Imaging deep carbon stocks with complex electrical conductivity

Mapping of soil organic carbon (SOC) is usually restricted to the top 100 cm of soils due to the limited penetration depth of standard soil sampling methods. This has resulted in current models underestimating SOC due to the unexplored deep carbon stocks. Moreover, standard methods only offer punctual data relying on interpolation to investigate extensive areas. We demonstrate here that subsurface 2D images of the complex electrical conductivity (CC) can delineate the presence and geometry of SOC reaching a depth of a few tens of meters below the surface. We show that an increase in the polarization effect at low frequencies ( < 5 Hz) is linearly related to high concentrations of SOC. We present measurements from a catchment with silty loam soils, where the geometry of a deep carbon stock (between 4 and 6 m depth) was identified by CC images, and validated through laboratory analysis of soil samples.