Estimating soil carbon sequestration potential with mid-IR spectroscopy and explainable machine learning

Abstract. Soil carbon sequestration refers to the process of capturing atmospheric carbon through plant photosynthesis and storing it in soil as organic carbon. The primary mechanism for carbon sequestration is via organic carbon molecules adsorbing onto mineral surfaces of the soil's fine fraction (clay + silt ≤ 20 μm), forming mineral-associated organic carbon (MAOC). Soil has a finite capacity to stabilise and sequester organic carbon, known as carbon saturation capacity, which depends on the proportion of reactive minerals in the soil. The difference between the current MAOC content and the carbon saturation capacity is referred to as the organic carbon saturation deficit (Cdef) or sequestration potential. Fourier-transformed (FTIR) mid-infrared (mid-IR) spectroscopy can simultaneously measure soil properties relevant to carbon stabilisation, organic carbon functional groups, clay and iron-oxide mineralogy and particle size. Therefore, we hypothesise that mid-IR spectroscopy can effectively and accurately estimate Cdef. Thus, we aim to (i) develop spectroscopic models to estimate the MAOC and Cdef of 482 Australian topsoil samples, (ii) model MAOC and Cdef using mid-IR spectra and an interpretable machine learning, and (ii) interpret the MAOC and Cdef models using the explainable artificial intelligence (AI) algorithm SHapley Additive exPlanations (SHAP). Using frontier line analysis, we fitted a function to the upper envelope of the MAOC vs clay + silt relationship to derive Cdef. We recorded mid-IR spectra of the samples and used the regression trees method CUBIST to model MAOC content and Cdef. We interpreted these models by examining the regression trees and using SHAP. The models were unbiased and estimated MAOC content with R2 of 0.86 and RMSE of 2.77 (g/kg soil), and Cdef with R2 of 0.89 and RMSE of 3.72 (g/kg soil). Model interpretation revealed Cdef estimates relied on negative interactions with absorptions from organic matter functional groups and positive interactions with absorptions from clay minerals. Our results show that mid-IR spectra can effectively estimate MAOC and soil Cdef, offering a rapid and cost-effective method for assessing and monitoring this critical soil function.