Mitigating the downstream effects of excess soil phosphorus through cultivar selection and increased foliar resorption

Elemental phosphorus has been categorized as a non-renewable resource that is crucial to global food security. This is largely due to the transport of phosphorus being dependent on aqueous transfer and therefore an inherent inability to return to upstream ecosystems. It is only through mining and transport of rock phosphate that agricultural land remains productive. Simultaneously, due to agricultural over-fertilization, phosphorus has been characterized as a pollutant in aquatic environments. Diffuse source run-off from high phosphorus soils continues to contribute to downstream eutrophication decades after nutrient management practices have been put into place. A potential solution involves planting high biomass-producing tree species along riparian areas. Trees belonging to the Salicaceae are ideal candidates as they have a wide geographical distribution in Canada and broad-scale applications, ranging from fibre production to biofuel feedstock to uses in phytoremediation. The objective of this thesis was to identify a commercially available tree genotype, be it poplar or willow, well suited for widespread planting in agricultural areas to limit nutrient enrichment of riparian ecosystems. Phenotypic differences in phosphorus storage and allocation were analyzed using ICP-AES and HPLC. Poplar varieties Tristis and Northwest demonstrated the highest capacity for luxury uptake with an estimated 3.7 – 3.9 mg P g-¹ when 2.2 mM soluble phosphate (100N:70P) was applied, with no measurable metabolic consequences. However, the majority of phosphorus was stored in leaves as phosphate and subsequently returned to the environment as autumnal senescence progressed. This led to the exploration of factors limiting phosphate translocation and resorption. Expression of an exogenous phosphate H⁺/H₂PO₄- symporter in poplar led to a small, but significant increase in phosphate resorption and a pronounced increase in sulfate resorption, leading to further questions surrounding anion efflux from the vacuole and the role of the tonoplast in limiting nutrient translocation. If resorption proficiency could be increased under the high nutrient loads found in productive lands, poplar genotypes with luxury consumption could be bred for improved resorption and used to reduce phosphorus entry into riparian ecosystems. Extrapolation of this information to crop species could lead to reduced fertilizer application and improved nutrient management of perennial production systems.Forestry, Faculty ofGraduat