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Microbial hydrolysis of urea and its subsequent nitrification in East Texas lignite mine spoil
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A functional nitrogen (N) cycle within mine spoils is necessary for successful long-term revegetation. To initiate successful revegetation of mined lands, large inputs of N fertilizers are required. Urea is gaining worldwide popularity as an N source due to its high N content (47.0 %) and economical price. Since little is known about the behavior of urea in the mixed overburden mine spoils of east Texas, a two-year study was conducted to determine rates of urea hydrolysis, and its subsequent nitrification and effects on microbial activity, at reclaimed sites of varied ages. Newly-leveled, I-, 4-, and 8-year-old spoil and an unmined soil at the Big Brown Mine in Fairfield, TX, were fertilized with 244 kg N ha-' of (NH4)2SO4 or urea. After fertilization, soils were periodically analyzed for microbial activity (arginine ammonification, AA; substrate induced respiration, SIR), ability to hydrolyze urea (UH), numbers of nitrifying bacteria, and nitrification potentials (Nps). Only soils from the unmined and newly- leveled sites failed to show increased AA rates. Vegetated mine spoils showed AA rates ranging from 1.2 to 2.3 mg NH 4 N kg-I soil h-1 within seven days. Rates of AA were similar in soils fertilized with (NH4)2SO4 or urea. Rates of SIR showed a similar pattern, being maximal in the 8-yr spoil and rates were similar in soils fertilized with (NH4)2SOl or urea. Rates of AA and SIR were highly correlated (r = 0.59 to 0.93). Revegetated mine spoils (4 to 8 yrs) showed UH rates as high, or higher than those of the unmined soil. Rates of UH were greatest at the 8-yr site (ranging from 3.0 to 3.5 mg urea-N hydrolyzed kg-' soil h-1) and lowest at the newly-leveled site (maxi-mum rate of 0.5 mg urea-N kg-' soil h-'). Rates of UH were similar in soils fertilized with (NH4)2SO4 or urea. Nitrification potentials in vegetated spoil (I to 8 yrs) were similar to, or greater than those of the unmined soil (ranging from 2.4 to 5.2 mg N03--N kg-' soil d-1). The newly-leveled site showed NPs below 2 mg N03--N kg-' soil d-1. Peak NPs were observed two weeks after fertilization and the (NH4)2SO4 and urea treatments showed similar rates. Nitrifying bacteria regained pre-mining levels within one year in revegetated mine spoil. Numbers ranged from loglo MPN 3 to 6. Soils from the I-, 4-, and 8-yr sites showed numbers similar to, or greater than, numbers in the unmined soil. Numbers of NH 4 +- and N02--oxidizing bacteria were similar in soils fertilized with either N source. Bacterial numbers increased one order of magnitude after the newly-leveled site was sprigged, suggesting an inoculation effect. Numbers of nitrifying bacteria and nitrification potentials were highly correlated (r = 0.75 to 0.94). This study demonstrated the rapid recovery, to pre-mining levels, of selected soil microbial activities and populations of nitrifying bacteria in mixed overburden. Ammonium sulfate and urea produced very similar responses for the parameters measured. Urea appears to be as suitable an N source as (NH 4)2SO4 in the revegetation process.
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Waggoner, Paul James (1993). Microbial hydrolysis of urea and its subsequent nitrification in East Texas lignite mine spoil. Master's thesis, Texas A&M University. Available electronically from
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