| | The Alberta Environmentally Sustainable Agriculture Program (AESA) Soil Quality Resource Monitoring Program has provided a data set containing soil organic carbon (OC) values and associated characteristics. These have been used to assess both the total amount, and changes in soil OC at these sites. The program consists of 42 sites distributed throughout the province and covering the major agricultural regions. Each site is intended to represent the ecodistrict where it is located. At each site, upper, mid and lower slope positions were sampled to evaluate the influence of topography on soil. The sites have been monitored since 1998. This monitoring included yield measurements and annual analysis of surface soil. These soil test results, combined with the total soil profile data from an initial pedological investigation, provide a unique opportunity to examine soil OC under a broad range of soils, climate and management practices.
The average carbon content of the sites, integrating the seven annual sets of analyses with the initial analysis, was 16.6 kg m-2 (166 t ha-1), but ranged from 8.3 kg m-2 (83 t ha-1) in the Mixed Grassland ecoregion to 25.1 kg m-2 (251 t ha-1) in the Peace Lowlands. There was no significant difference between slope positions although, for most ecoregions, there was a trend toward more OC at the lower slopes and less OC at the upper slope positions. The benchmark soils were compared to soil profiles from the Agricultural Regions of Alberta Soil Inventory Database (AGRASID) database. The difference between the benchmark values and the associated AGRASID values varied considerably, however, on average there was more OC in the surface metre of the benchmarks soils. This was primarily due to higher OC concentrations below the A horizon.
The seven years that this program has been in operation is a short time relative to the time scale of soil organic matter turnover, thus significant changes in OC were not necessarily expected. However, the results indicated a trend toward increasing OC within all ecoregions and slope positions, except for the upper slopes of the Aspen Parkland and Boreal Transition ecoregions. An averaged increase of 0.88 t ha-1 of soil OC was observed. This was after 12 soil profiles considered to be non-representative (because of extremely high rates of OC change) were eliminated from the total of 126 profiles (3 slope positions X 42 sites). The greatest increases appeared in the southern grassland regions and the Peace region. Although these trends were not statistically significant, there were a few significant differences in OC between years. These rates of increase are greater than would be expected based on other research studies in western Canada, or on the results of OC modeling. The reasons for the apparent changes could not be established based on the available information for these sites. Tillage and cropping practices did not appear to be a factor. There was some indication that increases in soil carbon were associated with drier weather conditions. |
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