Growth and assimilation of NH4+ and NO3− by Paxillus involutus in association with Betula pendula and Picea abies as affected by substrate pH*

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Abstract
Summary: The influence of pH on the growth and assimilation of 15N‐labelled ammonium and nitrate was studied in intact ectomycorrhizal systems consisting of Betula pendula Roth and Picea abies (L.) Karst. colonized with a common mycelium of Paxillus involutus (Batsch) Fr. The plants were grown together in Plexiglass observation chambers containing non‐sterile peat with three different pH values, 4.11, 5.1 and 6.1. The mycorrhizal mycelium was allowed to prow over a barrier into an area of peat from which plant roots Here excluded. Labelled 15NH4NOa was supplied, either as 13NH4NO3 or as NH415NO2, exclusively to the fungal mycelium. Shoots and roots were analyzed for 15N in total nitrogen while the mycelium was analyzed for 15N in NH41, NO3− and free amino acids. The 15N labelling pattern indicated that ammonium was immediately assimilated into amino acids, primarily glutamine, by the fungal mycelium at the uptake site. The amino acids were then translocated to the mycorrhizal roots. In contrast, nitrate‐N was not assimilated in the mycelium but rather transferred to the mycorrhizal roots as nitrate. Mycelial uptake and transfer of N to the spruce and birch seedlings were significantly higher for NH4‐N than for NH4‐N. No firm conclusions about pH effects on the preferential uptake of ammonium and nitrate could he drawn. However, pH had a pronounced effect on the mycelial growth of P. involutus which was hampered severely at pH 6.1 and to a lesser extent at pH 5.1.

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