Effects of ammonium and pH on growth of some ectomycorrhizal fungi in vitro

In The Netherlands, together with the vitality of trees, most of the associated ectomycorrhizal fungi are declining. This decline is strongly correlated with the deposition of ammonium sulphate. The effect of ammonium and pH on pure culture growth of ectomycorrhizal fungi originating from Douglas fir (Pseudotsuga menziesii) plantations was examined at a range of ammonium and pH levels. One isolate each of three ectomycorrhizal fungi, Laccaria bicolor, Lactarius rufus and L. hepaticus were selected and responded to an increase in the ammonium concentration from 1 to 10 mM, with a considerable increase in biomass production and a decrease in radial growth. During growth, acidification of the medium occurred. Addition of ammonium up to concentrations of 10 mM caused this acidification to increase considerably. Buffering the medium slightly affected the effect of ammonium upon biomass production; however, the radial growth production found in unbuffered medium was not apparent in L. bicolor and L. rufus. and was diminished in L. hepaticus. The sensitivity to medium acidification increased in the order L. rufus, L. hepaticus, L. bicolor. Both Lactarius isolates showed optimal radial growth at pH 4 0, whereas radial growth of L. bicolor increased with increasing pH values up to 6-6. Optimal biomass growth occurred at lower pH values of 3 0,4 0 and 4-8 for L. rufus, L. hepaticus and L. bicolor, respectively. In contrast with elevated ammonium concentrations, low pH had strong inhibitory effects on two of the fungi examined. That soil pH per se is a key factor in the decline of L. rufus and the increase of L. hepaticus is not supported by the pH response in pure culture.