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Previous Sludge Addition Effects on Nitrogen Mineralization in Freshly Amended Soil

Dep. of Agronomy and Horticulture

N. S. Urquhart

Dep. of Experimental Statistics, New Mexico State Univ., Las Cruces, NM 88003

* Corresponding author.

ABSTRACT

An experiment was conducted to determine the effects of previous sludge additions on N mineralization in a freshly amended soil. A secondary objective was to further delineate statistical models of N mineralization potential (N_o) and the mineralization rate constants k and h. The Glendale clay soil (fine-silty, mixed thermic, Typic Torrifluvents), with either no previous sludge amendment or sludge applied at the rate of 67.3 Mg ha^–1 in 1982 or 1983 in the field, was freshly amended with 30 g kg^–1 of gamma irradiated, dried, anaerobically digested sewage sludge in 1985. Triplicate samples of each soil (unamended, amended in 1982, and amended in 1983) and each sludge treatment (no amendment and freshly amended) were incubated aerobically for 16 wk at 0.01-MPa moisture potential and 35 °C. Periodically the soils were leached with 0.01 M CaCl₂, and the leachate was analyzed for inorganic and organic N. Previous sludge amendment had no effect on the mineralization of resh sludge. Soils previously amended in 1982 and 1983 leached 56 to 60 mg kg^–1 more NO^-₃ + NO^-₂ than unamended or freshly amended soils never amended with sludge. Total organic N mineralization averaged 65.4% and was independent of soil amendment history or fresh sludge amendment. Values of N_o, k, and h were estimated by nonlinear least squares for several nonlinear models. The best fit of the data resulted from modeling N mineralization as the sum of an exponential equation (of the rapidly decaying organic N fraction) and a constant (of the slowly decaying organic N fraction). Mineralization curves failed to flatten after 16 wk, suggesting that the experiment should have run longer before a two-exponential equation would have satisfactorily explained the data.

Contribution from the Dep. of Agronomy and Horticulture, New Mexico State Univ. This study was supported by the Dep. of Energy contract no. DE-AC04-76ET-33626 and De-AC04-83AL-21776. Journal Article no. 1314, Agric. Exp. Stn., New Mexico State Univ.

Received for publication December 1, 1986.