Model and measure: comparing soil organic carbon stocks quantification approaches at field scale in South Africa

Abstract

There is an ongoing debate about the most appropriate approach for quantifying changes in soil organic carbon stocks (SOCS). The aim of the study was to evaluate the performance of the Rothamsted carbon (RothC) modelling approach against the re-measured SOCS (actual) values at field scale in South Africa. The baseline assessment (2022) for two fields in South Africa was established beforehand. Afterwards, 50 samples from three depth increments, 0–5, 5–15 and 15–30 cm were collected, and the SOCS calculated (2024) for two study sites (Ottosdal and Vrede) using the re-measure approach and the modelling approach using the RothC model. The sampling cost was also estimated against the value of sequestered SOCS (2022–2024) and compared between the two approaches. The main finding from the study indicated that more samples being utilised during the modelling approach, using the RothC model, leads to the modelled SOCS values being closer to the actual re-measured values and ultimately more reliable SOCS measurements at field scale in South Africa. The spatial variation of soil organic carbon (SOC) content was the main factor influencing the total SOCS, sequestered SOCS and carbon credit values. The results indicated that the modelling approach using the RothC model slightly underestimated the total SOCS and sequestered SOCS compared to the actual re-measured values for both study sites. The industry-represented ‘One-observation’ did not have a consistent trend, which had significant financial implications using both approaches. Overall, utilising more samples either in the re-measure or modelling approach would provide more reliable carbon credit values. The study also found that the spatial variation of the SOC content must be taken into account at the onset of measuring the SOC content in the baseline year (first year), independent of whether the SOCS would be modelled or re-measured. The carbon sequestration industry must take the model limitations (such as field scale dynamics and sensitivity to short-term changes) into account and find ways to address these limitations of the RothC model at field scale.