Increased soil nitrate losses under mature sugar maple trees affected by experimentally induced deep frost

Abstract

This work reports and discusses data gathered during soil solution monitoring that was part of an experiment conducted in the Duchesnay Experimental Forest (Quebec, Canada) to study the effects of induced deep frost, superficial frost, and superficial frost plus drought on mature sugar maple trees (Acersaccharum Marsh.). Frost treatment was applied by preventing snow from accumulating under the canopy. Soil solution chemistry was modified when mature sugar maple trees declined after exposure to a severe deep frost. The first vegetation period after treatment showed that losses of NO₃⁻ below the rooting zone were greatly increased under affected trees. The leaching rate of NO₃⁻ and basic cations was directly related to the level of change in canopy dieback and transparency. A mean NO₃⁻ concentration of 630 μmol_c•L⁻¹ (53 times the controls; max. 4500 μmol_c•L⁻¹) was measured in soil solution under the deep frost treated trees. The leaching rate of K⁺ (18×, relative to the control) and Mn²⁺ (11×) was higher than that of Mg²⁺ (5×) and Ca²⁺ (2.6×). Acidification of the soil solution (50% more H⁺) as a result of intense nitrification caused an increase in aluminum concentration (5×) and a decrease in SO₄²⁻. The acidification during the year after treatment was equivalent to decades of atmospheric acid deposition. The seasonal mean of SO₄²⁻ did not differ between treatments, but there was evidence of a significant correlation between pH and SO₄²⁻ in soil solution. Concentration of NH₄⁺ was also enhanced but to a lesser degree (10×) than that of NO₃⁻. Specific conductivity was a good predictor of NO₃⁻, Ca²⁺, Mg²⁺, and total Al in soil solution. The ion balance shifted from an anion deficit to a strong cation deficit when NO₃⁻ concentrations were high. Superficial frost with or without induced summer drought did not cause any significant change in soil solution chemistry compared with the controls. These results indicated the necessity to consider perturbations induced by extreme climate conditions, like deep soil frost, for the interpretation of soil solution chemistry data in the context of acid deposition studies and forest health.