Organic Phosphorus (P) in Agricultural Soil and the Ability of Wheat to Use this as a P Source

Phosphorus (P) fertilisers are a finite resource which are becoming increasingly expensive and cause water pollution. Agricultural soils are thought to contain reserves of organic P which are a potential P source for crops. However, there is little understanding of the ability of crops to use this P source. If they could it would reduce reliance on P fertilisers. This thesis investigated the amount and identity of organic P in agricultural soils and the ability of wheat plants to access this P. To do this, solution 31P nuclear magnetic resonance spectroscopy was optimised and then used to analyse the forms and concentrations of P in several agricultural soils. Subsequently, the ability of a range of wheat varieties to access P in an abundant organic form (inositol phosphates) was assessed, both with and without mycorrhizal inoculation. This involved measuring P uptake from a radiolabelled organic P source, root traits which may facilitate P uptake and mycorrhizal colonisation. The agricultural soils all contained a substantial reserve of organic P (>23% of total soil P). Furthermore, wheat plants were able to access P from inositol phosphates and their ability to do this was dependent on their phosphatase enzyme activity and organic acid exudation. However, root system size was negatively correlated with root exudates suggesting a cost trade-off. Low level mycorrhizal colonisation did not improve P uptake from inositol phosphates or alter P uptake root traits. These findings suggest that the considerable pool of organic P in soil is a potential source for crops. The ability of wheat to access P in organic forms could be improved by enhancing their phosphatase enzyme activity and organic acid exudation. However, attention must be paid to cost trade-offs between root traits and, because of this, any negative effect that altering a trait has on other P uptake traits.