Use of a municipal sewage sludge as a source of fertilizer nitrogen : nitrogen availability and cadmium-zinc interactions

A mixture of primary and aerobic secondary municipal sewage sludge was obtained from Maryville, Tennessee to determine its suitability as a fertilizer for corn (Zea mays L.). In the field experiment, a check, an inorganic N fertilization rate (168 kg ha^-1 as NH₄NO₃), and three application rates of sludge (17, 34, and 51 Mg ha^-1 at 0.18 kg kg^-1 solids were compared for crop response at two levels of soil pH (pH < 5.5 and pH ≥ 6.0). Sludge additions increased silage and grain yields due to increased available N supplied by the sludge. The silage yields from the sludge application of 51 mg ha^-1 were similar to the inorganic N application rate of 168 kg ha^-1. Grain yields from the highest sludge application were not as great as from the inorganic N application, probably due to a lower availability of N from the organic source. When considering potential toxicity to the crop as well as to potential consumers, there were no harmful levels of Cu, Ni, Cd, and Zn present in leaf tissues or grain even with a moderately acidic soil (pH 5.2).

A greenhouse experiment was initiated to analyze the interaction of Cd and Zn in a soil, sludge, and plant system. Sludge obtained from Maryville, Tennessee was spiked with four rates of Cd and Zn in a completely randomized factorial arrangement and applied to soil at application rates of 7, 229, 451, and 673 μmol Cd kg^-1 and 0.9, 4.7, 8.5, and 12.3 mmol Zn kg^-1 The highest Cd and Zn additions resulted in yield increments of 11% and 20% respectively. The modest yield reduction due to Cd was attributed to the high percentage of fixed by the sludge and sequestered in the roots. Zinc effectively competed with Cd uptake as evidenced by a decrease in Cd content of roots with increased Zn additions. While decreasing Cd uptake, Zn had an effect of increasing Cd translocation from roots to tops. Cadmium, on the other hand, decreased Zn translocation from roots to tops. Estimations of Cd⁺² and Zn⁺² activities in soil solution were attempted using chelator and total soil solution methods. Calculated Cd⁺² and Zn⁺² activities proved to be overestimates of actual activities in soil solutions. Total soil solution Zn concentration and DTPA extractable Zn were good measurements of soil Zn availability.