The legacy of nitrogen pollution in heather moorlands: Ecosystem response to simulated decline in nitrogen deposition over seven years

Abstract

Eutrophication and acidification of heather moorlands by chronic atmospheric nitrogen (N) pollution, is of major concern within these internationally important ecosystems. However, in the UK and Western Europe generally emissions of NOy and NHx peaked during the 20th century. Due to the history and scale of atmospheric N pollution, the legacy of these high levels of N deposition, through accumulation in soil, may hinder or prevent ecosystem recovery. Effects of N pollution on heather moorland were investigated throughout the ecosystem including; the dominant plant species, Calluna vulgaris, the bryophyte and lichen community and the soil system using a long-term experiment simulating wet N deposition. We observed an increase in C. vulgaris height, shoot extension and canopy density, litter mineral N, total N concentration, N:P and C:N ratios in response to N addition. Bryophyte species diversity, bryophyte and lichen frequency and the frequency of two individual bryophyte species (Lophozia ventricosa and Campylopus flexuosus) were significantly reduced by N addition. We developed an N recovery experiment, using a split-plot design, on the long-term N treatment plots to investigate ecosystem response to a simulated decline in N deposition. Two years after cessation of N treatment the only ecosystem component that responded to the recovery experiment was C. vulgaris shoot extension, however after seven years of recovery there were significant declines in litter total N concentration and mineral N and an increase in litter C:N ratio. Although bryophytes and lichens form a close relationship with atmospheric N deposition these organisms did not show a significant response to the N recovery experiment, two years after cessation of N treatment. These data indicate that low nutrient ecosystems, such as moorlands, have the capacity to respond to declines in N deposition however the accumulation of pollution may hinder recovery of sensitive organisms, such as bryophytes and lichens.