Dry deposition of ammonia gas drives species change faster than wet deposition of ammonium ions: evidence from a long-term field manipulation

Although the effects of atmospheric nitrogen deposition on species composition are relatively well known, the roles
of the different forms of nitrogen, in particular gaseous ammonia (NH3), have not been tested in the field. Since 2002,
we have manipulated the form of N deposition to an ombrotrophic bog, Whim, on deep peat in southern Scotland,
with low ambient N (wet + dry = 8 kg N ha
1 yr
1) and S (4 kg S ha
1 yr
1) deposition. A gradient of ammonia
(NH3, dry N), from 70 kg N ha
1 yr
1 down to background, 3–4 kg N ha
1 yr
1 was generated by free air release.
Wet ammonium (NH4
+, wet N) was provided to replicate plots in a fine rainwater spray (NH4Cl at +8, +24,
+56 kg N ha
1 yr
1). Automated treatments are coupled to meteorological conditions, in a globally unique, field
experiment. Ammonia concentrations were converted to NH3-N deposition (kg N ha
1) using a site/vegetation specific
parameterization. Within 3 years, exposure to relatively modest deposition of NH3, 20–56 kg NH3-N ha
1 yr
1
led to dramatic reductions in species cover, with almost total loss of Calluna vulgaris, Sphagnum capillifolium and Cladonia
portentosa. These effects appear to result from direct foliar uptake and interaction with abiotic and biotic stresses,
rather than via effects on the soil. Additional wet N by contrast, significantly increased Calluna cover after 5 years at
the 56 kg N dose, but reduced cover of Sphagnum and Cladonia. Cover reductions caused by wet N were significantly
different from and much smaller than those caused by equivalent dry N doses. The effects of gaseous NH3 described
here, highlight the potential for ammonia to destroy acid heathland and peat bog ecosystems. Separating the effects
of gaseous ammonia and wet ammonium deposition, for a peat bog, has significant implications for regulatory bodies
and conservation agencies