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The possibility of using improved farming practices to mitigate the increase in atmospheric CO2 through soil carbon sequestration (SCS) reached international consensus during meetings leading to the Intergovernmental Panel on Climage Change (IPCC) First Assessment Report (1990). The rationale for this consensus was that by fostering the adoption of improved farming practices, it would be possible not only to enhance agricultural productivity but also to make soils act as sinks for atmospheric CO2. A significant amount of scientific and practical evidence has accumulated to date in support of this consensus (Kern and Johnson, 1993; Lal et al., 1995a, 1995b, 1998a, 1998b; Paul et al., 1997; Powlson et al., 1996). In chapter 23 of the IPCC Second Assessment Report, Cole et al. (1996) reported estimates of SCS potential that could be achieved during a 50- to 100-year span. Under a program of global adoption, the international team of authors estimated that approximately 40 Pg C could be removed from the atmosphere via SCS at rates of 0.4 to 0.8 Pg C y-1 depending on the length of sequestration considered. This global estimate was made based on the assumption that it was possible to recover about two-thirds of the 55 Pg C historically lost from the soil organic carbon (SOC) pool by implementing land use conversions (e.g., conversion of marginal agricultural land to permanent vegetation) and improved management practices (e.g., no-tillage agriculture).

When evaluated within a global environmental and energy framework that considered a portfolio of technologies (e.g., use of biofuels, improved energy efficiency, hydrogen production), SCS was found to be competitive particularly as an early starter of climate change mitigation technologies (Rosenberg and Izaurralde, 2001). By the mid-1990s, energy industries in western Canada became interested in analyzing the extent to which SCS could be used to offset industrial CO2 emissions. Thus, a group of Canadian energy industries sponsored, together with the federal and the provincial government of Saskatchewan, the Prairie Soil Carbon Balance Project (PSCBP), which successfully documented — province wide and at field scale — changes in SOC after 3 years of adoption of direct seeding (no tillage) practices (McConkey et al., 2000). The program was successful in demonstrating that it was possible to detect, with statistical power, changes in SOC as small as 1.2 Mg ha-1 occurring after the first 3 years of implementing a SCS practice.

Since SOC is a direct indicator of soil quality and fertility, soil scientists and agrologists have long been studying, monitoring, and mapping SOC under plot and field conditions (e.g., Jenkinson, 1988; Stevenson and Cole, 1999). However, if large-scale soil C offset projects were to be implemented, these would require methodologies to measure and monitor SOC changes that are not only applicable at a relatively low cost but also accurate enough to satisfy the requirements of an emerging carbon trading market. Thus, pilot and feasibility projects on carbon sequestration are being developed worldwide to learn about scientific and operational aspects of their implementation. The Food and Agriculture Organization (www.fao.org/ag/agl/agll/carbonsequestration) and the World Resources Institute (http://climate.wri.org/sequestra-tion.cfm) maintain online databases with descriptions of ongoing agricultural and forestry carbon sequestration projects. These databases reveal the existence of relatively few field pilot projects that include SCS as a main objective.

There are economic and technical facets that need to be resolved in order to advance the field of SCS from a feasible to a mature mitigation technology. Several economic aspects of SCS practices have to be evaluated, especially those dealing with their cost-effectiveness relative to other C sequestration practices (e.g., afforestation, reforestation). Marland et al. (2001) analyzed in detail the various policy and economic issues that should be evaluated in order to determine the success of SCS programs. McCarl and Schneider (2001) compared the economic competitiveness of various forms of greenhouse gas mitigation strategies including SCS, afforestation, and biofuels offset. These authors found SCS to be competitive with other mitigation options at low carbon prices.

As with any other natural variable, the detection of changes in SOC is often associated with uncertainty due to the interactive effects of many factors controlling its dynamic such as amount and quality of C input to soil, climatic, and edaphic conditions, as well as land use and soil management practices. Another concern arises from the fact that soils have a finite capacity to store SOC and, eventually, the C stored via SCS practices could be re-released to the atmosphere when policy and market incentives or personal decisions happen to favor the application of non-C-sequestering practices (e.g., return to an intensive tillage regime, conversion of marginal agricultural land from perennial vegetation to crop production).

These and other concerns will have to be addressed during the development of methods to measure and monitor SCS at the project level. It is likely, however, that if SCS practices were to be applied at a global scale, there would be a variety of methods adapted to local conditions. Thus, international efforts will be required to ensure the comparability of results across environments and methodologies. Regardless of the methods adopted, however, these are likely to have three components, namely, direct measurements, computer modeling, and remotely sensed monitoring. The objective of this chapter is to review advances in methodologies to measure SCS at the project level. We begin with a short discussion of the conceptual design proposed by Post et al. (1999) of a plan for monitoring and verifying SCS at regional scales and then follow this conceptual design with a discussion on methodological advances for measuring, modeling, and monitoring SCS at the project level.

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