, TDN, and TN were elevated for 6 yr between the fires and remained elevated for 9 yr after the second fire In contrast, the Northwest basin burned with a lower intensity and had no significant increase in annual concentrations of P or most forms of N in the first 3 yr after the fire; only NCV concentrations increased during this period. Despite the increases in export after wildfire, net retention of TN and TP over the 18-yr period was high. In the Northeast, East, and Northwest basins, retention of TN averaged 77, 80, and 87% compared with TP retention of 51, 67, and 84%.