Influence of emergent vegetation on nitrate cycling in sediments of a groundwater-fed river

Abstract

Emergent vegetation in river beds can play a significant role in nutrient cycling in riverine sediments. We analysed and compared pore water NO 3 -concentration gradients in the sediments of the River Leith, Cumbria, UK, in the presence and absence of emergent vegetation (dominated by Sparganium spp.). High resolution (1 cm interval), in situ, vertical profiles of NO 3 -to 30 cm depth were measured using deployment of diffusive equilibrium in thin films probes on four occasions from July to September 2010. We found significantly (p \ 0.05) lower NO 3 -concentration under vegetated sediments (VS) compared to those under adjacent un-vegetated sediments (UVS). Concentrations of dissolved oxygen, measured in pore water collected from multi-level piezometers at 10, 20, 25, 30 and 35 cm depths at the VS and UVS sties, were generally lower under VS (median concentration = 28 lM) than under UVS (median = 132 lM) and correlated significantly with NO 3 -concentration (Spearman's r = 0.74, p \ 0.05). Similarly, pore water dissolved organic carbon (DOC) concentration was 2.8 times higher under VS compared to UVS, and correlated negatively with NO 3 -concentration (Spearman's r = -0.39, p \ 0.05). Specific ultra-violet absorption at 254 nm (SUVA) and per cent aromaticity values of DOC were significantly higher under VS (p \ 0.05), suggesting that the DOC contained more complex (aromatic) compounds than DOC recorded under UVS. We suggest that, the higher quantity of DOC and its distinct SUVA and percentage aromaticity under VS may have supported faster dissolved oxygen consumption, with the creation of anoxic zones conducive for NO 3 -reduction mainly through denitrification. Metabolic uptake and immobilization of NO 3-by plants and microbes may have further contributed to lower NO 3 -concentrations under VS. As Sparganium spp. is a common plant growing in river beds in the UK, its role in NO 3 -cycling should be considered in attempts to accurately budget N cycling in river beds.

Diffusive equilibrium in thin films provides evidence of suppression of hyporheic exchange and large‐scale nitrate transformation in a groundwater‐fed river

“…This high electrical conductivity has been associated with elevated concentration of pore water solutes (including NO 3 − ) to a minimum depth of 100 cm. Moreover, pore water dissolved organic carbon was lower and dissolved oxygen was higher at site C compared with a vegetated sediment stretch (~8 m upstream of site C), suggesting limitation of nitrate reduction potential by upwelling water at site C (Ullah et al ., ). Putting this evidence together, it has been suggested that this sub‐reach is a zone of preferential flow of NO 3 − ‐rich groundwater (Binley et al ., ; Heppell et al ., ).…”

“…Nitrate concentrations increased above 5 cm sediment depth in probe 3 (black circles) at site C. This could be related to down welling surface water, mineralisation of nitrogen derived from the dieback of Bur‐reeds ( Sparganium spp .) (Ullah et al ., , ), and subsequent nitrification (Mortimer et al ., ). Of the three sample sites investigated, site C (riffle) is the most likely to experience HEF as a result of pumping exchange (Boano et al ., ) and/or changes in the hydrostatic head gradient across the riffle (Wondzell et al ., ).…”

“…Reach‐scale (200 m) observations of subsurface vertical and lateral water fluxes, together with riverbed geophysical surveys, have identified a sub‐reach (50 m) preferential discharge location characterised by direct connectivity to the underlying sandstone aquifer (Binley et al ., ) (see Supplementary Material 1). The concentrations of pore water anions (from 100 to 10 cm below the riverbed) in this sub‐reach (including NO 3 − ) are elevated with respect to the wider river reach, and show limited evidence of attenuation under baseflow conditions either by physical mixing or chemical transformation (Krause et al ., ; Heppell et al ., ; Ullah et al ., ; Lansdown et al ., ) (see Supplementary Material 2). In the absence of hydrological or chemical data for the upper 15 cm of sediments, Munz et al .…”

Critical control of flooding and draining sequences on the environmental risk of Zn-contaminated riverbank sediments

“…Probes were inserted for 72 h at the start of a flood and 72 h prior to the end of a flood. On removal, the gel was cut into 1 cm slices, and the method for gel cutting, storage and analysis was that used by Ullah et al (2014) and Byrne et al (2014). For ICP/OES analysis, the samples were diluted 1:10 with DIW.…”

“…For ICP/OES analysis, the samples were diluted 1:10 with DIW. The dilution factor based on the volume of DET strip diluted in 1 ml nitric acid (as carried out by Ullah et al 2012Ullah et al , 2014) with a further dilution ×10 with DIW was (×56). A blank DET probe underwent all analyses except deployment.…”