Microbial Populations in an Agronomically Managed Mollisol Treated with Simulated Acid Rain

Abstract

AbstractA fertile well‐buffered mollisol (Flanagan silt loam, fine montmorillonitic mesic Aquic Argiudoll) under cultivation with corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] was subjected to simulated rain of pH 5.6, 4.2, and 3.0, while moisture‐activated rain exclusion shelters provided protection from ambient rain. Soil was sampled to a depth of 3 cm on four dates throughout the 1985 growing season. The following microorganisms were enumerated by plate counts or most probable number: general heterotrophic bacteria, general soil fungi, free‐living N‐fixing bacteria, fluorescent pseudomonads, autotrophic ammonium‐oxidizing, nitrite‐oxidizing, and thiosulfate‐oxidizing bacteria. The ANOVA was used to determine the combined and individual effects of rain treatments, crop field, and sampling date. Crop field and sampling date affected microbial numbers more than rain treatments. Overall, rain treatment effects were limited to nitrite‐oxidizing bacteria; lower numbers occurred in the corn field in subplots treated with rain of pH 4.2 and 3.0, and in the soybean field treated with rain of pH 3.0. The trend was strongest in June and July. In the corn field in subplots treated with rain of pH 3.0, numbers of thiosulfate‐oxidizing bacteria were higher and numbers of general heterotrophic bacteria were lower; however, these trends were comparatively weak. Rain treatments caused essentially no decrease in soil pH, suggesting that acid rain constituents affect certain microbial populations without causing overt changes in pH. Because they appear to be unusually sensitive, nitrite‐oxidizing bacteria could be used as experimental indicators of changes in soil microbial communities subjected to acid rain.