The Conservation Reserve Program and Carbon Sequestration
CRP offsets at least 25% of agriculture's CO2 emissions.
Emissions of carbon dioxide (CO2) to the atmosphere are expected to continue to increase in the future and, along with other greenhouse gases, contribute to the potential for global climate change. The capture and incorporation into plant tissues of atmospheric CO2 by photosynthesis incorporates CO2-C into plant tops and roots. The subsequent incorporation (sequestration) of plant tissue C into soil organic matter as soil organic C (SOC) is among the best options for C storage in terrestrial ecosystems.
Adverse impacts of ongoing soil erosion in the U.S. resulted in legislative authority for the conservation reserve program (CRP) under the Food Security Act of 1985 (P.L. 98-198). The CRP is a voluntary program offering annual rental payments and cost-share assistance to establish long-term resource-conserving covers on eligible land. Placing cultivated or highly erodible land into permanent plant cover potentially increases the amount of atmospheric CO2-C captured and sequestered as SOC. The change in the SOC pool size is the net result of C additions minus C losses. Establishment of a permanent grass cover can increase the mass of C added into the soil, relative to what may be returned by traditional cropping systems, while lack of mechanical disturbance and absence of tillage decreases rates of SOC oxidation to CO2 and the rate at which CO2-C is returned to the atmosphere.
Increasing storage of C in vegetation and soil potentially offers significant accompanying benefits including: improved soil quality, sustainable productivity, decreased pollution of surface and ground waters by agricultural chemicals, reduced soil erosion, and less overall off-site environmental degradation. Carbon that is stored below ground is more permanent than plant biomass; however, it, too, can be easily lost by the adoption of unsuitable soil management practices. Historically, there has been little emphasis given to developing or implementing strategies for C sequestration. Rather, C sequestration was not considered at all or had a low priority, and losses of SOC occurred along with the release of large quantities of C to the atmosphere as CO2.
The CRP is a highly important land use in the west and especially within Colorado, as indicated by the following data. The current area in the CRP program is 33.8 million acres (www.fsa.usda.gov), an area equivalent to about 10% of all U.S. cropland. The CRP is not evenly spread across the U.S. For example, within the Great Plains and western Corn Belt (a 13 state region including TX, NM, CO, WY, MT, OK, KS, NE, SD, ND, MO, IA, and MN) there are 25.1 million acres, or 74.3 % of all CRP land in the U.S. In Colorado, the area under CRP is 2.2 million acres, an area that accounts for 6.5% of all CRP land in the U.S. and an area that is equivalent to 26.3% of all cropland within Colorado. Of 37 counties in CO with active CRP contracts, just six counties (Baca, Weld, Washington, Kiowa, Kit Carson, and Prowers) account for about 55 % of the CRP area within Colorado. Answers are needed about the accrued beneficial effects of CRP from placing land under permanent cover and benefits that may be lost by removal of land from CRP and returning it to cultivation, especially effects upon SOC sequestration.
Current literature documents rates of SOC sequestration under
the CRP by use of models. Such estimates indicate rates of C sequestration for
the western and central U.S. are <90 to 360 lbs/ac/yr of soil organic matter
and 220 to 1200 lbs/ac/yr of total below ground C, including roots. Some estimates
suggest that about 450 and 580 lbs C/ac/yr are sequestered under the CRP as
SOC in the 0 to 2 and 0 to 4 inch depths, respectively. Research reported in
1994 at five sites across TX, KS, and NE indicated that about 710 and 980 lbs
SOC/ac/yr were sequestered in the 0 to 15 and 0 to 120 inch depths under the
CRP. Research that returned cultivated fields in southeastern Wyoming to perennial
grasses showed increasing labile soil C pools; however, only a slight increase
in SOC was observed after six years in the CRP. Thus, there is a considerable
range reported in the literature for the amounts of SOC that can be sequestered
under the CRP.
A recent, well-documented, study was conducted by Unger (2001) in which "paired
comparisons" were used for determining the rate at which the SOC pool changed
over 10 years under CRP vs. cropped soils at eleven sites in TX. Unger's study
is especially valuable because of the number of sites studied within a relatively
small region of the Great Plains. His results showed SOC was sequestered at
an average rate of 830 lbs/ac/yr within the 0 to 8 inch depth. The rate at which
SOC pools change under the CRP as compared to cropped soils is likely a function
of climate, previous cropping history, current management practices, plant species
seeded on the CRP, topographic location, soil texture and mineralogy, and time.
Follett et al. (2001b) designed a study to provide broad regional information about the potential of using the CRP as a means to sequester atmospheric CO2-C in soil and to provide an estimate of the importance of the use of the CRP within the U.S. as a management option to address the issue of climate change. The area represented by the Follett et al. study statistically represented 13.9 million acres of land in the CRP or about 40 % of the current-total area of CRP in the U.S. Using a paired plot design, fourteen sites that had been in the CRP a minimum of five years were sampled across a matrix of three soil temperature regimes and three soil moisture regimes found in the Great Plains and western Corn Belt. Estimates of annual rates of SOC sequestration by the CRP were fairly wide and, not unexpectedly, included both negative as well as positive values. The range observed was due to differences among climatic regimes studied, difficulties associated with paired-sampling designs, and that CRP grass stands and cropped fields often had different ownership (and likely different management) even though soil and landscape factors were well matched. Irrespective, a high statistical confidence (>95%) was achieved for sequestration rates obtained (500, 660, and 810 lbs SOC/acre/yr in the 0 to 2, 0 to 4, and 0 to 8 inch depths, respectively). These estimates compare well with those from 1994 at five sites in TX, KS, and NE (710 and 980 lbs SOC/acre/yr sequestered in the 0 to 16 and 0 to 120 inch depths) and with the eleven sites in Unger's (2001) study (average of 830 lbs/acre/yr within the 0 to 8 inch depth).
Assuming that 500 to 800 lbs of SOC/ac/yr are sequestered across the 33.8 million acres of CRP land in the U.S., between 8.5 and 13.5 million tons of SOC are sequestered annually within the U.S. All U.S. agriculture has been reported to emit about 47.3 million tons of C/yr and, thus, the CRP can be estimated to offset from 25 to perhaps 40% of agriculture's CO2 emissions, in addition to other environmental benefits attributed to the CRP.
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