Critical Load of Sulphur and Nitrogen for Terrestrial Ecosystems in Alberta, Canada:  Sensitivity to input data.

Yayne-abeba Aklilu1, Laura Blair2, Gordon Dinwoodie3, Julian Aherne4, Naomi Tam5 and Sunny Cho6

The steady state mass balance model (SSMB) was used to calculate the critical loads of acidity: the maximum critical loads for sulphur (CLmax(S)) and nitrogen (CLmax(N)) deposition and the minimum critical load for nitrogen deposition (CLmin(N)). These critical loads were calculated for terrestrial ecosystems in the province of Alberta.  While SSMB is relatively simple to use, the challenge in using this model lies in estimating the required input especially for large study area such as a province (661,848 km2).  Base cation weathering and appropriate application of chemical criteria are two such critical inputs. For this work, the soil texture approximation method and soils information from a national soil database was used to determine base cation weathering.  The chemical criteria, base cation to aluminum ratio, used to determine acid neutralizing capacity leaching was applied by land use.  Higher CLmax(S) and CLmax(N) (> 600 eq ha-1yr-1) resulted for areas of the province with deeper soil and higher clay content providing the conditions which can result in higher base cation weathering. CLmax(S) and CLmax(N) less than 200 eq ha-1yr-1 were determined for areas with sandy acidic soils  as well as soils with higher organic content. The distribution of CLmin(N) reflects biomass nitrogen content and removal of harvested biomass. For the most part CLmin(N) was less than 100 eq ha-1yr-1 with some areas having a CLmin(N) less than 50 eq ha -1yr-1. Areas with CLmin(N) greater than 50 eq ha-1yr-1 contain managed forests which have notable nitrogen removal rates from biomass harvest and removal.  Exceedance of critical loads can indicate areas of potential ecosystem impact and areas that need focused deposition monitoring.  Exceedances and sensitivity of these exceedances are explored using modelled sulphur and nitrogen deposition and calculated critical loads of acidity.  The findings are used to better inform the precipitation monitoring network for the province.


1Government of Alberta,
2Government of Alberta,
3Government of Alberta,
4University of Trent,
5Government of Alberta,
6Government of Alberta,