2.3 Application of analytical representations on predicting N-cycling.
Hyporheic zone denitrification ranges between 1-200% of denitrification in the stream water column (Harvey et al., 2013). The Denitrification in streambed sediments depends on the presence of nitrate, labile organic carbon, and anaerobic zones within the sediment bed (Gomez-Velez et al., 2015; Zarnetske et al., 2012). In the downwelling zone, the movement of oxygen rich water from the stream into the sediments favors aerobic conditions which are generally (with the exception of anaerobic microzones (e.g., Zarnetske et al., 2011, 2012) unfavorable to denitrification (Azizian et al., 2015). As water parcels continue their journey through the hyporheic zone (i.e., at relatively long \(t^{*}\)) heterotrophic respiration of organic matter (associated with in-sediment microbial biofilms) leads to a progressive depletion of oxygen and an eventual shift to nitrate reducing (i.e., denitrifying) conditions (Azizian et al., 2015; Harvey et al., 2013; Zarnetske et al., 2011, 2015).
The boundary between aerobic and anaerobic zones is delineated by the Damköhler number for aerobic respiration (\(\text{Da}_{\text{rp}}\)) (Zarnetske et al., 2012). It is defined as the ratio between the median residence time of all flow paths (\(t_{50}\)) and the characteristic time necessary for oxygen consumption (\(\tau_{\text{rp}}\sim 1\) h; Gomez et al., 2012; Gomez-Velez et al., 2015; Zarnetske et al., 2011). At \(\text{Da}_{\text{rp}}\) < 1, aerobic conditions are dominant and denitrification is inhibited, while at\(\text{Da}_{\text{rp}}\) >> 1 anaerobic conditions prevail and denitrification is likely (i.e., provided that the other two requirements, the presence of nitrate and labile organic matter, is satisfied) (Zarnetske et al., 2012). When flow paths exhibit residence times longer than a characteristic denitrification timescale (\(\tau_{\text{dn}}\sim 10\) h; Gomez-Velez et al., 2015; Harvey et al., 2013; Zarnetske et al., 2015), they tend to be fully anaerobic, and all or most of the nitrate is removed (\(\text{Da}_{\text{dn}}\ \)>> 1). Correctly representing the residence time scales in the hyporheic zone, and therefore the Damköhler Numbers for aerobic respiration and denitrification, can lead to more accurate assessments of the link between the hyporheic zone RTD and key ecological functions, such as stream metabolism, generally, and denitrification, in particular (Mulholland et al., 2009).
The hydraulic and morphological properties of the Embarras River (Table 2) located 15 km south of Urbana-Champaign, east-central Illinois, USA (Sukhodolov et al., 2006) was considered in this study to demonstrate how the analytical framework introduced in this paper can be applied for predicting Da. This river was chosen due to the existence of a dunes-like bedform as its predominant morphology.