Nitrogen Mineralization Potential and Nitrogen Transformations of Sludge‐Amended Soil

Abstract

AbstractAn experiment was conducted to determine values for the mineralization potential (No), rate constant (k), and rate of mineralization for soils freshly amended with sewage sludge, and to determine the influence of soil type and sludge rate on these values. A secondary objective was to determine if the sum of two exponential functions could better model N mineralization than a single exponential functions, as is normally used. A clayey soil (Glendale, a Typic Torrifluvent) and a sandy soil (Latene, a Typic Calciorthid) were amended with three rates (0, 15, and 30 g kg−1) of gamma irradiated, dried, anaerobically digested sewage sludge. The Glendale soil was mixed with sand (1:1 by weight) to increase aeration. Triplicate 80‐g samples of each soil mixture were incubated aerobically for 32 weeks at 0.01 MPa moisture tension and 35°C. Before incubation and periodically during incubation, the soils were leached with 0.01M CaCl2 and the leachate analyzed for NH+4, NO‐2, and NO‐3. No and k were estimated by a nonlinear least squares iterative statistical method. Cumulative (NO‐3 + NO‐2)‐N at the end of 32 weeks was dependent on sludge rate, but not on soil type or the soil type‐sludge rate interaction. Cumulative (NO‐3 + NO‐2)‐N for the 0, 15, and 30 g kg−1 treatments were 60.9, 137.4, and 182.6 mg (NO‐3 + NO‐2)‐N kg−1, respectively. Mineralization rates decreased rapidly the first two weeks, followed by a slower decrease with time. Rates were higher in sludge‐amended soil than in unamended soil and averaged 0.6, 1.8, and 2.2 mg (NO‐3 + NO‐2)‐N kg−1 week−1 for the 0, 15, and 30 g kg−1 treatments, respectively, after 32 weeks. Several methods of estimating No and k were attempted. The best estimates for sludge‐amended soil were obtained by modelling N mineralization as the sum of two exponentials. Total N mineralization (organic N loss, x̄ (mean) = 63%) was more than twice that of net N mineralization (NO‐3 + NO‐2‐N accumulated, x̄ = 27%). Total and net mineralization were greater in the Glendale soil (x̄ = 68% and 30%, respectively) than in the Latene soil (x̄ = 58% and 24%, respectively). Total mineralization increased with sludge addition, but net mineralization decreased with sludge addition. Denitrification losses probably accounted for the discrepancies between total and net mineralization. Denitrification increased with sludge addition and was greater in the Glendale than the Latene soil.