There is great interest in the status of soil organic carbon (SOC) as an indicator of soil health and as a measure of removal of the greenhouse gas, carbon dioxide, from the atmosphere.

Each ton of SOC in the soil represents a past removal of 3.67 tons of carbon dioxide from the atmosphere so there is much interest from the private and public sector in increasing investment into practices the increase SOC for greenhouse gas mitigation.

The Prairie Soil Carbon Balance Project (PSCB) was initiated by the Saskatchewan Soil Conservation Association to establish a system to monitor SOC on commercial farm fields across Saskatchewan that were converted from conventional management to direct seeding and continuous cropping in 1997.

Under this project a network of 136 commercial farm fields in Saskatchewan was established with initial measurement of SOC in fall 1996 with the plan to track SOC change with repeated samplings. A small benchmark, 16 x 7 ft, is in each field, locatable with a buried marker. Within each benchmark at each sampling, a composite sample of six soil cores was taken to 16 inches below the surface in 4-inch increments. For each sampling, a new set of six cores were taken offset from other samplings by 20 to 40 inches from the previous coring locations.

The mass of SOC is estimated from analysed SOC concentration and the soil density. The network was resampled in 1999 (136 fields), 2005 (121 fields), 2011 (80 fields), and 2018 (90 fields). The number of fields differed between samplings due to changes with involvement of co-operators over the project duration.

An unexpected finding was there was massive spatial variability of SOC within the benchmark. Therefore, when the new six cores were taken at each sampling, by chance, the six cores could sample soils with high relative SOC or soils with low relative SOC. This creates a random difference between samplings due to spatial variability.

With another sampling, the physical offset for the new six cores creates introduces another difference between measurements. To avoid systematic variability due to analysis, comparison was made between SOC for the current sampling and archived 1996 soil that were both analysed at the same time in the same laboratory.

The first major finding of the PSCB was that it is not possible to use one benchmark to reliably estimate SOC change due to the spatial variability of SOC. Nevertheless, the average differences for many fields in the PSCB network are reliable measures that can be detected from differences due to chance alone.

To about a 1-ft depth, the change in SOC from 1996 was 0.69 ton/acre in 1999, 0.16 ton/acre in 2005, 1.36 ton/acre in 2011, and 0.99 ton/acre in 2018. The change to 2018 amounts to about 5% of initial SOC in 1996. Although these changes are modest, they conclusively show that SOC in increasing on direct seeded commercial farm fields in Saskatchewan.

The smaller change from 1996 for 2005 sampling was related to the depressing effect on SOC of widespread droughts over the 2001-2003 period. The second major finding of the PSCB was that SOC increase lessened as initial SOC in 1996 increased. This occurred for all depths and across soil zones.

A third major finding was that SOC was increasing at deeper depths than expected based on past research on small plots. In soils with low SOC at depth, there were important gains in SOC at depth regardless of the depth of soil profile development.

The measured changes from the PSCB generally agree with the estimates of SOC change that are contained in Canada’s national inventory of greenhouse gas emissions and removals. More sitespecific modelling agreed better with national than with observations.

We are indebted to the many farm cooperators who have made the PSCB project possible. We need the co-operators’ further assistance to collect data on the management history of the fields so that we can investigate the effect of management on SOC behaviour.

The PSCB project has provided some new and unique information about the behaviour of SOC on commercial farm fields throughout Saskatchewan. We now have confirmation that the fields are increasing in soil carbon and that has market value.