Prairie Soil Carbon Balance Project

By Garry Mayerle, P Ag

Conservation Agrologist

The Prairie Soil Carbon Balance Project was initiated to provide scientific verification that Saskatchewan farmers who have adopted direct seeding are storing carbon ( C ) in their soil. After 3 years the project has shown conclusively that direct seeding does build soil C and therefore can significantly reduce greenhouse gases.

Soil carbon is directly related to soil organic matter, the stuff that gives our soil its colour, quality, tilth, ability to resist erosion, and nutrient supplying characteristics. An important conversion to remember is that soil organic matter is 58% carbon. Once this relationship is understood, it is easy to follow the progression from building this organic matter to increasing soil carbon. Scientifically acceptable results are important as we seek to get international powers to accept our soils as a recognized sink in the greenhouse gas issue.

The most prolific greenhouse gas is carbon dioxide (CO₂). A simple "chemical conversion" that shows this progression is that 1 tonne of C can produce 3.67 tonnes of CO₂. Plants take CO₂ out of the atmosphere and break it down into carbon and oxygen through the process of photosynthesis. The carbon is built into plant structures. In our annual cropping system, much of this plant structure is returned to the soil every year. Conservation practices, such as direct seeding, maintain more of this carbon in our soil than traditional production systems. The net effect is that carbon has effectively been taken out of the atmosphere and stored or sunk into our soils.

The Prairie Soil Carbon Balance Project involved 4 different levels of trials. The first level was evaluating the changes in soil carbon content as fields were converted to direct seeding. The goal was to establish enough sites to give good statistical data from every combination of soil type, texture, and regional climate in the agricultural area of the province. A second level of trials was designed similarly but also included a 2 to 10 acre strip of tilled field for comparison. The purpose was to reduce other variables that go along with low disturbance seeding. The third level was carbon measurements at different landscape positions on fields that had recently been converted to direct seeding. The 4^th level of trials compared carbon stored in land that had already been directed seeded for a number of years to across-the-fence-line conventionally farmed land. Another component of the project established later evaluated carbon storage on native and tame forages.

Four groups of people were involved in making this project happen. The first important group was the 150 cooperating farmers who had fields that fit the criteria for establishing carbon monitoring sites. This next group is the SSCA staff who found the producer cooperators, collected cropping histories and background information and the annual biomass samples at the level 2 sites. The more technical side of the project was carried out by theAgriculture and Agri-Food Canada and University of Saskatchewan research scientists and technicians. Dr. Brian McConkey of Swift Current Research Station headed up a great deal of this section of the project. His crew did the research, soil testing, biomass determinations, and data manipulation. GEMCo, a consortium of Canadian energy producing companies, provided the part of the funding that got the project going.

SSCA was involved in 3 levels of the project. In the first 2 levels, 137 sites were established. The level 1 & 2 fields were chosen because they were being converted to direct seeding as the project was starting. Each level 1 site was a benchmark sampling site where 3² soil cores were taken to a 16² depth and divided into 4² segments, which were then analyzed to determine carbon content. The actual changes in soil carbon in each 4² x3² core would be very small. Think about a ½ tonne of C added to 26 tonnes already in the soil spread out over 43,560 sq.ft and the difference in C content in 6 3² soil cores looks pretty small.

The coring procedure was set up to eliminate as much as possible variables other than the effects of direct seeding that might change soil carbon content. The sites were 2m x 5m in size and were located in a level spot to minimize any effects of erosion. 6 soil cores were taken each time a coring was done. The initial cores were taken in the fall of 1996. They established the initial soil carbon content. After 3 years of direct seeding, they were sampled again in the fall of 99. Each site was located within 5 meters using a compass. The exact location of the microsite was determined with a buried electromagnetic marker. Measurements were also adjusted to account for soil density differences. The importance of these procedures is that any measured change in carbon in the core is based on direct seeding and not on natural variability due to a change in location.

The purpose of the 22 level 2 trials was to track and recognize soil carbon changes that come along with production changes associated with direct seeding such as reduced fallow, increased use of fertilizer, or rotation adaptations. There was more in depth analysis on these sites. 3 micro sites were established in each of the tilled and directed seeded treatments with approximately 40 plus meters between them. Within a week of swathing each year, square meter yield samples were taken at each of these sites. 3 samples were taken from each microsite. The samples were dried, weighed and threshed. Both yield and biomass comparisons could then be made between the tilled and direct seeded sites.

The 4th level of trials was completed near the beginning of the project. There were 9 paired farm comparisons in this level of the project. SSCA was involved in lining up these cooperators. Dr. McConkey reported on these comparisons at the last annual conference.

RESULTS:

One of the significant results is the yield comparison on the level 2 trials. See fig. 2. Direct seeding increased total biomass production as compared to the tilled site production. A significant amount of this increase was in above ground residue. The importance of this in terms of stored C is that direct seeded crops take more CO₂ out of the atmosphere and so have more C to store. As far as yield goes the trend was to a slight increase in yield although the figure was not statistically significant. McConkey states, "On these fields the farmers had crop rotations involving a mix of grain legumes, oilseed, and cereal crops. Direct seeding provided a general productivity advantage to all these crops."

Gains in soil C may not be profitable for producers yet but the likelihood of a day when they will be rewarded for stored carbon seems imminent. On averag,e across the province, soil to a depth of 12² gained ½ tonne of C per ac during the first 3 years of direct seeding.

Regional differences in stored carbon were most obvious between the drier and the wetter areas of the province. The drier area can be characterized as semiarid. This is open prairie area where grasses predominated in the past. In these areas an average of 1/3 of a tonne of C /ac was stored in the top 12² of soil during 3 years of direct seeding. In comparison in the subhumid area of the parkland where trees flourished in the past almost 2/3s of a tonne of C /ac was stored. See table 1. The expectation is that as soil clay content increases more C should be stored. In this project soil texture did not play any clear role in the amount of soil C stored. It is important to remember that this data is just from the first three years of a new management practice.

TABLE 1Average soil C change during first 3 years of direct seeding for all level 1 & 2 sites dividing province into wet and dry soil climatic zones.

One of the most significant results of the project is the demonstration of a method for confirming soil C changes with the introduction of new management practices that is not cost prohibitive. Setting up this quantification and verification project has strengthened support for including soil sinks on agricultural land as a method of reducing the concentration of CO₂ in the atmosphere.