RESIDUAL NITROGEN AVAILABILITY FROM SOILS TREATED WITH SEWAGE-SLUDGE IN A FIELD EXPERIMENT

The paper reports on a study that determined the residual nitrogen availability from sewage sludges applied to a Hubbard sandy loam soil. In Study I, three sludge types - anaerobically digested, aerobically digested, and mixed primary-settled and waste-activated were applied over a 3-year period. Soil samples were collected from the field experiment 2 years after the final sludge application. Study II samples were obtained from plots of a field experiment in which an anaerobically digested sludge has been applied 4 year earlier. Several biological and chemical characteristics were examined as indexes of N availability and are discussed in the paper. It was found that the sludge application rate was more important than the sludge type in affecting N mineralization. The sludge-treated soils from Study II had lower and less varied index values than those from Study I. The evaluation of N-availability indexes was based on the degree of correlation between the index values and N uptake by irrigated, field grown maize (Study I) or sorghum-sudangrass grown in the greenhouse (Study II).