Agroecosystems /
Carbon Sequestration
Research Focus
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Nebraska Phase I Progress Report
Background
During the last century, human activities, such as burning fossil fuels, have dramatically increased the concentration of greenhouse gases (GHGs) in the atmosphere. GHGs trap heat inside the atmosphere much like the way glass traps heat inside a greenhouse (Figure 1). Without these gases, the earth would be too cold for human habitation (U.S. Global Change Research Program, 2000). However, the effects of the human-induced increase in GHG concentrations are uncertain. Many scientists believe that increased atmospheric GHGs will result in unpredictable and potentially severe changes to the Earth’s climate with unknown impacts on weather patterns, sea levels, cropland production, and national economies (IPCC, 1996).
Figure 1: The Greenhouse Effect
GHGs are produced naturally in the environment and have resided in the atmosphere since well before the age of industrialization when humans began to contribute additional amounts to the atmosphere. Three GHGs that are of primary concern include carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). This study concentrates on CO2, which is the most prevalent GHG in terms of quantity in the atmosphere and has the greatest overall effect on warming. However, on a molecule-for-molecule basis, N2O has the greatest warming potential, followed by CH4 and then CO2. CO2 levels have risen substantially over the past century as evidenced by the long-term record of ice cores and atmospheric measurements shown in Figure 2 (Neftel et al., 1994; Keeling, et al., 2000).
Figure 2: Atmospheric CO2 (1800-Present)
The continual cycling of carbon through the earth’s atmosphere and terrestrial biota make up an important part of the global carbon cycle (Figure 3, Schlesinger, 1991; Schimel, 1995).
Figure 3: The Global C Cycle
CO2 is released into the atmosphere as a product of respiration, the process used by plants, animals, and microorganisms to gain energy for bodily functions. Humans, through industrial activities, have added CO2 to the atmosphere due to the burning of fossil fuels (coal, natural gas, and oil). CO2 is removed from the atmosphere during photosynthesis when plants convert it into biomass, including leaves, branches, stems, and roots. This biomass carbon will eventually be returned to the atmosphere upon the death and decomposition of the organism. In the interim, it is sequestered or retained on the land as dead plant and animal material that is broken down by microorganisms and incorporated into the soil. Carbon can remain in soils for thousands of years, effectively storing or sequestering CO2 from the atmosphere (Figure 4).
Figure 4: C Cycle In Agriculture EcoSystems
Agricultural soils contain substantial amounts of carbon, typically 20 to 80 tonnes per hectare in the top 20 cm. However, relative to their native ecosystem levels, most agricultural soils are depleted in carbon, having lost 30-50% of their original carbon levels due to changes associated with production agriculture and past management practices (Figure 5).
Figure 5: Soil C Trajectories
Historically, agricultural practices often resulted in reduced inputs of carbon through plant residues and increased losses via decomposition and erosion (Paustian et al. 1997a). Lower productivity, particularly prior to the 1950s, and greater removal of crop residues decreased the amount of plant material that could potentially add carbon to the soil (Figure 6).
Figure 6: Past Agricultural Practices
More intensive tillage, allowing microorganisms to break down more organic matter and encouraging soil erosion, increased losses of soil carbon.
Through improved agricultural practices, farmers can increase carbon storage in soils (Paustian et al., 1997, 1998, 2000; Lal et al., 1998). Conservation tillage (e.g., no-till or reduced till) helps protect soil carbon from microbial attack by preserving a more stable aggregate structure and also helps to decrease soil erosion. Better residue management enhances carbon input to soil by leaving more plant material in the fields for conversion to soil organic matter. Improved cropping rotations can also enhance soil productivity by increasing the amount of plant material that becomes soil organic matter. Winter cover crops add additional residues to the soil and help decrease soil erosion and nitrogen losses. An effective option for increasing carbon storage in the soil is to set aside land in long-term, permanent cover, such as the Conservation Reserve Program (CRP) as well as in conservation buffers (e.g. filter strips, grassed waterways). This leads to higher amounts of soil organic matter because there is reduced soil disturbance and more plant material incorporated into the soil by the perennial biomass (Figure 7).
Figure 7: Improved Agricultural Practices
The United States is involved, both nationally and internationally, in efforts to stabilize atmospheric GHG concentrations at a level that would prevent dangerous interference with the Earth’s climate. Title XVI of the Energy Policy Act of 1992 addresses global climate change, and Section 1605(b) specifically mandates the development of procedures for the voluntary reporting of GHG emission reductions. Agriculture has shown that the voluntary application of conservation practices can provide sustainability and protection of natural resources.
Over the last 60 years, the NRCS, working through 3,000 local conservation districts, have provided technical assistance and funding to farmers who implement soil and water conservation practices. Many of these practices utilize permanent vegetation and crop residues to increase soil organic matter, which are also providing a benefit of removing CO2 from the atmosphere and sequestering C in the soil. These management practices have been implemented according to NRCS standards and specifications, and are recorded in NRD records as verifiable documentation of their existence and location.
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