Denitrification Capacity and Denitrifying Bacterial Populations in Soils of the Lower Mississippi River Valley.

During the last 200 years over 75% of the original riparian forests of the Lower Mississippi River Valley (LMRV) 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.  Restriction fragment length polymorphism (RFLP) analysis using nirS gene indicated that the community structure of denitrifying bacteria was different among different land uses.  Out of 68 RFLP types, only 6 types were common in all soils.  SRB soil contained the highest number of RFLP types (41 types) whereas AG soil had the lowest number (25 types).  The nirS genes detected from those soils fell into two phylogenetic branches on the basis of their comparative sequences.  One group was close to the nirS genes from uncultured bacteria originating from several environmental sources; the other group was novel.