Nitrogen net mineralization and dynamics following whole-tree harvesting and winter windrowing on clayey sites of northwestern Quebec

Abstract

Concerns over decreases in soil nitrogen reserves and productivity following the removal of logging residues (windrowing, shearblading and piling) have been raised by numerous researchers. Medium-term impacts of this practice on soil N reserves and availability and on indices of organic matter quality were assessed for balsam fire ( Abies balsamea (L.) Mill.), white birch ( Betula paperyfera Marsh.) and white spruce ( Picea glauca (Moench) Voss) stands growing on dry to fresh clayey sites in northwestern Quebec, Canada. Unharvested control stands, whole-tree harvested cutovers and windrowed sites were compared. Fifteen years following harvesting and windrowing, forest floor Kjeldahl N concentrations and content and forest floor in situ net N mineralization rates (undisturbed closed top cores incubation) were affected by harvesting but not by windrowing. No differences in mineralization constant, potentially mineralizable N and cumulative mineralized N (526 day incubation period) were found between treatments, suggesting that treatment differences in field net N mineralization rates were the result of interactions between residual ecosystem structures such as forest floor, coarse woody debris and vegetation and meteorological conditions. If these trends persist over time, it could signal that, while whole-tree harvesting does not have a direct effect on soil organic matter quality, long-term impacts on N dynamics could result from changes in ecosystem structures. Slash removal following whole-tree harvesting did not have any additional negative impact.