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Response of Nitrification and Nitrifying Bacteria in Mine Spoil to Urea or Ammonium Sulfate

Department of Soil and Crop Sciences, Texas A&M University, College Station, TX 77843-2474

*Corresponding author (d-zuberer{at}tamu.edu).

ABSTRACT

Reclaiming lands mined for lignite in east Texas requires large inputs of N fertilizers. However little is known about the relative effects of NH⁺₄-N vs. urea-N on nitrifying bacteria and rates of nitrification in soils undergoing reclamation. We conducted a field study to determine nitrification potentials, enumerate nitrifying bacteria and to determine the relationship between the two in mixed-overburden. Newly leveled and 1- to 8-yr-old spoils near Fairfield, TX, were fertilized with 244 kg N ha^-1 of (NH₄)₂SO₄ (AS) or urea and concentrations of NH⁺₄, NO^-₂, and NO^-₃ in the overburden were monitored. Nitrification potentials and most probable numbers (MPN) of NH⁺₄- and NO^-₂-oxidizing bacteria were measured. Nitrification potentials were lowest in fresh spoil and increased with age of site. Nitrification rates at the 1-yr-old site were equivalent to those at an adjacent unmined site. Nitrification of NH⁺₄-N and urea-N occurred at similar rates in the field (means 2.6 and 2.7 mg NO^-₃-N kg^-1 soil d^-1, respectively) but there tended to be more NO^-₃ in spoil fertilized with urea. Nitrifiers regained pre-mining levels within one year ranging from log₁₀ MPN 3 to 6 g^-1. Nitrifiers were least abundant at the fresh site (log₁₀ 3) but were similar at the other sites ranging from log₁₀ 5 to 6. Numbers of nitrifying bacteria and nitrification potentials were highly correlated (r = 0.75 to 0.94). Urea and AS produced similar responses for the parameters measured, indicating urea to be as suitable an N source as AS in the reclamation process.

Received for publication February 15, 1995.