Transit‐Time and Temperature Control the Spatial Patterns of Aerobic Respiration and Denitrification in the Riparian Zone

Abstract

Nitrate pollution is a widespread problem in many catchments with intense agricultural activities. Denitrification is a redox process that removes nitrate from the aquatic system via its transformation to nitrogen gas. Denitrification is difficult to assess at larger scales since it depends on multiple factors, such as solute concentrations, temperature variations, and also the time that water resides in the subsurface, where reactions can take place. To evaluate how these factors can influence denitrification, we employed a coupled modeling approach representing the riparian zone of a 4 th order stream in central Germany. We found that temperature variations strongly regulate the process and that during the winter the aerobic (oxygen rich) zone around the stream expands, which further inhibits denitrification in the near stream groundwater. However, even in the winter denitrification occurs, but at larger distance from the stream where oxygen has been depleted sufficiently. With increasing temperature, the influence of other factors on denitrification and on the redox zonation around the stream decreases. Coupled numerical models can provide further insights into the occurrence and interrelations of the multiple processes controlling water quality patterns in river corridors.

Soil denitrification rates are more sensitive to hydrological changes than restoration approaches in a unique riparian zone

“…Our finding is in line with the previous studies reporting that periodic inundation in riparian zones led to an increase in denitrification rate (Jensen et al, 2017; Tomasek et al, 2019). One reason for this result is the cycles of wetting and drying, which can increase soil reserves of organic carbon (Table 2) and provide the electron donor for nitrate reduction via denitrification (Hamonts et al, 2013; Nogueira, Schmidt, Brunner, et al, 2021; Nogueira, Schmidt, Trauth, et al, 2021; Trauth et al, 2015). This result was further supported by the direct and positive relationship between SOM and denitrification rates in the PLS‐PM (Figure 5).…”

“…The larger denitrifier gene abundance, the higher denitrification rates, which can be supported by the PLS-PM. The other possible reason is that the extreme flooding zone presents mainly oxic conditions due to the good connectivity with stream (Nogueira, Schmidt, Brunner, et al, 2021), while the regions further away present more anoxic environment which is optimal for denitrification.…”

Advancing measurements and representations of subsurface heterogeneity and dynamic processes: towards 4D hydrogeology

“…Field deployable optode-based oxygen sensors have enabled high-resolution measurements of oxygen concentrations in time (Diem et al 2014, Vieweg et al 2016) and space (Brandt et al 2017) allowing for robust assessments of respiration dynamics at the GW-SW interface (Vieweg et al 2016). Based on such data, the strong temperature dependence of aerobic respiration rates has been demonstrated (Diem et al 2014), which may dominate turnover rates compared to the effects of variable transit times (Nogueira et al,2021a). Similar effects were found to affect complex spatio-temporal patterns of riparian denitrification, which seem to be jointly controlled by hydraulically driven variability in exchange fluxes and transit times, supply of organic carbon as an electron donor from stream water and riparian sediments and seasonal temperature variations (Trauth et al 2018, Lutz et al 2020, Nogueira et al 2021b.…”

“…Besides high-resolution data sets of key variables like oxygen concentration, it is often the combination of these rich data sets with innovative methods for analysis and modeling (e.g. Diem et al 2014, Lutz et al 2020, Nogueira et al 2021a) that advances our mechanistic understanding of the processes and feedbacks that define the functionality of GW-SW interfaces.…”

Understanding the Hydrogeochemical Response of a Mountainous Watershed Using Integrated Surface‐Subsurface Flow and Reactive Transport Modeling

“…The analysis of transit and residence time distribution was used to quantify surface-subsurface mixing (Yang et al, 2018). Transit times controlled the spatial patterns of aerobic respiration and denitrification in the riparian zone, finely resolved in a small watershed model (Nogueira et al, 2021). The work of Liggett et al (2015) highlighted the XU ET AL.…”