Carbon Cycling During Sewage Sludge Decomposition in Soils

The objectives of this study were to determine the extent and kinetics of sludge decomposition in soil, to evaluate the effects of sludge addition on microbial degradation of soil organic matter, and to determine the distribution of residual sludge organic C in soil organic matter fractions. Synthetic sewage sludge was prepared by anaerobically digesting a mixture of ¹⁴C-labelled Candida utilis cells and inorganic salts. The prepared sludge had a composition which was similar to that of many anaerobically-digested municipal sludges. Synthetic sludge was added to soils and ¹⁴CO₃ activity and CO₂ production were measured periodically during 336 days of incubation. At the end of incubation, the distribution of residual ¹⁴C in soil organic matter fractions was determined. Sludge decomposition was rapid during the first 28 days of incubation and then decreased to a slower, nearly constant rate for the remainder of the period. At the end of 336 days of incubation, 46% of synthetic sludge organic ¹⁴C was evolved as CO₂. In comparison, 26 to 42% of added organic C was apparently evolved as CO₂ from soils treated with municipal sludges and incubated for 130 days. These findings suggest that a fraction of anaerobically-digested sludge was readily decompossible when added to an aerobic soil; however, the major portion of sludge organic matter (55 to 80%) was resistant to decomposition. There was a linear relationship between cumulative ¹⁴CO₂ evolution and the log of incubation time. From this relationship the turnover time of synthetic sludge in soil was calculated to range from 343 to 890 years. Decomposition of sludge in soil increased the degradation of native soil organic matter by approximately 100% over a period of 336 days. At the end of 168 days of incubation, 62% of the residual sludge organic ¹⁴C added in sludge was recovered from soils as humic and fulvic acids.