INFLUENCE OF FLY-ASH ON SOIL MICROBIAL ACTIVITY AND POPULATIONS

Incubation studies were conducted to assess the influence of various combination levels of alkaline (pH 12.2) powerplant fly ash and sewage sludge on soil microbial activity and numbers. A Glynwood (fine, illitic, mesic, Aquic Hapludalf) silt loam soil was mixed with 0, 5, 10, or 20% (w/w) ash and 0 or 5% composted sewage sludge. Respiration was strongly depressed in the 10 and 20% ash treatments over 28 d. Sludge addition improved respiration in all ash treatments except the 20% treatment. Total bacterial, actinomycete, and fungal counts in the soil typically decreased with increasing ash content. Counts were depressed by 57, 80, and 86%, respectively, at the 20% ash application rate. Sludge application increased microbial numbers but all populations were lower at the highest ash rates compared to the untreated control. Soil phosphatase, sulfatase, dehydrogenase, and invertase were inhibited as ash treatment levels increased. Catalase activity was not significantly affected by ash concentration. Addition of the sludge to the ash-soil mixtures generally enhanced enzyme activity. The results indicate that high rates of fly ash to soils may hinder normal decomposition and nutrient cycling processes. This may be partially alleviated, however, by coapplication of a readily oxidizable organic substrate such as sewage sludge.