Throughfall chemistry of ponderosa pine exposed to wet and dry acidic deposition, drought, ozone, and nitric acid

Throughfall chemistry was studied in ponderosa pine (Pinus ponderosa Laws.) seedlings exposed to wet and dry acidic deposition, ozone (O3), drought, and nitric acid (HNO3) in open-top chambers in the Sierra Nevada of California from 1988 to 1990. Seven acid rain events (pH 3.5, 4.4, and 5.3) occurred during each growing season. Acidity of the simulated rain treatments had little effect on net throughfall fluxes except for greater net SO4(2-) content in throughfall of the pH 3.5 treatment. Electrical conductivity and NO3- and NH4+ content of throughfall were significantly higher in the NF150 treatment (1.5 X ambient 03) than in the charcoal-filtered (CF) or non-charcoal-filtered (NF) treatments in all 3 years. Throughfall acidity in the NF150 treatment was higher than that in the CF or NF treatment during the last 2 years of the study. Effects of the NF150 treatment on throughfall chemistry were attributed primarily to deposition of HNO3 from higher N oxides produced by the air-supplied 03 generator. The concentration of HNO3 in the NF150 chambers was typical for forests in the United States but only 26% as high as that at Tanbark Flat in the Los Angeles Air Basin. We estimated that co-generated HNO3 contributed 0.5% of the total N in current-year foliage of seedlings in the NF150 treatment. Irrigation treatment did not have a significant effect on throughfall chemistry except for a slightly greater decrease in throughfall pH in the well-watered plants compared with the drought-stressed plants in the pH 5.3 rain treatment in 1989. Absolute values for electrical conductivity and NO3- and NH4+ content in throughfall tended to be higher with greater particulate dry deposition (45-55% removed versus 90% removed by filtration) in 1989 and 1990, but differences were not statistically significant.