Sammanfattning av doktoranduppgiften
Long-term phosphorus supply in soils – Evaluation of P-AL and the significance of stable P forms
The overall aim of this project is to improve our knowledge about the P exchange between the soil solution and the solid phase by the use of isotope techniques, common chemical extractions, sink techniques and analysis of archived soil samples and historical data from the Swedish long-term fertility experiments. From the results, a model will be developed and used to create a tool aimed to refine the current Swedish fertilisation recommendations.
Most analyses included in this project will be performed on data obtained from soil taken in autumn 2015 from six experiments in the southern and central parts of the Swedish long-term fertility experiments. Soil will be taken from treatment A3 (highest level of N, no P or K) and D3 (highest level of N, P and K). To be able to investigate any change in the labile and stable P pools as a response to the different fertilization levels over time selected archived samples will be analysed.
The ability of the two common soil P tests, AL (Egnér) and Olsen, to quantify the amount of labile P in a range of typical Swedish soils is assessed by isotopic dilution methods. The rate of P desorption is studied by use of continually exchanged iron impregnated filter papers in soil suspensions. The results from the isotope technique will be used for parametrization of the kinetic Freundlich equation. The possibility that this equation can extrapolate the isotopically exchangeable pool (E) over much longer time periods will be investigated. The results from the kinetic Freundlich equation will be combined with results from chemical analyses of archived soils and be used to derive estimates for the rate of exchange between the labile P pool and the less easily available, "stable" inorganic P pool. This will allow the prediction of the long-term availability of P present in the stable pool. Results from different P extractions performed on archived soil will show the change of oxalate-extractable P and labile pools over time, and we will then be able to estimate the amount of stable P in equilibrium with a certain dissolved P concentration in the soil solution.
Collected data and information will be used to derive models and mathematical relationships to more accurately predict the size of the plant-available P pool and the long-term fluxes of P in the stable pool - labile pool - plant system. Crop yield data will be used to calculate the balance between P addition and removal and Freundlich relationships will be used to predict P binding and release. Actual diffusion rates will be constrained by the isotope experiment and the change of extractable pools in the archived samples. A detailed analysis of earlier published models will be performed, and the models will be investigated to see if they can describe the results from this study. The derived models will then be simplified and used to develop a simple tool for improved fertilization recommendation.