Carbon Sequestration and Agricultural Land Uses
On February 13 2020 Susantha Jayasundara, Research Associate in the School of Environmental Sciences at the University of Guelph, gave a presentation at the Northern Ontario Ag Conference entitled ‘Impact of Pastures and Forages on Soil Organic Carbon in the Great Clay Belt of Northern Ontario.”
In this presentation Jayasundara discussed his research into the impacts that different agricultural land uses have on the amount of organic carbon stored in the ground in several farms along Highway 11 around Kapuskasing. His preliminary results showed that pastured fields sequester approximately 30% more carbon than either arable cropping fields or forested areas.
Soil organic carbon is a measurable component of soil that includes organic matter from plants or animals. When plants grow they draw carbon from above ground and extend it into roots below ground. When those plants die and the roots decompose that carbon remains in the earth and improves the soil in many ways. For example, the microbes involved in decomposition of organic material release nutrients that plants need to grow. The microbes also release by-products of the decomposition that bind soil components together into aggregates, giving healthy soil its structure and allowing for improved air, water, and root spread. Finally, organic material provides a food source for any number of beneficial organisms that live in the soil.
This is part of the reason why having a cover crop growing in a field is better for than leaving a field uncovered, because that cover crop growth contributes to the carbon content of the soil.
A 2010 study reported that, while the vast majority of global carbon is stored in oceans, soil stores more carbon than both vegetation and atmosphere combined. It is possible that, if better understood, sequestering carbon in soil could play an important role in countering the carbon emitting impact of agriculture and play a role in combatting climate change.
To this end, Jayasundara conducted soil tests on six farms in the Kapuskasing area to see what the organic carbon levels were in soil from different land use areas. He tested forested areas, arable fields used for cropping, and permanent pasture fields. He took several 60 centimetre long soil cores from each of these areas and conducted a number of analyses on them, including assessment of soil organic carbon concentrations, nitrogen volume, soil texture and characteristics, and other chemical properties.
The results showed that, in general, permanent pasture fields and forested areas have higher carbon concentrations than arable cropping fields. Those varying carbon concentrations were also mostly confined to the top 15 centimetres of soil, meaning that below the top layer the use of the land had little effect on the carbon content. Adding a degree of complexity to the research is the fact that the density of the soil also varied, making it difficult to truly compare the soil carbon content. Perhaps because of compaction from years of being worked, the arable cropping fields had a higher level of soil density than the pasture and forest areas, and the density of soil changes the amount of carbon stored, simply because there is more soil in a core from a denser area than a less dense area.
Compensating for the varying densities of soil, the results showed that on average, soil under pasture fields stored 30% more organic carbon than soil under forests or arable crop fields. Forests and arable cropping fields were more or less comparable with each other in terms of organic carbon stores.
Some other variables also play a role in the volume of carbon that can be stored in soil. For example, a study in 1995 assessed the soil organic carbon in different fields of the Kapuskasing Research Farm. The land had been cleared in 1942, and some fields had drainage tile installed in 1944. Both the tiled and un-tiled fields were used to grow a cereal-forage rotation until 1995, when they were tested. It was found that the tile-drained fields had 51% more carbon stored than the still-forested land that had not been cleared, while the un-tiled fields had slightly less carbon stored than the forested land. The hypothesized reason for the difference is that the tile drainage allows for better root growth, and therefore better overall plant productivity and carbon storage.
Overall, the amount of organic carbon in soil depends on the balance of carbon inputs and carbon losses. Some factors that increase carbon in the ground are to use fields as pastures for grazing animals, or diversifying crop rotations to include perennial forages. In crop rotations, returning crop residue to the soil will also increase carbon content.
Some things that reduce the carbon in soil include overgrazing fields, heavy tilling, allowing soil erosion, and leaving land fallow.
Increasing the amount of carbon in soil, besides playing a role in countering climate change, also has immediate positive effects on the field in question. Higher carbon in soil is beneficial to the plants in the field and will result in higher yields, as well as making the field more resilient to drought and flood.
Though Jayasundara’s research is not complete, his preliminary conclusions are that soil under permanent pastures store 30% more soil organic carbon than arable cropping land or forests. Building on these findings, his future work will look into the impact of pasture management practices on reducing the environmental impact of livestock production, among other things.