Effects of NH4 + on the growth and K+ (86Rb) uptake of various ectomycorrhizal fungi in pure culture

As a result of considerable deposition of ammonium sulphate on vegetation and soil the NH₄/K ratios in the soil water extracts of pine forests in the Netherlands are increasing. Increasing the NH₄/K ratio from 1 to 40 in the solid nutrient medium of isolated ectomycorrhizal fungi will in general result in a 60 to 80% inhibition of the lateral growth. On increasing the NH4/K ratio the biomass of most of the fungi examined shows a maximum at ratios of approximately 10 to 20, although the colony diameter at those ratios remains rather small. NH₄ ⁺ also exerts a pronounced effect on the uptake rate of Rb⁺ inRhizopogon luteolus.At low Rb concentrations the inhibitory effect of NH₄ ⁺ is maximal, whereas on increasing the Rb+ concentration the inhibitory effect of NH₄ ⁺ is abolished. This means that NH₄ ⁺ competitively inhibits the Rb⁺ uptake inR. Iuteolus.