P depletion and activity of phosphatases in the rhizosphere of mycorrhizal and non-mycorrhizal cucumber (cucumis sativus L.)

An experiment was set up to test the ability of arbuscular mycorrhizal (AM) roots and hyphae to produce extracellular phosphatases and to study the relationship between phosphatase activity and soil organic P (Po). Non-mycorrhizal cucumber and cucumber in symbiosis with either of two mycorrhizal fungi were grown in a sandy loam-sand mixture in three-compartment pots. Plant roots were separated from two consecutively adjoining compartments, first by a 37 m mesh excluding roots and subsequently by a 0.45 m membrane excluding mycorrhizal hyphae. Soil from the two root-free compartments was sectioned in a freezing microtome and analyzed for extracellular acid (pH 5.2) and alkaline (pH 8.5) phosphatase activity as well as depletion of NaHCO3-extractable inorganic P (Pi) and Po. Roots and mycorrhizal hyphae depleted the soil of Pi but did not influence the concentration of Po in spite of increased phosphatase activity in soil influenced by roots. Phosphatase activity at both pH values was highest in soil influenced by uncolonized roots, but this was attributed to higher root length densities as compared to mycorrhizal roots. Mycorrhizal hyphae showed no influence on soil phosphatase activity in spite of high hyphal length densities (> 22 m cm−3). Hyphae were also able to deplete soil of Pi beyond the membrane interface.