Effects of harvesting, prescribed burning and wildfire on soil chemical properties in Northwestern Ontario

Soil chemistry responses to disturbances were quantified and compared at two dates by sampling the organic, 0-5 cm and 5-10 mineral soil layers in a northwestern Ontario boreal mixedwood site. The disturbance treatments were as follows: (1) Control, 70 year old mixedwood stand dominated by white spruce (Picea glauca (Moench) Voss) and balsam fir (Abies balsamea L. Mill ); (2) Harvest, full- tree logged cutovers in winter; (3) Prescribed Burn, cutovers which were burned in early spring; (4) Reburn, cutovers which were prescribed burned and subsequently experienced a wildfire; (5) Wildfire, crown fire in vegetation similar to that of the Control. The severity level of the three fire treatments, as determined by depth of organic layer mineralized, was lowest for the Wildfire, moderate for the Prescribed Bum and high for the Reburn. The hypotheses under investigation were that the magnitude of change in soil chemical properties is greater following fire than full-tree harvesting and increasing fire severity increases the degree of change in soil chemical properties. Organic layer soil reaction was significantly higher by one pH unit, in the three fire treatments than in the Harvest treatment for June and in the Harvest and Control treatments in August. All three fire treatments had a significantly higher amount of available phosphorus than the Harvest and Control treatments in the organic layer sampled in June. Elevated amounts of calcium and potassium in the organic layer sampled in June of the fire treatments resulted in significant differences between them and the Harvest treatment while the difference in magnesium was significant among the fire treatments and Harvest and Control. These significant differences among treatments in available phosphorus and cation levels did not persist to the August collection period. There were no significant differences among fire treatments for the organic layer sampled in June but variables that the ordination suggested were responsible for separating the fire treatments included organic matter content and total nitrogen. Cation exchange capacity was significantly higher in the Wildfire and Control treatments than in the Prescribed Bum, Reburn and Harvest treatments for the organic layer sampled in August. The cation exchange capacity and total nitrogen for the Wildfire increased significantly between June and August and may be the result of a delay in the addition of ash and particulate organic matter to the forest floor from combustion which occurred in the tree crowns. Organic matter content, as estimated by loss on ignition, was significantly lower in the Reburn treatment compared to the Control in August; suggesting greater volatilization of carbon occurred in this treatment than in the other two fire treatments