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

Ullah, Sami and Zhang, Hao and Heathwaite, A. Louise and Heppell, Catherine and Lansdown, Katrina and Binley, Andrew and Trimmer, Mark (2014) Influence of emergent vegetation on nitrate cycling in sediments of a groundwater-fed river. Biogeochemistry, 118 (1-3). pp. 121-134. ISSN 0168-2563

Abstract

Emergent vegetation in river beds can play a significant role in nutrient cycling in riverine sediments. We analysed and compared pore water NO3 (-) 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 NO3 (-) 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 NO3 (-) 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 mu M) than under UVS (median = 132 mu M) and correlated significantly with NO3 (-) 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 NO3 (-) 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 NO3 (-) reduction mainly through denitrification. Metabolic uptake and immobilization of NO3 (-) by plants and microbes may have further contributed to lower NO3 (-) concentrations under VS. As Sparganium spp. is a common plant growing in river beds in the UK, its role in NO3 (-) cycling should be considered in attempts to accurately budget N cycling in river beds.