Soil N cycling in harvested and pristine Boreal forests and peatlands

The heterogeneous Boreal Shield forest in Canada is one of the most extensive pristine forests remaining in the world and is being intensely harvested. We studied the spatial variability of organic and inorganic N cycling processes in three Boreal Shield catchments in northwestern Ontario for 2 years before and 1 year following clearcutting. Net N mineralization rates were similar among upland conifer, upland deciduous and peatland stands, ranging from negligible to 150 mg kg−1 in the forest floor/peat soils and −30 to 40 mg kg−1 in mineral soils of the upland stands over the growing season. Net nitrification rates were generally negative, <10% of net mineralization rates, and similar among the landscape units. Reciprocal transplants of forest floor/peat and mineral soil from the uncut and cut stands indicated that changes in environmental conditions in the clearcut influenced net N mineralization by 50-fold and nitrification rates by nine-fold in the peatlands but not the coniferous uplands. Net inorganic N cycling rates measured the 1st year following clearcutting were within the natural range of variability, which is consistent with previous studies in northern coniferous and aspen forests. In contrast with the literature however, no difference in soil dissolved organic N mobilization rates (peatland stand range: 0.2 to 4.8 mg kg−1 d−1; upland coniferous stand range: −0.1 to 2.3 mg kg−1 d−1) were found between uncut and recently clearcut stands.