Reduced forms of phosphorus in temperate agricultural soils

Kehler, Anchen and Haygarth, Philip and Blackwell, Martin and Tamburini, Federica (2023) Reduced forms of phosphorus in temperate agricultural soils. PhD thesis, Lancaster University.

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Abstract

Climate predictions in temperate regions suggest that autumn and winter rainfall events are likely to increase in both duration and total amounts of precipitation. This is likely to increase the spatial and temporal occurrence of reducing environments in soils. Unlike more typical aerobic soil systems, these anaerobic systems facilitate conditions in which alternative oxidation states of vital elements needed for a healthy soil system present themselves. This therefore introduces the likelihood of the establishment of alternative chemical equilibria, involving lower oxidation state compounds, including the element P, which in recent years has gained attention as being an immediate element of concern. Currently. There is a lack of information about the abundance, availability and utilisation of such nutrient sources within biological systems; and further to that, the chemical processes that control mobilisation and immobilisation of reduced chemical species. This research investigates the reduced phosphorus compound, N-(phosphonomethyl) glycine, also known as glyphosate and its breakdown product Aminomethyl phosphonic acid (AMPA). Both compounds are phosphonate compounds, a reduced group of phosphorus compounds in the oxidation state +3. This research was conducted to better understand their biochemistry and dynamics within the soil system as soil contaminants in agricultural soils. An extensive literature review was conducted, gathering existing data to investigate the multiple pathways and transformations that the reduced P compounds and groups likely utilise in the soil system. A conceptual model was produced, demonstrating this and highlighting one of the largest agricultural inputs being from the reduced P group, the phosphonates. A study was undertaken to asses which soil treatments phosphonate compounds are most likely to be found in, with 31P NMR data demonstrating that grassland and wetland sites have a higher likelihood to contain phosphonate compounds, both containing them at a concentration of 0.1mg kg-1, most likely due the land management practice allowing for longer term establishment of stable ecosystems. Further investigation looked at the possibility that within the soil system, phosphonates, such as those identified with NMR, may be cycled through micro-organisms to prevent their inevitable build up through the application of AMPA, the primary metabolite of glyphosate. Using microbiological methods and PCR analysis, four species were identified as capable of utilising AMPA for growth, including Schwanniomyces polymorphus, Saitozyma podzolica, Trichosporon sp. S1-8 and Yersiniaceae bacterium. There was no indication that land management had an impact on species presence with a capability to utilise phosphonate compounds for growth and metabolism. This data demonstrates that their active genes present within soils allow for cycling of reduced P compounds that will be able to adapt to a changing and potentially more extreme soil ecosystem as the climate changes. Through a batch study, the soil chemical dynamics of glyphosate was used to determine adsorption/desorption of the compound and its impact on soil inorganic P. Data demonstrated that, glyphosate displaced inorganic P when in contact with soils, which has far-reaching implications for agricultural sites that have frequent glyphosate treatment for weed management. The action of micro-organisms however appears to reduce this effect, potentially through utilisation of the glyphosate, therefore preventing is release into solution. The research presented in this thesis also identified links between soils that contain phosphonate compounds and the presence of micro-organisms that are capable of utilising them, with grassland and wetland sites being the only land management type to contain phosphonates as well as successfully isolate micro-organisms that thrived without the addition of AMPA to a nutrient media. The findings of this study highlight the many interactions that phosphonate compounds, primarily through the study of glyphosate, have the ability to be involved in and impact on the soil ecosystem. They have the ability to be indirectly and directly environmentally damaging due to their chemical properties, but additionally are source of nutrients for certain soil micro-organism species; all of which highlight the importance for their consideration in soil biogeochemical cycling.