Limiting Factors Of Nitrate Removal in Mesoscale Denitrifying Wood Chip Bioreactors

Abstract

The use of woodchip denitrifying bioreactors holds promise as a simple, efficient and cost-effective system to reduce nitrate loads in agricultural runoff through stimulation of microbial denitrification. While bioreactor performance at colder temperatures and under varying hydraulic residence time (HRT) has been investigated, the correlation to functional microbial communities has not been studied in great detail. In this study, we quantified denitrifying functional gene copy numbers throughout the course of a three-month study within mesoscale [1.83m x 0.3m x 0.61m] denitrification bioreactors that were operated at three temperature regimes and two HRT. We found that with increasing temperature and HRT there was a significant increase in percent nitrate removed. Temperature and HRT had little effect on nirK and nosZ clade I gene copy numbers throughout the length of the bioreactors, however, they had a significant effect on 16S rRNA and nosZ clade II gene copy numbers at the inflow locations of the bioreactors. Utilizing a hydraulic flow model developed for the denitrifying bioreactors in this study, a decrease in nitrate concentration along the length of the reactors was calculated. We correlated a decrease in 16S rRNA, nirK and nosZ clade II gene copy numbers to the decrease in nitrate concentrations predicted by the hydraulic flow model. Our results suggest that at temperatures of 14.5°C and 12-hr HRT, denitrifying bioreactor microbial communities are limited by variables other than temperature and HRT. It is suggested that carbon availability is the most likely limiting factor, indicating a need to further investigate the role of both denitrifying and decomposing communities in denitrifying bioreactors. These findings contribute to a better understanding of the microbial functional communities in denitrifying wood chip bioreactors, allowing us to optimize the design and performance of these reactors in agricultural midwestern states.