Mineralization of Nitrogen in Sewage Sludges

Abstract

Mineralization of organic N is one of the principle factors governing the annual amount of sewage sludge applied to agricultural cropland. The mineralization of sludge organic N was studied in a Fincastle silt loam soil treated with 24 sewage sludges obtained from sewage treatment plants in several regions of the United States. The mineralization of sludge organic N was determined during a 16‐week incubation by analysis of inorganic N in (i) leachates from soil‐sludge‐sand mixtures and (ii) subsamples of unleached soil‐sludge mixtures. The nonleached procedure gave slightly higher estimates of mineralizable N in most sludges (average of 17 and 16% N mineralized by nonleached and leached procedures, respectively). The approximate amounts of mineralizable N in sludges expressed as a percentage of organic N were: raw and primary, 25%; waste‐activated, 40%; anaerobically digested, 15%; and composted, 8%. Inorganic N released from sludges by autoclaving in 0.01M CaCl₂ or extracting with H₂SO₄ + KMnO₄ was not highly correlated with N mineralization. Sludge organic N content was a reasonable predictor of mineralization of sludge N added to soils. The following regression equation was found to describe the mineralization of sludge organic N in soil: % of added sludge N mineralized = 6.37 × % sludge organic N + 4.63 (r = 0.77). To calculate the appropriate sludge application rates for agronomic crops, the percentage of sludge N mineralized during the initial cropping season should be related either to the process used for sludge treatment or to the concentration of organic N in the sludge.