Soil salinity management using a Field Monitoring System (FMS) in tsunami-affected farmlands in Miyagi, Japan

The March 11, 2011 M9.0 megathrust earthquake off the East Coast of Japan generated a devastating tsunami that inundated over 1300 km of the Pacific Coast. It reached approximately 5 km inland in some areas of Miyagi, Japan (Earthquake and Reconstruction Division of Miyagi Prefecture, 2014). The tsunami caused extensive damage to thirteen thousand hectares of farmland, which included collapsed houses, buildings, and many types of infrastructure (Chiba et al., 2014; Roy et al., 2015). Since the disaster, according to a master plan initiated by the Ministry of Agriculture, Forestry and Fisheries (MAFF) of Japan, various recovery/remedial works have been carried out (MAFF, 2011). A 1:5 soil to water extract electrical conductivity (EC1:5) test is recommended in MAFF guidelines to evaluate soil salinity levels. Some studies reported that natural rainfall helped to reduce the salinity levels (EC1:5 < 0.6 dS m) in the plow layer (up to 20 cm) of paddy fields with cracks near the soil surface (Chague-Goff et al.; 2012, JIID, 2013; Terasaki et al., 2015). However, Chiba et al., (2014) reported that leaching effects originating from natural rainfall alone was insufficient for rice growth in tsunami-affected regions, where severe subsidence occurred. In most cases, downward infiltration could accomplish sufficient salt exclusion through drainage canals in paddy fields (Chiba et al., 2015). However, the EC1:5 method © 2021 Institute of Agrophysics, Polish Academy of Sciences I. TOKUMOTO et al. 228 would be a time and labour consuming choice with which to conduct a long-term investigation into the desalinization process. Instead, pore water electrical conductivity (ECw) estimated using volumetric soil water content (θ) and bulk soil EC (ECb) are a useful way of monitoring soil salinity conditions. Recently, θ and ECb measurements have become available through the use of commercial soil moisture sensors such as time domain reflectometry (TDR) (Noborio et al., 2001; Miyamoto et al., 2015) and time domain transmissions (TDT) (Miyamoto et al., 2013; Hirashima et al., 2020). These measurement techniques allow for the attainment of in situ θ and ECb data simultaneously. A Field Monitoring System (FMS) developed by Mizoguchi et al., (2012) can be used to facilitate agricultural production recovery on damaged lands through the real time monitoring of θ and ECb. The FMS includes two central systems: field measurements and a monitoring system. The FieldRouter (FR) allows for the collection of in situ data and field photos through Bluetooth, which are then sent to a data server over the Internet. For monitoring high ECb in the soil through FMS, the TDT sensor (Acclima) is more affordable than the TDR sensor because of the SDI-12 protocol, which is a standard for interfacing data recorders with microprocessor based sensors (SDI-12 support group, 2012). Additionally, the TDT sensor is a low cost, high precision method used for θ estimation, and it also has a lower user ability requirement for TDT waveform analysis (Blonquist Jr. et al., 2005). The advantage of FMS with TDT sensors is to obtain accurate θ and ECb in the root zone because both values are required to estimate ECw for the evaluation of the desalinization process. For example, the relationship between θ, ECb and ECw was described by the Rhoades model (Rhoades et al., 1976): ECb = ECwθTcoef + ECs , (1) where: Tcoef is interpreted as the soil specific transmission coefficient to account for changes in tortuosity within the electrical current flow path in response to changes in θ, and ECs is the bulk surface conductivity of soil particles. The soil specific transmission coefficient may be expressed by: Tcoef = aθ + b , (2) where: a and b are empirical parameters, depending on soil types. This theoretical model describing the dependence of ECb on ECw and θ (e.g., Rhoades et al., 1976; Rhoades et al., 1989; Mualem and Friedman, 1991; Malicki and Walczak, 1999) is used to estimate ECw for a given combination of the measured ECb and θ in a particular soil (e.g., Mallants et al., 1996; Risler et al., 1996; Muñoz-Carpena et al., 2005). In other words, the close dependence of ECb on θ makes it difficult to understand the desalinization process without ECw when θ changes in natural conditions (Seki et al., 2019). Hence, the analysis of salt leaching in fields using ECw must be significant. Also, ECw is usually proportional to chloride ion (Cl) concentration, which influences crop yield (Abe et al., 2017). To assist in the process of the recovery of agricultural production, this study aimed to evaluate the impact of flooded leaching and natural rainfall on the desalinization process using FMS with TDT sensors in Higashimatsushima of Miyagi, where severe subsidence occurred. Using the Rhoades model, the relationship between θ, ECb, and ECw was revealed in the laboratory, research efforts were then focused on the decline in in situ ECw following two weeks of application of the flooded leaching method with topsoil addition. Additionally, Cl concentration profiles were estimated based on the ECw results for different crop management methods. MATERIALS AND METHODS Field measurements of the FMS were conducted at a site of the tsunami-damaged farmlands of Higashimatsushima (38°25'38.6"N 141°14'46.4"E) in Miyagi, Japan, in 2014 and 2015 (Fig. 1). In Higashimatsushima, 40% of the entire tsunami-damaged area of 3 600 ha consisted of farmland. In the region, subsidence was 60 cm, and organic rich and muddy sediments dominated the soil type. The groundwater level (GWL) was shallow, and the soil became quickly saturated after rainfall events. In 2013, debris was removed, and a drainage pumping station started operation. Irrigation water was delivered to outlets through underground pipelines and was supplied to the paddy fields. At the end of October 2014, a treatment consisting of spreading 10 cm of topsoil was carried out to lower the groundwater depth. The average saturated hydraulic conductivity (Ks) and dry bulk density (ρb) were measured using undisturbed soil core samples taken from a layer between 10 and 45 cm, the results were 1 cm d and 1.1 g cm, respectively. Fig. 1. The map of our research site: Inundated areas of the Tohoku earthquake and massive tsunami in Higashimatsushima, Miyagi (Association of Japanese Geographers, 2011) (a), and the zoom map near our research site (b). The regions are characterized by a warm humid temperate climate (average monthly temperatures ranged from 1.6°C to 24.2°C from 1981 to 2010), and the average annual SALINITY MANAGEMENT WITH FMS 229 rainfall is approximately 1 100 mm. The rainy season is the best season to reduce soil salinity. Through a previous field survey of the rainy season in July 2013 (Chiba et al., 2014), it was established that EC1:5 was 4 dS m at depths between 10 and 40 cm. In September 2014, EC1:5 dropped below 2.3 dS m, but it was still difficult to grow crops such as rice (Oryza sativa L. cv. Hitomebore) and soybeans (Glycin max var. Tanrei) on the farmland. In addition, the long-term high salinity caused cracks near the subsoil surface (Fig. 2). Fig. 2. Picture of cracks on the soil surface before mounting the topsoil layer. In November 2014, a study was begun in Higashimatsushima to monitor θ, ECb, and soil temperature using FMS with TDT sensors (Acclima) (Fig. 3). Three TDT probes were installed at depths of 15, 30, and 50 cm under the added soil layer. TDT measurements were recorded every hour in a model CR800 data logger (Campbell Scientific, Logan, UT) and uploaded to the data server at noon through the FR (X-ability). GWL was measured using a CTD sensor (Decagon Devices), hooked up with an Em50 data logger (Decagon Devices). The CTD sensor was placed in a well at a depth of 80 cm from the soil surface. Fig. 3. Diagram of the FMS for remote sensing technique to measure environmental factors (θ, ECb, etc.) at our research site. Regarding ECb, a temperature correction equation was used to obtain ECb values at 25°C (ECb25) (US Salinity Laboratory Staff, 1954):