Residual Nitrogen Availability from Soils Treated with Sewage Sludge in a Field Experiment

In this experiment we determined the residual nitrogen availability from sewage sludges applied to a Hubbard sandy loam soil. In Study I, three sludge types—anaerobically digested (119, 237, and 466 metric tons/ha), aerobically digested (244 metric tons/ha), and mixed primary-settled and waste-activated (493 metric tons/ha)—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 had been applied (63, 125, and 200 metric tons/ha) 4 years earlier. Several biological and chemical characteristics were examined as indexes of N availability. Cumulative N mineralization during aerobic incubation was approximately linearly related to time¹/² for all treatments. Index values for Study I samples varied widely, hut in general, reflected the quantity of sludge added: total soil C (1.05 to 4.66%), total soil N (0.092 to 0.608%), indigenous inorganic N (12 to 266 µg/g), N mineralized during 52 weeks of aerobic incubation (87 to 884 µg/g) and during 16 weeks of anaerobic incubation (14 to 525 µg/g), nitrogen mineralization potential, Nₒ (126 to 1,010 µg/g), and autoclave-extractable N (76 to 441 µg/g). 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 (Zea mays L.) (Study I) or sorghum-sudangrass (Sorghum vulgare sudanese L., var. ‘Trudan V’) grown in the greenhouse (Study II). Each index was useful in determining relative differences of N availability for this soil having received greatly differing organic N treatments from sewage sludge. Nitrogen mineralized during short and long periods of incubation correlated with plant N uptake equally well. The chemical indexes of N availability were as reliable as more time-consuming incubation procedures for determining the N-supplying capacity of soils.