Spectral Induced Polarization of Biochar in Variably Saturated Soil

Abstract

Core Ideas Water retention and spectral induced polarization were measured for biochar in sand. Water retention curves could be fitted by using a dual porosity model. Residual water content was found to increase with the addition of biochar. The polarization of sand–biochar mixtures was much larger than that for sand. Spectral induced polarization may be suitable for monitoring biochar in the field. Biochar is considered a promising soil amendment, but an effective method to detect and characterize the spatial distribution and temporal dynamics of biochar in soil is still missing. The aim of this study is to investigate the ability of the spectral induced polarization (SIP) method for the noninvasive detection of biochar in unsaturated sandy media. In particular, a pure sand and two sand–biochar mixtures with 1 and 2% biochar made from pine (Pinus spp.) wood by pyrolysis at 800°C were investigated. The measured SIP spectra as a function of saturation were interpreted by fitting a Cole–Cole model to the low‐frequency part of the SIP measurements. The porous nature of the biochar particles strongly affected the SIP response of the partially saturated sand–biochar mixtures. Due to the high residual water content of the biochar in a dry background, the relationship between bulk electrical conductivity and water saturation was nonlinear in a log–log representation. This nonlinear behavior could adequately be explained with a dielectric mixing model that considered the drainage of the biochar particles. Both the measured phase and chargeability of the sand–biochar mixtures showed a complex dependence on water saturation. This was attributed to the decrease in polarization strength of the biochar particles with desaturation and the simultaneous increase in phase of the sand background. Overall, the results of this study suggest that field SIP measurements may be a promising tool for the characterization and monitoring of biochar amendments to agricultural soils.

Moisture–Conductivity Calibration for Electrical Imaging of Horticultural Substrate

“…As the water content increases, the pore water connectivity is improved, and thus, the conductivity increases [17,19,20]. The results in this study indicate that the magnitude of complex conductivity and the in-phase conductivity follow good power-law relations with the VWC, and these results are consistent with those in the previous works [11][12][13][14][15]. Moreover, the results in the previous studies also show a relatively regular relationship [12,15] or good power-law function [11,13,14] that the quadrature conductivity increases with the increase in saturation (or VWC), but this work does not show good results that are consistent with those in the previous studies, probably due to the following reasons.…”

“…The related equivalent circuit parameters can be found in Table 2, and the corresponding VWCs and dry densities are shown in Table 1. Overall, the characteristics for the change in subŝ σ with the increase in θ are similar to those for meaŝ σ , i.e., the increase in θ primarily leads to the obvious and regular increase in subs σ and  subs σ , and the conductive effect related to the substrate pore network filled with water plays a leading role; these results are relatively consistent with those in the previous studies [13][14][15]. The substrate is at the volumetric water content of 0.190 cm 3 /cm 3 and dry density of 0.397 g/cm 3 .…”

“…The related equivalent circuit parameters can be found in Table 2, and the corresponding VWCs and dry densities are shown in Table 1. Overall, the characteristics for the change inσ subs with the increase in θ are similar to those forσ meas , i.e., the increase in θ primarily leads to the obvious and regular increase in |σ subs | and σ subs , and the conductive effect related to the substrate pore network filled with water plays a leading role; these results are relatively consistent with those in the previous studies [13][14][15]. To investigate the effect of the VWC θ on the substrate conductivity subŝ σ , several sets of conductivity (or resistivity) spectra of the substrates at nearly equal dry densities and different VWCs were picked out, and the spectral analysis and the comparison of equivalent circuit parameters on each set of spectra were conducted.…”

“…In addition, to minimize the difference between the density of boxed samples and that of an imaged domain, relatively complex and strict packing work is also required [3,4]. In another laboratory strategy [7,8], or the related SIP studies of variably saturated soils [6,12,15], the gradual desaturation (or saturation) and related conductivity measurements of an undisturbed (or disturbed) sample can be conveniently performed, but a relatively complex system, using a specially designed cell and an air injection apparatus, is demanded. In a more practical field strategy, each imaged domain or investigated site must be equipped with the related conductivity and moisture sensors.…”

“…At present, there are some published works focusing on the complex conductivity (or spectral induced polarization (SIP)) measurements of soils in the frequency range below several tens of kHz. In these studies, the effect of the water saturation, which can be related to the VWC through the porosity, on the complex conductivity were investigated, and it was shown that the magnitude of complex conductivity, the in-phase conductivity, and even the quadrature conductivity follow power-law relations with the saturation [11][12][13][14][15]. Additionally, in some other works focusing on soil moisture measurements, the related electrical measurements were performed in wider frequency ranges of up to several MHz, and despite the presence of the effects of electrode polarization, the related results show good regular relations at different frequencies between the measured resistivity (or impedance, conductivity) and the water content [16][17][18].…”