Calculating soil carbon (or any other element/soil nutrient) stocks involves measuring the amount of carbon stored in the soil, which can help us understand the potential of soils to sequester carbon and mitigate climate change.

Soil Carbon Stock (Mg C ha⁻¹) = Soil Carbon Concentration (g C kg⁻¹) × Bulk Density (g cm⁻³) × Soil Layer Thickness (cm) × 0.1

Note that the conversion factor of 0.1 is used to convert the units from g C cm⁻² to Mg C ha⁻¹ (Batjes, 1996). To calculate soil carbon stocks, follow these steps:

  1. Define the area of interest: Determine the boundaries of the area where you want to calculate soil carbon stocks. This could be a plot, field, or landscape.
  2. Sample collection and soil profile description: Collect soil samples from the area of interest. You can use a soil auger, a soil core sampler, or other sampling tools. Record the depth of each sample, as well as any other relevant information about the soil profile, such as texture, structure, and color (Schoeneberger et al., 2012).
  3. Bulk density determination: Measure the bulk density of the soil samples, which is the mass of soil per unit volume (Blake and Hartge, 1986). This can be done using the core method, clod method, or other methods suitable for your soil type.
  4. Soil carbon concentration analysis: Analyze the soil samples to determine the concentration of organic carbon. This can be done using various methods, such as dry combustion (Nelson and Sommers, 1996), wet oxidation (Walkley and Black, 1934), or infrared spectroscopy (Reeves III, 2010).
  5. Sum the carbon stocks: Add up the carbon stocks of all the soil layers to get the total soil carbon stock for the area of interest.
  6. Measuring Sequestered Carbon: Methods should be similar (location, depth) in the pre- and post-sequestration measurements, it can take 6-10 years for measurable changes to be observed (Smith, 2019).

Remember that the precision and accuracy of your calculations will depend on the number of samples, the sampling design, and the quality of the laboratory analyses. When calculating soil carbon stocks, it is important to consider factors that can influence the accuracy and representativeness of your results. These factors include:

  • Spatial variability: Soil properties, including carbon content, can vary significantly within a small area. To account for spatial variability, you should collect samples from multiple locations and depths within your area of interest.
  • Temporal variability: Soil carbon stocks can change over time due to natural processes and human activities, such as land-use changes, soil management practices, and climate change. To capture temporal variability, you can repeat the sampling and analysis process at different times, and compare the results to assess changes in soil carbon stocks over time.
  • Sampling design (see below): A well-designed sampling strategy can help you obtain representative samples and more accurate estimates of soil carbon stocks. Consider using a BACIP sampling design or other appropriate sampling methods to account for variability in soil properties within your area of interest (Burt, 2004).
  • Soil depth: The distribution of soil carbon can vary with depth. It is essential to sample multiple soil layers to capture the vertical distribution of carbon in the soil profile. You may also want to consider the specific depth intervals relevant to your research question or management objective.
  • Laboratory analysis: The choice of analytical method for determining soil carbon concentrations can influence the accuracy and precision of your results. Be sure to select a method that is suitable for your soil type and research objectives, and ensure that the laboratory performing the analysis follows standardized procedures and quality control measures.

BACIP monitoring design

To show that specific management interventions (such as livestock exclosures) actually have a significant impact on soil carbon sequestration we recommend using Balanced Alternate Control Impact Pair (BACIP) designs. BACIP is a type of experimental design used in environmental and ecological research to assess the impacts of treatments or interventions on ecosystems or communities (Green 1979; Smith 2002). These designs involve two key elements: spatial replication and temporal replication (Underwood 1994).

Spatial replication: BACIP designs involve selecting two sites, one control and one impact site, which are matched for their environmental characteristics (Smith 2002). The control site remains untouched, while the impact site receives the treatment or intervention.

Temporal replication: Measurements are taken at both sites before and after the treatment or intervention. This allows researchers to determine if any observed changes are due to the treatment or other factors (Green 1979; Smith 2002).

Advantages of BACIP designs include:

  • The ability to account for temporal variability in environmental systems can be crucial when assessing ecological impacts (Underwood 1994).
  • The ability to control for spatial variability, by comparing the impact and control sites (Green 1979).
  • Increased statistical power, as each impact-control pair acts as a replicate (Smith 2002).

Limitations of BACIP designs include:

  • The need for well-matched control and impact sites can be difficult to find (Smith 2002).
  • The assumption is that control and impact sites respond similarly to external factors other than the treatment (Green 1979).
  • Potential difficulties in interpreting results if multiple factors change simultaneously (Underwood 1994).

References:

  • Batjes, N.H. (1996). Total carbon and nitrogen in the soils of the world. European Journal of Soil Science, 47(2), 151-163.
  • Blake, G.R., & Hartge, K.H. (1986). Bulk Density. In A. Klute (Ed.), Methods of Soil Analysis, Part 1: Physical and Mineralogical Methods (pp. 363-375). Soil Science Society of America.
  • Burt, R. (2004). Soil survey laboratory methods manual. Soil Survey Investigations Report No. 42, Version 4.0. United States Department of Agriculture, Natural Resources Conservation Service, National Soil Survey Center.
  • Green, R. H. (1979). Sampling design and statistical methods for environmental biologists. John Wiley & Sons.
  • Nelson, D.W., & Sommers, L.E. (1996). Total carbon, organic carbon, and organic matter. In D. L. Sparks (Ed.), Methods of Soil Analysis, Part 3: Chemical Methods (pp. 961-1010). Soil Science Society of America.
  • Reeves III, J.B. (2010). Near- versus mid-infrared diffuse reflectance spectroscopy for soil analysis emphasizing carbon and laboratory versus on-site analysis: Where are we and what needs to be done? Geoderma, 158(1-2), 3-14.
  • Schoeneberger, P.J., Wysocki, D.A., Benham, E.C., & Soil Survey Staff. (2012). Field book for describing and sampling soils, Version 3.0. Natural Resources Conservation Service, National Soil Survey Center.
  • Smith, E. P. (2002). BACI design. In A. H. El-Shaarawi & W. W. Piegorsch (Eds.), Encyclopedia of Environmetrics (Vol. 1, pp. 141-148). John Wiley & Sons.
  • Underwood, A. J. (1994). On beyond BACI: sampling designs that might reliably detect environmental disturbances. Ecological Applications, 4(1), 3-15.
  • Walkley, A., & Black, I.A. (1934). An examination of the Degtjareff method for determining soil organic matter, and a proposed modification of the chromic acid titration method. Soil Science, 37(29-38).