Nitrogen availability of anaerobic swine lagoon sludge: Sludge source and temperature effects.
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2001-11-26
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Increased numbers of swine producers in North Carolina will be removing sludge from their lagoons in the next few years, mainly due to an increase in lagoons exceeding sludge capacity. Information on availability of nitrogen (N) in the sludge is needed to make improved recommendations about its use as a nutrient source for crops. The objectives of this study were to investigate possible affects related to lagoon sludges from different companies and operation types and to evaluate effects of seasonal temperatures and various application dates on the availability of N in lagoon sludge. Two separate incubation studies and one greenhouse study were conducted to quantify the N availability of the sludge. Sludges were mixed with a Wagram soil (loamy, siliceous, thermic Arenic Kandiudult) and incubated for one year at fluctuating seasonal temperatures based on four application dates (Feb. 26, June 4, Sept. 3, and Dec. 3). A second incubation experiment was conducted using sludges from three different company and operation-types. Samples were analyzed monthly for nitrate and ammonium. These sludges were also applied as the primary N source for bermuda grass, which was grown in the greenhouse, harvested and analyzed for total N. Operation effects were not detected in the incubation and greenhouse experiments. Company effects were detected in the incubation experiments yet considered to be negligible because differences were only significant (p < 0.05) at weeks 0, 2, 3, and 8. A quadratic plateau curve fit to N mineralization data for all sludge sources (rˆ2 = 0.52) demonstrated that most of the active organic N was mineralized after 8 weeks of incubation. Nitrogen availability for all sludges averaged 45 percent after 8 weeks for the incubation study, but only 20 percent for the 14-week greenhouse study. This may have been related to inconsistent moisture throughout the soil in pots. In the incubators with fluctuating temperatures, NH4 remained in the soil for 4 months in the simulated winter application and for only 1 month for the simulated fall and summer applications, illustrating a direct influence of temperature changes on nitrification. Sludge N availability was fit to a nonlinear regression model for a first order reaction as follows: Nt = No (1 - eˆ(-kt)) + Nos where: Nt = total inorganic N concentration, over time (mg N/kg); No = potentially available organic N (mg N/kg); k = first order rate constant (monthˆ-1); t = time (month); and Nos = inorganic N concentration when time = 0. Rate constants (k) increased between simulated applications as follows: fall (0.07) < winter (0.075) < spring (0.22) < summer (0.36). Sludge applied during simulated winter temperatures released N at a relatively constant rate, as compared to simulated summer temperatures, which increased rapidly during the first 6 months, then stabilized to allow minimal increase of mineralized N for the remainder of the incubation.Predicted N availability for all temperature treatments after one year of incubation averaged 74 percent of the total N applied, supporting agronomic recommendations of 60 percent first-year plant-available N for incorporated swine lagoon sludge (NCCES, 1997). Year-long coefficients are unable to provide N availability information for short time length for growing seasons. To account for this, N availability for each month after sludge application was estimated using the first order equations for each simulated application date.
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MS
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Soil Science
