UPTAKE OF P-32 FROM LABELED ORGANIC-MATTER BY MYCORRHIZAL AND NONMYCORRHIZAL SUBTERRANEAN CLOVER (TRIFOLIUM-SUBTERRANEUM L)

An experiment was designed to study whether hyphae and colonized roots of arbuscular mycorrhiza have more direct access to P in organic matt er than roots of non-mycorrhizal plants. Soil supplied with 0, 15 or 4 5 mg P kg(-1) was uniformly mixed with P-32-labelled organic matter at four levels (0, 1, 2 and 5 g kg(-1)) and inoculated with a mycorrhiza l fungus or left uninoculated. Pots were incubated at 60% of field cap acity for one week prior to sowing of clover, and plants were harveste d after a growth period of 23 days. Mycorrhizal colonization increased shoot dry weight, P concentration and P-32 uptake at all P levels. Sp ecific activity in plants was consistently higher than in correspondin g soil. This indicates that the added P-32 never reached an equilibriu m with inorganic P in the soil. P mineralized from organic matter thus had a residence time in the soil solution short enough to partially a void isotopic exchange and adsorption. Mycorrhizal colonization influe nced specific activity of P-32 in plants from three of the nine combin ations of P and labelled organic matter: At the lowest level of P the specific activity was highest in non-mycorrhizal plants, and at the in termediate level of P there was one treatment where mycorrhizal plants had the highest specific activity. These differences are discussed. P lant dry weight and P concentration did not respond to addition of org anic matter, though soil extracts consistently contained higher amount s of inorganic P as a result of organic matter addition. The results s uggest that mycorrhizal plants at an early growth stage utilize a subs tantially higher amount of P released from organic matter than non-myc orrhizal plants. This mycorrhizal advantage does not seem to be relate d to a mycorrhizal influence on mineralization.