McLaughlin, Michael JohnDegryse, FienMontalvo Grijalva, Daniela Fernanda2017-06-292017-06-292014http://hdl.handle.net/2440/106338Low phosphorus (P) availability limits plant growth in many soils, particularly in Andisols and Oxisols, due to their large content of minerals that strongly sorb P (e.g. Al/Fe oxyhydroxides, allophane). Because of the strong P retention, P fertilizer requirements are high in these soils. Strategies to increase the efficiency of P fertilizers – and reduce P rates needed to obtain maximal yield – remain key to reducing the pressure on limited rock phosphate reserves. To develop management practices or fertilizer formulations that enhance P availability and fertilizer efficiency in strongly P-sorbing soils, a better understanding of the chemical reactions of P in these soils is needed. This work aimed (i) to examine the chemical behaviour of soil P and added P to plant uptake in strongly P-sorbing soils and (ii) to compare the effect of different P fertilizer types (granular/fluid/nano-sized) as a strategy to increase the efficiency of P fertilizers. A laboratory incubation experiment was conducted to evaluate the diffusion and lability of P from granular and fluid fertilizers applied to Andisols and Oxisols using the isotopic dilution technique and a novel visualization method. In all soils, fluid fertilizers enhanced P diffusion, but not P lability, i.e. the amount of added P that remained in isotopically exchangeable form. In the Oxisols, a greater percentage of added P remained isotopically exchangeable when added as granular monoammonium phosphate (MAP) (41% labile) than when added as fluid MAP (25% labile). In the Andisols, no significant difference was observed in the percentage of labile P between both fertilizer types (circa 25% labile). Given these results, it was hypothesized that there would be no agronomic benefit from the application of fluid P fertilizer in these soils. A subsequent pot trial was conducted to assess the uptake of P by wheat (Triticum aestivum) from granular and fluid fertilizers using the indirect isotopic dilution method in two Andisols, two Oxisols, and a calcareous soil (where fluid P has been proven more effective). This pot trial indeed showed no significant difference in dry matter yield, P uptake and the percentage of P derived from the fertilizer in the plant (%Pdff) between granular and fluid MAP in the Andisols or Oxisols, while there was a significant increase with fluid fertilizer in the calcareous soil. Hydroxyapatite nanoparticles (Ca₁₀(PO₄)₆(OH)₂, n-HAP) were also tested as a potential P fertilizer, based on the hypothesis that nano-sized particles can potentially move in the soil and reach the plant roots through the transpiration flow. Because of the strong adsorption and subsequent fixation of soluble P in this type of soils, nanoparticulate P could potentially have a benefit over soluble fertilizers. Column studies showed some leaching (5%) of n-HAP in the Andisol but very little in the Oxisol. In contrast, bulk-sized HAP did not move in either of the soils. A pot trial using the isotopic dilution procedure evaluated P availability for wheat from n-HAP, bulk-sized HAP, and triple superphosphate. For Andisols and Oxisols, P uptake and %Pdff differed significantly from P treatments as follows: TSP > n-HAP > bulk-HAP. Thus, while sparingly-soluble fertilizer in nanoparticulate form (n-HAP) performed better than its bulk counterpart, it was less efficient than soluble fertilizer (TSP). It was hypothesized that the difference between n-HAP and bulk-HAP was due to the difference in rate of dissolution, but that the n-HAP has no direct effect on the uptake and only contributes via dissolution. The pot trial showed that n-HAP did not have an agronomic benefit over soluble granular fertilizers, but the possible contribution of nanocolloidal P to P uptake was still further investigated in hydroponic experiments. Phosphorus bioavailability is related to its concentration and speciation in the soil solution. Free orthophosphate is the form of P taken up by plants; but colloidal P constitutes an important fraction of total solution P in oxide- or allophane-rich soils and its bioavailability has not been previously considered. The uptake of P by wheat seedlings was measured from radiolabeled non-filtered (colloid-containing) and 3-kDa filtered (colloid-free) soil-water extracts from Andisols and Oxisols. In the Andisol extracts, P uptake was up to seven-fold higher in the non-filtered solutions than in the corresponding 3-kDa filtered solutions. It is hypothesized that labile humic/fulvic-Fe/Al-P complexes increased the diffusive transport flux of free P to the roots. In the Oxisol extract, no difference in P uptake between both solutions was observed. Also, the diffusional flux of P measured with the diffusive gradient in-thin films (DGT) method was larger in the nonfiltered than in the 3-kDa filtered solutions. These results are the first observation that natural colloidal P is not inert and can contribute to plant P uptake. This work has shown that increasing soil available P and fertilizer efficiency in soils where strong adsorption reactions control P availability is very challenging. However, the observed contribution of colloidal P to plant P uptake for Andisols is a finding that may lead to the development of new management practices to enhance the release of P-containing colloids into solution as a complimentary strategy to P fertilization in these strongly Psorbing soils. Although in this study hydroxyapatite nanoparticles offered no advantage over conventional soluble P fertilizers for plant growth, this does not imply that nano-sized P fertilizers can be ruled ineffective. The addition of labile nanocolloidal P that is mobile in soil and contributes to P uptake is still a worthwhile fertilizer strategy to investigate.andisoloxisol33P isotopic dilutionfluid fertilizerphosphorushydroxyapatite nanoparticlescolloidal phosphorusP adsorptionResearch by PublicationTheses10.4225/55/5955a36fa7514