Mapping cation exchange capacity using a quasi-3d joint inversion of EM38 and EM31 data

Abstract

Cation exchange capacity (CEC, cmol(+) kg−1) is a measure of the capacity of soil to retain and exchange cations. However, it is expensive to sample and directly measure across a heterogenous field and at different depths. To add value to limited data, proximally sensed apparent soil electrical conductivity (ECa, mS m−1) from electromagnetic (EM) instruments has been coupled to CEC at each depth through a linear regression (LR) model. In this study, LR between ECa and depth specific CEC was compared with a LR developed between true electrical conductivity (σ, mS m−1), inverted from ECa, and CEC from various depths, including topsoil (0–0.3 m), subsurface (0.3–0.6 m), shallow subsoil (0.6–0.9 m) and deeper subsoil (0.9–2.1 m). We estimate σ using quasi-3d (q-3d) inversion software (EM4Soil) considering inversion of EM38 and EM31 ECa either alone or in combination (joint inversion), in horizontal (ECah) and vertical (ECav) modes, and EM38 at two different heights (i.e. 0.2 or 0.4 m). The calibration results showed LR between ECa and depth specific CEC in the topsoil (R2 = 0.31), subsurface (0.37) and shallow subsoil (0.52) was unsatisfactory. Stronger LR could be established for deeper subsoil CEC (> 0.60). However, a single LR could be developed between CEC at all depths with σ (R2 = 0.72) estimated by jointly inverting EM38 (0.2 m) and EM31 ECa in both modes using a forward model (CF), inversion algorithm (S2) and small damping factor (λ = 0.03). A leave-one-out-cross-validation showed CEC prediction was precise (RMSE, 2.39 cmol(+) kg−1), unbiased (ME, -0.01 cmol(+) kg−1) with good concordance (Lin’s = 0.82). To improve areal prediction closer spaced transects are required, while to improve vertical resolution of prediction we recommend the use of a single-frequency multi-coil array DUALEM-421.