Nitrogen pollution and the ecology of heather moorland
Heather moorlands are of great ecological value and as such are recognised as
internationally important habitats. A large proportion of European heather moorland is
concentrated within the UK, covering approximately 2-3 million ha" of the UK upland
environment. Enhanced levels of N pollution have led to concerns about the eutrophication of
many natural and semi-natural ecosystems in the UK, including heather moorland. This study
aimed to investigate the response of a moorland ecosystem to artificially enhanced N
deposition and the interaction of P with increasing N input. A further aim was to investigate
the potential for moorland ecosystem recovery from enhanced N deposition. The research
presented within this thesis was part of an ongoing long-term N manipulation experiment
established in 1989 on a heather moorland in Wales. This study focussed on a N
manipulation experiment established at the site in 1998, where N was added at treatment
levels of 0, 10,20,40 and 120kg ha" yr". In addition to this there were Nand P combination
treatments, with P added at a rate of 20kg ha" yr". A N recovery experiment was established
on the 1998 experimental plots, whereby treatment with N to one half of each plot ceased in
2003, in order to investigate potential for recovery from enhanced N deposition.
Cal/una vulgaris growth was stimulated by increasing total N input as was the foliar N
concentration. The stimulation of C. vulgaris growth indicates that this species was N limited
at ambient levels of deposition. Bryophyte and lichen frequency and cover and bryophyte
species diversity were significantly reduced by artificially enhanced N deposition. The foliar N
concentration of the dominant moss species Hypnum jutlandicum was elevated with
increasing N input. Peat and litter nutrient levels also responded positively to N input, with
significant increases in N concentration and exchangeable N. The increase in exchangeable
N with N addition demonstrates the increase in bio-available N with enhanced N deposition in
moorland ecosystems. The P acquiring enzyme phosphatase and the lignin degrading
enzyme phenol oxidase showed no consistent response to N input.
The different components within the moorland ecosystem showed varying levels of N, P or N
and P co-limitation. The dominant plant species C. vulgaris was clearly N limited at ambient
levels of N deposition, as indicated by significant growth stimulation and foliar N accumulation
with increased N input. However, P addition did cause a positive growth response in this
species, albeit weaker than the response to N input, suggesting a certain degree of Nand P
co-limitation. The bryophyte and lichen community were clearly P limited, with P addition
significantly increasing bryophyte and lichen frequency and species diversity. Soil
microorganisms also displayed some evidence of P limitation as the addition of P, even at
ambient N deposition, significantly reduced the activity of phosphatase.
The moorland showed little evidence of recovery 2 years after N treatment had ceased.
However, there was a significant reduction in litter exchangeable NH4, C. vulgaris total foliar N
concentration and C. vulgaris shoot extension. Liverwort cover had increased in those plots
where N treatment had ceased. The data from the N recovery experiment suggests that
although there was some evidence of recovery, the legacy of N deposition to the experiment
may persist for some time.
A regional survey in 2005 of moorland sites in Scotland, Wales and the Peak District, was
used to investigate whether any of the responses to artificially enhanced N deposition at the N
manipulation experiment could be used as bio-indicators of N deposition at a regional scale.
Of the bio-indicators tested, litter exchangeable Nand C. vulgaris N:P ratio showed a
significant positive association with modelled N deposition. A significant negative association
was observed between litter phenol oxidase activity, bryophyte species richness and N
deposition. Litter total N concentration and phosphatase activity and C. vulgaris and H.
jutlandicum total foliar N concentration did not have a significant association with N
deposition. The potential bio-indicators identified are from varying components of the
moorland ecosystem (i.e. soil system, bryophytes and C. vulgaris) and consequently may not
only provide an indication of ecosystem N status but also overall moorland health in response
to varying levels of N deposition.