Effects of seasonal variation and land cover on riparian denitrification along a mid-sized river

Abstract

Urban areas contribute disproportional nitrogen (N) loads to downstream aquatic ecosystems resulting in potential hypoxic 'dead' zones. Riparian areas along streams and rivers reduce inorganic N concentrations through denitrification, an anaerobic microbial process. Our study objective was to investigate the denitrification potential of riparian areas with differing land cover composition along the Licking River in Kentucky, USA À a tributary of the Ohio River. For one year we collected monthly samples from four sites along a 60 km reach of the Licking River. We experienced substantial drought conditions in the first half of our study period followed by record precipitation and flooding in the second half of our study period. Land cover surrounding the sample sites was classified into distinct classes hypothesized to contribute to N loads. We found flooding increased denitrification potential, water nitrate concentration, and soil organic matter, while drought conditions increased soil ammonium concentrations. Our site with the greatest impervious surface had the highest denitrification potential, and soil and water ammonium concentrations. However, we determined that denitrification potential was mostly driven by soil organic matter content and only slightly by soil nitrate among all study sites. Our data demonstrate that riparian zones of mid-sized rivers in urban areas can be integral in removing excess N during flood events and can do so year-round provided sufficient N and carbon resources are present.

The potential for nitrate removal during infiltration: Mapping with machine learning informed by field and laboratory experiments

“…The model also predicted elevated NR p in association with former and current wetland and estuary-adjacent soils, and along the lower reaches of small drainages (Figure 3a). These results are consistent with findings that riparian zones, floodplains, and wetlands are often important for denitrification and other nutrient cycling processes due to episodic flooding and carbon-rich soils (Arango & Tank, 2008;Hopfensperger et al, 2014). High NR p was often co-located with high soil organic carbon (Figure S6), consistent with other studies of denitrification potential (Hopfensperger et al, 2014;Inwood et al, 2007;Newcomer et al, 2012).…”

“…Additionally, urban areas surrounding the city of Watsonville show elevated potential nitrate removal (Figure 3a and c). Urban areas are often associated with moderate to high [NO 3 ] in runoff from sanitary sewer and septic systems and atmospheric deposition coupled with fast hydrologic flow paths that limit nitrogen processing (Hopfensperger et al, 2014; Kaushal & Belt, 2012). Watsonville is surrounded by considerable agricultural development, creating a complex urban‐agricultural interface where land uses, conditions, and processes are likely to be mixed.…”

“…Higher soil organic carbon and soil residence times tend to be associated with more nitrate removal via denitrification, whereas initial nitrate concentration and clay content have more complex relationships with nitrate removal (Figure 2). These and other factors can combine in highly non‐linear ways leading to complicated dependencies which may be less apparent when looking at the spatial distribution of individual predictors (e.g., Figure S6) (Boyer et al, 2006; Gift et al, 2010; Hopfensperger et al, 2014; Kaushal et al, 2011).…”

Denitrification along the Stream‐Riparian Continuum in Restored and Unrestored Agricultural Streams

“…Greater riparian soil moisture likely influenced oxic status and exposure of denitrifying microbial communities to NO 3 − . Higher soil moisture and flooding has been shown to increase denitrification potential in riparian zones (Roley et al, 2012b; Hopfensperger et al, 2014). Since soil moisture was lower in the growing season at our sites, anoxic conditions may have failed to develop as extensively, leading to reduced DEA.…”

Effects of Revetments on Nitrification and Denitrification Potentials in the Urban River–Riparian Interface

“…Initially, we selected promising explanatory variables based on the existing literature and Pearson correlation analysis results. Subsequently, CMIN/DF, P, GFI, AGFI, NFI, and RMSEA were used to assess the quality of the SEM model-fitting data [37,38].…”