In agricultural land areas, no-tillage (NT) farming systems have been practiced to replace intensive tillage practices such as, moldboard plow (MP), chisel plow (CP), and other systems to improve many soil health indicators, and specifically to increase soil organic carbon (SOC) sequestration and reduce soil erosion. Numerous approaches to estimate the amounts and rates of SOC sequestration as a result of a switch to NT systems have been published, but there is a concern regarding protocol for assessing SOC especially for different tillage systems. Therefore, the objectives of this paper are to: (i) define and understand concepts of SOC sequestration, (ii) quantify SOC distribution and the methodology of measurements, (iii) address soil spatial variability at field- or landscape-scale for potential SOC sequestration, and (iv) consider proper field experimental design, including pretreatments baseline for SOC sequestration determination. For SOC sequestration to occur, as a result of a treatment applied to a land unit, all of the SOC sequestered must originate from the atmospheric CO2 pool and be transferred into the soil humus through land unit plants, plant residues, and other organic solids. The SOC stock present in soil humus at end of a study must be greater than the pretreatment SOC stock levels in the same land unit. However, one should recognize that a continuity equation showing drawdown in atmospheric concentration of CO2 may be difficult, if not impossible, to quantify. Therefore, SOC sequestration results of paired comparisons of NT to other conventional tillage systems with no pretreatments SOC baseline, and if the conventional system is not at a steady state, will likely be inaccurate where the potential for SOC loss exists in both systems. To unequivocally demonstrate that the SOC sequestration has occurred at a specific site, a temporal increase must be documented relative to pretreatment SOC content and linked attendant changes in soil properties and ecosystem services and functions with proper consideration given to soil spatial variability. Also, a standardized methodology that includes proper experimental design, pretreatment baseline, root zone soil depth consideration, and consistent method of SOC analysis must be used when determining SOC sequestration.
