Soil microbial community responses to fly ash amendment as revealed by analyses of whole soils and bacterial isolates

Due to its silty texture and plant nutrient content, coal fly ash may prove a valuable amendment to coarse-textured soils. Its effects on soil chemical and physical properties in the field have been studied, but little is known regarding effects on soil microbial communities. In this study, field plots were amended with fly ash at rates of 0 or 505 Mg fly ash ha−1 and subsequently cropped to a fallow–corn–wheat rotation or continuous fescue. Twenty months later, microbial responses to the fly ash were assessed by analyzing the fatty acid composition and carbon substrate utilization potential of microbial communities and aerobic heterotrophic bacteria isolated from the field plots cropped to wheat and fescue. Differences in whole-soil fatty acid profiles from amended and non-amended soils were found. Soils amended with fly ash were enriched in fatty acid 16:1ω5c, and elevated quantities of 17:0 cy and 16:1ω7c were present in fly ash-amended soils cropped to fescue and wheat, respectively. Fatty acid profiles also were affected by cropping system. Extracts from wheat-cropped soils were enriched in 17:1ω7c, while those from fescue plots had greater amounts of 18:2ω6c and 18:1ω9c. Carbon substrate utilization patterns of microbial communities were affected by cropping system but not by fly ash amendment; communities from soils cropped to wheat utilized more carbon substrates than did communities from fescue-cropped soils at the soil dilution tested. Studies of bacterial isolates revealed that Arthrobacter species dominated the culturable, aerobic heterotrophic population, accounting for 25–42% of the total number of isolates recovered from the field plots. Percentages of unidentified isolates also were significant and ranged from 27 to 45% of isolate totals. Effects of fly ash on soil isolates were detected within species of Arthrobacter, with reduced numbers of A. protophormiae in soils amended with fly ash relative to non-amended soils. Overall, the structure of the culturable, aerobic heterotrophic population did not reflect that of the soil community, as fatty acids reported to be markers for Gram-positive organisms were not the major community fatty acids. Enhanced crop growth and soil texture, pH, and nutrient content as a result of fly ash amendment may explain why no detrimental effects to the microbial community were found. Instead, whole-soil fatty acid data indicates that fly ash amendment may benefit fungi and Gram-negative bacteria relative to other components of the soil microbial community.