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To establish sustainable agricultural practices, monitoring soil water and solute distributions in actual field conditions is important. Because the estimation of electrical conductivity (EC) using time domain reflectometry (TDR) is considered to be affected by Andisols because of their unique dielectric properties, in this study, we investigate the potential for the continuous monitoring of EC and nitrate concentrations using TDR in an Andisol field. The study was performed from December 2007 to August 2008. To estimate soil solution EC (ECw), the relationship among ECw, apparent EC (ECa) and volumetric water content (θ) for an Andisol was investigated, and three models (Rhoades model, Extended Rhoades model and Hilhorst model) were applied to describe this relationship. The Rhoades model was found to be reasonably accurate in describing the ECw-ECa-θ relationship, while the Hilhorst model showed reasonable agreement with the experimental data. In the field experiment, ECw values estimated using TDR with the Rhoades model correlated with those obtained from solution samples and a linear regression between ECw and soil nitrate concentrations was obtained from the field experimental data. The use of this regression with ECw estimated from TDR measurements, together with the Rhoades model, is considered a useful tool for the continuous monitoring of soil nitrate concentrations in Andisol fields under transient conditions. Discipline: Agricultural engineering Additional key words: soil physics, solute transport * Corresponding author: e-mail teruhito@affrc.go.jp Received 28 April 2014; accepted 15 December 2014. Introduction The contamination of groundwater due to the leaching of excess fertilizer is a common problem in upland field areas of Japan. To establish sustainable agricultural practices, estimating the displacement of water and solutes that occur at a depth below the plant root zone during a given period is important. Time domain reflectometry (TDR) has become an established and reliable means to determine the volumetric water content (θ) and apparent electrical conductivity (ECa) (e.g. Noborio, 2001). TDR can rapidly measure both θ and ECa in the same soil volume and has hence been used to monitor solutes. In addition, some researchers have applied TDR to estimate nitrate concentrations in soil solutions (Nissen et al. 1998, Das et al. 1999, De Neve et al. 2000). Although Andisols exhibit unique dielectric properties that may affect the estimation of electrical conductivity (EC) when using TDR, their implications for soil EC determination have received little attention. Accordingly, few works are available to evaluate the use of TDR for studying solute transport (Vogeler et al. 1996, Muñoz-Carpena et al. 2005). Furthermore, very little information is available to compare values of soil solution EC (ECw) based on TDR measurements with values obtained using conventional methods, i.e. soil coring or solution samplers under field transient conditions (Heimovaara et al. 1995, Caron et al. 1999). Accordingly, the main objective of this study was to test the applicability of TDR measurements in assessing the temporal dynamics of EC and nitrate concentrations in Andisol fields. To this end, we evaluated three models (Rhoades et al. 1976, Rhoades et al. 1989, Hilhorst 2000) to determine their efficiency in describing the ECw-ECa-θ relationship for Andisols. We also performed a field experiment to compare ECw based on TDR measurements with those obtained from solution samplers. In addition, we obtained regression to predict nitrate concentrations from ECw values.

Monitoring Electrical Conductivity and Nitrate Concentrations in an Andisol Field Using Time Domain Reflectometry