S1048: Assessment of the Carbon Sequestration Potential of Common Agricultural Systems on Benchmark Soils Across the Southern Region Climate Gradient
Statement of Issues and Justification
Problem StatementWith the recent advent of the Chicago Climate Exchange (CCX, 2006), soil carbon (C) credits, which can be accrued by the agricultural community through the use of conservation-tillage and grassland restoration practices, can be bought and sold in a stock-exchange-like market (Schneider and McCarl, 2003) to off-set carbon dioxide (CO2) emissions to the atmosphere from large companies and utilities. The agricultural community has a competitive advantage in the carbon economy because agricultural soils, if properly managed, can sequester appreciable amounts of atmospheric C as soil organic carbon (SOC). Conversion from conventional- to conservation-tillage practices provides a management mechanism for increasing soil carbon stocks. Marketing C credits can represent an added economic incentive to producers to continue using or convert to more environmentally friendly and sustainable agricultural management practices. Additionally, perennial biomass energy crops, many of which are grasses and may be a future focus of US producers, also have the potential to sequester SOC, adding an economic dimension to dedicated energy crops (Lemus and Lal, 2005; Liebig et al., 2005; Omonode and Vyn, 2006; Tillman and Lehman 2006). For the carbon economy to be politically-, legislatively-, and economically-viable, science-based methods for monitoring and verifying SOC changes must be accurate, sensitive, and practical. However, soil C sequestration rates have been developed for C credit accrual in upper-mid-western states, but not for southern states.
In a study conducted throughout Canada, VandenBygaart et al. (2003) documented that C sequestration potential was generally greater for soils with a low SOC pool. Similarly, southern soils (i.e., Alfisols and Ultisols), particularly those of the Mississippi River Delta region and other areas throughout the southern US, may have a greater potential for C sequestration than upper-mid-western soils (i.e., Alfisols and Mollisols) due to the generally lower soil organic matter (OM)/SOC contents. However, southern US soils may also respond differently to imposed management practices than Canadian soils. Considering the economic component to soil C sequestration, if C credits for the southern US are set based on the soil C sequestration potential of upper-mid-western soils, producers will likely not be fully economically compensated for their conservation practices if soil C sequestration potentials are indeed greater in the southern US than in the upper mid-west.
In addition to initial SOC level, soil C sequestration depends on soil texture (Ihori et al., 1995; Percival et al., 2000; Brye and Kucharik, 2003), landuse or agricultural management system (West and Post, 2002) and time (i.e., consistent duration of current landuse or agricultural management system; Potter et al., 1999; Brye et al., 2002a; Brye and Kucharik, 2003; Tolbert et al., 2002; Post et al., 2004). Therefore, it is essential to develop appropriate soil C sequestration potentials for southern soils, across varying soil textures and cropping systems, so that agricultural producers can maximize economic benefits for employing more-sustainable, conservation-tillage practices and contribute to decreasing the rising atmospheric CO2 concentrations.
Furthermore, momentum is currently increasing to find alternative sources of fuel. Perennial crops that produce a large amount of above-ground biomass are being targeted as one potential bio-fuel source that could be dedicated for energy production by combustion. Though it has been shown that perennial bio-fuel crops, particularly the grasses, will increase soil C storage (Tolbert et al., 2002), little to no data exists demonstrating the soil C sequestration potential associated with perennial bio-fuel crops in the soils of the southern US.
Justification
The scientific community knows several important things about the Earths atmosphere and how it has changed in the last century or so: 1) atmospheric CO2 concentrations, as well as concentrations of other greenhouse gases, where greenhouse gases are those [i.e., CO2, methane (CH4), ozone (O3), nitrous oxides (NOx), and water (H2O)] that tend to absorb radiant heat energy and trap it within the Earths atmosphere before that heat energy escapes to space, are increasing; 2) as a result of increased greenhouse gas concentrations, the entire Earth has experienced a warming trend of between 0.5 and 1.7 oF in the last 100 years (USEPA, 2006); 3) human activity, particularly the burning of fossil fuels among other mechanisms, is changing the composition of the Earths atmosphere and exacerbating the greenhouse effect and global warming (IPCC, 2001; USEPA, 2006). Other significant mechanisms affecting the composition of Earths atmosphere include land-use change and agriculture.
Some serious ramifications exist if increasing greenhouse gas concentrations and global warming continue. A potential positive impact might be increased photosynthetic rates and productivity from some plant species in the presence of an atmosphere with a greater CO2 concentration. However, the potential negative consequences are far more numerous and include the partial melting of ice caps, flooding of coastal lands, major disruptions of weather patterns, which some may argue are occurring already, lower agricultural yields, species extinctions, and a proliferation/expansion of disease vectors, among others. Though some is already known about what is happening and what might happen if trends continue at current rates, many questions remain regarding the specific effects of land-use change and other human activities (e.g., agriculture) on the Earths climate that will require much further scientific exploration (USEPA, 2006).
Although not as extensive as the burning of fossil fuels, agricultures role with increasing greenhouse gas concentrations is significant given the amount of arable land present in some states and throughout the contiguous US. Animal agriculture is responsible for CO2 and CH4 emissions, while cultivated row-crop agriculture as a specific land use is responsible for CO2 (i.e., soil respiration), CH4 [i.e., methanogenesis from anaerobic soil conditions generally associated with rice (Oryza sativa L.) production], and NOx (i.e., denitrification from water-logged or prolonged saturated soil conditions) emissions. Although CH4 is ~ 30 times more effective than CO2 at trapping heat energy in the atmosphere, CO2 emissions to the atmosphere from soil respiration are much more widespread and temporally consistent than agriculturally related CH4 emissions.
Soil respiration is the combination of root and microorganism respiration, both of which are affected by soil moisture and temperature conditions. In the context of row-crop agriculture, cultivation (i.e., tillage), as a common residue and soil management practice, loosens and aerates the soil, at least temporarily, and tends to stimulate the oxidation of soil OM, hence increasing soil respiration (Doran, 1980; Fortin et al., 1996; Reicosky 1997; Curtin et al., 2000). This general relationship between tillage and soil surface CO2 flux (i.e., soil respiration) has also been demonstrated for silt-loam Alfisols in the Mississippi River Delta region of eastern Arkansas, where soil surface CO2 fluxes over a 2-year period were 38 % greater under conventional tillage (CT) than under no-tillage (NT) in a wheat (Triticum aestivum L.)-soybean [Glycine max (L.) Merrill] double-cropped production system (Brye et al., 2006a). Therefore, it is certainly possible to reduce CO2 emissions to the atmosphere and increase C in the soil simultaneously within row-crop agriculture simply by altering residue and general soil management strategies.
The build-up or accrual of C from the atmosphere can occur in soil, biomass (i.e., forests), and oceans and is known in general as carbon sequestration. Since soil is estimated to contain nearly double the amount of OC contained in the atmosphere (Schlesinger, 2000), it is believed that soil can act as a significant C sink because of soils known responsiveness to modification (i.e., various residue and general soil management strategies; Baker et al., 2007). Lal et al. (2003) projected soil C sequestration rates of 24 to 40 Mt C yr-1 if widespread adoption of conservation-tillage practices, generally defined as any tillage practice that leaves enough crop residue to cover 30 % of the soil surface, occurred throughout the US. In a global analysis based on 67 long-term experiments, West and Post (2002) reported that 57 (" 14) g C m-2 yr-1 could be sequestered in the soil by conversion from CT to NT. Similarly, Post and Kwon (2000) reported a mean global C sequestration rate of 33.2 g C m-2 yr-1 upon conversion from agriculture to grasslands. In contrast, VandenBygaart et al. (2003) also reported SOC increases following conversion to no-tillage, but only for western Canada, while similar land-use conversion in eastern Canada had no effect on SOC accretion. Due to the variable spatial results reported in the literature, a comprehensive survey throughout the southern US climate gradient is warranted.
As an ecosystem function, soil C sequestration generally does not continuously increase. Soil C sequestration rates have been shown to decrease over time (Silver et al., 2000). West and Post (2002) estimated that peak C sequestration rates would be achieved within 5 to 10 years upon conversion from CT to NT and that a new equilibrium soil C concentration would be achieved within 15 to 20 years with little soil C increase thereafter. Soil C sequestration potential has also been linked to general soil taxonomy and the size of the SOC pool. VandenBygaart et al. (2003) demonstrated that soil C sequestration potential decreased as the SOC pool increased. This may be particularly significant throughout much of the southern US which has rather low levels of SOC due to a long history of cultivation. Brye et al. (2006b) documented a 2-fold greater increase in SOC content under NT than CT over a 2-year period in a wheat-soybean double-cropped production system in east-central Arkansas, which translated into sequestration rates of 84.5 and 162 g SOC m-2 yr-1 in the top 10 cm of soil. In contrast, an undisturbed native tallgrass prairie on a silt-loam Alfisol in east-central Arkansas sequestered 45.7 g SOC m-2 yr-1 in the top 10 cm over a 14-yr period from 1987 to 2001 (Brye et al., 2004b). These few assessments of soil C sequestration in eastern Arkansas support the conclusions of VandenBygaart et al. (2003) and demonstrate that soil C sequestration potential is greater for soils with a low SOC pool, which characterizes many of the soils located throughout much of the southern US. In addition, in a 2008 survey of soils in the Suwannee River watershed in GA, SOC values for the top 15 cm ranged from 0.39 to 2.52% C with an average of 0.79% for both conventionally-tilled and no-tillage managed farms. Assuming that the low-SOC soils could be increased to the high-SOC values as a result of conservation tillage, a maximum potential SOC increase of 550% (or 5354 g m-2) could be realized. A more conservative estimate using the average SOC value would suggest a potential increase of 104% (or 1012 g m-2). Thus, southern US soils may have a greater potential for soil C sequestration than upper-mid-western soils due to the generally lower soil OM/OC contents. Soil C sequestration will also likely be greatly impacted by the current bio-fuel/bio-energy initiative.
A steady land-use change in the agricultural sector from traditional food/fiber crops toward dedicated bio-energy crops is foreseeable. Whereas the US energy security remains a major concern, other important issues such as soil C sequestration and climate mitigation add new immediacy to conversion of traditional crops to bio-fuel crops (Walsh et al., 2003). Politicians and economists advocate capitalizing on emission and sequestration of greenhouse gases in the form of a C-trading market (Freibauer et al., 2004). Because bio-energy crops will enhance the potential of SOC accumulation (McCarl and Schneider, 2001; Tilman et al., 2006), the projected C credit, estimated from $5 to more than $20 per metric ton (Causarano et al., 2006), will contribute a new dimension to the economic viability of bio-energy crops and farm incomes. The problem is that there is little scientific knowledge regarding the potential expected change in SOC when a soil is converted from a row crop or grazing pasture to a perennial bio-fuel crop. The information that is available focuses on native grasses [i.e., switchgrass (Panicum virgatum L.] and some perennial bio-fuel stocks (i.e., Miscanthus). To confound the lack of information, SOC sequestration data for the southern US are minimal, and few of the many SOC sequestration studies in the literature address the variable impact of climate and soil properties on SOC sequestration potential.
Though many soil responses to a particular treatment are easily quantifiable within a relatively short period of time, to be accurately assessable, the ecosystem function of soil C sequestration requires more than just one or two growing seasons. Inter-annual variability of soil C is often larger than the actual increases (or decreases) in soil C from year to year. Thus, accurate assessments of soil C sequestration will require more than 1 or 2 years between samplings.
Broader Project Impact
This proposal outlines a multi-state, regional project that is designed to improve our understanding and knowledge regarding the complex relationships among climates, soils, land uses, and C. The specific objective is to assess the soil carbon sequestration potential of common agricultural and natural ecosystems of varying ages on benchmark soils across the southern region climate gradient. This objective can be achieved relatively easily in the context of current research activities being conducted by all participating individuals despite not all individuals presently working on soil C sequestration or the bio-fuel issue. This project will generate essential, scientifically based field data to support accurate projections of soil C sequestration potentials across the climate gradient of the southern US. The project will build on published research by the regional soil physics group (S-124, S-185, S-225, and S-257) characterizing the physical properties of southern benchmark soils (Dane et al., 1983; Nofziger et al., 1983; Quisenberry et al., 1987; Romkens et al., 1986; Romkens et al., 1985; Bruce et al., 1983; Cassel, 1985; Scott, 1999). This project will also represent the first multi-state regional project of its kind throughout the southern US integrating research from Texas to Florida. Aside from the USDA-ARS GRACEnet Project led by Dr. Ron Follett, the only other similar multi-state regional project, NC1017 entitled Carbon Sequestration and Distribution in Soils of Eroded Landscapes , is in the north-central US including participating states of IL, IN, IA, KS, MN, MO, ND, OH, SD, and WI and the USDA-ARS National Soil Tilth Laboratory.
The Southern Association of Agricultural Experiment Station Directors has a current list of 33 Priorities for Multistate Research Activities. The proposed Regional Project will address aspects of eight (8) of these priority areas including: 1) Multiple uses of agricultural lands, 2) Environmentally benign agricultural operations, 3) Nutrient management in agricultural systems, 4) Air, soil, and water resources conservation and enhancement, 5) Natural resource and ecosystem management, 6) Environmental policy and regulations, 7) Integrated and sustainable agricultural production systems, and 8) Bioenergy and alternative fuels from agricultural products.
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