Quantity and Diversity of Denitrifying Genes in Different Land-Use Impacted Soils of the Lower Mississippi River Valley.

During the last 200 years over 75% of the original riparian forests of the Lower Mississippi Valley has been converted to other land uses, mainly agriculture. This conversion has radically changed the biogeochemical functionality of the altered ecosystems, in particular their ability to retain and process N.  This study focused on assessment of land use impact on denitrifying bacterial populations that play an important role in removing nitrogen from the Lower Mississippi River Valley.  The population size of denitrifying bacteria was quantified via real-time PCR technique targeting denitrifying genes, namely nitrate reductase gene (narG), nitrite reductase gene (nirK and nirS), and nitrous oxide reductase gene (nosZ) in soils from forested wetland (FR), cultivated agricultural land (AG), and restored conservation reserve site (SBR).  The copy number of narG, nir (nirK+nirS), and nosZ per gram soil ranged from 1.1 x 10⁶ to 1.2 x 10⁷, 8.8 x 10⁶ to 4.0 x 10⁷, and 8.7 x 10⁶ to 3.5 x 10⁷, respectively. The highest copy number of narG and nir genes was observed in FR soils while SRB soils contained the most abundant nosZ copy number.  The gene copy number of 16S rRNA genes, presumably indicating total bacterial size, ranged from 8.9 x 10⁸ to 6.9 x 10⁹. The nirK and nirS genes were PCR amplified, cloned, and sequenced from DNAs extracted from soils  of FR, AG, and SBR site.  Most sequences from the soils were not closely related to previously observed genes, indicating that these soils contained novel nirK and nirS sequences.  Unique community structures were observed at each site, suggesting that the land use has effected on the development of denitrifying bacteria.