Nitrogen mineralization in an Alfisol amended with thermoelectrical industry by-products

In thermoelectric industry, synthetic gypsum and coal ash are generated as industrial by-products. The objective of this work was to evaluate the effect of different mixtures of synthetic gypsum and coal ash on N mineralization in an Alfisol. Nitrogen mineralization was evaluated by soil incubations under controlled conditions and potential mineralization using the nonlinear first order kinetics model Nm_t = N₀ [1 - ℮^(-kt)], where Nm_t is the mineral N accumulated at a specific time, N₀ is the potentially mineralizable N, k is the mineralization rate, and finally t is the incubation time. Treatments consisted of different mixtures of synthetic gypsum, coal ash and urea (M₁: 50% synthetic gypsum, 50% coal ash, M₂: 50% synthetic gypsum, 35% coal ash, 15% urea, M₃: 45% synthetic gypsum; 40% coal ash, 15% urea, M₄: 65% synthetic gypsum, 20% coal ash, 15% urea, M₅: 55% synthetic gypsum, 30% coal ash, 15% urea). Each mixture was applied to the soil at 2 t ha^-1. The results shown the highest concentration (p < 0.05) of net mineral N (N_MN) applying M₃ (166.14 mg kg^-1), while the lowest N_MN (p < 0.05) was found applying M₅ (98.30 mg kg^-1). Treatment M₁ presented the highest N₀ value (579.0 mg kg^-1) respect to treatments with urea in its composition, with exception of M₃ (523.4 mg kg-1). Where the highest and lowest k were determined in M₅ (k = 0.44990) and M₃ (k = 0.04470), respectively. Mixtures of synthetic gypsum, coal ash and urea increased N mineralization over mixtures without urea. Respect N potential mineralization, in some cases N₀ and k values would respond more to the fit of the mathematical model employed than to the biological process.