Modelling and mapping critical loads and exceedances for the Georgia Basin, British Columbia, using a zero base-cation depletion criterion
Link to Publication Critical load (CL) and exceedance maps of sulphur (S) and nitrogen (N) for upland soils were generated for the Georgia Basin, British Columbia, Canada, by synthesizing available data layers for atmospheric deposition, climate (precipitation, temperature), soil, site classification and elevation. Critical loads were determined using the steady-state mass-balance model and a criterion based on zero-tolerance for further base-cation depletion. The resulting CL values were generally lowest on ridge tops and increased towards valleys. Critical load  exceedance ranged from 13% of the Georgia Basin under wet deposition to 32% under modelled total (wet and dry) deposition. Moreover, exceedance increased by an additional 10% when considering upland areas only for the Georgia Basin. Significant portions of the Georgia Basin are predicted to experience exceedance-enhanced base-cation depletion rates above 200 eq ha–1 y–1 and turn-over times to a final new base saturation state within 200 years under continued atmospheric S and N deposition.
Assessing N inputs and outputs for Canada’s forest

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According to the State of the Forest Report 2009 (Natural Resources Canada 2009), total forest coverage in Canada amounts to 403 million ha, and annual tree biomass production across Canada amounts to 410 billion tons. About 0.22 % of this amount would be N (Arp et al. 2008). For this production to be sustainable, forests need a primary N supply amounting to 887,000 N tons per year. This amount is almost met by the estimated annual dry and wet atmospheric deposition rate of 813,000 N tons per year. Under steady–state input-output assumptions, the atmospheric N so captured would be returned to the atmosphere on account of slow and occasional but fast N remineralization processes. The slow process refers to litter decay and N cycling, involving litter fall, and gradual organic matter decomposition, N mineralization, nitrification, leaching and denitrification. Soil-based N2O emissions due to denitrification are the result of excess NO3-N transfer from uplands to wet areas below through soil leaching and run-off. The fast component refers to recurring fires, which mostly consumes forest canopies, leaving charred stems behind. The amount of N returned so returned amounts to 411,000 tons per year, i.e., 44% of the total amount atmospheric N deposition. Harvesting removes about 150,000 N tons per year from the forested areas, i.e., 19% of the atmospheric N deposition. Denitrification is estimated to return 327,000 N tons per year to the atmosphere (26.3%) owing to anaerobic NO3-N to N2O and N2 transformations. In addition, there are stream-based N exports from forested areas, amounting to (i) 32,500 NO3-N tons per year, and (ii) 159,000 DON-N tons per year. This report also gives an assessment of biomassrelated N inputs and outputs by province and territory, and by forest type: hardwoods (HW), mixed woods (MW) and softwoods (SW). The numbers quoted are based on average numbers and calculations by province and territory, as provided by the State of Forest Report (Natural Resources Canada, 2009). A more thorough approach would build these averages from provincial and territorial inventories regarding forestry, hydrology, climate, and topography.