Nitrogen Mineralization Potentials of Revegetated Lignite Overburden in the Texas Gulf Coast

Fresh mine spoil is virtually devoid of organic matter (OM) and N. The needed N is commonly added as inorganic fertilizer; however, sewage sludge additions could supply both OM and N. This study was conducted to compare the mineralizable N resulting from inorganic fertilizer and sludge applications to overburden. Nitrogen mineralization potentials were determined from laboratory data on a pre-mined native soil (Elmina fine sandy loam) and 4-yr old reclaimed mixed overburden that received 180 kg N ha⁻¹ yr⁻¹ as NH₄NO₃ using the Stanford and Smith model (1972). In addition, the effect of anaerobically digested sewage sludge (sludge) on N-mineralization potential of overburden was evaluated on samples collected from field plots. Treatments were as follows: 0 kg N ha⁻¹, 212 kg N ha⁻¹ as NH₄NO₃, 106 kg N ha⁻¹ as NH₄NO₃ plus 106 kg N ha⁻¹ as sludge, and 212 kg N ha⁻¹ as sludge applied to 13.8 m² plots of overburden. Soil and spoil samples (0–15 cm deep) were collected from these plots 2 weeks, 26 weeks, and 52 weeks after amendment. Mineralization rates were determined using columns containing 0.2 kg of soil or treated overburden plus 0.1 kg of sand which were incubated for 18 weeks at 35°C. These columns were leached with saturated CaSO₄ and a N-free nutrient solution at 0, 2, 6, 10, 14, and 18 weeks and were extracted to a tension of −0.066 MPa after each leaching. The leachate was analyzed for NH⁺₄ and NO⁻₃ by colorimetric methods. Application of 56 Mg sludge ha⁻¹ significantly (α = 0.05) increased the total N 26 and 52 weeks after application to levels present in the 4-yr old mineral fertilized reclaimed spoil. Sludge application also resulted in a sustained release of plant available N for 52 weeks after application. Mineralization potentials 2 weeks after the plots were treated were ranked as unamended overburden = NH₄NO₃ treated overburden < native spoil = 4-year old reclaimed overburden < NH₄NO₃ plus sludge treated overburden < sludge treated overburden.