Effect of variable retention harvesting and stand type on soil nitrogen availability in boreal mixedwood forests.

摘要

Clear-cut harvesting has been shown to increase the nitrification rate and lead to elevated levels of soil nitrate in some forests. This change in nitrogen (N) cycling and availability could negatively affect regeneration, future site productivity as well as surrounding ecosystems. Variable-retention (VR) harvesting has been proposed as a more environmentally acceptable alternative to clearcutting. VR harvesting retains a portion of live trees in the cutblocks and it is assumed that this will create conditions more similar to uncut forests and mitigate the negative effects of clearcutting on the N cycle. In boreal mixedwood forests, maintenance of a deciduous component and management of mixed stands is currently encouraged, partly based on the assumption that deciduous trees facilitate faster N cycling and higher N availability. The evidence of this phenomenon is, however, equivocal. In boreal mixedwood forests in northern Alberta, I assessed whether the uncut deciduous-dominated forests had higher N availability and faster N cycling than coniferous-dominated forests, and whether N availability was related to the proportion of deciduous trees. I also assessed the effects of clearcutting and VR harvesting on soil N availability in boreal mixedwood forests and characterized pathways and rates of nitrification. Uncut stands of deciduous forests had higher availability of ammonium but did not have faster N cycling and higher nitrate levels as hypothesized. Clearcutting had little effect on soil N availability in all forest types. Net and gross rates of N mineralization and nitrification were not altered and N availability was not elevated by clearcutting. VR-harvested sites did not differ either from clearcuts or uncut stands. Despite very low soil nitrate concentrations, nitrification was an active process at all sites, even in uncut coniferous stands. Nitrification was carried out mainly by heterotrophs and both nitrate production and consumption increased with labile C addition. Clearcutting in these boreal forests did not create a soil environment dominated by nitrate at any site. VR harvesting of western boreal mixedwood forests may not be justified based on nutritional concerns as the effects of clearcutting on N availability were minimal.