NITROGEN MINERALIZATION AFTER SEWAGE SLUDGE APPLICATIONS TO FOUR CORN CROPS

The available N in sewage sludge-amended soils is one of the restrictive factors for residue application in great amounts. This criterion must be considered in regulations for agricultural use of sewage sludge to avoid environmental pollution of soil and water bodies. The availability of mineral N from native soil organic-N is not considered in the calculation of the maximum rate of sewage sludge application, when these residues are applied for the first time, and this procedure is usual in the mineral fertilizer recommendations. Continuous sewage-sludge applications to soils may however cause cumulative residual effects on mineral N generation, which would imply in specific regulations for re-applications in the same area. The purpose of this study was to verify the potential availability of mineral N in a Latossol under corn previously treated with sewage sludge. The soil samples (0-0.20 m) for this study were collected in a field experiment in Jaguariuna (Sao Paulo State, Brazil), between 1999 and 2002, where the treatments consisted of four consecutive applications, with a total application of 0; 14,716; 29,432; 58,864 and 117,728 kg ha(-1) of urban sludge from the Franca sewage treatment plant Franca and 0; 22,700; 45,400; 90,800 and 181,600 kg ha(-1) of industrial and urban sludge from the Barueri sewage treatment plant. After sampling, the treatments were incubated for 15 weeks in a laboratory. At the beginning of incubation, a residual effect of previous applications was observed on organic N and nitrate accumulation. At the end of the incubation period, the net inorganic N accumulation increased in the soil and pH had decreased, both proportional to the rates of organic N applied previously as sludge in the field. The N mineralization potential estimated by the single exponential model was 28 mg kg(-1) for soil without sludge, and ranged from 28 to 100 mg kg(-1) for soil treated with Franca sludge, and from 40 to 113 mg kg(-1) for soil treated with Barueri sludge. It was concluded that N residual effects must be considered prior to sewage sludge re-application to a same soil. The N mineralization potential of soil and sewage sludge as well as nitrate accumulation in the soil profile have to be determined to calculate adequate sludge rates.