The role of ammonium and nitrate retention in the acidification of lakes and forested catchments

The relative contribution of HN0₃ to precipitation acidity in eastern Canada has increased in recent years leading to some concern that the relative importance of NO^–₃ deposition in acidification of terrestrial and aquatic ecosystems may increase. To gauge the extent of this impact, annual mass balances for N0^–₃ and NH⁺⁴ were calculated for several forested catchments and lakes in Ontario. Retention of NH⁺₄ (R_NH4) by forested catchments was consistently high compared to retention of NO₃^– (R_NO3) which was highly variable. Retention of inorganic nitrogen was influenced by catchment grade and areal water discharge. In lakes, the reciprocals of retention of N0^–₃ and NH⁺₄ were linearly related to the ratio of lake mean depth to water residence time (z/; equal to areal water discharge), and retention did not appear to be a function of degree of acidification of the lakes. Net N consumption-based acidification of lakes, defined as the ratio of annual NH; mass to N0^–₃ mass consumption, was negatively correlated with / and N consumption-related acidification was most likely to occur when – was < 1.5 m yr^–1.If retention mechanisms are unaffected by changes in deposition, changes in deposition will still result in changes in surface water concentrations although the changes will be of similar proportions. Therefore, NO^–₃ saturation should not be defined by concentrations alone, but should be defined as decreasing long-term, average NO^–₃ retention in streams and lakes in response to long-term increases in NO^–₃ deposition. Analysis o f survey data will be facilitated by grouping lakes and catchments according to similar characteristics.