Inertia in an ombrotrophic bog ecosystem in response to 9 years' realistic perturbation by wet deposition of nitrogen, separated by form.

Wet deposition of nitrogen (N) occurs in oxidized (nitrate) and reduced (ammonium) forms. Whether one form drives vegetation change more than the other is widely debated, as field evidence has been lacking. We are manipulating N form in wet deposition to an ombrotrophic bog, Whim (Scottish Borders), and here report nine years of results. Ammonium and nitrate were provided in rainwater spray as NH4 Cl or NaNO3 at 8, 24 or 56 kg N ha(-1)  yr(-1) , plus a rainwater only control, via an automated system coupled to site meteorology. Detrimental N effects were observed in sensitive nonvascular plant species, with higher cumulative N loads leading to more damage at lower annual doses. Cover responses to N addition, both in relation to form and dose, were species specific and mostly dependent on N dose. Some species were generally indifferent to N form and dose, while others were dose sensitive. Calluna vulgaris showed a preference for higher N doses as ammonium N and Hypnum jutlandicum for nitrate N. However, after 9 years, the magnitude of change from wet deposited N on overall species cover is small, indicating only a slow decline in key species. Nitrogen treatment effects on soil N availability were likewise small and rarely correlated with species cover. Ammonium caused most N accumulation and damage to sensitive species at lower N loads, but toxic effects also occurred with nitrate. However, because different species respond differently to N form, setting of ecosystem level critical loads by N form is challenging. We recommend implementing the lowest value of the critical load range where communities include sensitive nonvascular plants and where ammonium dominates wet deposition chemistry. In the context of parallel assessment at the same site, N treatments for wet deposition showed overall much smaller effects than corresponding inputs of dry deposition as ammonia.

Carbon isotope variations in a plantation of Sitka spruce, and the effect of acid mist.

Intra- and inter-tree variations in 13C/12C ratios were studied within a single clone plantation of 20-year-old Sitka spruce, some of which were treated with mist simulating acidic cloud water. For groups of trees of similar height and the same treatment, sampled at the same whorl height, δ13C values for current year needles showed variations (1 SD) of between 0.2 and 0.7‰. The variations reflect the seasonally averaged influences, on intercellular CO2 concentrations, of slight variations in the microhabitat within a group. For a typical intra-group variation of 0.4‰ one may be able to distinguish between groups whose mean intercellular CO2 concentrations differ by only 8 ppm. Acid misting resulted in a lowering of δ13C values by c. 0.7‰ (significant at the P≤0.05 level). This reflects higher intercellular CO2 concentrations for acid misted trees, which can be interpreted in terms of their having assimilation rates c. 10% lower than those of control trees, and might explain the observed reduction in stem growth for acid-misted trees. Without careful attention to sampling strategy, however, these small inter-tree δ13C variations can be easily masked by the much larger intra-tree variations with height. Large gradients of increasing needle δ13C with height, of c. 0.5‰ m-1, were observed in two untreated trees of different total height. The gradient was similar for both trees so, though δ13C values of both trees were identical close to their leaders (-27‰), the taller tree displayed much lower values close to the ground (-31‰). The gradients are believed to reflect lower light levels close to the ground, rather than the accumulation of respired CO2 in the atmosphere. The different height response of stems versus needles, reflected by an increase in δ13Cstems-δ13Cneedles with height (for cellulose), is discussed in terms of stem photosynthetic recapture of internally respired CO2.