Dissolved Gas Analysis in a Denitrifying Bioreactor to Quantify Denitrification and Green House Gas Emissions.

Denitrifying bioreactors (DNBRs) are an emerging mechanism to mitigate the impact of excess reactive nitrogen by harnessing the activity of ubiquitous denitrifying soil microorganisms. Although achievable nitrate removal in DNBRs, calculated as the difference between outlet and inlet concentrations, is often reported in case studies, direct measurement of denitrification is warranted to establish and improve the efficiency in these managed systems. Direct measurement of denitrification through dissolved gas analysis enables estimation of green house gas emissions from DNBRs produced via incomplete denitrification and other microbial respiration, a negative consequence of implementation that must be minimized through design and management practices. The potential to enhance denitrification while simultaneously reducing green house gas emissions through amendment with biochar, a novel organic carbon medium in DNBR research, was examined in an inline bioreactor fed by agricultural tile drainage consisting of two separate treatment compartment to assess the effect of a 10% biochar addition to traditional woodchip media on bioreactor performance. Dissolved N₂, O₂, Ar, CO₂, CH₄, and N₂O in water samples collected from the DNBR was quantified using a GC/MS analytical method developed for this study. N₂ attributable to denitrification was distinguished form atmospheric using naturally occurring Ar as an inert tracer. The ratio N₂O:N₂ is examined with respect to factors including redox potential, pH, and reaction progress and compared between the two treatments.  Additionally, the indirect emission factor for N₂O, calculated as the ratio of N₂O-N with initial NO₃^--N, is reported and compared to the current IPCC value for agriculture. Nitrate removal measured as the change in NO₃^- concentration over time is compared with denitrification and N₂O:N₂ to assess the validity of the assumption that denitrification is responsible for nitrate removal and inform interpretation of previous research and suggest a methodological approach to future DNBR evaluation.