Determination of soil bulk density dynamic in a Vertisol during wetting and drying cycles using combined soil water content and thermal property sensors

Abstract

The dynamics of soil bulk density (BD) play an important role in soil water, heat, and gas transport, which is more complicated in Vertisols due to the shrinking and swelling process. Previous studies showed BD can be estimated from soil thermal conductivity (λ) and water content (θv) measurements. Thus we comprehensively evaluated the applicability of the commercial sensors (5TE/TP01) in determining soil BD dynamic affected by wetting and drying cycles in a Vertisol. The repacked and intact soil cores with various BD and θv values were used to evaluate the accuracy of the two sensors for determining BD. We also assessed the potential of two thermal greases (MX-4 and TFX) in improving probe-soil contact and in determining BD dynamic of the Vertisol during wetting and drying (WD) cycle by different BD estimation models (Tian-de Vries model and Lu model). Results showed that the 5TE/TP01 combination gave accurate BD estimates with RMSEs of 0.0179–0.120 Mg m−3. For the WD cycle experiment, soil shrinkage led to a large bias in BD estimation. Thermal grease enhancing contact between the probes and soil matrix improved the accuracy of BD estimates, in which the TFX grease performed better (RMSE: 0.031–0.082 Mg m−3) than the MX-4 grease (RMSE: 0.067–0.204 Mg m−3). In regard to BD estimation models, the Lu model gave more accurate BD estimates (RMSE: 0.031–0.082 Mg m−3) than the Tian-de Vries model (RMSE: 0.057–0.187 Mg m−3). The high bias of BD estimation during WD cycles mainly occurred at the dry end side of soil shrinkage and at the beginning of wetting, which may result from soil cracking and the hysteresis of soil swelling. Our results demonstrate that the commercially available sensors in measuring soil water content and thermal property (e.g., 5TE/TP01) with the aid of thermal grease is a promising method to estimate the BD dynamics in Vertisols.