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Nitrogen Transformations in Soils Previously Amended with Sewage Sludge

Lab. of Microbial Ecology, Div. of Applied Sciences, Harvard Univ., Cambridge, MA 02138

E. A. Paul

Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824

* Corresponding author.

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 yr 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.

Contribution from the Dep. of Plant and Soil Biology, Univ. of California, Berkeley.

Received for publication June 20, 1988.