The effect of acid rain on the growth and nutrient content of two species of hardwood tree seedlings, and on the pH, microflora and nutrient content of the soil

Abstract

<p>The increasing acidity of precipitation may cause foliage damage, decrease plant growth and leach essential nutrients from the soil. The purpose of this study was to determine the sensitivity of two hardwood tree species and three soils to simulated acid rain. The value of infecting tree seedlings with mycorrhizal fungi before transplanting into acid-leached soils was also examined;Dundas silt loam, Hayden loam and Luther loam supporting coleus and sorghum were leached three times each week for 20 weeks with a mixture of H(,2)SO(,4) and HNO(,3) adjusted by dilution to pH 5.5, 4.0 or 2.5. Ninety seedlings each of green ash (Fraxinus pennsylvanica Marsh.) and silver maple (Acer saccharinum L.) were transplanted into the soils and were misted twice weekly with the acid solutions;Ash seedlings misted with the pH 2.5 solution were shorter, had a smaller total leaf area and produced less leaf and stem material than ash seedlings misted with pH 5.5 solution. In contrast, the growth and weight of the maple seedlings increased in response to the pH 2.5 treatments. Leaves of the ash exposed to the pH 2.5 treatment were covered with numerous small lesions, and the cuticular wax was partially worn away. No lesions were observed on the leaves of the maple. Nitrogen concentrations in the leaves, stems and roots of both species increased with increasing acidity, while phosphorus concentration in the leaves decreased. Potassium and calcium levels were unaffected by the acidity of the rain. Soil type and preinfection with mycorrhizal fungi had little effect on plant growth or nutrient content;Soils became increasingly acidic as the acid treatments continued. Numbers of fungi and bacteria in the soil decreased in the pH 2.5 treatment after 20 weeks, but were not significantly different from the pH 5.5 treatment after 52 weeks. Nitrogen and iron concentrations in soils treated with pH 2.5 solutions increased, while potassium concentrations decreased. Phosphorus and calcium levels were unaffected by the solution acidity;The growth of the maple seedlings increased in response to the fertilizing effect of nitrogen in the acid solutions. The extensive foliage damage which occurred on the ash seedlings very likely reduced the photosynthesizing ability of the plants. The resulting decrease in carbohydrates masked the beneficial effect of increased nitrogen availability.</p>