Effects of Fire on Soil Nutrients in Clearcut and Whole‐tree Harvest Sites in Central Michigan

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Abstract
Surface‐mineral soil samples from adjacent northern red oak (Quercus rubra L.)‐bigtooth aspen (Populus grandidentata Michx.) sites harvested by contrasting methods were evaluated for available Ca, Mg, K, P, and total N before and after a wildfire. Abundant slash from clearcutting contributed to significant and persistent increases in Ca, Mg, K, and N following fire. Sample data grouped by variations in estimated surface burn intensity revealed no significant differences, indicating that slash windrows did not appreciably localize nutrient increases. Minimal residues from whole‐tree harvest released smaller quantities of Ca, Mg, and K, and total N exhibited no significant change. Significant increases in soil Ca, Mg, K, and P at both sites occurred within a month after burning. Five months after the fire, soil Ca, Mg, K, and P at both sites generally decreased, in some cases to prefire levels. Cation leaching losses from the surface soils, monitored by porous cup lysimeters at the 1‐m depth, increased within 2 months after the fire, but losses appeared to stabilize within 5 months. Leaching losses of Ca were significantly greater in the clear cut site. Although short‐term soil nutrient changes following fire were generally positive, the long‐term effects on site quality remain in question due to the probable net loss of organic matter and nutrients through volatilization and accelerated leaching.