Changes in soil carbon and nitrogen cycling along a 72-year wildfire chronosequence in Michigan jack pine forests

We investigated the changes in soil processes following wildfire in Michigan jack pine (Pinus banksiana) forests using a chronosequence of 11 wildfire-regenerated stands spanning 72 years. The objective of this study was to characterize patterns of soil nutrients, soil respiration and N mineralization with stand development, as well as to determine the mechanisms driving those patterns. We measured in situ N mineralization and soil respiration monthly during the 2002 growing season and used multiple regression analysis to determine the important factors controlling these processes. Growing-season soil respiration rates ranged from a low of 156 g C/m2 in the 7-year-old stand to a high of 254 g C/m2 in the 22-year-old stand, but exhibited no clear pattern with stand age. In general, soil respiration rates peaked during the months of July and August when soil temperatures were highest. We used a modified gamma function to model a temporal trend in total N mineralization (total N mineralization=1.853-0.276xagexe-0.814xage; R2=0.381; P=0.002). Total N mineralization decreased from 2.8 g N/m2 in the 1-year-old stand to a minimum value of 0.5 g N/m2 in the 14-year-old stand, and then increased to about 1.5 g N/m2 in mature stands. Changes in total N mineralization were driven by a transient spike in N turnover in the mineral soil immediately after wildfire, followed by a gradual accrual of a slow-cycling pool of N in surface organic horizons as stands matured. Thus, in Michigan jack pine forests, the accumulation of surface organic matter appears to regulate N availability following stand-replacing wildfire.