Cumulative Effects of Simulated Acid Rain on Soil Chemical and Microbial Characteristics and Conifer Seedling Growth

Abstract

AbstractEffects of simulated acid rain treatments of HNO3 and H2SO4, in the mole ratio of 3:1 and pH 5.6, 4.5, 4.0, and 3.0, were tested on seedlings of Douglas‐fir (Pseudotsuga menzesii [Mirb.] Franco), sugar pine (Pinus lambertiana Dougl.), and ponderosa pine (Pinus ponderosa Laws) growing in A‐horizon samples of two forest soils. Two wet periods of 34 and 27 weeks duration, separated by a dry period of 18 weeks, simulated climatic conditions common in the Sierra Nevada, California. Representative results are presented for soil supporting Douglas‐fir, plus results of growth and shoot‐N for all three species. Effects differed between soils but were usually increased from one wet period to the next, and markedly decreased with soil depth. Effects commonly appeared in the pH 3.0 treatment only. Little or no effects were noted for seedling growth, visual symptoms of foliar damage, N concentration in shoots, or for the general soil microbial assays of CO2 evolution and microbial biomass. There were notable decreases in exchangeable cations and soil pH, and increases in soil Al, Mn, Fe, and Zn. Effects on N transformations in soil were inconsistent, but in the pH 3.0 treatment there were some significant increases in mineralizable N and denitrification potential, and decreases in nitrification potential. Similar results were obtained for N transformations in one of the same forest soils supporting ponderosa pine in an earlier experiment of only 12 weeks duration, but at greater acidity of pH 2.0.