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

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 CO2 was greater than the carbon equivalents due to reduction of O2, NO3- and SO42- 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 3.6 mol N m-2 y-1 was mineralised in the sediment, of which 11% could be accounted for by the measured efflux of NH4+. The remainder could be accounted for by the N demand from primary production (67% macrophytes/biofilm; 36% phytobenthos). Copyright © 2009 John Wiley & Sons, Ltd.