Contribution by two arbuscular mycorrhizal fungi to P uptake by cucumber (Cucumis sativus L.) from ³²P-labelled organic matter during mineralization in soil

An experiment was set up to investigate the role of arbuscular mycorrhiza (AM) in utilization of P from organic matter during mineralization in soil. Cucumber (Cucumis sativus L.) inoculated with one of two AM fungi or left uninoculated were grown for 30 days in cross-shaped PVC pots. One of two horizontal compartments contained 100 g soil (quartz sand : clay loam, 1:1) with 0.5 g ground clover leaves labelled with P-32. The labelled soil received microbial inoculum without AM fungi to ensure mineralization of the added organic matter. The labelling compartment was separated from a central root compartment by either 37 mu m or 700 mu m nylon mesh giving only hyphae or both roots and hyphae, respectively, access to the labelled soil. The recovery of P-32 from the hyphal compartment was 5.5 and 8.6 % for plants colonized with Glomus sp. and G. caledonium, respectively, but only 0.6 % for the non-mycorrhizal controls. Interfungal differences were not related to root colonization or hyphal length densities, which were lowest for G. caledonium. Both fungi depleted the labelled soil of NaHCO3-extractable P and P-32 compared to controls. A 15-25 % recovery of P-32 by roots was not enhanced in the presence of mycorrhizas, probably due to high root densities in the labelled soil. The experiment confirms that AM fungi differ in P uptake characteristics, and that mycorrhizal hyphae can intercept some P immobilization by other microorganisms and P-sorbing clay minerals.