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Biogeochemical and climate drivers of wetland phosphorus and nitrogen release: Implications for nutrient legacies and eutrophication risk

This paper describes a study of the dynamics and processes of nutrient cycling and release for a lowland wetland‐pond system draining Wytham Wood ECN site in Oxfordshire. The authors used ECN's long-term water chemistry and meteorological datasets for the site.
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Jarvie, H.P., Pallet, D.W., Schafer, S.M., Macrae, M.L., Bowes, M.J., Farrand, P., Warwick, A.C., King, S.M., Williams, R.J., Armstrong, L., Nicholls, D.J., Lord, W.D., Rylett, D., Roberts, C. and Fisher, N. (2020), Biogeochemical and climate drivers of wetland phosphorus and nitrogen release: Implications for nutrient legacies and eutrophication risk. J. Environ. Qual.. Accepted Author Manuscript. doi:10.1002/jeq2.20155

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Abstract

The dynamics and processes of nutrient cycling and release were examined for a lowland wetland‐pond system, draining woodland in southern England. Hydrochemical and meteorological data were analyzed from 1997 to 2017, along with high‐resolution in‐situ sensor measurements from 2016 to 2017. The results showed that even a relatively pristine wetland can become a source of highly‐bioavailable phosphorus, nitrogen and silicon during low‐flow periods of high ecological sensitivity. The drivers of nutrient release were primary production and accumulation of biomass, which provided a carbon source for microbial respiration and, via mineralization, a source of bioavailable nutrients for phosphorus and nitrogen co‐limited microbes. During high‐intensity nutrient‐release events, the dominant nitrogen‐cycling process switched from denitrification to ammonification, and a positive‐feedback cycle of phosphorus and nitrogen release was sustained over several months during summer and fall. Temperature controls on microbial activity were the primary drivers of short‐term (day‐to‐day) variability in phosphorus release, with subdaily (diurnal) fluctuations in phosphorus concentrations driven by water‐body metabolism. Inter‐annual relationships between nutrient release and climate variables indicated ‘memory’ effects of antecedent climate drivers, through accumulated legacy organic matter from the previous year's biomass production. Natural flood management initiatives promote the use of wetlands as ‘nature‐based solutions’ in climate‐change adaptation, flood management, and soil and water conservation. This study highlights potential water‐quality trade‐offs, and shows how the convergence of climate and biogeochemical drivers of wetland nutrient release can amplify background nutrient signals by mobilizing legacy nutrients, causing water‐quality impairment and accelerating eutrophication risk.

  • Combines 20‐yr nutrient monitoring and use of high‐resolution in‐situ sensors
  • Concurrent release of Soluble P, NH4‐N and Si during summer and fall
  • Nutrient release linked to climate drivers of biomass accumulation and turnover
  • High‐intensity nutrient release events reflected legacies of previous years’ biomass
  • Highlights water‐quality tradeoffs of wetlands for soil and water conservation

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