ORGANIC NITROGEN MINERALIZATION RATE FOLLOWING THE FIFTH SEWAGE SLUDGE APPLICATION IN AN OXISOL

In agricultural systems treated with sewage sludge, the management ought to seek an adequate N supply to plants, while avoiding nitrate enrichment because of its potential to pollute subsurface water bodies. Thus, when determining the maximum amounts to be applied to a specific crop, some sewage sludge and soil properties should be known. One is the fraction of organic N of the waste that will be mineralized (NMF) during the crop cycle. This quantity, combined with mineral N in the sludge, determines the amount of N in sewage sludge that will be available during the growing season. This study aimed to determine whether previous applications of sewage sludge affect the NMF of the residue recently applied to a Dark Red Dystroferric Latosol (Oxisol). The soil had been previously treated in four successive maize crops, with four rates and two types of sewage sludge, one of urban origin (Franca, SP) and the other from urban-industrial sources (Barueri, SP). The study of N mineralization was installed in laboratory in a randomized blocks design with three replications and lasted 15 weeks. In the fifth application, sewage sludge doses of equivalent to 160, 320, 640, and 1,280 mg kg(-1) organic N were studied. On seven dates, the pH and the ammonium-N and nitrate-N concentration were determined. The NMF was calculated with data obtained for the net mineralization measured 105 days after the start of incubation. The performance of the two sludge types was similar, with a high initial mineralization rate. The ammonium levels remained low after the start until the end of incubation and there was a linear increase in nitrate levels during the same period. Nitrate accumulation peaked at the highest sewage sludge dose applied. The potential for mineralization, estimated by the simple exponential model, varied from 58 to 284 mg kg(-1) N. No effect of previously applied waste on the mineralization fraction of recently applied organic N by sewage sludge to the Oxisol was observed, with a reliability of 95 %. Results showed that for reapplications to the same area, sewage sludge rates must be lower than the doses calculated for a single application, due to accumulations of organic and inorganic-N in the soil.