Evaluation of the time-domain reflectometry (TDR)-measured composite dielectric constant of root-mixed soils for estimating soil-water content and root density

The effects of dense plant roots on the time-domain reflectometry (TDR)-measured composite dielectric constant, εc, and soil-water content, θTDR, were investigated by laboratory experiments with root-mixed sand and loam soil. TDR overestimates εc in the root-mixed sand while it underestimates εc in the root-mixed loam soil. The TDR-measured water contents, θTDR, in these two root-mixed soils provide neither the actual soil-water content, θ (excluding volume occupied by the roots and water within the roots), nor the bulk soil-water content, θb (neglecting water in the roots, but including the root-occupied volume in the bulk volume), or the bulk total water content, θt (water in the soil plus that in the roots comprises the total water) (please see nomenclature for definitions). The effects of the roots on TDR measurement are more pronounced at low soil-water content than at high soil-water contents. A 4-phase dielectric mixture model, consisting of the volume fractions of the soil minerals, roots, water, and air, describes the composite dielectric constant εc well. The dielectric mixture model can estimate the actual soil-water content θ and the root density, vr (volume fraction), from εc. This model also estimates the dielectric constant of the soil minerals, εs, and the root density vr simultaneously provided that the bulk soil-water content is known. The exponent of the dielectric mixture model, α, and the root density are moderately sensitive to the dielectric constant of the soil minerals. An accurate estimate of εs is therefore preferred for evaluating the composite dielectric constant in the root-mixed soils. The root density is relatively insensitive to α. The dielectric mixture model also accurately estimates the root density and bulk soil-water content simultaneously. The actual water content, θ, in the root-mixed soil is calculated by [θ=(θb/(1−vr))].