INCREASED SOIL NITRATE LOSSES UNDER MATURE SUGAR MAPLE TREES AFFECTEDBY EXPERIMENTALLY-INDUCED DEEP FROST

This work reports and discusses data gathered during soil solution mon itoring that was part of an experiment conducted in the Duchesnay Expe rimental Forest (Quebec, Canada) to study the effects of induced deep frost, superficial frost, and superficial frost plus drought on mature sugar maple trees (Acer saccharum Marsh.). Frost treatment was applie d by preventing snow from accumulating under the canopy. Soil solution chemistry was modified when mature sugar maple trees declined after e xposure to a severe deep frost. The first vegetation period after trea tment showed that losses of NO3- below the rooting zone were greatly i ncreased under affected trees. The leaching rate of NO3- and basic cat ions was directly related to the level of change in canopy dieback and transparency. A mean NO3- concentration of 630 mu mol(c) . L(-1) (53 times the controls; max. 4500 mu mol(c) . L(-1)) was measured in soil solution under the deep frost treated trees. The leaching rate of K+ ( 18X, relative to the control) and Mn2+ (11X) was higher than that of M g2+ (5X) and Ca2+ (2.6X). Acidification of the soil solution (50% more H+) as a result of intense nitrification caused an increase in alumin um concentration (5X) and a decrease in SO42-. The acidification durin g the year after treatment was equivalent to decades of atmospheric ac id deposition. The seasonal mean of SO42- did not differ between treat ments, but there was evidence of a significant correlation between pH and SO42- in soil solution. Concentration of NH4+ was also enhanced bu t to a lesser degree (10X) than that of NO3-. Specific conductivity wa s 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 defi cit when NO3- concentrations were high. Superficial frost with or with out induced summer drought did not cause any significant change in soi l solution chemistry compared with the controls. These results indicat ed the necessity to consider perturbations induced by extreme climate conditions, like deep soil frost, for the interpretation of soil solut ion chemistry data in the context of acid deposition studies and fores t health.