Why is it difficult and costly to directly measure soil carbon in agricultural landscapes?

It is argued that worldwide efforts to shift agricultural land management to practices which sequester excess atmospheric carbon may only provide a small percentage of the sequestration capacity that is required to mitigate climate change. (Schlesinger, 2022). Although the carbon sequestration capacity of agricultural soils may be marginal, the co-benefits provided by managing agricultural soils to sequester carbon are significant. Co-benefits include improved soil function (or soil health), improved production system resiliency, and reduced soil, air, and water pollution.

From land management, scientific, and policy perspectives, it can be important directly measure soil carbon levels. Soil carbon levels are quantified using Soil Organic Carbon (SOC). SOC is a measurable fraction of Soil Organic Matter (SOM) (a commonly measured soil property) (Moorberg and Crouse, 2017). While SOM comprises a small percentage of most soil mass (between 2-10%), it plays an important role in the biophysical function of soils. SOC refers to only the carbon component of SOM (Government of Western Australia, 2021).

Measuring SOC at the field and landscape scales poses significant methodological and cost challenges. Sampling methodologies must be temporally and spatially representative and SOC must be measured at the correct depth in the soil profile. At the same time, the large numbers of soil samples required for proper temporal and spatial representation and relatively deep soil cores would incur significant labor costs for sample collection and lab analysis. A World Bank carbon sequestration guidebook states that methodological challenges and high costs remain a significant barrier to “implementing transparent, accurate, consistent, and comparable methods for [the] measurement” of carbon sequestration (2021, 7).

Over the short term SOC levels tend to fluctuate throughout the year based on seasonal fluctuations in precipitation (Abram 2020). Over the longer term (such as five or ten year intervals), SOC levels can be slow to change, even with shifts to carbon sequestration-focused land management practices (Department of Agriculture and Food, Western Australia 2013; World Bank 2021). SOC measurements must be timed in a manner that allows for accurate quantification over both seasonal and multi-year time scales.

SOC changes can be highly variable across fields and landscapes and are closely tied to biotic and abiotic factors such as historical and present-day farm management practices, soil type distribution, landscape position, and climate (Tautges et al. 2019; Morais, Teixeira, and Domingos 2019). SOC measurements must accommodate for spatial variability at the field and landscape scales.

To account for the spatial and temporal variability described above, it is recommended that SOC measurements be taken at a depth of 30cm or even deeper (Slessarev et al. 2021). Soil samples taken at proper depths require vehicle-mounted hydraulic probes (Franzen 2018).

Sampling SOC at temporally and spatially representative intervals at proper depths at the field and landscape scales requires large numbers of soil samples (NC State Extension 2017). The testing of soil samples for SOC is related to SOM and uses common soil lab equipment. SOC can be measured by most commercial soil labs. However, measuring SOC in lab settings is a labor-intensive process, especially when large numbers of samples are involved (FAO 2019).

The direct measurement of SOC at the field and landscapes scales can be useful for scientific, land management, and policy purposes. However, the methodological and cost barriers posed by representative soil sampling raises questions about the practicality of using SOC as an indicator for measuring the effectiveness of carbon sequestration-focused agricultural practices.

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