Oxygen flux implications of observed nitrogen removal rates in subsurface-flow treatment wetlands

Nitrification, an oxygen-requiring microbial process, is generally considered the rate-limiting step for N removal in subsurface-flow constructed wetlands treating organic wastewaters. We used a simplified model of sequential N transformations and sinks to infer required rates of oxygen supply at 5 stages along experimental wetland mesocosms supplied with four different organic wastewaters with contrasting ratios of COD: N and forms of N. Mass balances of water-borne organic, ammoniacal and nitrate N, and plant and sediment N uptake showed average net rates of N mineralisation ranging from 0.22-0.53 g m^-2 d^-1, nitrification 0.56-2.15 g m^-2 d^-1, denitrification 0.47-1.99 g m^-2 d^-1 (60-84% of measured N removal) and plant assimilation 0.28-0.47 g m^-2 d^-1. The nitrogenous oxygen demand (NOD) required to support the observed nitrification rates alone was high compared to expected fluxes from surficial and plant-mediated oxygen transfer. In the presence of high levels of degradable organic matter (COD removal rates up to 66 g m^-2 d^-1), heterotrophs with significantly higher oxygen affinities and energy yields are expected to out-compete nitrifiers for available oxygen. Problems with commonly held assumptions on the nature of coupled nitrification-denitrification in treatment wetlands are discussed.