Brian McConkey, Baochang Liang, Wayne Lindwall, and Glenn Padbury

Agriculture and Agri-Food Canada, Semiarid Prairie Agricultural Research Centre,

Swift Current, SK, S9H 3X2

Why should I care about Soil Organic Carbon?

When plants grow they remove carbon dioxide from the atmosphere to make leaves, stems, roots, and grain. Plants are about 45% carbon by weight on a dry basis. The non-harvested plant parts return to the soil and become soil organic matter. Soil organic matter is about 58% carbon by weight. This carbon that is part of soil organic matter is called soil organic carbon (SOC). (Actually, there is no easy way to measure soil organic matter so scientists measure the SOC and divide by 58% to estimate the amount soil organic matter).

On a worldwide basis there is more than twice as carbon tied up in SOC and recent plant residues than there is as carbon in CO2 in the atmosphere.

Since all SOC was CO2 in the atmosphere, any land use management that increases SOC will remove CO₂ from the atmosphere. In this way soil acts as a "sink" for CO₂. This process of removing CO₂ from the atmosphere and storing it in the soil is termed carbon sequestration. Every pound of SOC represents 3.7 pounds of CO₂ removed from the atmosphere.

It is important to remember that SOC is also a key indicator of soil quality and health and is important to the cycling of nutrients and producing favorable soil structure.

CO₂ is a greenhouse gas. This means that CO₂ absorbs radiation at wavelengths that other major gases in the atmosphere (nitrogen gas, oxygen gas, and water vapor) do not absorb. Therefore, instead of being radiated into outer space, radiation at these wavelengths is absorbed by the greenhouse gases and re-radiated back to the earth, causing warming of the earth’s surface. The world’s rising consumption of fossil fuels (coal, petroleum, and natural gas) is greatly raising the atmospheric CO₂ concentration.

As a result of the concern about climate change from rising concentration of greenhouse gases in the atmosphere, many people in the world have persuaded their governments to support measures to reduce the buildup of greenhouse gases in the atmosphere. This has resulted in several international conferences, including that held at Kyoto in December 1997, where the industrialized nations have pledged to reduce greenhouse gas emissions to below 1990 emissions levels. No mortal can predict the future climate but there is sufficient concern about climate change that many people are willing to incur costs now to reduce greenhouse gas emissions in the hope that will lower the risk of catastrophic climate change in the future.

As shown in Table 1, SOC is made up a diverse range of materials - some living and some well decomposed. Soil organic matter can be divided into many parts or fractions based on chemical or physical properties but no fractionation scheme is universally accepted. However, most soil scientists will at least conceptually divide SOC into an active, younger component that plays an important role in nutrient cycling as it is broken down by soil microbes and a less active, older component that plays a role in exchange reactions and physical structure but which itself is only very slowly broken down by soil microbes. In the Northern Great Plains up to 80% of the SOC is the well-decomposed materials that are very stable. This large proportion of more decomposed SOC gives the soils their dark color and explains why this area has more SOC than soils of many other areas.

In prairie soils, most SOC exists in the upper 4 to 12 inches of soil. Most changes in SOC, such as that due to adoption of direct seeding, occur in the upper few inches of soil.

Table 1. SOC fractions

How does agriculture affect SOC?

When soil was first broken by Europeans for crop production, SOC decreased rapidly. There were several factors that accounted for this initial reduction: tillage broke up the soil and exposed much more soil organic matter to microbial decomposition, fallow periods promoted microbial breakdown of SOC by leaving soil moist without any new plant material additions, erosion of topsoil removed soil organic matter, and annual crops typically produced less residue than perennial crops (remember the roots!). When SOC was decreasing in the years after conversion to arable agriculture, great quantities of CO₂ were released into the atmosphere that added to that released by burring fossil fuels. Generally, soil scientists believe much of the cropland in North America is now in approximate equilibrium under conventional management practices (ignoring losses of SOC in eroded sediment).

What is this SOC "equilibrium" and how can I increase SOC in my soils?

Equilibrium exists when additions of new organic matter from plants just balances that lost by microbial breakdown of soil organic matter. When SOC is at equilibrium, the total amount of SOC remains about the same from year to year and as much CO₂ is removed from the atmosphere as is returned.

There are two basic ways to increase SOC:

1) increase organic carbon returned to soil

or

2) decrease microbial decomposition of soil organic matter

Some practices accomplish both. For example, converting cropland to perennial forage typically increases SOC. Perennial forages usually add more carbon to the soil than annual crops (roots!) and decrease the microbial decomposition because there is no soil disturbance once established and because the growing perennial vegetation keeps the surface soil drier for a longer period than annual crops.

Tilled fallow is an example of a practice that works in the opposite direction and greatly decreases SOC. No new plant residues are returned during fallow and the repeated soil disturbance and moist soil conditions greatly increases microbial decomposition of soil organic matter.

Direct seeding increases SOC primarily by decreasing microbial decomposition of organic materials because there is less soil disturbance. Any increase in plant residue production from direct seeding would also contribute to increasing SOC.

Therefore, practices that increase SOC include:

1. putting perennial vegetation on marginal land
2. shelterbelts, woodlots
3. increasing crop residue production through better management such as improved fertilization or varieties
4. reduction in fallow
5. reduction is soil disturbance (tillage)
6. high intensity-short duration grazing that allows better regrowth of pasture vegetation.

While SOC is increasing due to a change in management change, CO₂ is being removed from the atmosphere. Eventually, however, the SOC will eventually reach a new equilibrium some time after the management change was imposed. When the new equilibrium is reached, there will again be balance between new carbon additions from plants and microbial breakdown of soil organic matter. At that time, there will be no net effect of atmospheric CO₂ and carbon sequestration stops.

The amount SOC at equilibrium depends on many factors:

The exact amount of SOC at equilibrium depends on the interaction of these factors.

It is important to note that if the appropriate equilibrium SOC amount for your management practices is below existing SOC, SOC will be lost as CO₂ until the new equilibrium is reached. Hence, sequestered carbon can be lost if management practices that are less conserving of SOC are adopted, such as could occur when land is sold and/or the farm operator changes.

How much SOC can I sequester with direct seeding?

The potential amount of carbon sequestration is the difference between the current SOC and SOC when it reaches the appropriate equilibrium. In areas where grass was the native vegetation, SOC under native grass is a workable estimate on the maximum amount of SOC that could be ever expected to be sequestered. However, it is unlikely that the SOC that accumulated after many millennia under native grass would ever be achieved again with annual crops, even with good fertilization.

Table 2 gives SOC measured for across-the-fence neighboring farm comparisons in Saskatchewan. Regardless of land use, the equilibrium SOC amounts are lower in the semiarid climate due to lower crop residue production than in the subhumid climate. Neither of the no-till farms has yet achieved the amount of SOC that existed before the land was broken. This table also demonstrates that there is nothing unnatural about carbon sequestration - carbon sequestration is simply returning to the soil some of the carbon that was lost since original breaking.

Topography has a major effect on SOC distribution on the landscape with SOC typically increasing from the top to the bottom of the slope.

Clay soils that have lost a lot of SOC due to past poor management have greater opportunity to sequester SOC in the short term than coarser textured soils.

For purposes of carbon sequestration, it is now standard practice to consider all carbon near the land surface including large roots, surface litter or residue (including standing stubble). In the past, soil scientists excluded surface residues and large roots that reduced the apparent amount of C sequestered. Table 3 shows the impact including the surface litter on carbon sequestered.

Table 2. 0-8 inch SOC (ton/acre) under different land use and different climates in Saskatchewan

Table 3. 0-6 inch SOC and carbon in surface residues in fall, 1994 for rotations and tillage system started in 1981 on long-term conventional-till wheat fallow.

How much is SOC worth?

Soil organic matter has a value to agricultural land in terms of improving soil structure and nutrient cycling independent of any value associated with reduction in atmospheric CO₂. Nevertheless, if agricultural soils are accepted as a tradable CO₂ sink, then emitters of greenhouse gases will be willing to buy the SOC to offset their emissions.

The dollar value of SOC will be determined in the marketplace based on the competing costs of achieving those emission reductions from technological improvements that lower emissions directly or from competing sink offsets such as growing forests. Estimates of the value of SOC range from less than CAN$1 to CAN$400 per ton. Current trades in emission reduction offset are toward the low end of that range. The higher end of the range is unlikely because it implies drastic effects on the economy. For example, at CAN$400 per ton, a power company could cost effectively buy feed grains to burn instead of coal to make electricity (such biomass fuels add no new CO₂ to the atmosphere).

Farmers are also emitters of greenhouse gases - the major ones being CO₂ from fossil fuels, nitrous oxide (N₂O) from inefficiencies in nitrogen use, and methane (CH₄) from ruminants (cattle and sheep) and manure storage and disposal. Therefore, in a situation where there are taxes on emissions, farmers may need their SOC increases to offset their own emissions, especially if they have a substantial cattle operation.

How important is carbon sequestration to overall greenhouse gas emissions?

Carbon sequestering practices on agricultural land could have a large impact on North American greenhouse gas emissions. For example, using conservative estimates of carbon sequestration potential, carbon sequestration could make up 8 to 18% of Canada’s greenhouse gas emissions by 2008, depending on the extent of adoption of carbon sequestering practices.

What will a system to quantify SOC credits look like?

The Kyoto Protocol on greenhouse gas emissions requires a transparent (i.e. clear, scientifically valid) and verifiable system to quantify any CO₂ sinks. With support from a consortium of Canadian industrial greenhouse gas emitters (GEMCo), a pilot project to develop such a system is being undertaken in Saskatchewan.

The basis of the system is a computer model of SOC, which will be a refined version of the CENTURY model. Estimates from this model will be made for thousands of soil-climate-cropping system combinations, these SOC estimates multiplied by the appropriate area, and totaled to produce regional estimates of SOC. There are about 140 benchmarks established across Saskatchewan (see below) to verify estimates of carbon change. The same general system could be used to develop SOC estimates and changes for an individual farm, although the system would have no rigorous method of verification for an individual farm unless the farm happened to have one of the benchmarks on the major soil type present on the farm.

In addition, we have benchmark microsites established on the upper slope, midslope, and lower slope position at six fields throughout Saskatchewan on land recently converted to direct seeding to provide measurements of how landscape position affects soil organic carbon change. We are also resampling of selected long-term (>10 year) cropping and tillage system experiments within the prairies to provide more complete observations of how management has affected soil organic carbon contents and other soil quality indicators over time. We have 10 comparisons of long-term (> 6 year) tillage system effects using adjacent paired fields throughout Saskatchewan. These comparisons give an opportunity to investigate tillage effects on the soil quality for soil types and landscapes not represented in existing research plots. These comparisons also include erosion effects that are also minimized on small and level research plots. At each paired-field site, soil quality under native grass is also measured.

SOC is typically one of the most spatially variable soil attributes so is difficult and thus costly to quantify on a field or larger area basis from actual measurement (likely greater cost than the SOC is worth). Benchmarks (specific small areas that are sampled over time), like used in the Saskatchewan project, remove a lot of the effects of spatial variability. However, benchmarks are really only valuable for verifying the model, the model still has to be used to estimate SOC in the rest of field outside of the benchmark area.

From the perspective of SOC for emissions reduction credits, it is the change in SOC that is important, not the total amount. This means with direct measurement only carbon sequestered after initial sampling could be counted as, without a model, there would be no way to estimate SOC before initial sampling. Further, only SOC changes relative to some baseline would be considered as an offset credit. So to measure SOC changes directly, you would also have to measure that baseline. Although we used neighboring farm comparisons in the Saskatchewan, these are only supplements to, not replacements for, more exacting measurements using benchmarks and research plots. Importantly, to use the neighboring farm approach as the sole determinant of SOC changes, it would be necessary to account for all SOC lost to erosion on both fields. This is a difficult, almost impossible, to do by direct measurement if there has been appreciable wind erosion. Also, the neighboring field would have to have had the identical amount of SOC when you converted to direct seeding - an uncertain assumption to make. It is highly unlikely that carbon sequestration offset credits determined by neighboring farm comparisons would be accepted.

What is happening in international negotiations regarding carbon sequestration?

At Kyoto, Canada was the only vocal supporter of agricultural soils as sink for CO₂ and succeeded in having agricultural soils not explicitly excluded as an eligible sink. In subsequent scientific negotiations at Bonn in June, 1998, several other countries, including the U.S., joined Canada in support of agricultural soils as sinks. The European Union has been vigorous opponents of agricultural soils as sinks because they claim that it is an unfair advantage for North America, sequestered carbon is fragile and difficult to quantify, and carbon sequestration only delays the reduction in fossil fuel burring. Nevertheless, at a more recent international climate change meeting in Buenos Aires, there was growing international support for considering agricultural soils as a CO₂ sink. A decision on the eligibility of agricultural soil sinks by the international community is expected in 2000 or 2001.

What about agricultural greenhouse gas emissions?

If agricultural soils are accepted as greenhouse gas sinks then the entire greenhouse gas balance needs to be considered.

Methane (CH₄) is generally seen as the most important farm-related greenhouse gas emission. CH₄ is produced mostly in the rumen of cattle and sheep with some also produced in wet manure whether stockpiled or recently spread. The importance of CH₄ is related to the fact that it is a more potent greenhouse gas than CO₂. One pound of CH₄ is considered the same as 21 pounds of CO₂ from the perspective of the greenhouse effect. Through better feed and manure handling it is possible to reduce CH₄ emissions somewhat but most CH₄ emissions have to be considered as unavoidable part of livestock rearing.

Nitrous oxide (N₂O) is generally considered the second most important farm-related greenhouse gas emission. The amounts of N₂O released are relatively small but N₂O is a very potent greenhouse gas. One pound of N₂O is considered the equivalent to about 310 pounds of CO₂. N₂O is produced by microbes during denitrification when nitrate and readily decomposable organic matter are present without oxygen - most commonly occurring when the soil is saturated, even for a short time, with water. Usually only a small portion of nitrate that is denitrified transforms to N₂O with most nitrate ending up as nitrogen gas (N₂) that is not a greenhouse gas. However, when temperatures are slightly above freezing, most nitrate is transformed into N₂O. Regardless of the form of nitrogen fertilizer, all free nitrogen in the soil will be eventually converted to nitrate. To reduce N₂O emissions, it is important to minimize the amount of nitrate in the soil, especially at time of spring thaw. Practices such as fallow are particularly bad for N₂O emissions because the soil typically wet at time of spring thaw with abundant soil nitrate that accumulated during the fallow year from the break down of soil organic matter. It is also important to minimize the amount of soil nitrate in areas of the field are subject to even short-term flooding during heavy rains. Some denitrification, and hence some N₂O emission, also occurs within wet manure and has also been linked to incorporation of legume residues into the soil.

CO₂ is a relatively small proportion of farm-related greenhouse gas emissions. Nitrogen fertilizer is made from natural gas and often represents the largest single farm-related CO₂ emission although actual emissions occur during manufacture. Therefore, improving the efficiency of nitrogen fertilizer use will accomplish the largest reduction in farm-related CO₂ emissions. Note that improving nitrogen use efficiency could also reduce N₂O emissions. Fossil fuels used for machinery, vehicles, and space heating are typically the second largest farm-related CO₂ emission. Fossil fuel use associated with machinery manufacture and maintenance and for buildings manufacture and maintenance are the typically the third largest source of farm-related CO₂ emissions although most of these emissions occur off-farm.