CommentaryTillage and soil carbon sequestration—What do we really know?
Introduction
Concerns about rising atmospheric CO2 levels have prompted considerable interest in recent years regarding the sink potential of soil organic carbon (SOC). The world's soils are estimated to contain 1500 Gt of SOC, roughly double the amount of C in the atmosphere (Schlesinger, 2000). And while that total pales in comparison to the 38,000 Gt of C contained in the world's oceans, it attracts attention because it is potentially responsive to modification. Indeed, though fossil fuel combustion has been the major cause of increasing CO2 in the atmosphere, land modifications have been a significant contributor. Some have estimated that in United States, many soils have lost 30–50% of the C that they contained prior to cultivation (Kucharik et al., 2001).
Much of the blame for this loss of C has been assigned to the practice of plowing the soil (Reicosky, 2003), and tilled soils are viewed by many as a depleted C reservoir that can be refilled. Lal et al. (1998) estimate that United States croplands have lost 5 Gt C, an average of 36 t ha−1, and suggest that much of this can be restored over a 50 year period with appropriate management. The primary practice that is mentioned is conservation tillage, broadly defined as any tillage method that leaves sufficient crop residue in place to cover at least 30% of the soil surface after planting (Lal, 2003). It has been argued that widespread adoption of conservation tillage within United States could sequester 24–40 Mt C year−1 (Lal et al., 2003). These statistics have been projected globally to estimate that conversion of all croplands to conservation tillage could sequester 25 Gt C over the next 50 years, marking it as one of the key global strategies for stabilizing atmospheric CO2 concentrations (Pacala and Socolow, 2004). This view has attained general acceptance, to the extent that some farmers now receive payments from coal-burning utilities in emissions-trading arrangements brokered through the Chicago Climate Exchange, in return for practicing conservation tillage. Payments are based on the premise that conservation tillage sequesters the equivalent of 0.5 t CO2 acre−1 year−1, or ∼0.3 t C ha−1 year−1. Our objective was to answer the question: how solid is the evidence for C sequestration in conservation tillage systems?
Section snippets
Soil sampling studies
The standard method for assessment of C sequestration has been soil sampling of long-term tillage trial plots. Multi-year experiments are necessary because annual changes in SOC are spatially variable and generally small relative to background SOC. A recently published review of such studies (West and Post, 2002) concluded that conversion of conventional tillage to no-till sequesters an average of 0.57 ± 0.14 t C ha−1 year−1. No-till, in which the soil is left undisturbed from harvest to planting, is
Gas exchange studies
Changes in soil C can in principle be inferred from continuous measurement of net ecosystem CO2 exchange (NEE) between the land surface and the atmosphere provided other C additions or losses (e.g. harvested grain) are properly credited. The instrumentation to conduct such measurements has only recently become available, so long-term data over contrasting tillage systems are just now being assembled. These measurements are subject to their own experimental difficulties and uncertainties (
Alternative explanations for SOC loss following cultivation
Because soils have lost so much C since tillage began, the idea that a reduction in tillage would sequester C seems plausible. However, this may be a case of confusing causation with correlation. The conversion of pre-settlement forests and grasslands to agriculture involved other changes beyond mechanical disturbance of the soil that may have had far more impact on SOC. Perhaps the most obvious difference between today's agricultural lands and the ecosystems that preceded them is that
Conclusions
This discussion should not be construed as a defense of the plow. There are many good reasons to reduce tillage: no-till and other conservation tillage systems can protect soils against erosion (Gebhardt et al., 1985), reduce production costs (Al-Kaisi and Yin, 2004), and decrease the consumption of fossil fuels (Phillips et al., 1980). These benefits have been well documented, and are in themselves sufficient to justify the promotion of conservation tillage strategies. However, the widespread
References (40)
- et al.
Stepwise time response of corn yield and economic return to no tillage
Soil Tilllage Res.
(2004) - et al.
Examining strategies to improve the carbon balance of corn/soybean Agriculture using eddy covariance and mass balance techniques
Agric. Forest Meteorol.
(2005) Long-term tillage effects on cool-season soybean in rotation with barley, soil properties, and carbon and nitrogen storage for fine sandy loams in the humid climate of Atlantic Canada
Soil Tillage Res.
(2005)- et al.
Soil organic nitrogen in a Minnesota soil as related to tillage, residue, and nitrogen management
Soil Tillage Res.
(2006) - et al.
Soil water dynamics, physical properties and corn and wheat response to minimum and no-tillage systems in the southern Pampas of Argentina
Soil Tillage Res.
(2005) - et al.
Eddy covariance flux corrections and uncertainties in long-term studies of carbon and energy exchanges
Agric. Forest Meteorol.
(2002) - et al.
Carbon fluxes in the rhizosphere of winter wheat and spring barley with conventional versus integrated farming
Soil Biol. Biochem.
(1995) - et al.
Annual carbon dioxide exchange in irrigated and rainfed maize-based agroecosystems
Agric. Forest Meteorol.
(2005) The effect of organic matter on the bulk and true densities of some uncultivated podzolic soils
J. Soil Sci.
(1973)- et al.
Subsidence of agricultural lands in the Sacramento-San Joaquin delta, California: role of aqueous and gaseous carbon fluxes
Water Resour. Res.
Red clover and tillage influence on soil temperature, water content, and corn emergence
Agron. J.
Conservation tillage
Science
Effect of three conservation tillage practices on soil temperature and thermal properties
Soil Sci. Soc. Am. J.
Measurements and modeling of carbon and nitrogen cycling in agroecosystems of southern Wisconsin: potential for SOC sequestration during the next 50 years
Ecosystems
Global potential of soil carbon sequestration to mitigate the greenhouse effect
Crit. Rev. Plant Sci.
The Potential of U.S. Croplands to Sequester Carbon and Mitigate the Greenhouse Effect
Achieving soil carbon sequestration in the United States: a challenge to policy makers
Soil Sci.
Soil strength properties under four tillage systems at three long-term study sites in Indiana
Soil Sci. Soc. Am. J.
Corn seedling root growth as affected by soil physical properties
Agron. J.
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