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Acid Rain and Soil Microbial Activity: Effects and Their Mechanisms¹

ABSTRACT

In this investigation, our aim was to determine if acid rain affects soil microbial activity and to identify possible mechanisms of observed effects. A Sierran forest soil (pit 6.4) planted with Ponderosa pine seedlings was exposed to simulated rain (pH 2.0, 3.0, 4.0, and 5.6) with ionic composition reflecting that found in northern California, corresponding to 15 cm of precipitation over a 12-week period. After exposure, the soils were collected in two samples (top 1 cm and 4 to 5 cm), which were assayed separately for respiration and enzyme activities (urease, phosphatase, dehydrogenase, and arylsulfatase). Changes in microbial activity were most significant in surface soils. Only the pH 2.0 input caused inhibition of both respiration and enzyme activities. The overall microbial response to the pH 3.0 and 4.0 acid regimes was one of stimulation, although the response of individual enzymes was more varied. In surface soils receiving the pH 3.0 input, for example, urease activity was unaffected while arylsulfatase and phosphatase were increased and decreased respectively. Therefore, individual microbial processes will have different sensitivities to acid rain. Although changes in C-availability in the exposed soils are documented, changes in the supply of N are evaluated as the major mechanism through which simulated acid rain affects soil microbial activity.

Key Words: acid inputs • anionic composition • carbon availability • nitrogen availability

¹ Contribution of the Dep. of Plant and Soil Biology, University of California, Berkeley, CA 94720. This study was supported by a subcontract from the EPA/NCSU Acid Precipitation Program, EPA Cooperative Agreement CR806912 between the U.S. Environmental Protection Agency and North Carolina State University.

² Postdoctoral Associate, Assistant Professor, and Associate Professor, respectively, University of California, Berkeley.

Received for publication May 3, 1982.

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