MICROBIAL ECOLOGY AND NITRIFICATION IN SOILS AS AFFECTED BY NITROGEN SOURCE, TEMPERATURE AND NITRAPYRIN
Abstract
Anhydrous ammonia (NH(,3)) is one of the most important N fertilizers used for crop production in the United States. However, little is known about the effects of NH(,3) addition with or without nitrapyrin (a nitrification inhibitor) on the ecology of soil microflora. Furthermore, the effects of cool temperatures or the interaction of cool temperatures and nitrapyrin on populations of nitrifying bacteria has not been studied. Therefore, field and laboratory studies were conducted to investigate the effects of NH(,3) and other N sources, cool temperatures, and nitrapyrin on the microbial ecology in several soils. In a field study, NH(,3) addition initially reduced the Nitrosomonas and Nitrobacter populations in soil, but the populations increased markedly within 3 weks after N application. The sensitivity of soil microorganisms to NH(,3) followed the order: fungi > actinomycetes > bacteria. The sensitivity of the major genera of soil fungi to NH(,3) followed the order: Fusarium > Pythium > Trichoderma > Rhizoctonia > Gliocladium. The effects of NH(,3) on the fungal and actinomycete populations, although reduced, were still evident after 48 weeks. Nitrapyrin reduced the Nitrosomonas population in soil for 3 weeks, whereas nitrapyrin was effective in inhibiting nitrification for 10 weeks. Application of nitrapyrin with NH(,3) poses no long term threat to the soil microflora populations. In a laboratory study, nitrapyrin was most effective in inhibiting nitrification when applied with (NH(,4))(,2)SO(,4) and UAN (urea-ammonium nitrate) fertilizers and least effective when applied with NH(,4)OH. Nitrapyrin addition minimized, but did not eliminate, the increase in the Nitrosomonas population in the soils treated with UAN and NH(,4)OH. Nitrapyrin had a limited effect on the Nitrosomonas population in soils amended with (NH(,4))(,2)SO(,4). In a laboratory study, moderate nitrification rates (1-2 (mu)g NH(,4)('+)-N oxidized/g/day) were obtained in soils receiving NH(,4)OH-N at simulated fall temperatures of < 10(DEGREES)C. The addition of nitrapyrin significantly reduced the nitrification rate and Nitrosomonas population for as long as 188 days in samples treated with NH(,4)OH-N at simulated fall temperatures of 13(DEGREES) to 4(DEGREES)C. However, the activity of Nitrosomonas (NH(,4)('+) oxidized/cell/day) in samples receiving NH(,4)OH plus nitrapyrin was very high presumably because the inhibitor eliminated inefficient strains of the bacteria. A synergistic effect of low temperature and nitrapyrin addition resulted in low nitrification rates and decreased Nitrosomonas populations in soil.
Degree
Ph.D.
Subject Area
Agronomy
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