Qualitative and quantitative mapping of biochar in a soil profile using hyperspectral imaging

• Soil profile and ground soil samples were scanned with NIR hyperspectral camera.
• Biochar content was mapped using multivariate analysis.
• Validation was carried out using measured volumetric biochar concentrations.
• NIR hyperspectral imaging can provide information on the distribution of biochar in soil.

NIR (near infrared) hyperspectral reflectance imaging has been carried out with the aim of identifying biochar particles and mapping biochar concentrations in a soil monolith (12 cm × 12 cm × 30 cm), which was extracted from the top 30 cm of an agricultural field amended with biochar. Hyperspectral images were obtained directly on one vertical side of the soil monolith and also on ground soil samples extracted from the same side after scanning. Median spectra from the ground samples were used to calibrate a PLS regression model of the C content in the samples and this model was applied to predict the distribution of the biochar in the soil profile. Spectral angle mapper (SAM) was also applied as a comparative technique to predict the biochar distribution, using reference spectra from pure unground biochar. The analysis of the hyperspectral measurements was validated by conducting chemical analysis of the total C and 13C signature on the ground samples extracted from the scanned side of the monolith. The results indicate that biochar can be qualitatively identified in soil profiles using hyperspectral NIR images. Quantifying the amount of biochar, however, is challenging on the pixel level due to the rough surface of the soil profile, non-homogenous distribution of the biochar and varying moisture conditions.