Original Development of a Radiative Transfer Model for Remote Sensing of Soil Carbon Content in Nigerian Agricultural Landscapes

Soil organic carbon is a critical indicator of land health, agricultural productivity, and terrestrial carbon cycling, yet continuous spatial mapping of soil carbon across Nigeria's diverse agricultural landscapes remains challenging due to the limitations of conventional point-sampling approaches. Hyperspectral remote sensing offers a physics-based pathway to spatially continuous soil carbon estimation, but radiative transfer models linking soil optical properties to carbon content specifically for Nigerian tropical soil types are not yet established. This research develops an original radiative transfer model for predicting soil reflectance spectra as a function of organic carbon content, mineral composition, particle size distribution, and moisture across Nigerian Alfisol, Ultisol, and Vertisol soil taxonomic classes distributed across the South West, North Central, and North East geopolitical zones respectively. The theoretical model extends the Hapke radiative transfer theory for particulate media by incorporating original analytical expressions for the imaginary refractive index component attributable to soil organic matter chromophores, derived from laboratory diffuse reflectance measurements on synthetic soil mixtures with controlled organic carbon content spanning 0.1 to 8.5 percent. Model inversion is formulated as a Bayesian optimization problem and applied to Hyperion and PRISMA hyperspectral satellite data over 45 calibration sites with co-located laboratory carbon measurements. Results demonstrate that the physics-based model inversion achieves root mean square error for soil organic carbon prediction of 0.42 percent, compared to 0.89 percent for empirical partial least squares regression calibrated on the same dataset. Original contributions include derivation of the soil organic matter spectral absorption function for tropical humic acid compositions prevalent in Nigerian soils. Keywords: radiative transfer, soil organic carbon, remote sensing, hyperspectral, Nigeria