Background/Objectives

Nature-based carbon sequestration (NBCS), employing various regenerative land management activities, is considered to hold great potential for carbon capture and storage within soils.  However, empirical data for evaluation of actual carbon accrual capacities and rates are not systematically presented and do not reflect recent developments.  These data are important for the planning and execution of soil carbon storage projects, both to forecast carbon gains and to predict return on investment in the emerging carbon credit market. With increasing attention on NBCS as a climate solution, it requires an updated evaluation of the achievable level of carbon sequestration based on land management methods.  In addition, because the various carbon registries employ different statistical methodologies to quantify carbon accrual, there is considerable uncertainty as to the number of carbon credits that may be awarded for any given project.

Approach/Activities

To address this information gap, we have compiled information on 129 project sites where soil carbon accrual measurements have been conducted.  These data quantify soil carbon accrual under various regenerative land management practices, categorized into grassland management, management within the Conservation Reserve Program (CRP), cropland management, grazing management, and combined crop-livestock systems.  To estimate the amount of carbon credits that could potentially be awarded to these projects, we have applied two statistical methods used by carbon registries: the statistical method specified in the Verra standard (Verra method), and a conventional method based on the confidence interval of difference in population means of two independent groups (difference-between-means (DBM) method).

Results/Lessons Learned

We present statistical distributions of the annual carbon sequestration rates (Metric tonnes [Mg] CO₂ ha^-1 year^-1) achieved by each land management category.  These data confirm that, in general, the carbon sequestration rates are rather modest, with an overall median rate of 1.1 Mg CO₂ ha^-1 year^-1, and median rates across categories ranging from 0.30 to 1.58 Mg CO₂ ha^-1 year^-1. These rates correspond to an annual percent change in soil carbon mass of less than 0.01% per year, based on a typical soil bulk density (1.55 g/cm³) and assumed carbon accrual within the top 30 cm. Comparing accrual rates observed in this study with those published in the early 2000s, higher accrual rates were observed herein for grazing management due to the inclusion of rotational grazing practices and for grassland management potentially due to croplands restored having certain degree of degraded soil.  The modest mass of soil carbon accrual is challenging to measure and susceptible to significant variability among sampling locations; consequently, carbon registries rely upon statistical methods to estimate and certify carbon credits based on soil sampling and testing results. In most cases, the amount credited is less than the absolute difference in mean soil carbon values before and after the land management project, by margins ranging from 2 to 99%. In some cases, Verra method would award all absolute difference in means as creditable. Comparing the Verra-based carbon credits with DBM-based credits, the DBM method provided fewer carbon credits for 55% of the sites, yet this method reduced the overall spread or variation of the credits across the dataset.  The information presented in this study will support planning-level estimates of the soil carbon gains that may potentially be achieved by climate-smart practices on agricultural lands and, at the same time, demonstrate the effect of the statistical methodologies employed by carbon registries for certification of carbon accrual.