Uptake of ³²P from labelled organic matter by mycorrhizal and non-mycorrhizal 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 matter than roots of non-mycorrhizal plants. Soil supplied with 0, 15 or 45 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 mycorrhizal fungus or left uninoculated. Pots were incubated at 60% of field capacity for one week prior to sowing of clover, and plants were harvested after a growth period of 23 days. Mycorrhizal colonization increased shoot dry weight, P concentration and P-32 uptake at all P levels. Specific activity in plants was consistently higher than in corresponding soil. This indicates that the added P-32 never reached an equilibrium with inorganic P in the soil. P mineralized from organic matter thus had a residence time in the soil solution short enough to partially avoid isotopic exchange and adsorption. Mycorrhizal colonization influenced specific activity of P-32 in plants from three of the nine combinations of P and labelled organic matter: At the lowest level of P the specific activity was highest in non-mycorrhizal plants, and at the intermediate level of P there was one treatment where mycorrhizal plants had the highest specific activity. These differences are discussed. Plant dry weight and P concentration did not respond to addition of organic matter, though soil extracts consistently contained higher amounts of inorganic P as a result of organic matter addition. The results suggest that mycorrhizal plants at an early growth stage utilize a substantially higher amount of P released from organic matter than non-mycorrhizal plants. This mycorrhizal advantage does not seem to be related to a mycorrhizal influence on mineralization.