Overstory #66 - Carbon Sequestration

Editor's Note

Agroforestry systems can have a beneficial influence on the global climate. This edition of The Overstory introduces how conserving soils and planting trees can slow or reverse the release of carbon into the atmosphere.

What is the Greenhouse Effect?

Greenhouse gases such as carbon dioxide are a natural and essential component of the Earth's atmosphere. Atmospheric gases such as water vapor, carbon dioxide, carbon monoxide, methane, and ozone, absorb heat, and keep heat from radiating away from earth into outer space. This effect is much like the way glass traps heat in a greenhouse, therefore the natural warming of Earth by its atmosphere is called the "greenhouse effect."

Carbon dioxide is the most important greenhouse gas, accounting for about half of the greenhouse effect. The natural concentration of greenhouse gases (GHG) has been essential to life as we know it on earth, creating the average temperature of 15 degrees Celsius (59 degrees Fahrenheit). Without the naturally occurring greenhouse effect, the average temperature would be minus 18 degrees Celsius (0 degrees Fahrenheit)!

However, human activities such as burning fossil fuels, the chemical industry, and agriculture and land use changes are increasing the amount of greenhouse gases, especially carbon dioxide, in the atmosphere. Human activities have increased GHG and raised global temperature 0.5 degrees Celsius over the past 100 years. As a result of human-induced increases in greenhouse gasses, the temperature is projected to increase by 1 to 5 degrees Celsius during the next 100 years. While this may not sound like much, the impact could be very dramatic, if not catastrophic, on the climate. For each one degree Celsius increase in temperature, vegetation zones may change dramatically, moving toward the poles by 200 to 300 km. Each one degree Celsius of global warming will also increase water evaporation, leading to about 2% greater mean global precipitation.

Increased greenhouse gases may lead to extremes in heat waves, droughts and floods (NCRS 2000). The increased frequency of floods and droughts is significant over the last 100 years, correlating with human use of fossil fuels and drastic land use changes from forest/grassland to cropland and urban development. Increased greenhouse gases means more energy (heat) is available in the atmosphere. This energy is released in the form of intense storms and changing weather patterns causing floods, droughts, and increased fires.

Carbon Balance on Agricultural Lands

Agricultural producers can help counteract climate change by increasing the storage (or "sequestering") of carbon on agricultural lands. Both soil and vegetation act as carbon sinks, reducing the amount of carbon dioxide in the atmosphere (NRCS 2000). Wise stewardship practices can mean more carbon is sequestered in an agroforestry system than is lost to the atmosphere.

Keeping topsoils intact maintains soil quality and reduce carbon emissions into the atmosphere. Increasing vegetative cover and planting trees is also an important way to store carbon. While usually adopted for other benefits, conservation practices that conserve soil and increase vegetation also increase carbon storage.

These include:

• installing permanent vegetation buffers, such as windbreaks, contour hedgerows, and riparian buffers
• converting marginal agricultural land to perennial grassland or forest
• using conservation or no-till cultivation systems
• increasing fertilizer and water use efficiency
• increasing cropping intensity
• managing woodlands to conserve soil and increase biomass
• incorporating trees into agricultural operations through agroforestry
• using cover crops (NRCS 2000 and Gavenda 2000).

Soil Quality and Global Warming

The role of soils in carbon storage has been somewhat overshadowed by tree-planting efforts. While planting trees is important to increase carbon storage, conserving soils is essential. Soils are the largest non-fossil land-based organic carbon reservoir on Earth.

Global soil carbon content is:

• three times as much as in terrestrial plants and animals;
• twice the amount in the atmosphere;
• a third of the carbon in fossil fuels.

Soil erosion is a major cause of soil organic carbon loss and increasing greenhouse gas emissions. This takes place by:

1. exposing carbon locked within soil aggregates,
2. mineralizing carbon by oxidation and microbial processes, and
3. decreasing the soil's ability to support vegetation by lowering soil fertility, losing water as runoff, decreasing plant-available soil water, burying or flooding crops, and other erosion related effects.

Carbon dioxide additions to the atmosphere are caused not only by burning fossil fuel in agricultural activities, but also by soil organic carbon decomposition, and vegetation burning. Grassland and forest soils tend to lose 20-50% of the original soil organic carbon within the first 50 years of cultivation. Erosion, leaching, methane production, volatilization, and mineralization (decomposition of complex organic compounds to inorganic forms) lead to carbon loss from the soil.

Some soil processes lead to carbon storage. These include:

• deep rooting (plants or trees forming roots deep into the soil)
• humification (the formation of organic matter into humus)
• aggregation (the clustering of soil particles), and
• movement within the soil (by soil organisms and microorganisms).

Agroforestry to Sequester Carbon

Trees on farms or pastures for reforestation or agroforestry are planted for a number of benefits. They also store carbon. Some examples include (NAC 2000):

Silvopasture - Timber/grazing systems managed on the same area of land can increase net carbon storage. When both the tree and grass components are properly managed, an increase in net carbon storage vs. pasture of forest alone can be achieved.

Windbreaks – Vegetation windbreaks store carbon while also protecting soils farmsteads, livestock, roads, people, soils, and crops from wind. Additional CO2 reductions may also result from improved water use efficiency.

Forested Riparian Buffers - Trees grow rapidly in riparian zones due to favorable moisture and nutrient conditions. When suitable trees and shrubs grow in these moist environments they also filter out excess nutrients, pesticides, animal wastes, and sediments coming from adjacent agricultural or urban activities.

Short Rotation Woody Crops (SRWC) - Low prices for traditional crops have increased the interest of farmers in fast-growing woody crops for fuel and fiber. SWRC systems provide a way of increasing on-farm income, while also being designed to treat agricultural, livestock, or municipal wastes. The rapid growth of SRWC results in high rates of nutrient uptake and large amounts of carbon storage over rotation lengths as short as 5 to 15 years. Net carbon benefits are realized if the wood fiber is used for solid wood products or fuel.

In some regions, agreements are in place that allow nations or corporations to offset their greenhouse gas emissions by buying credits from farmers who increase their stores of carbon in the soil or in trees (NRCS 2000). The potential market for these carbon credits could be beneficial to agricultural producers.

Conclusion

Agricultural producers play an important role in efforts to slow or reverse the release of carbon into the atmosphere. Soil conservation is essential to this effort, as soils are a tremendous organic carbon reservoir. Planting trees and increasing vegetation can aid in conserving soil, while providing other farm benefits and increasing carbon storage.

References

Gavenda, Bob. 2000. Soils and Carbon Sequestration. Kona Soil and Water Conservation District Field Tour, Pu'u Waawaa, North Kona, February 5, 2000. USDA-NRCS, Kealakekua, Hawaii.

USDA NRCS (United States Department of Agriculture Natural Resources Conservation Service). 2000. Growing Carbon: A New Crop that Helps Agricultural Producers and the Climate Too. Available from: the Soil and Water Conservation Society Tel: 1-888-526-3227.

NAC (United States Department of Agriculture National Agroforestry Center). 2000. Working Trees for Carbon Cycle Balance/Agroforestry: Using trees and shrubs to produce social, economic, and conservation benefits. Gary Kuhn, USDA National Agroforestry Center, East Campus - UNL, Lincoln, NE 68583-0822, 402-437-5178.

Acknowledgments

The authors wish to thank Gary Kuhn of USDA National Agroforestry Center for giving valuable feedback on this edition of The Overstory.

Related Editions to The Overstory

• The Overstory #61--Effects of Trees on Soils
• The Overstory #42--Improved Fallow
• The Overstory #29--Tropical Green Manures/Cover Crops