Hydrological controls on DOC : nitrate resource stoichiometry in a lowland, agricultural catchment, southern UK
Catherine M. Heppell1, Andrew Binley2, Mark Trimmer3, Tegan Darch1,2, Ashley Jones1,2, Ed Malone1,2, Adrian L. Collins4, Penny J. Johnes5, Jim E. Freer5, and Charlotte E. M. Lloyd61School of Geography, Queen Mary University of London, Mile End Road, London E1 4NS, UK 2Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK 3School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK 4Sustainable Soils and Grassland Systems Department, Rothamsted Research, North Wyke, Okehampton, EX20 2SB, UK 5School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, UK 6School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK
Received: 19 Jan 2017 – Accepted for review: 26 Feb 2017 – Discussion started: 07 Mar 2017
Abstract. The role that hydrology plays in governing the interactions between dissolved organic carbon (DOC) and nitrogen in rivers draining lowland, agricultural landscapes is currently poorly understood, yet important to assess given the potential changes to production and delivery of DOC and nitrate arising from climate change. We measured DOC and nitrate concentrations in river water of six reaches of the lowland River Hampshire Avon (Wiltshire, southern UK) in order to quantify the relationship between Baseflow Index (BFI) and DOC : nitrate molar ratios across contrasting geologies (Chalk, Greensand and clay). We found a significant positive relationship between nitrate and Baseflow Index (p < 0.0001), and a significant negative relationship between DOC and Baseflow Index (p < 0.0001), resulting in a non-linear negative correlation between DOC : nitrate molar ratio and Baseflow Index. In the Hampshire Avon, headwater reaches which are underlain by clay and characterised by a more flashy hydrological regime are associated with DOC : nitrate ratios > 5 throughout the year, whilst groundwater-dominated reaches underlain by Chalk, with a high Baseflow Index have DOC : nitrate ratios in surface waters that are an order of magnitude lower (< 0.5). Our analysis also reveals significant seasonal variations in DOC : nitrate transport and highlights critical periods of nitrate export (e.g. winter storm events in sub-catchments underlain by Chalk and Greensand, and autumn events in drained, clay sub-catchments) when DOC : nitrate molar ratios are low, suggesting low potential for in-stream uptake of inorganic forms of nitrogen. Future work should determine whether the results reported here are transferable to other agricultural, lowland catchments, and seek to understand the generalised hydrological controls on the availability of DOC transported through such landscapes.
Heppell, C. M., Binley, A., Trimmer, M., Darch, T., Jones, A., Malone, E., Collins, A. L., Johnes, P. J., Freer, J. E., and Lloyd, C. E. M.: Hydrological controls on DOC : nitrate resource stoichiometry in a lowland, agricultural catchment, southern UK, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2017-30, in review, 2017.
Catherine M. Heppell et al.
Catherine M. Heppell et al.
Hampshire Avon: Daily discharge, stage and water chemistry data from four tributaries (Sem, Nadder, West Avon, Ebble).