Commentary
Tillage and soil carbon sequestration—What do we really know?

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Abstract

It is widely believed that soil disturbance by tillage was a primary cause of the historical loss of soil organic carbon (SOC) in North America, and that substantial SOC sequestration can be accomplished by changing from conventional plowing to less intensive methods known as conservation tillage. This is based on experiments where changes in carbon storage have been estimated through soil sampling of tillage trials. However, sampling protocol may have biased the results. In essentially all cases where conservation tillage was found to sequester C, soils were only sampled to a depth of 30 cm or less, even though crop roots often extend much deeper. In the few studies where sampling extended deeper than 30 cm, conservation tillage has shown no consistent accrual of SOC, instead showing a difference in the distribution of SOC, with higher concentrations near the surface in conservation tillage and higher concentrations in deeper layers under conventional tillage. These contrasting results may be due to tillage-induced differences in thermal and physical conditions that affect root growth and distribution. Long-term, continuous gas exchange measurements have also been unable to detect C gain due to reduced tillage. Though there are other good reasons to use conservation tillage, evidence that it promotes C sequestration is not compelling.

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

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