Impact Type: Deposition of pollutant
Deposition of ammonia, nitrate and other forms of nitrogen from the atmosphere could be an important source of nitrogen in some upland catchments where intensive agricultural activity is absent. Detailed nitrogen budgets, however, do not exist, so the relative inputs from atmospheric deposition are unknown. In such cases, increasing nitrogen inputs from atmospheric sources are likely to have ecological impacts. In most lowland rivers and burns, nitrogen inputs from catchment land-use, not deposition from the atmosphere, are likely to be much more significant (Strong et al. 1997, Smith & Stewart 1989, Foy et al. 1982).
A critical load cannot be given for nitrogen, as quantitative relationships between biology and nitrogen concentrations are poorly understood. The nitrogen to phosphorus ratio can be important, with a molar ratio of around 16:1 (7:1 by weight) being the threshold between N- and P-limitation (Wetzel 2001). Impacts could be assessed by deviation from a 'natural' ratio for an individual site. The nitrogen concentration and load in rivers, particularly those in agricultural catchments, have been rising for a few decades (Zhou et al 2000, Gibson et al. 1992). The main source of this nitrogen is inorganic fertilisers (Strong et al. 1997, Smith & Stewart 1989).
|Critical Load/ Level|
No comparable habitat with established critical load estimate available