Abstract
Phosphorus, an essential nutrient for life, excess phosphorus is a common ecological concern in watercourses worldwide leading to eutrophication and pollution of natural water systems. In the UK, eutrophication of freshwater systems is one of the most common cause of water bodies not achieving good ecological status by the Environment Agency. The Somerset Levels and Moors (SLMs) condition status is described, by Natural England and the Environmental Agency, as ‘unfavourable declining’ due to the phosphate levels being up to three times higher than the Common Standards Monitoring Guidance targets of 0.1 mg/l total phosphate. Housing development on the SLMs has become implicated due to their phosphate discharge, so Natural England and planning authorities are working towards ‘phosphate neutrality’. There is a lack of long-term research nationally on the biogeochemical interactions of phosphate, nitrate, and iron on a peat-based agricultural wetland; the factors affecting the temporal but also spatial distribution of phosphate, nitrate, and iron; and how restoration activities on a peat-based wetland impact these concentrations. Iron can bind to bioavailable phosphates which can also affect the bioavailable pool available for phosphate exchange between surface water and peat. This thesis fills the knowledge gap through investigations on the case studies at Moorlinch SSSI and Greylake RSPB sites located on the SLMs, on a local scale with results having national application to drive further research and drive policy change in regards to water quality issues and wetland restoration. Firstly, a biodiversity evaluation of both sites was undertaken pre and post wetland restoration as a point of reference from which to evaluate how the creation of wetland features, changes in land management, and increasing water levels will help improve the biodiversity on a lowland agricultural peat-based wetland. Moorlinch SSSI is granted this protection status for its wet grasslands and ditch systems. The restoration that has taken place enhances this status because part of the site is now transitioning to a tall fen, swamp, and open water mosaic which will have co-benefits for biodiversity and ecosystem services, such as the open water value to wading birds, improvements in water quality, flood management and peat soil restoration. The restoration efforts on Moorlinch SSSI had an overall increase in biodiversity and net gain (29% total gain habitats and watercourses combined within <30years).Secondly, an investigation was completed into the biogeochemical exchange of phosphate and nitrate in the groundwater and surface water on rewetted peat soil on the SLMs. This research has demonstrated that bioavailable phosphate was controlled on a spatial and temporal basis, by the biogeochemical interactions of nitrate, phosphate, and iron, influenced by oxygen levels, microbial activity and water level. Soil disturbance and increased soil oxidisation during the wetland restoration was shown to remobilise phosphates (>8mg/l) into the surrounding water and newly created pools (1.5mg/l) and scrapes during the first few months post restoration. This release of phosphate has been reported to cause eutrophication events when phosphates reach levels above 3mg/l, however eutrophication was not observed during this phase of post restoration due to the time of year being in winter seasons (3-9oC) and high water levels (3-14cm) on site; giving insight into implementing these findings into wetland creation and restoration management plans on phosphate contaminated soil.Thirdly, a controlled laboratory investigation into peat soil phosphate storage on Moorlinch SSSI and its exchange during flooding, drying, and rewetting was completed to quantify exchange rates and allow estimations of recovery time after remediation. Temperature, redox potential, dissolved oxygen, microbial activity, and water levels all influenced the release of bioavailable phosphate from the peat soils. The results of the experiments showed the top 30cm layer of peat soil holds the highest percentage of the legacy phosphorus contamination on Moorlinch (p<0.05) with the top 30cm ranges between 500-1000mg/kg in total phosphorus concentration; <4mg/l (approximately 0.37%) of SRP was being exchanged with the surrounding surface water (p<0.05). This suggested that flushing the soil would take a very long time to achieve a water quality standard of <0.1mg/l and creating a phosphorus limiting environment. This research therefore indicates that a different method of phosphate removal is needed on the Somerset Levels to reduce the legacy of phosphate pollution from the system within a realistic time period.Finally, an investigation regarding the potential use of native wetland plant species as paludiculture crops in the uptake and removal of phosphorus from lowland peat-based wetland on the Somerset levels and Moors. Wetland plants growing on Moorlinch and Greylake had phosphorus removal potential such as 18.0 kg/ha/yr (Typha latifolia), 9.6 kg/ha/yr (Phragmites australis), 5.2 kg/ha/yr (mixed wetland grass) and 2.8 kg/ha/yr (Phalaris arundinacea). Currently, paludiculture with crops, such as Typha latifolia, are not competitive with the existing agricultural land use of dairy production, due to high cultivation costs and low revenues, but with improved farming technology and emerging concept of carbon offset credits they could have potential to become a profitable and bring ecological beneficial biomass production. The wetland plants that naturally grow on the SLMs could potentially be used for paludiculture and for the phosphorus mitigation to improve the overall water quality on the Somerset Levels as a large-scale phosphorus removal approach. Products from Typha latifolia crop can be used in biofuels, building materials, and fiber cables, a renewable resource that potentially has many industry uses. paludiculture trails on the Somerset Level should investigate whether cropping more than once throughout the year would be beneficial in removing more phosphorus off site and still hit profitable targets. The question remains what will happen once peatlands become nutrient deficient, will they be re-created into fen peatlands or would paludiculture trials add fertiliser into the system to keep farming the land; something to consider adding to the management plan for The Somerset Levels and Moors.
Keywords
peatland, water quality, Phosphate, Nitrate, wetland
Document Type
Thesis
Publication Date
2025
Embargo Period
2025-03-18
Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License
Recommended Citation
Underwood, N. (2025) An investigation of the biogeochemical exchange of the macro plant nutrients phosphorus and nitrogen on rewetted peat soil on the Somerset Levels and Moors.. Thesis. University of Plymouth. Retrieved from https://pearl.plymouth.ac.uk/gees-theses/462
