Use of Molecular and Isotopic Techniques To Monitor the Response of Autotrophic Ammonia-Oxidizing Populations of the β Subdivision of the Class Proteobacteria in Arable Soils to Nitrogen Fertilizer

摘要

This study examined the effects of NH4NO3fertilizer on the size and activity of nitrifying, autotrophic, ammonia-oxidizing populations of the β subdivision of the classProteobacteria in arable soils. Plots under different long-term fertilizer regimes were sampled before and after NH4NO3 additions, and the rates of nitrification were determined by 15N isotopic pool dilution assays. Ammonia-oxidizing populations in the plots were quantified by competitive PCR assays based on the amoA and ribosomal 16S genes. Prior to fertilizer addition, ammonium concentrations and nitrification rates in the plots were comparatively low; ammonia-oxidizing populations were present at 104 to 105 gene copies g of soil−1. Three days after the application of fertilizer, nitrification rates had risen considerably but the size of the ammonia-oxidizing population was unchanged. Six weeks after fertilizer treatment, ammonium concentrations and nitrification rates had fallen while the ammonia-oxidizing populations in plots receiving fertilizer had increased. The rapidity of the rise in nitrification rates observed after 3 days suggests that it results from phenotypic changes in the ammonia-oxidizing bacterial population. Associated increases in population sizes were only observed after 6 weeks and did not correlate directly with nitrifying activity. Phylogenetic analyses of PCR products from one of the plots revealed a population dominated byNitrosospira-type organisms, similar to those prevalent in other soils.