"Hotspots" and "Hot Moments" of Denitrification in Urban Brownfield Wetlands

The influence of hydrology and soil properties on disproportionately high ("hot") rates of nitrate (NO₃⁻) removal via denitrification has been relatively well established. It is poorly understood, however, how the unique soil characteristics of brownfield wetlands contribute to or hinder denitrification. In this study, we examined drivers of "hot" denitrification rates over time ("hot moments") and space ("hotspots") in a watershed located on an unrestored brownfield in New Jersey, USA. We carried out measurements of denitrification over 9-day sequences during three seasons in sites with the same vegetation (Phragmites australis) but different soils (fill material, remnant marsh soils, flooded organic-ric̄h soils). Denitrification rates above the 3rd quartile value of the data distribution were defined as "hot" and the most important drivers of these rates were determined using mixed models. Porosity and NO₃⁻ availability were the strongest spatial and temporal predictors, respectively, of high denitrification rates, with coarse-textured, unflooded fill materials unexpectedly supporting the highest rates. These results suggest that pore-scale hydrology is a more complex controller of wetland denitrification than previously thought. Course-textured, unflooded soils have high fractions of air-filled pores relative to flooded soils, leading to more endogenous NO₃⁻ production, and less diffusion constraints than fine-textured soils, leading to higher NO₃⁻ availability to denitrifiers in suboxic pores. Laboratory studies confirmed denitrifiers were limited by NO₃⁻ availability. However, denitrification rates in all soils matched or exceeded atmospheric NO₃⁻ deposition and stormwater NO₃⁻ loading at the site, suggesting that brownfields may play an important role in NO₃⁻ removal from urban stormwater.