Soil microbial activity as an indicator of soil fertility: Long-term effects of municipal sewage sludge on an arid soil

The utility of a selection of soil microbial assays for predicting the effects of land application of municipal sewage sludge on long-term soil fertility was evaluated in a 4 yr study with one unfertilized control plot, and plots with anaerobically digested sewage sludge applied at the optimal rate (based on N requirements) for cotton growth and three times the optimal rate. These loading rates were 8.0 and 24tha^-1 yr^-1 (dry wt) during 4 yr. The soil was a Pima clay loam (Typic Torrifluvent) which was planted to Upland cotton (Gossypium hirsutum L.). After 4 yr of annual sludge application and annual growth of cotton crops, it was found that sewage sludge application had had no significant effect on various measured soil physical and chemical properties other than an increase in available PO₄-P. Total heavy metal contents were not affected by sludge application, but some DTPA-TEA-extractable metals (Zn, Cu, Pb and Ni) increased significantly with sludge treatment. Results also indicated that 4 yr of sewage sludge application had had no significant adverse effect on soil microbial populations or activity. 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 CO₂ evolution. In some cases, there was a significant elevation of a few of the measured variables (dehydrogenase activity and CO₂ evolution). Cotton lint yields in year 4 of the study were not significantly affected by treatment, but plant stand was significantly decreased with higher sludge application. As there was no significant association (Pearson product-moment correlation coefficient or the Kendall ρ b, as applicable) between measurements of microbial activity and cotton plant growth, this study lends no support to using soil microbial activity as measured here as a predictive index of soil fertility as affected by land application of sewage sludge. However, this study does illustrate that long-term applications of sludge to arid southwestern desert soils does not adversely affect microbial populations or activity.