The influence of anaerobic conditions and redox on phosphorus loss from waterlogged soils : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Diffuse phosphorus (P) loss from agricultural land contributes to surface water quality degradation. To mitigate losses and protect waterways it is important to describe all the ways that P can be mobilised and transported from land. The main aim of this thesis was to investigate P loss associated with the reductive dissolution of iron (Fe) and manganese (Mn) oxides in the soil, due to waterlogging and reducing conditions. The overall hypothesis was that anaerobic and reducing conditions in soils contribute significantly to potential annual P losses. A combination of laboratory analysis and field work was conducted in New Zealand and Ireland. The first objective was to quantify the potentially reducible phosphorus (P) component in the lab for a range of stored soil samples, and the distribution of reducible P pools relative to known soil maps. In New Zealand, eight stored soils were tested in the laboratory (Andisol, Cambisol, Vitric andisol, Ferrasol, Luvisol, Gleysol, Arenosol/Fluvisol, and Acrisol), and five stored soils were tested in the laboratory from Ireland (Cambisol, Gleysol, Luvisol, Phaeozem, and Podzol). Current models use oxic soil tests, which may not represent anaerobic conditions. Anoxic water extractable P (anoxic WEP) and sodium-bicarbonate-dithionite extractable P (dithionite-P) tests were developed to predict soil P vulnerable to reductive dissolution and potential loss under anaerobic conditions. In New Zealand and Ireland, anoxic WEP and dithionite-P varied by soil order and land use, and anoxic WEP was greater than oxic WEP¬, which showed the short-term impact of soil anoxia on P release. Models predicting anoxic WEP and dithionite-P at the 1:50,000 scale in New Zealand found relatively small proportions of agricultural land were enriched in dithionite-P (31% >85 mg kg-1) or anoxic WEP (3% >0-0.291 mg L-1). The second objective was to determine if redox reflects changes in P and redox-sensitive components, with attention to the length of time that the soil is saturated, P fertiliser treatment, and temperature. The second laboratory experiment was an incubation comparing the rate and extent of P release across contrasting soil textures (clay loam, silt loam, sandy loam), three long-term P fertiliser levels, at two temperatures (3oC v.s. 18oC), and two oxygen levels (oxygen < 0.5 mg L-1 v.s. > 7 mg L-1). The mean dissolved reactive P (DRP) concentration and its rate of release increased with fertiliser application, temperature, and in two soils, anoxic conditions - commensurate with the depletion of nitrate (NO3-) and the reductive dissolution of Fe and Mn. The release of P was complete within 24 hours, which showed that the reaction, and potential for enhanced P loss, could occur within a day of saturation. The third objective was to observe trends in the release of P and other redox-sensitive species into soil solution under saturated conditions, during the drainage season. A field experiment monitored a New Zealand soil profile with 16 unsaturated zone samplers installed down to 20 and 80 cm below ground level, from May to September of 2017 and 2019. They were installed to observe P and Fe release as the profile wets up or dries out. Events that saturated soil caused reducing conditions that released up to 77% and 96% greater P and Fe, respectively, than average over the rest of the sample period. Artificial saturation experiments in the laboratory and in the field used the same soil as the field experiment and showed that soils treated with NO3- released up to 86% less DRP and 98% less Fe than the soil that received no N. This showed that the reduction reaction was buffered by the presence of NO3-. The fourth objective was to determine the role of redox processes in the release of P and other redox species within and across drainage and runoff events. Flow-weighted artificial drainage and runoff samples were collected from four hydrologically-isolated plots on the same slope in Ireland in 2009, 2018 and 2019. They were analysed for P and redox sensitive components and assessed throughout seasons and storm events. At the site used, dissolution of P and Fe dissolution had a stronger relationship in reducing conditions. Additionally, the reaction was not season specific. The main drivers of Fe and P release were the extent of waterlogging in the soil, and levels of NO3-. In conclusion, an area that is prone to saturation excess could have increased P loss due to reductive dissolution. The research presents two updated soil test methods and predictive equations that could be used to improve estimations of P loss from poorly drained areas and specific soil types. Laboratory data gives evidence that soil texture, P fertility levels, temperature and oxygen conditions affect the amount and rate of WEP released within 24 hours of waterlogging. Field data shows that the relationship between P and Fe can be seen in soil solution as well as at the drainage output. The present work also shows that waterlogging and NO3- levels are key drivers in the occurrence of P and Fe dissolution from soil. Therefore, the research highlights periods where warm temperatures and high moisture conditions coincide as high risk. Having a high P/low NO3- system, particularly in sites that are prone to saturation events may exacerbate P release into waterways in the future. The influence of waterlogging and anaerobic conditions on Fe and P should be integrated into current critical source area models and used to inform potential mitigation strategies.