The phosphorus transfer continuum under climate change

Abstract

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.