Diagnostic indicators of elevated nitrogen deposition

Tissue N content of mosses, which has been shown to be an indicator of enhanced N, was studied at a range of locations dominated either by wet or dry deposited and oxidised and reduced forms of N. Tissue N responded differently to wet and dry deposited N. For a 1 kg ha−1 y−1 increase in N deposition, tissue N increased by 0.01% at wet deposition sites but by 0.03% at sites dominated by dry deposited NH3. Tissue N at wet deposition sites responded more to concentrations of NO3− and NH4+ in precipitation (r2 0.63) than to total N deposition (r2 0.27), concentration explaining 66% of the variation in tissue N, wet deposition 33%. The study clearly concludes that tissue N concentration in mosses provides a good indication of N deposition at sites where deposition is dominated by NH3, and is also valuable in identifying vegetation exposed to large concentrations of NH4+ or NO3−, in wet deposition dominated areas, such as hilltops and wind exposed woodland edges. Moss tissue N is a good indicator of enhanced N deposition at sites close to point sources of NH3, but of limited diagnostic value in wet deposition dominated areas.