Nitrogen mineralization in a tropical soil amended with sewage sludges

Mineralization of organic N contained in sewage sludges is a key factor in determining the rates of these materials to be applied to agricultural soils. This work aimed at quantifying the potential of mineralization of the organic N in an Oxisol treated with two anaerobically-digested sewage sludges (one derived from strictly urban sewage-Franca, Sao Paulo State, Brazil -, and the other from a mixture of urban and industrial sewages-Barueri, Sao Paulo State). The sludges were applied at four rates: 1.5, 3, 6 and 12 g kg(-1) (Franca), and 4, 8,16 and 32 g kg(-1) (Barueri), and the incubation period, was 15 weeks. Net inorganic N accumulation in the soil measured at the end of the incubation period was proportional to the rate of organic N applied as sludges. The potential of mineralization estimated by the single exponential model for the soil without sludge was 24 mg kg(-1) of N, and ranged from 44 to 265 mg kg(-1) of N, for the sludge-treated soil. The fraction of potential mineralization of the organic N contained in the sludges was estimated in 31%. N mineralization was slower in the soil treated with the two higher rates of the Barueri sludge. Sewage sludges caused soil acidification; Franca sludge was more effective in this way than Barueri sludge.