1 Risø National Laboratory for Sustainable Energy, Technical University of Denmark2 unknown3 Department of Chemical and Biochemical Engineering, Technical University of Denmark
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.
Plant and Soil, 1995, Vol 172, Issue 2, p. 221-227