DIRECT MOLECULAR BIOLOGICAL ANALYSIS OF AMMONIA OXIDIZING BACTERIA POPULATIONS IN CULTIVATED SOIL PLOTS TREATED WITH SWINE MANURE

The application of pig slurry, high in ammonia-nitrogen concentration, to agricultural land is a practice whose effect on soil microbial com munities is pearly understood. The autotrophic ammonia-oxidising bacte ria are an integral component of the nitrogen cycle in soil, and their activity will be affected by addition of nitrogenous fertilizer. Mole cular biological techniques were applied to the direct detection and i dentification of ammonia-oxidiser populations in cultivated soil plots treated with different amounts of pig slurry. Members of the genus Ni trosospira were shown by 16S rDNA-directed PCR to be present in both u namended and amended soils, regardless of the quantity of pig slurry a pplied. In contrast, members of the genus Nitrosomonas were detected b y the same approach only in those soil plots that had received high lo adings of slurry. The fidelity of amplification products was always co nfirmed by oligonucleotide probing. In addition, we used high stringen cy PCR and confirmatory gene probing to detect the presence of the amm onia monooxygenase gene (amoA) of Nitrosomonas europaea directly in al l amended soil samples, with hybridization signal intensities that inc reased with the amount of pie slurry applied to plots. Nitrosomonas eu ropaea amoA DNA could not be detected in soil from the untreated plot. These data support the view that nitrosospiras are ubiquitous as impo rtant members of nitrifying populations in the environment. The direct detection of nitrosomonad DNA only in amended soils supports the hypo thesis that these nitrosomonads become highly competitive under condit ions analogous to laboratory enrichment cultures.