Is soil sampling appropriate for quantitative carbon accounting for biochar? An experimental investigation to assess soil carbon accumulation

[Geoengineering News]

https://www.sciencedirect.com/science/article/pii/S0961953425009481

Authors: David Chiaramonti, Giulia Lotti, Francesca Tozzi, David Casini, Francesco Primo Vaccari, Hamed Sanei, Michaela Luconi, Marco Buffi 

31 October 2025

https://doi.org/10.1016/j.biombioe.2025.108537

Highlights

•REDII-IR prescribes soil sampling to assess Soil Carbon Accumulation in case of biochar.

•Three 25 m2 consecutive plots were studied in a biochar-added area and SCA assessed via soil sampling.

•Arbitrary selection of sampling points is a major reason for deviation between expected and measured SOC, together with the micro amount of material used in lab instrument.

•Statistically non-significant differences were observed in SOC and BD measurements, relevant for setting SOC baseline (not necessary for biochar).

•Carbon savings/credits assessment from biochar addition cannot be based on soil sampling.

Abstract

Biochar, a major CDR method with significant co-benefits to agriculture, is listed as a sustainable agricultural method for SCA in sustainable biofuel regulations. In Europe, this is accounted via the esca factor (REDII-IR), while at international level this is considered through the Fsca factor. Fsca is analogous to esca in REDII, with similar, even if not identical, requirements (ICAO, for SAF). RED-II requires soil sampling to quantitatively assess the SCA from biochar addition: instead, ICAO CORSIA, as well as the draft incoming EU-CRCF (for voluntary carbon removals), require full characterization of biochar, incorporation in soil and third-party auditing during deployment (ICAO), but not necessarily soil sampling. This study presents experimental evidence evaluating the adequacy of current soil sampling protocols for the quantitative accounting of carbon saving/removals from biochar application to soil. The findings demonstrate that these protocols have intrinsic limitations, even when applied within a narrowly defined (75 m2), homogeneous, and controlled area. Key issues include the arbitrary selection of sampling locations, the limited quantity of material analysed by standard laboratory instrumentation, and the statistically insignificant variation observed in SOC and BD measurements. Measured SOC figures were inconsistent with the amount of carbon introduced through biochar amendment: the SOC content of the biochar-amended soil plot was larger than the one actually introduced and thus expected to be retrieved via analytics. This observation is attributed to the spatial heterogeneity of soil characteristic, and statistical significance of measured samples, in addition to the physical challenge of blending homogeneously a solid amendment (biochar) in a the solid soil phase, a limitation that cannot be entirely overcome even when employing conventional and appropriate tillage methods.

These results also raise broader concerns regarding the use of conventional soil sampling protocols for establishing SOC baselines in other (i.e. non biochar-based) carbon farming approaches. The observed high variability in carbon stock measurements hardly matches the precision required for assigning economic value. To address these shortcomings, an integrated approach combining rigorous experimental design with validated modelling frameworks is necessary to ensure scientifically robust and quantitatively defensible allocation of greenhouse gas (GHG) mitigation benefits and carbon savings/credits.

Source: ScienceDirect