169 © The Ecological Society of America www.frontiersinecology.org T errestrial carbon (C) offsets – sequestration of organic C in soils or biomass brought about by intentional changes in land management – offer the possibility of reducing the concentration of atmospheric carbon dioxide (CO 2 ), offset- ting emissions from other sectors. However, their inclusion in US climate and international C policy is controversial. Forest growth and agricultural soil organic C (SOC) seques- tration currently offset about 13% of US fossil-fuel emissions (US EPA 2008) and, given policies to promote adoption of land-management practices that sequester C, that percent- age could increase in the future (Pacala et al. 2007). In agri- cultural lands, terrestrial SOC offsets can be realized rela- tively quickly, through existing technology (Figure 1); in many cases, such options could cost less than the expense associated with direct emission reductions, providing flexibil- ity in meeting emission targets or restrictions (Smith et al. 2007). However, the Kyoto Protocol’s Clean Development Mechanism has demonstrated that in the absence of well-for- mulated regulations on allowable offsets, billions of US dol- lars can be spent obtaining offsets with doubtful claims of actual greenhouse-gas reductions (Wara 2007). The debate about the legitimacy of terrestrial offsets versus their utility pits public interest groups, who demand genuine emission reductions, against the sectors likely to face emission caps, who desire a variety of options for achieving emission reduc- tion targets at the lowest possible cost. Policies that permit offsets have attempted to resolve this tension by restricting the size of the offset market. For exam- ple, the American Clean Energy and Security Act of 2009 limits market participation of terrestrial C offsets to two bil- lion tons of CO 2 equivalents – ie emissions of other green- house gases, such as methane and nitrous oxide, can be con- verted to CO 2 equivalents by accounting for differences in radiative forcing and lifetime in the atmosphere (these calcu- lations allow comparability of the impacts of different green- house gases, or emission reduction strategies). Restrictions can also limit the range of allowable land-management prac- tices (as specified in the Kyoto Protocol), may allow some greenhouse gases but not others, and can stipulate verifica- tion procedures that reduce participation. However, limiting the breadth and diversity of market participants alone will not necessarily enhance the reliability of offsets. Robust, transparent, and accurate, yet inexpensive, meth- ods for verifying terrestrial C offsets are critically important REVIEWS REVIEWS REVIEWS Measuring and monitoring soil organic carbon stocks in agricultural lands for climate mitigation Richard T Conant 1,2* , Stephen M Ogle 1 , Eldor A Paul 1,3 , and Keith Paustian 1,3 Policies that encourage greenhouse-gas emitters to mitigate emissions through terrestrial carbon (C) offsets – C sequestration in soils or biomass – will promote practices that reduce erosion and build soil fertility, while fos- tering adaptation to climate change, agricultural development, and rehabilitation of degraded soils. However, none of these benefits will be possible until changes in C stocks can be documented accurately and cost-effec- tively. This is particularly challenging when dealing with changes in soil organic C (SOC) stocks. Precise meth- ods for measuring C in soil samples are well established, but spatial variability in the factors that determine SOC stocks makes it difficult to document change. Widespread interest in the benefits of SOC sequestration has brought this issue to the fore in the development of US and international climate policy. Here, we review the challenges to documenting changes in SOC stocks, how policy decisions influence offset documentation requirements, and the benefits and drawbacks of different sampling strategies and extrapolation methods. Front Ecol Environ 2011; 9(3): 169–173, doi:10.1890/090153 (published online 8 Jun 2010) In a nutshell: • Dependable methods for measuring terrestrial carbon offsets are critically important for developing a legitimate terrestrial carbon offset market • Existing methods for quantifying soil organic carbon (SOC) concentration in samples are well established and have a high analytical precision • Quantifying changes in SOC stocks relies on a set of mea- surements that are extrapolated in various ways to represent a larger geographic area • The main challenge in documenting plot-level changes in SOC stocks is in designing an efficient, cost-effective sam- pling and SOC stock estimation system • The most dependable SOC stock estimation systems will be those that are accurate, flexible, and inexpensive, and that can most easily integrate new data and knowledge 1 Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO * (conant@nrel.colostate.edu); 2 School of Global Environmental Sustainability, Colorado State University, Fort Collins, CO; 3 Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO