Carina Leale HBESc thesis abstract

Boreal forest soils represent globally significant stores of carbon, and to a lesser degree nitrogen, yet there is little consensus about how they are altered by natural and anthropogenic disturbances. Disturbance by wildfire and forest harvesting is widespread, yet there are few investigations comparing the impacts of these disturbances on soil nutrient cycling, and even fewer on compound disturbance, such as salvage logging. A chronosequence approach was used to investigate the impacts of full-tree harvesting, wildfire, and compound disturbance i.e. post-fire salvage logging on soil carbon and nitrogen stores, pH and carbon to nitrogen ratios overtime. Stable carbon and nitrogen depth profiles were examined to identify the dominant mechanisms driving the observed trends following disturbance. The findings suggest that full-tree harvesting and wildfire impact carbon and nitrogen stores in fundamentally different ways, with stores being reduced after wildfire and increased after harvest. However, the differences observed were confined primarily to the organic and upper mineral soil, and the changes diminished with time. Results from this study suggest that impacts of post-fire salvage logging on carbon and nitrogen stores resembles that of fire, but that recovery is much slower making the practice potentially more harmful than harvesting and wildfire alone. Additional exploration into the long-term effects of compound disturbance on carbon and nitrogen stores is needed if salvage logging is to continue to become a justifiable practice. The driving mechanism of ¹³C enrichment with depth was inconclusive, with both mixing and kinetic fractionation during humification appearing to be equally dominant at these sites. Rayleigh distillation analysis indicated no differences in overall depth enrichment among the sites. Similarly, examining ¹⁵N enrichment profiles did not contribute to developing a process-based understanding of nitrogen cycling in these soils. This research demonstrates a need for further investigation into the usefulness of stable isotope depth profiles to indicate mechanisms responsible for storage change on a variety of soil types and textures following disturbance.