Effect of harvest gap formation and thinning on soil nitrogen cycling at the boreal–temperate interface

In mixedwood forest, different types of commercial thinning that generate different gap sizes are being tested as alternatives to clearcutting to create forest stands with an irregular structure that would emulate the pre-industrial forests. The main goal of this study was to investigate the soil nitrogen (N) dynamics in response to two partial harvesting treatments, used alone or in combination: commercial thinning creating tree-size canopy gaps and harvest gaps creating 0.05 ha gaps. In a 30 year old balsam fir (Abies balsamea (L.) Mill.) stand, both treatments, alone or in combination, as well as unharvested controls were compared on replicated 0.75 ha plots. The most noticeable changes following treatments were observed in gaps, while commercial thinning did not significantly influence any of the parameters assessed. In gaps, increases in N mineralization rates and mineral N concentrations and proportions (NO 3 − -N and NH 4 + -N) relative to dissolved organic N were observed. Our results suggest that these changes are caused by the increase in soil temperature and water content. In these forests, the response threshold of the N cycling regarding the size of the intervention would therefore be located between the removal of one or a few trees (one to three stems, 6–12 m 2 ) and a gap of 500 m 2 . Other studies conducted in different climate and forest types have shown that this threshold could be of equivalent or of a smaller size. These findings will contribute to optimizing our management strategies regarding partial cuts or small-scale clearcuts.