Nitrogen exists as the 2 stable isotopes 14N and 15 N. (For detailed descriptions of the use of stable isotopes in ecology, see Hogberg, 1997; Handley and Raven, 1992). Isotopic composition is expressed in terms of a values δ parts per thousand (‰) difference from the standard, which is N2 in air: . δ15N = [(15/14 sample-15/14 standard)/15/14 standard)]*103. δ15N values are measures of the amounts of heavy and light N isotopes in a sample (15N/14N), a positive value indicating 15N enrichment and a negative value the reverse. Stable isotope studies can help to identify the source and fate of N added to the environment by anthropogenic activities. There is evidence that the 2 major forms of atmospheric N, NOx and NHy have different δ15N signatures, reflecting their different origins. Ammonia emissions from volatilisation processes tend to 15N negative, while NOx emissions from vehicles tend to be 15N positive. (NH3 emissions from vehicles may also be positive, though further research is required on this). Monitoring of the 15N signal can therefore be used to provide an indication of the emission source of (combustion processes or volatilisation) of the nitrogen.
Studies have shown a close correlation between +δ15N and traffic exposure, for higher plants (pers comm. S. Power and T Collins) and mosses (Pearson et al. 2000). Moss samples collected near motorways or busy roads had mainly positive signatures ranging from +6 to -1‰. Power and Collins demonstrated a significant relationship between δ15N signature in Calluna vulgaris, distance from London and from nearest 'A' trunk roads and also background NO2 in London. Signatures were more negative, ranging from just 0 to -9‰.
Ammonia emitted from agricultural practices has very negative δ15N values, as the ammonia volatilised is preferentially enriched with the lighter 14 N. Hence δ15N values of moss species sampled upwind and downwind of a poultry farm in southern Scotland, were shown to reflect the deposition of negative ammonia and its uptake into plant tissue (Harrison et al. 1999) with values from -6.8‰ upwind to -11.5‰ downwind.
Suitability to indicate atmospheric concentrations:
Suitability to indicate atmospheric depositions:
Although the evidence produced is limited, the relationships demonstrated between δ15N and N deposition are strong and can be used for a range of species. However, the method is best suited to very high levels of nitrogen supply, where a particular source (with given isotopic signature) dominates.
Suitability to indicate environmental impacts:
Insufficient evidence available, but unlikely to be suitable.
Sensitivity to other factors:
A wide range of other factors affect isotopic fractionation of nitrogen species. Wet and dry deposition may also be fractionating processes.
Long term (typically >1 to several years).
Insufficient information is available on practical aspects such as sampling season etc., but it is likely that constraints similar to those governing total tissue N would also apply here. In general the method has not been adequately used or tested to provide any uncertainty measure.
Expertise in field:
Sampling requires training in species identification particularly of bryophytes or lichens, which many Agency staff may have already received. When age or growth phase of the plant or foliage is important, these must be identifiable by staff.
Ideally, samples should be collected, cleaned (removal of litter, other species etc) dried (either by oven or in the air) and transported to an analytical laboratory.
Expertise in laboratory:
Measurement of δ15N requires highly expensive equipment and specialist technicians and costs per sample can be high. Interpretation of results would require the services of an expert.
Cost (per unit sample): £10-100
Cost Comment: For 3 species, 3 replicates per species and 3 locations per site.
Agency StaffSample collection, cleaning, drying and packaging for 27 samples -4 days
Analytical laboratoryPreparation - milling (at least 0.7 mm sieve size, or ball mill), mixing, tubbing (and supply of sample tubs), labelling tubs etc... £10* per sample + VAT (if applicable).
Analysis - N-15 - LN2 milling, Mass Spec £17* per sample + VAT (if applicable)
Total per sample: ˜£27
Total per 27 samples: ˜£729