The objective of this study was to determine the effect of application of increasing doses of an alkaline biosolid on carbon (C) and nitrogen (N) stocks, as well as on the organic matter quality of an Oxisol cultivated with eucalyptus, after five years of biosolids application. The study was conducted at the Experimental Station of ESALQ/USP, in Itatinga County, São Paulo State, Brazil. The experiment was initiated in March 1998 on an area where a seven-year-old Eucalyptus grandis plantation had been harvested and substituted by a new one planted under the minimum cultivation system. Four months later, anaerobic digested biosolid with original moisture content was applied over the soil surface and in-between plant rows, with no posterior incorporation. Five treatments were evaluated: (a) Control; (b) Mineral Fertilization with N, P, K, B and Zn (Mineral Fert.); (c) 10 t ha-1 of biosolids + K (10 t ha-1 + K); (d) 20 t ha-1 of biosolids + K (20 t ha-1 + K); e (e) 40 t ha-1 of biosolids + K (40 t ha-1 + K). Soil samples were collected in layers 0-5, 5-10, 10-20, 20-30, 30-60 cm deep in September 2003, after five years of biosolids application. Total C concentrations and soil apparent density were determined for all depths. In samples collected at a depth of 20 cm the pH, organic C fractions based on oxidation degrees, soluble sugars, protein, lipids, hemi cellulose, cellulose and lignin, potential CEC (at pH 7.0), and actual CEC (at natural pH) were determined. After five years of biosolids application, no differences were observed among the treatments in relation to the total C and N concentrations and stocks, soil density, and organic C fractions. Mean stocks of C were 26.52; 7.96; 10.31; 12.88; and 31.19 t ha-1; and N 0.97; 0.39; 0.99; 0.63; and 0.34 t ha-1 for the soil layers 0-5, 5-10, 10-20, 20-30, and 30-60 cm, respectively. About 50 % of the total C was in the labile compartment, which is typical for areas with species and/or management practices that favor the return of plant residues back into the soil. Among the organic compounds, only lignin presented treatment-related changes. The treatments 40 t ha-1 + K and mineral fertilizer enriched the soil organic matter (0-5 cm) with lignin in comparison to the others. This effect was attributed to the larger deposition of eucalyptus leaves in these two treatments and the recalcitrant nature of lignin. The results of CEC indicate that the quality of soil organic matter had not changed after five years of biosolids or mineral fertilizer application, at least in relation to this property. CEC (at natural soil pH) was more influenced by soil pH values than by the amounts of C.
sewage sludge; Eucalyptus grandis; CEC; organic carbon fractionation; approximated analyses; organic compounds
