Green Infrastructure: Why Can't We Effectively Treat Nitrogen, and What Are the Microbes Doing?

Abstract:

Green infrastructure (GI) is growing in popularity, but we are failing to remove nutrients and are missing the target to protect water quality. GI has shown mixed abilities to reduce N, and alarmingly export N in some conditions. The goal of this project was to investigate the microbial processes controlling N cycling (mainly denitrification) within stormwater basins and how these processes influence water quality.

We examined two stormwater basins in Ithaca, NY; one slow draining ‘wet’ site and one quick draining ‘dry’ site. Multiple lines of evidence were used to investigate N cycling within these systems: (1) metagenomics investigation of functional genes, (2) water quality field monitoring, and (3) denitrification lab-scale assays. Water quality monitoring was paired with microbial functional genes; differences in N reductions within the basins were related to N cycling genes. To determine if influent stormwater pollutants affect denitrification, a controlled field manipulation experiment was also conducted. Soil from each site, and a control site without stormwater influence, were exchanged. Potential denitrification was measured weekly via denitrification enzyme assay (DEA).

Soil DNA was sent for sequencing on July 27, 2015; results are expected in September 2015. The stormwater effects on denitrification using the exchanged soils is ongoing until September 8, 2015. Preliminary results show the wet site has considerably higher denitrification potential compared to the dry site, 2.00 and 0.27 mg N kg^-1 hr^-1, respectively. Initial water quality monitoring indicate nitrate mass loads are reduced following treatment within the basin. However, nitrate concentrations remain almost constant, indicating that volume reduction, and not microbial treatment, is “removing” nitrate.