The response of a heterogeneous upland boreal shield catchment to a short term NO3- addition

Boreal Shield rocky ridges at the Experimental Lakes Area, northwestern Ont ario, contain two plant/soil communities with contrasting N cycles. Picea m ariana-Pinus banksiana "forest islands" are N limited whereas the lichen, m oss, and grass community (or "lichen patches") on the surrounding bedrock o utcrops appear intrinsically N saturated. The potential for this landscape to retain a N input of eightfold ambient levels was tested with a 2-y addit ion of 40 kpa N ha(-1) y(-1) as NaNO3 to one small catchment (0.40 ha). The elevated N input was poorly retained by the whole catchment during snowmel t. However, during the growing season, N retention in the treated catchment remained as efficient as in references. Forest islands and bedrock surface s responded in opposite fashions to the elevated N input. By the second yea r of N addition, bedrock surfaces no longer retained additional N inputs. I n contrast, N-amended and reference forest islands retained a similar propo rtion of N inputs, indicating that forest islands did not become N saturate d. The response of the whole catchment to N addition was more similar to fo rest islands than bedrock surfaces. Even if forest islands only cover a sma ll proportion of catchment area, they can have a strong impact on whole cat chment element export because most of the water must move through at least one island before leaving the system. Because the different components of t he boreal shield landscape are hydrologically connected, N saturation may o ccur as a cascading effect in this ecosystem. Monitoring boreal shield land scapes by using outlets at the lower end of the hydrological cascade can fa il to detect the impacts of perturbations such as increased N deposition on upper components.