Initial recovery of soil carbon and nitrogen pools and dynamics following disturbance in jack pine forests: A comparison of wildfire and clearcut harvesting

Abstract

Forests naturally maintained by stand-replacing wildfires are often managed with clearcut harvesting, yet we know little about how replacing wildfire with clearcutting affects soil processes and properties. We compared the initial recovery of carbon (C) and nitrogen (N) pools and dynamics following disturbance in jack pine ( Pinus banksiana) stands in northern Lower Michigan, USA, by sampling soils (Oa+A horizons) from three “treatments”: 3–6-year-old harvest-regenerated stands, 3–6-year-old wildfire-regenerated stands and 40–55-year-old intact, mature stands ( n=4 stands per treatment). We measured total C and N; microbial biomass and potentially mineralizable C and N; net nitrification; and gross rates of N mineralization and nitrification. Burned stands exhibited reduced soil N but not C, whereas clearcut and mature stands had similar quantities of soil organic matter. Both disturbance types reduced microbial biomass C compared to mature stands; however, microbial biomass N was reduced in burned stands but not in clearcut stands. The experimental C and N mineralization values were fit to a first-order rate equation to estimate potentially mineralizable pool size (C 0 and N 0) and rate parameters. Values for C 0 in burned and clearcut stands were approximately half that of the mature treatment, with no difference between disturbance types. In contrast, N 0 was lowest in the wildfire stands (170.2 μg N g −1), intermediate in the clearcuts (215.4 μg N g −1) and highest in the mature stands (244.6 μg N g −1). The most pronounced difference between disturbance types was for net nitrification. These data were fit to a sigmoidal growth equation to estimate potential NO 3 − accumulation (Nit max) and kinetic parameters. Values of Nit max in clearcut soils exceeded that of wildfire and mature soils (149.2 vs. 83.5 vs. 96.5 μg NO 3 −–N g −1, respectively). Moreover, the clearcut treatment exhibited no lag period for net NO 3 − production, whereas the burned and mature treatments exhibited an approximate 8-week lag period before producing appreciable quantities of NO 3 −. There were no differences between disturbances in gross rates of mineralization or nitrification; rather, lower NO 3 − immobilization rates in the clearcut soils, 0.20 μg NO 3 − g −1 d −1 compared to 0.65 in the burned soils, explained the difference in net nitrification. Because the mobility of NO 3 − and NH 4 + differs markedly in soil, our results suggest that differences in nitrification between wildfire and clearcutting could have important consequences for plant nutrition and leaching losses following disturbance.