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 chemica l and physical properties in the field have been studied, but little is kno wn regarding effects on soil microbial communities. In this study, field pl ots were amended with fly ash at rates of 0 or 505 Mg fly ash ha(-1) and su bsequently cropped to a fallow-com-wheat rotation or continuous fescue. Twe nty months later, microbial responses to the fly ash were assessed by analy zing the fatty acid composition and carbon substrate utilization potential of microbial communities and aerobic heterotrophic bacteria isolated from t he 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 16:1 omega 5c, and elevated quant ities of 17:0 cy and 16:1 omega 7c were present in fly ash-amended soils cr opped to fescue and wheat, respectively, Fatty acid profiles also were affe cted by cropping system. Extracts from wheat-cropped soils were enriched in 17:1 w7c, while those from fescue plots had greater amounts of 18:2 omega 6c and 18:1 omega 9c. Carbon substrate utilization patterns of microbial co mmunities were affected by cropping system but not by fly ash amendment; co mmunities from soils cropped to wheat utilized more carbon substrates than did communities from fescue-cropped soils at the soil dilution tested. Stud ies 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 uni dentified isolates also were significant and ranged from 27 to 45% of isola te 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 so il community, as fatty acids reported to be markers for Gram-positive organ isms were not the major community fatty acids. Enhanced crop growth and soi l texture, pH, and nutrient content as a result of fly ash amendment may ex plain why no detrimental effects to the microbial community were found. Ins tead, whole-soil fatty acid data indicates that fly ash amendment may benef it fungi and Gram-negative bacteria relative to other components of the soi l microbial community. (C) 2001 Elsevier Science Ltd. All rights reserved.