Effects of Different Sewage Sludges on Some Chemical and Biological Characteristics of Soil

Chemical and biological characteristics of an aerobically digested and three anaerobically digested sewage sludges from metropolitan Syracuse, N.Y. were analyzed. The aerobic digest had a higher organic fraction (ash 39%, carbohydrate 14%), more nitrogen (total N 4.2%), and more phosphorus (total P 1.71%) than the three anaerobic digests (ash 61–80%, carbohydrate 4–6%, total N 0.6–1.3%, and total P 0.75–1.71%). Potassium concentrations were low, and heavy metals varied among sludges. In the two sludges taken directly from centrifuges, nematode density was low which contrasted to the high density of nematodes in sludge from drying beds. Enchytraeidae were found only in the aerobically digested sludge. These sludges were applied separately onto pulverized soil (Teel silt loam) in randomized plots within a glasshouse. Samples were taken at intervals for 260 days and analyzed for chemical and biological components. Rates of change in ash, carbohydrate, total N, and total P were proportional to the original magnitude of these parameters, highest rates being exhibited by samples from plots amended with the aerobic digest; the aerobic digest lost 48% of its organic content in 162 days. Few significant changes over time were noted in available P, CEC, Cu, Zn, and K for a given plot type. The earthworm Eisenia foetida was placed in the plots after 90 days since earlier experiments had shown that two of the fresh anaerobic sludges killed the worms. Earthworms were thriving 170 days after introduction. There were no significant differences between plot types for catalase, peroxidase, or tyrosinase because of high intraplot variation. Total and viable bacterial populations were higher in the sludge amended plots than in arable soils, and declined with time. Mite and collembola populations were low in the sludge-amended soil but similar to populations in arable soils. Those plots, which were amended with sludges originally deficient in nematodes, showed a rapid increase in these worms, which were bacterial feeders and predominantly Rhabditidae. Enchytraeids increased in all sludge-amended plots at a slower rate than the nematodes. The plots converged over time with respect to biological and chemical characteristics.