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 Ontario, contain two plant/soil communities with contrasting N cycles. Picea mariana–Pinus banksiana”forest islands” are N limited whereas the lichen, moss, and grass community (or “lichen patches”) on the surrounding bedrock outcrops appear intrinsically N saturated. The potential for this landscape to retain a N input of eightfold ambient levels was tested with a 2-y addition of 40 kg 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 snowmelt. However, during the growing season, N retention in the treated catchment remained as efficient as in references. Forest islands and bedrock surfaces responded in opposite fashions to the elevated N input. By the second year of N addition, bedrock surfaces no longer retained additional N inputs. In contrast, N-amended and reference forest islands retained a similar proportion of N inputs, indicating that forest islands did not become N saturated. The response of the whole catchment to N addition was more similar to forest islands than bedrock surfaces. Even if forest islands only cover a small proportion of catchment area, they can have a strong impact on whole catchment element export because most of the water must move through at least one island before leaving the system. Because the different components of the boreal shield landscape are hydrologically connected, N saturation may occur as a cascading effect in this ecosystem. Monitoring boreal shield landscapes by using outlets at the lower end of the hydrological cascade can fail to detect the impacts of perturbations such as increased N deposition on upper components.
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