16S rRNA and Denitrifier Gene Abundance Differ Between Organic and Conventional Cropping Systems in the Southeast.

Emissions of the trace gas nitrous oxide (N₂O) play an important role in global climate change and stratospheric ozone depletion. N₂O emission by denitrification is the net result of the balance between production and reduction of N₂O by denitrifying bacteria. The aim of this study was gain a better understanding of how long-term farming systems management impacts denitrifier microbial population abundance. We compared the effects of organic and conventional long-term management over time on total microbial (16S rRNA) and a functional gene affiliated with nitrate reduction (nirS) abundance by using quantitative real-time PCR. We compared five time points within the cropping cycle, including pre cover-crop kill, post cover-crop kill, post manure application, a rain event, and post corn harvest. The abundance of 16S rRNA genes ranged from 10⁹ to 10¹⁰ target copies g^-1 of soil whereas nirS genes were found at 10⁶ to 10⁸ target copies g^-1 of soil. Analysis of both genes revealed no significant differences of microbial population abundance within each gene among various times and treatments. Data suggested soil type to be a greater driver of nirS and 16S rRNA abundance than management system.