Uptake of phosphorus from different sources by Lotus pedunculatus and three genotypes of Trifolium repens .2. Forms of phosphate utilised and acidification of the rhizosphere

Abstract

Improvements in the ability of legumes to utilise applied phosphate (P) could lower fertiliser requirements. Earlier studies have found differences between legume species and genotypes of Trifolium repens in their ability to take up P from P sources that vary in solubility. Acidification of the rhizosphere may account for different P utilisation, especially from reactive phosphate rocks. To investigate differences in the ability of legumes to acidify their rhizosphere and extract various forms of P, Lotus pedunculatus and 3 genotypes of white clover that varied in their ability to respond to added P were grown in root study containers with either unfertilised soil, or soil fertilised with either monocalcium phosphate (MCP) or North Carolina phosphate rock (NCPR). After 6 weeks of plant growth, the containers were dismantled and 10 thin soil slices were taken 0-8 mm from the rhizoplane. These slices were fractionated for various P forms and pH was measured. All 4 legumes, especially lotus, acidified their rhizosphere. This resulted in negligible depletion of the small, H2SO4-P-i fraction (Pi, inorganic phosphorus) of unfertilised and MCP-fertilised soil, but caused direct dissolution of NCPR. The predominant forms of P depleted in the MCP-fertilised soil were the resin-Pi and NaOH-Pi forms, whereas in the NCPR treatment the H2SO4-Pi fraction was depleted. No significant depletion of any P fraction was observed in the unfertilised soils as plants were very small and P change in the soil slices was below that which could be measured using the experimental technique. Suggestions are made on how the experimental technique could be improved.