Decomposition of Sewage Sludge Compost in Soil: III. Carbon, Nitrogen, and Phosphorus Transformations in Different Sized Fractions

Abstract

AbstractA laboratory incubation study was conducted to determine the rate and extent of decomposition of sewage sludge compost in soil amended with compost fractions of various particle sizes. Cured compost was passed through a 6‐mm screen and further fractionated with a 1‐mm sieve. The three fractions of compost (<6 mm, 1 to 6 mm, and <1 mm) were mixed with a loamy sand at a rate of 89.6 metric tons/ha (dry wt) and incubated at 25°C. Rates of mineralization and decomposition were determined by monitoring CO2 and NH3 evolution and measuring changes in the organic and inorganic fractions of C, N, and P with time. A lime‐treated soil was included as an appropriate control because the compost was produced from a lime‐stabilized undigested (raw) sewage sludge.Decomposition of the three compost fractions, as determined by CO2 evolution, was directly related to the amount of carbon in the compost‐soil mixtures. Cumulative CO2 evolution was lowest from the compost that passed the 1‐mm sieve and had the lowest C/N ratio. Approximately 8% of the compost C from all fractions evolved as CO2 in 45 days of incubation. The quantity of N mineralized ranged from 3 to 13% for the different fractions, and the extent of N mineralization was inversely related to the C/N ratio. Ammonia evolution paralleled N mineralization in these mixtures. The amount of extractable P was nearly the same for all fractions and did not change during incubation. Although the <1‐mm fraction contained a greater amount of total P than the other two fractions, it contained a smaller amount of extractable P. When the pH was adjusted to 6.6, decomposition of the native soil C increased 82%, but neither soil N mineralization nor the amount of extractable P was affected.