Impact of precommercial thinning in balsam fir stands on soil nitrogen dynamics, microbial biomass, decomposition, and foliar nutrition

Precommercial thinning is being tested in Quebec as a preventive silvicultural treatment to reduce vulnerability of young balsam fir (Abies balsamea (L.) Mill.) stands to spruce budworm (Choristoneura fumiferana (Clem.)) damage and to shorten rotations. As part of a larger study of ecosystem response to thinning, we have examined the impact of this treatment on soil nitrogen dynamics, microbial biomass, cellulose decomposition, and foliar nutrition across a range of drainage conditions (good, imperfect, and poor). In the first year after thinning, initial early season ammonium (NH4+-N) pools in the mineral horizon were significantly higher in the thinned plots (P = 0.019), while net nitrogen mineralization (NH4+-N plus NO3--N) decreased in these same plots (P = 0.052). The thinning treatment significantly increased microbial biomass nitrogen (Nmic) in the organic horizon (P = 0.051). Simple regression analysis indicated the importance of soil temperature in controlling Nmic. Decomposition of cellulose substrate in the organic horizon was significantly increased by thinning, and mass loss was related to soil temperature. Increased decomposition and nutrient availability after thinning were reflected in improved N, P, and K nutrition in current, 1- and 2-year-old balsam fir needles. The temporal extent of this improved fertility will be verified by longer term monitoring.