POTENTIAL MECHANISMS OF ACID MIST INJURY TO RED SPRUCE

One-year-old red spruce seedlings (Picea rubens Sarg.) were exposed to six simulated mist treatments in open-top chambers, between 25 May an d 8 November 1988. Mists were applied twice weekly to foilage and soil , with each application equivalent to 2 mm precipitation, applied over 30 min. The treatments contained pairwise combinations of H+, NH4+, S O4(2-) and NO3- ions at concentrations of 1.6 or 3.2 mol m-3, with one treatment including all four ions. Effects of individual ions and ion combinations on visible injury, growth and nutrition were determined. After four applications of H2SO4 at 3.2 mol m-3 (pH 2.5), current yea r needles turned red-brown. Subsequent application of H2SO4 at a reduc ed concentration of 0.5 mol m-3 (pH 3.0) resulted in no further increa se in foliar injury symptoms. No foliar injury symptoms developed on s eedlings receiving HNO3 (pH 2.5), NH4NO3 or (NH4)2SO4 treatments. Twen ty-five per cent of needles were damaged on seedlings receiving H2SO4 + NH4NO3 (pH 2.5), but only after 24 applications of mist, suggesting that either NH4+ or NO3- ions ameliorated the effect of H2SO4. There w as a stimulation in shoot and root biomass in those treatments contain ing N, with or without H2SO4. Contrast analysis indicated a positive g rowth response to acidity. Significant uptake of S and N occurred in r esponse to treatment. This experiment indicates that it is the simulta neous presence of H+ and SO4(2-) in occult precipitation that has the greatest potential for inducing foliar injury. The presence of NO3- wa s found to ameliorate the toxic effects. Potential mechanisms underpin ning these observations are discussed.