## 3 Materials and Methods
### Time-Domain Reflectometry  
Time-domain reflectometry determines the dielectric permittivity in soil. A TDR probe is composed of a couple of parallel rods linked to a voltage generator and an oscilloscope. An electromagnetic wave is generated and the signal runs along the length of the probe, being reflected when it reaches the end. The time a wave needs to run along a rod is inversely proportional to the bulk dielectric permittivity \(\varepsilon_{r}\)', according to Equation (8). The result is an average value of permittivity of the sensed volume; generally, the first centimeters near the sensor are the most influent, although, theoretically, the electromagnetic field propagates indefinitely. Some studies have also tried to analyze in more detail the reflected signal of a TDR measurement to identify zones with different permittivity situated along the length of the rods [35]. A piece of advice from all manufacturers is to pay attention in ensuring that the rods of TDR sensors have good contact with the soil; if air gaps are present around the rods, the sensibility of the instrument is drastically reduced. Summarizing this, water content is efficiently measured by a TDR probe that evaluates the real component of complex dielectric permittivity. Other properties,  
\begin{table}
\begin{tabular}{l l} \hline
**Parameter** & **Value** \\ \hline pH (-) & 7.32 \(\pm\) 0.04 \\ EC (uS/cm) at 25 \({}^{\circ}\)C & 165 \(\pm\) 5 \\ Bicarbonate (mg/kg) & 66.9 \(\pm\) 10.8 \\ Ammonia (mg/kg) & 2.18 \(\pm\) 0.11 \\ Nitrate (mg/kg) & 68.0 \(\pm\) 0.4 \\ Chloride (mg/kg) & 26.2 \(\pm\) 0.3 \\ Sulfate (mg/kg) & 211 \(\pm\) 3 \\ \hline \end{tabular}
\end{table}
Table 1: Results of soil characterization.  
\begin{table}
\begin{tabular}{l l} \hline
**Parameter** & **Value** \\ \hline Diameter of columns (cm) & 13.8 \\ Soil mass (kg) & 3.4 \\ Soil particle size (mm) & 0.15–2 \\ Porosity (\(-\)) & 0.4 \\ Volumetric water content (V/V) & 0.20 \\ Volumetric diesel content (V/V) & 0.19 \\ C/N ratio (W/W) & 450 \\ \hline \end{tabular}
\end{table}
Table 2: Main characteristics of mesocosms.  
like salinity, mostly influence the imaginary component and, consequently, the electrical conductivity. These sensors were selected for their ability to simultaneously measure dielectric permittivity and electrical conductivity, and to provide a temperature correction of measurements.  
The TDR probes, as explained in Equation (6), measure an apparent dielectric permittivity because they are sensitive to soil properties linked to the imaginary part of dielectric permittivity. To calculate the real part, they measure, independently from \(\varepsilon_{r}\)' measurement, the voltage attenuation of a known non-polarizing waveform at 100 kHz. This voltage ratio ranges from 1 in non-conductive media to about 17 in highly conductive media. The voltage ratio is correlated to the electrical conductivity, which is linked to the imaginary component of the dielectric permittivity through Equation (3). It is assumed that at 100 kHz the imaginary component only represents losses due to conduction phenomena and, therefore, the dielectric losses (\(\varepsilon_{r}\)\") are negligible.  
The probe, manufactured by Campbell Scientific (model: CS 655), composed of two 12-cm long rods, was inserted vertically into the soil. The measurements gave average values of the sensing volume of the probe. The recording was performed every 10 min along the entire duration of the experiment (105 days). A simple scheme of the acquisition instruments is shown in Figure 1.