Nitrogen transformations in soils previously amended with sewage sludge

Abstract

This short-term (10-d) incubation experiment established the rates of nitrogen (N) transformations occurring in sludge-amended and nonamended soil. Utilizing a nitrification block (C2H2) with (15NH4)2SO4, first-order rate constants were calculated for N immobilization, ammonification, nitrification, and denitrification. These rate constants were compared to values obtained after a long-term (87-wk) incubation performed on soils sampled from the same field plots. The short-term rates of ammonification were still higher than the controls 4 year after the last sludge addition. Sludge applications over an 8-yr period (180 Mg ha−1 yr−1) reduced soil nitrification potential compared to the controls when spiked with 15N. Denitrification did not cause a significant loss of N during either a short- or long-term incubation period. The microbial biomass in the sludge-amended soil contained more N, which resulted in a microbial C/N ratio of approximately 4:1 vs. 5:1 for the controls. Initial (short-term) N immobilization rate constants were 0.43 for the sludge-amended and 0.35 for the nonamended soil.