Testing for Carbon Stocks [Part 3: Soil Analysis for Carbon Stocks]

In soil analysis for carbon stocks, the actual analysis of the soil core is a crucial step in calculating carbon stocks. A soil’s bulk density determination can be approached one of two ways: simply or comprehensively.

Bulk Density Determination

Standard Bulk Density

Standard or “simplified” bulk density involves a straightforward calculation where the total dry mass of the soil core (minus the water content) is divided by the core volume. This method treats the entire soil sample as a single entity, without distinguishing between fine soil particles and coarse fragments. While this approach is simple and effective for homogeneous soils, it can be less accurate for soils containing significant amounts of coarse material (>2 mm), such as gravel or organic debris, which do not contribute to soil fertility or carbon sequestration.

Comprehensive Bulk Density

In contrast, comprehensive bulk density takes a more detailed approach by separating the soil into <2 mm and >2 mm fractions. This approach is based on the Verra and BCarbon testing protocols. The fine fraction (<2 mm) typically holds most of the soil’s organic matter and is the primary focus for carbon sequestration and fertility. The coarse fraction (>2 mm) is considered separately, as it does not contribute to the soil’s productive properties. Additionally, water content is accounted for in both methods, but comprehensive bulk density provides a more accurate representation of the soil’s true bulk density by focusing on the mass of the fine fraction and excluding the coarse fragments, giving a clearer picture of soil properties that are crucial for management practices and carbon stock assessments.

Drying Methods and Their Impact on Soil Testing

For most of our soil prep, we use warm room drying at 40-45°C to minimize any potential alterations in soil properties. However, many prefer air-drying for carbon stock determination. Research has shown that there are no statistically significant differences in organic carbon values between air-drying and warm-room drying methods (around 45°C). A 2015 Oklahoma State thesis (Mueller 2015) also supports this, demonstrating that drying at both room temperature (25°C) and 45°C yielded similar results for soil organic carbon, with no significant differences found between the treatments (Figure 1). Interestingly, the soils tested for OC.

However, soils with higher initial moisture or organic carbon content tend to show greater variability after drying. These factors can influence how much carbon is lost during the process. Air-drying provides more time for microbial activity. This potentially leads to increased nitrate production and greater carbon degradation. Warm-room drying is faster and minimizes these microbial effects. In another study, Wilson et al. (2009) further validated that drying at around 40°C has negligible impact on bulk density and carbon density values, making it a reliable method for both parameters.

Inorganic Carbon Determination

Last year, we transitioned to a dual approach for carbon determination, where we continue to use a LECO analyzer to measure total carbon in soil samples, but now determine inorganic carbonates using a carbon dioxide gas analyzer to quantify the gas released from the reaction of soil carbonates with hydrochloric acid (Diagram 5).

This dual-method approach maintains the precision of organic carbon analysis while significantly improving efficiency in carbonate determination. Research supports the accuracy and operational advantages of using infrared gas analysis (IRGA) for inorganic carbon, particularly in high-throughput environments where speed and consistency are essential.

Calculating Soil Carbon Stocks

Once we have the necessary measurements, soil carbon stocks are calculated using the following formula:

The conversion factor is typically 0.1. This conversion, along with this formula, integrates the soil organic carbon percentage, bulk density, and soil depth to express carbon stocks in megagrams of carbon per hectare (Mg C/ha).

Conclusion

Testing soils for carbon stocks is a critical step in participating in carbon stock programs. Accurate measurement of soil carbon, combined with robust soil health practices, not only contributes to climate change mitigation but also offers significant economic and environmental benefits. If you’re considering joining a carbon stock program, start by testing your soil’s carbon levels today. Your efforts can make a lasting impact on both your land and the planet.

By employing comprehensive testing methods and reliable drying techniques, we ensure accurate soil carbon assessments, enabling effective participation in these valuable programs.