The phosphorus transfer continuum under climate change
Creators: name: Forber, Kirsty Jessica
Publisher: name: Lancaster University
Year: 2018

description: Phosphorus (P) is a key nutrient in governing crop growth, and its fate
      and behaviour in the environment is critical to water quality. Climatic
      changes such as hotter, drier summers and warmer, wetter winters will
      cause changes to the movement of P across the land-water continuum
      yielding potentially detrimental impacts to water quality which underpin
      many ecosystem services. This thesis uses the ‘P transfer continuum’ as a
      framework to discuss and explore the possible impacts of climate change to
      P in the environment. I use the three National Demonstration Test
      Catchment (DTC) platforms (Eden, Cumbria; Wensum, Norfolk; and Avon,
      Hampshire) which are representative of typical catchment typologies and
      agricultural activities in the UK to frame this work. Potential changes to
      the P transfer continuum are essential to consider and react to if we are
      to improve water quality in order to preserve ecosystem services into an
      ever uncertain future.
      
      Estimations of dry period characteristics (duration and temperature) under
      current and predicted climate (determined using data from the UK Climate
      Projections (UKCP09) Weather Generator tool) were used to design a
      laboratory experiment to examine whether changes in the future patterns of
      drying/re-wetting will affect the amount of soluble reactive phosphorus
      (SRP) solubilised from soil. This study is focused on the second tier of
      the P transfer continuum: mobilisation via solubilisation. For three UK
      soils critical breakpoints (6.9-14.5 d) of drying duration have been
      identified; before the breakpoint an increase in SRP loss with the number
      of dry days was observed; after this point the amount of SRP lost
      decreased or stayed fairly constant. It is likely that longer periods of
      dry days followed by rapid re-wetting events will not yield more SRP via
      solubilisation than at the breakpoint. However, because the frequency of
      longer dry periods will increase under climate change, the solubilisation
      of SRP from soil (-1 to +13%) will also change.
      
      Using the Hydrological Predictions in the Environment (HYPE) model for
      three distinct UK DTC catchments, and Extended End-Member Mixing Analysis
      (EEMMA), I explore how climate change (UKCP09 scenarios) might impinge on
      catchment total phosphorus (TP) retention and sensitivity, which is
      determined by catchment characteristics to P input pressures. This study
      encompasses the all tiers of the P transfer continuum: source,
      mobilisation, transfer (or delivery), and impact. Under a high emissions
      scenario (2080s), an increase of catchment TP retention was predicted in
      three UK catchments. I conclude that catchment sensitivity to climate
      change should be accounted for in determining appropriate water quality
      targets that can be effectively delivered via catchment stakeholders and
      government.
      
      I use the Newby Beck (Eden DTC) sub-catchment as a critical example of how
      anthropogenic point sources of P can alter the retention of P even at the
      headwater scale where agricultural diffuse sources dominate. I use
      bi-weekly sampling of P, chloride (Cl- ) and flow (Q), load apportionment
      modelling (LAM) and mass balance, alongside sediment sampling to
      investigate retention at the headwater scale. I found that although
      diffuse sources contributed to more of the TP load, point sources dominate
      more frequently and are therefore proportionally perhaps more important in
      terms of continuous downstream water quality. Under climate change the
      transfer of diffuse sources is likely to increase, therefore it might be
      hypothesised that climate change will yield extremes between nutrient
      quality in summer (high concentrations, low flows) and winter (high loads,
      high flows). This calls for policy and regulation to reflect the urgency
      of the impacts of climate change on the riparian health in rural headwater
      communities.
      
      I conclude by discussing the implications of climate change on the P
      transfer continuum. I highlight the possible risks of climate change
      exacerbating, rather than changing, the processes described in the P
      transfer continuum for the Newby Beck catchment. My findings, in addition
      to those from the NUTCAT team, call for climate change to be taken
      seriously in forming new effective policies which preserve the health of
      UK water bodies, the sustainability and profitability of UK agriculture,
      the enjoyment and amenity value of our water courses, and avoid large
      financial costs into the future. I therefore provide a new framework which
      can be used to aid the challenges which surround the preservation of water
      quality into the future.