SOIL NUTRIENT DYNAMICS IN A BOREAL MIXEDWOOD CUTOVER FOLLOWING THE APPLICATION OF HEXAZINONE

Abstract

Large-scale human disturbances, primarily harvesting and site preparation, are recent and increasingly frequent activities in the Boreal Mixedwood, and potential impacts on long-term site productivity are unknown. A 7-yr experiment was carried out in a clear-cut Boreal Mixedwood forest to assess the effect of the herbicide hexazinone on soil nutrient dynamics. A completely randomized design with three herbicide treatments (0, 2, and 4 kg of active ingredient (a.i.) per hectare applied as Pronone 10G) and five replicates per treatment was established. The surface organic horizon (L-H) and three mineral horizons (Ah, Ae, and Btnj horizons) were sampled for five years: one year prior to the herbicide application and during the first, second, third, and sixth year after the herbicide was applied. Littertraps and lysimeters were installed and samples collected for the first three years of the study. Total biomass decreased and N concentrations increased in litterfall for at least 2 yr following the application of hexazinone. Other elemental concentrations were affected only in the year of application. Concentrations of ammonium (NH4+-N) and nitrate (NO3−-N) in the L-H of the hexazinone treatments were higher relative to the control. Extractable phosphorus (P), sulfur (S), and potassium (K) in the L-H of the herbicide treatments were lower by as much as 25%. These differences persisted for the duration of the study. No changes in extractable calcium (Ca) and magnesium (Mg) were found. Elevated concentrations of NH4+-N and K were measured in the Ah and Ae mineral horizons of the highest herbicide treatment, but total soil nutrient pools remained unaffected by the herbicide treatments. Reduced plant uptake of nutrients and the indirect impact of changes to the vegetation on soil mineralization–immobilization processes resulted in the differences in the available nutrient concentrations of the L-H horizon. The short-term results (6 yr) following the hexazinone application indicate that there was little change to the total nutrient pools and that the potential loss of nutrients at this site was minimal.