Mineralization of Nitrogen from Sewage Sludge and Sludge Compost

An incubation study was conducted to determine N mineralization from sewage sludge and sludge compost. Digested and raw sludges and compost derived from these sludges were freeze‐dried and applied to a silt loam soil at 0, 454, 907, and 1,814 kg N/ha. The soil‐sludge mixtures were incubated at 35°C for various time intervals and periodically leached and analyzed for total N, NH4‐N, and NO3‐N. A separate study using K15NO3 was conducted to evaluate N transformations, such as immobilization and denitrification. The overall data indicate that sewage sludge and sludge compost differ markedly in their initial net mineralization patterns. Amendments high in available C (raw sludge) are biologically very active and significant quantities of NO3‐N can be lost by denitrification and immobilization soon after addition. The N in amendments that have been stabilized by removal of readily oxidizable C (digested sludge) gave net mineralization patterns similar to native soil organic N, although large amounts of N can be released from digested sludge. The N in amendments that have been stabilized by composting with woodchips is not easily mineralized. With time these initial differences among sources decrease and the mineralization rates become more alike, although the time needed to reach this point depends on the quantity of amendment added.