Evidence of Nitrate Removal “Hot Moments” During Flow and Nitrate Pulses in a Denitrification “Hot Spot”

Highlights There must be denitrification ‘hot moments’ during nitrate load pulses to explain observed nitrate removal in watersheds. We tested this hypothesis using high-frequency nitrate data in a laboratory woodchip bioreactor. Nitrate volumetric removal rates can be multiplied by several folds during pulses of flow, nitrate, and of both combined. This highlights the likely determinant role of hydrodynamics in biogeochemical activity in denitrification ‘hot spots.’ Abstract. After decades of research on the conditions, factors, and drivers of nitrate removal in edge-of-field denitrification ‘hot spots’ such as riparian buffers (RB), there is still a gap in our understanding of how and when nitrate removal occurs. Following rainfall events, all natural subsurface treatment systems at the edge-of-field are subject to pulses of flow and nitrate fluxes. This implies that the majority of the nitrate load occurs within a minority of the time. The short exposure time of water and nitrate in these denitrification ‘hot spots’ during these pulses should largely limit their capacity for nitrate removal, unless the volumetric removal rates, i.e., the mass of nitrate removed per unit volume of soil per unit residence time, are largely increased during these pulses. Reversing the argument, for denitrification ‘hot spots’ to significantly remove nitrate in pulsed flux systems, flow and nitrate flux pulses must coincide with ‘hot moments’ of biogeochemical activities to compensate for short exposure times. This phenomenon should in fact be prevalent in nature and be experimentally verifiable. One of the characteristics of flow pulses is the increase of velocity and of nitrate concentrations in subsurface flow. However, it is unclear whether these have positive or negative impacts on denitrification. To investigate this, we conducted a series of experiments over a 4-month period in a 0.3 m × 0.3 m × 2.4 m-long horizontal lab woodchip bioreactor, a practice known and designed to be a denitrification hotspot. We quantified the nitrate volumetric removal rates during pulses of flow, nitrate concentrations, and the combination of both and compared them to baseline conditions. This was made possible by collecting hourly nitrate and dissolved organic carbon (DOC) concentrations at twelve locations along the bioreactor over extended periods of time. The results demonstrate that the removal rates during pulses were two to four times greater than the removal rates during baseline conditions. These findings strongly support our hypothesis that biogeochemical ‘hot moments’ during flux pulses may exist in nature in denitrification ‘hot spots,’ which must be confirmed in the field in other edge-of-field systems such as riparian buffers. Keywords: Edge-of-field treatment systems, High frequency sampling, Pulsed load of flow and nitrate, Riparian buffers, Woodchip bioreactor.