PHOSPHORUS DEPLETION AND PH DECREASE AT THE ROOT SOIL AND HYPHAE SOIL INTERFACES OF VA MYCORRHIZAL WHITE CLOVER FERTILIZED WITH AMMONIUM

To study phosphorus (P) depletion and soil pH changes at the root-soil interface (rhizosphere) and at the hyphae-soil interface, mycorrhizal and non-mycorrhizal white clover (Trifolium repens L.) plants were grown for 7 wk in two sterilized soils (Luvisol and Cambisol) in pots comprising five compartments: a central one for root growth, two adjacent compartments, separated from the central compartment by a nylon net of 30-mu-m mesh size, for growth of vesicular-arbuscular (VA) mycorrhizal [Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe] hyphae (hyphal compartments), and two outer compartments, separated from the hyphal compartments by a 0.45-mu-m membrane, which neither roots nor hyphae could penetrate (bulk soil compartments). Phosphorus was supplied as Ca(H2PO4)2 at a rate of 50 mg P kg-1 soil in the root compartment and 150 mg P kg-1 soil in the hyphal and bulk soil compartments. Nitrogen was supplied as (NH4)2SO4 at the rate of 300 mg N kg-1 soil uniformly to all compartments. In both soils, shoot dry weight and P uptake were much higher in mycorrhizal plants compared with non-mycorrhizal plants. Hyphae of VA mycorrhizal fungi contributed 70 % (Cambisol) or 80 % (Luvisol) to total P uptake of mycorrhizal plants. In the hyphal compartments, concentrations of both H2O-extractable soil P (Cambisol and Luvisol) and NaHCO3-extractable soil P (Luvisol) were decreased drastically. Soil P depletion profiles developed not only at the root-soil interface (rhizosphere), but also at the hyphae-soil interface and extended several millimetres from the hyphae surface into the soil. Likewise, the soil pH was decreased at the root-soil interface, in the hyphal compartment and also at the hyphae-soil interface. The results demonstrate that, similarly to roots, hyphae of VA mycorrhizal fungi have the ability to form a P depletion zone and a zone of altered pH in the adjacent soil. Thus, as well as at the root-soil interface, soil conditions at the hyphae-soil interface may also differ considerably from conditions in the bulk soil.