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 adde d 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 m ycorrhizal 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 ergoste rol (per mg fresh weight) were significantly greater in fine hair root s than in thicker roots and also higher in surface horizons than deepe r in the soil core. In May, the only significant effect of nitrogen ap plication was found in the fine roots in the surface (0-15 mm) soil. I n this fraction, ergosterol was significantly higher in the plots whic h 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 r oot fraction were not changed by the nitrogen additions. No changes we re observed in the soil nutrients (total N and P, extractable base cat ions) or surface pH, but nitrogen fertilization did stimulate shoot gr owth, flowering and litter production. Nitrogen concentrations in livi ng 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 re latively 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.