EFFECT OF COMPOSTED SEWAGE-SLUDGE AMENDMENT ON SOIL-NITROGEN AND PHOSPHORUS AVAILABILITY

Municipal sewage sludge previously composted with sawdust (CSS) was ap plied to an eutric sandy cambisol at rates of 7.5, 15.0, 22.5, and 30 g.kg(-1). Incubation and pot experiments were conducted to evaluate CS S effectiveness on nitrogen (N) and phosphorus (P) soil availability a nd on plant nutrition. The CSS rates did not increase soil mineral N a nd had little effect on organic P and on labile forms of P. Efficiency of total applied P was 17% for the soil labile forms and 4.8% for the resin extractable fraction. In contrast, CSS significantly increased hydroxide extractable inorganic P and nonextractable soil P fraction. The major portion of the increment on nonextractable forms was at the expense of HCl extractable P fraction [calcium (Ca)-bounded], dominant on the original CSS. Thus, chemical rather than biological reactions lead to the redistribution of CSS-borne P to more firmly held forms af ter its application to the soil. Ryegrass dry matter yield, N content, and N uptake did not increase in CSS-treated soils. Plant P content i ncreased at the second harvest, but the effect was nil in the subseque nt harvest. Total P uptake increased from 14.1 to 20.2 mg.pot(-1), but percentage P recovery by ryegrass was modest, averaging 2.5% of the C SS-borne P. Results suggest that moderate application of CSS to agricu ltural systems are inadequate for crop growth but may contribute to nu trient recycling without environmental risks related to N and P loss.