| Summary: | This study measured the effects of four years of municipal, anaerobically digested sewage sludge application on long-term soil microbial activity in a Pima clay loam (Typic Torrifluvent) growing Upland cotton (Gossypium hirsutum L.). Treatments were one unfertilized control, sludge applied at optimum rates for plant growth (based on N requirements), and sludge applied at three times the optimum rates (a total of 8.01 and 24.2 Mg ha⁻¹ (dry weight over four years)). Soil microbial activity was measured by viable heterotrophic plate counts for bacteria, actinomycetes and fungi; acridine orange direct counts for bacteria; the dehydrogenase assay; and carbon dioxide evolution analysis. As the high sludge treatment significantly reduced cotton plant stand and significantly stimulated some parameters of microbial activity (dehydrogenase activity and CO₂ evolution), soil microbial activity may not serve as a reliable predictive index of plant response to sludge-applied pollutants.
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