Imaging the deep carbon stocks with complex electrical conductivity 

Abstract. Due to the limited penetration depth of standard sampling methods, mapping of soil organic carbon (SOC) is usually restricted to the top 100 cm of soils. Hence, current models underestimate 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 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 demonstrate a positive correlation between imaginary CC at low frequencies (< 5 Hz) and SOC content permitting a quantitative interpretation of geophysical images. 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.