Intercomparison of soil organic carbon stocks measured from conventional and in situ sampling techniques at different spatial scales

Soil organic carbon (SOC) is a key property of soil quality in arable soils and can play a central role in the voluntary carbon credit market by improving soil health, future food security and mitigating against climate change. The adoption of regenerative agricultural practices are considered one solution to achieve increases in SOC sequestration rates. However, the spatiotemporal dynamics of SOC mean the changes in SOC attributed to management practices are often difficult to detect across different spatial scales and over short temporal periods. Thus, rapid, cost-effective methods for quantifying carbon are sought by key stakeholders in both academia and industry for accurate accounting of SOC stocks. A field trial experiment was conducted in 2023 on five arable fields in Denmark to compare SOC stocks measured between: (i) the conventional sampling method and (ii) a portable, handheld visible near infrared (NIR) spectrometer. The conventional sampling method used a hydraulic corer to extract soil in each field (n=9) split into three depth increments (0-15, 15-30 and 30-60 cm). The samples were analysed for %SOC by the dumas dry combustion method whilst bulk density was measured volumetrically from the intact core upon extraction. For the spectroscopy method, soil was first extracted using a handheld auger (0-30 cm) which was subsequently homogenised and soil properties (%SOC, %Clay content and bulk density) were measured in situ by the scanner. The SOC concentration of these in situ soil samples were also measured by dry combustion analysis. We found differences in cumulative SOC stocks between the conventional and NIR methods attributed to variation in how SOC concentration was measured (directly vs. dry combustion) and less so to variation in bulk density. Clay contents were also similar between the conventional sampled soils and NIR soils, whilst the SOC concentrations measured by dry combustion were similar for both conventional and NIR soils. Overall, the results highlight that portable vis-NIR spectroscopy could be a scalable solution for monitoring SOC stocks in arable soils.