Microbial responses to coal fly ash under field conditions

Coal fly ash may be a valuable soil amendment because it contains plant nutrients and liming agents and has a silty texture that can improve the water-holding capacities of sandy, drought-prone soils. Short-term laboratory studies have indicated that addition of unweathered fly ash to soil can stress microbial populations and their activities, but effects of fly ash addition at the field scale are not known. In this study, field plots received 0 or 505 Mg fly ash ha(-1) (incorporated by conventional tillage to a depth of 40 cm) and were subsequently cropped to a fallow-corn (Zea mays L.)-wheat (Triticum aestivum L.) rotation or continuous fescue (Festuca arundinacea Schreb,). Twenty months later, during the wheat phase of the rotation, the plots were sampled (0-15 cm) and assayed for activity of soil enzymes (dehydrogenase, alkaline phosphatase, arylsulfatase, and denitrifying enzymes); numbers of aerobic heterotrophs, ammonium oxidizers, denitrifiers, and bradyrhizobia; and N mineralization, nitrification, and denitrification potentials. Nitrification potentials doubled in ny ash-amended soils, and numbers of denitrifiers were 200-fold higher in fescue-cropped, fly ash-amended soils relative to fescue-cropped, non-amended soils. No other large differences in microbial populations or activities were found. The lack of detrimental effects on microorganisms in the field was possibly due to reductions in fly ash's soluble salt and trace element concentrations with time, the mild alkalinity of the ny ash used in this study, and the positive responses of crops to fly ash amendment.