CONTRIBUTION BY 2 ARBUSCULAR MYCORRHIZAL FUNGI TO P-UPTAKE BY CUCUMBER (CUCUMIS-SATIVUS L) FROM P-32 LABELED ORGANIC-MATTER DURING MINERALIZATION IN SOIL

An experiment was set up to investigate the role of arbuscular mycorrh iza (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 comp artment 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 t he hyphal compartment was 5.5 and 8.6 % for plants colonized with Glom us sp. and G. caledonium, respectively, but only 0.6 % for the non-myc orrhizal controls. Interfungal differences were not related to root co lonization or hyphal length densities, which were lowest for G. caledo nium. Both fungi depleted the labelled soil of NaHCO3-extractable P an d P-32 compared to controls. A 15-25 % recovery of P-32 by roots was n ot 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 in tercept some P immobilization by other microorganisms and P-sorbing cl ay minerals.