Fungal biomass in roots and extramatrical mycelium in relation to macronutrients and plant biomass of ectomycorrhizal Pinus sylvestris and Alnus incana

We studied the effects of macronutrients on the production and distribution of fungal biomass and plant biomass in ectomycorrhizal (Paxillus involutus (Fr.) Fr.) or non-mycorrhizal Pinus sylvestris L. and Alnus incana (L.) Moench. Fungal biomass was measured as ergosterol content in roots and extramatrical mycelium. Alnus incana was nodulated with Frankia. All six macronutrients were varied according to a two-level fractional factorial design. The plants were grown in pots during two growing periods in a growth chamber. Levels of N, P and sometimes K and interactions between them, had highly significant effects, whereas Ca, Mg and S had no significant effects. The production of extramatrical mycelial biomass peaked when P was low and other nutrients were high. This investment in extramatrical mycelium resulted in a 660 % higher biomass in mycorrhizal compared with nonmycorrhizal P, sylvestris at this nutrient regime. The proportion of fungal biomass in roots was stable in P. sylvesris but more variable in A. incana. Alnus incana grew less when mycorrhizal then when non-mycorrhizal. The growth responses to mycorrhiza and to the different nutrient treatments were evident at the end of the first growing period, Non-mycorrhizal P, sylvestris did not respond to P limitation by a production of proportionally more roots. This might be a reflection of an obligate dependency on mycorrhiza for effective P uptake. By contrast, the root/shoot ratio in both mycorrhizal and non-mycorrhizal P. sylvestris decreased strongly in response to increased N. The opposite root/shoot response was found in Alnus incana, and the ratio decreased strongly in response to increased P and increased in response to increased N.