Stress responses of Calluna vulgaris to reduced and oxidised N applied under ‘real world conditions’

Effects and implications of reduced and oxidised N, applied under ‘real world’ conditions, since May 2002, are reported for Calluna growing on an ombrotrophic bog. Ammonia has been released from a 10 m line source generating monthly concentrations of 180–6 μg m −3, while ammonium chloride and sodium nitrate are applied in rainwater at nitrate and ammonium concentrations below 4 mM and providing up to 56 kg N ha −1 year −1 above a background deposition of 10 kg N ha −1 year −1. Ammonia concentrations, >8 μg m −3 have significantly enhanced foliar N concentrations, increased sensitivity to drought, frost and winter desiccation, spring frost damage and increased the incidence of pathogen outbreaks. The mature Calluna bushes nearest the NH 3 source have turned bleached and moribund. By comparison the Calluna receiving reduced and oxidised N in rain has shown no significant visible or stress related effects with no significant increase in N status. Exposure to NH 3 reduces stress resistance and increases visible damage in mature Calluna.