THE EFFECT OF REPEATED NITROGEN-FERTILIZATION ON MYCORRHIZAL INFECTION IN HEATHER [CALLUNA-VULGARIS (L) HULL]

The effects of repeated nitrogen fertilization over three years (1989-92) on mycorrhizal infection of Calluna vulgaris (L.) Hull growing in peat soils in north Wales are described. Solutions of NH4NO3 were added at regular intervals (10 to 20 times annually) to provide 0, 40, 80 or 120 kg N ha(-1) a(-1) above background deposition. An estimate of mycorrhizal biomass in washed roots from soil cores sampled in May and July 1992 was gained by determining the concentration of ergosterol, a fungal metabolite, in ethanol extracts. The concentrations of ergosterol (per mg fresh weight) were significantly greater in fine hair roots than in thicker roots and also higher in surface horizons than deeper in the soil core. In May, the only significant effect of nitrogen application was found in the fine roots in the surface (0-15 mm) soil. In this fraction, ergosterol was significantly higher in the plots which received 80 kg N ha(-1) a(-1) plots than in all the other treatments. However, in July the ergosterol concentration in the fine, surface root fraction were not changed by the nitrogen additions. No changes were observed in the soil nutrients (total N and P, extractable base cations) or surface pH, but nitrogen fertilization did stimulate shoot growth, flowering and litter production. Nitrogen concentrations in living shoots and litter were raised as a result of the N inputs while the levels of other main nutrients P and K were not altered. Given the relatively small changes measured in the amount of mycorrhizal infection in the recent experiment, this measurement may be a poor indicator of excess atmospheric nitrogen deposition to heathlands soils.