Replacement of wildfire by whole-tree harvesting increases nitrification and nitrate movement in jack pine forest soils

Abstract

Forest types that were once naturally maintained by stand-replacing wildfires are typically managed with clearcut harvesting; however, we know relatively little about how well clearcutting mimics the effects of stand-replacing wildfire on the availability and cycling of nitrogen (N) in forest soils. We compared net N mineralization, nitrification and NO3− movement in soils from young (2–9 y) jack pine (Pinus banksiana) stands that had been regenerated by either stand-replacing wildfire or clearcut harvesting. We located four pairs of burned and harvested stands that were otherwise similar in age, soil type and geographic location. Net N mineralization, measured both in situ and in the laboratory, was nearly 2 times higher in young stands of clearcut origin compared to stands of wildfire origin. Differences between disturbance types were much more pronounced for net nitrification, which was more than 20 times greater, on average, in clearcut-origin sites. Burned sites rarely exhibited any net nitrification in field incubations, and at nearly 2/3 of the sampling times burned sites exhibited net immobilization of NO3−. This lack of nitrification occurred despite the fact that we always observed net N mineralization across all of our in-situ incubations in wildfire-origin stands. Finally, we observed nearly four times greater NO3− movement through the soil profile in clearcut stands, as evidenced by capture on ion-exchange resins at a depth of 25cm. Together, these results clearly indicate that replacement of wildfire disturbance by clearcut harvesting promotes nitrification and NO3− movement in jack pine ecosystems of northern Lower Michigan. The exact mechanism underlying this response in our sites remains uncertain; however, it appears that some factor that restricts net nitrification in young stands of wildfire origin is not present in clearcut-origin stands. Our finding of increased nitrification and NO3− movement in the years following clearcutting could lead to increased losses of N and base cations, and could also favor plant species with greater preference for NO3− as a N source.