Carbon and nitrogen cycling in a vegetated lowland chalk river impacted by sediment

Abstract

Abstract:We investigated the accumulation and biogeochemical cycling of organic matter beneath Ranunculus plants in a lowland river. Organic carbon accumulated beneath the plants at a mean rate of 20 mmol C m 2 h 1 . Annual gross primary production for both Ranunculus and its biofilm, and the microphytobenthos, could account for 26% of the carbon accumulated. The remainder was attributable to organic carbon in both suspended particulate matter (77%) and that associated with sands saltating along the bottom (33%). Maximum carbon oxidation occurred in spring and early summer and declined thereafter. The efflux of CO 2 was greater than the carbon equivalents due to reduction of O 2 , NO 3 and SO 4 2 measured at the surface, which suggested a significant contribution to carbon oxidation from the subsurface and some oxidation via alternative electron acceptors. The peak in carbon oxidation could not be accounted for by either rising temperature or primary production but tracked the quality of recently deposited allochthonous organic matter. The ratio of carbon oxidation to total organic carbon accumulation suggested that 19% of the organic matter deposited was remineralised on an annual basis, although this reached 58% in June. We calculate that a total of 3Ð6 mol N m 2 y 1 was mineralised in the sediment, of which 11% could be accounted for by the measured efflux of NH 4 C . The remainder could be accounted for by the N demand from primary production (67% macrophytes/biofilm; 36% phytobenthos).

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

“…This difference in DOC may have been due to root exudates and decomposition of dead plant material contributed by the emergent vegetation. Moreover, emergent and submerged macrophytes in river beds have also been found more effective than bare sediment patches in trapping fine sediments from fluvial sources ) and exogenous particulate organic matter (Sand-Jensen 1998;Trimmer et al 2009;O'Hare et al 2012). The trapped fine sediments with particulate organic matter could have been an additional source of the relatively higher DOC delivery into the underlying pore water through surface water down welling during high flow events (Gu et al 2008;Byrne et al 2013).…”

“…The trapped fine sediments with particulate organic matter could have been an additional source of the relatively higher DOC delivery into the underlying pore water through surface water down welling during high flow events (Gu et al 2008;Byrne et al 2013). S. erectum patches across 47 river reaches in the UK contained 1.8 times higher organic matter than bare river bed sediments (O'Hare et al 2012), which indicates higher potential for dissolved oxygen consumption through microbial respiration (Sobczak and Findlay 2002;Trimmer et al 2009) and release of DOC to serve as an electron donor during NO 3 -reduction (Forshay and Dodson 2011).…”

“…Consideration of plant impacts on nutrient cycling in the context of plant-microbe interaction in soils and sediments is critical for assessing the nutrient and pollution attenuation potential of ecosystems (Clarke 2002;Schneider and Melzer 2004;Jackson et al 2008;Ullah and Moore 2009). Such processes are potentially important in riverine ecosystems, but as the vegetation growing in river beds tends to be patchy, its role in nutrient cycling is poorly studied (Sand-Jensen 1998;Groffman et al 2005;Trimmer et al 2009;Jackson et al 2008;Forshay and Dodson 2011). Thus understanding the role of vegetation in influencing nutrient cycling is critical for accurate budgeting of nutrient cycling in the hyporheic zone (Groffman et al 2005;Forshay and Dodson 2011).…”

“…Nitrogen cascading into riverine sediments from upwelling groundwater, may be attenuated in the hyporheic zone through immobilization (sorption, and plant and microbial uptake) (Howard-Williams et al 1982;Clarke 2002;Strauss and Lamberti 2002), and through reduction to N 2 gas by denitrification before the reactive N joins river flows (Pretty et al 2006;Krause et al 2009;Trimmer et al 2009;Stelzer et al 2011;Forshay and Dodson 2011;Harrison et al 2012). In addition to research looking at the influence of classic geomorphic features of rivers on nutrient cycling in riverine sediments, a number of researchers have also noted the importance of evaluating the influence of different vegetation types on nutrient cycling including those growing in riverbeds (Templer et al 1996;Wigand et al 2001;Clarke and Wharton 2001;Groffman et al 2005;Trimmer et al 2009;Lefebvre et al 2006;Forshay and Dodson 2011).…”

Modelling flow and inorganic nitrogen dynamics on the Hampshire Avon: Linking upstream processes to downstream water quality

“…As much of the groundwater-fed Hampshire Avon is dominated by aquatic Ranunuculus spp. (a primary reason for the Designation of Special Area of Conservation) and because the sediments beneath such macrophytes have previously been shown to be biogeochemical hotspots (Trimmer et al, 2009), we also investigate the scale of nitrate loss that may be associated with denitrification in reactive vegetated sediments and the likely impacts of such vegetated sediments on fluxes of DIN leaving the catchment. A strategy for nitrogen reduction using a Nitrate Vulnerable Zone (NVZ) has been assessed using the model.…”