Mobilization and transfer of nutrients from litter to tree seedlings via the vegetative mycelium of ectomycorrhizal plants

The ability of the mycorrhizal fungus Paxillus involutus to mobilize nitrog en and phosphorus from discrete patches of beech (Fagus sylvatica), birch ( Betula pendula) and pine (Pinus sylvestris) litter collected from the ferme ntation horizon of three forest soils, and to transfer the nutrients to col onized B. pendula Roth seedlings, was investigated in transparent observati on chambers. The mycelium of P. involutus foraged intensively in all three types of litter, leading to a significant decline in their phosphorus conte nts after 90 d. Over the same period only one of the litter types, beech, s howed more than a 10% loss of its N contents. Exploitation of the litter le d to invigoration of the vegetative mycelium of the fungus throughout the c hambers as well as to significant increases of biomass production and leaf area in seedlings grown in the plus litter (+ L) relative to those in minus litter (- L) systems. The yield increases were associated with gains in wh ole plant tissue content and concentration of P, but in content only in the case of N. Calculations suggest that a major proportion of the phosphorus lost from litter originated in its organic fraction. The possible basis of the discrepancy between values of N loss from litter and gain by the plant is discussed and the extent to which the distinctive pattern of nutrient mo bilization is a feature peculiar to this fungus-plant combination is consid ered. It is concluded that nutrient mobilization from natural organic subst rates in the fermentation horizon of forest soils may be a key function of the vegetative mycelium of mycorrhizal systems. The need for experimental a nalyses of a greater range of fungus-plant partnerships is stressed.