Alpine Tundra Soil Bacterial Responses to Increased Soil Loading Rates of Acid Precipitation, Nitrate, and Sulfate, Front Range, Colorado, U.S.A.

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Abstract
To determine if the soil bacterial community exhibited a response to perturbations from acid rain, the effects of increased soil loading rates of acid precipitation, nitrate, and sulfate on the alpine tundra soil bacterial community were investigated under field conditions along Niwot Ridge, Colorado. During late June and July three concentrations, in a total volume of 10 liters representing 2.5, 5, and 10 times the normal soil loading rates of H2SO4/HNO3, SO42-/NO3- and NO3- were applied in an aqueous solution weekly to separate 1-m2 test plots. After three applications samples were collected from each test plot, untreated control plots, and control plots where an equivalent amount of water was added. Using one-way analysis of variance it was determined that increased soil loading rates of acid were significantly (p .ltoreq. 0.02) associated with decreases in the total soil bacterial population, bacterial diversity, populations of nitrifiers, and populations expressing lipolytic activity compared to the water control. In contrast, significant increases in the actinomycete population, fungal population, and populations expressing proteolytic activity were associated with an increase in acid soil loading rates. Populations of nitrifiers, and nitrogen fixers were significantly inhibited by increased soil loading rates of NO3- and NO3-/SO42- mixtures. Denitrifiers and organisms exhibiting proteolytic activity increased with addition of NO3- to the soil. The results indicated that the bacterial community exhibited an immediate dosage-dependent response to this type of perturbation.