*3.2. Column-Leaching Experiments*

The nitrate-leaching concentrations recorded during experiment 1 are shown in Figure 4a. The first day of leaching was marked by elevated leaching rates of 96.70, 116.82,

and 184.82 mg/L for C1, C2, and C3, respectively. The measured concentration indicated that leaching exceeded the initially loaded concentrations and imply that nitrogen was already present in the soil samples from field fertigation activities. It was also evident that Soil 3 had a higher nitrogen content. On the second day, there was an evident decrease in NO<sup>3</sup> <sup>−</sup> concentration in C3 (77.52 mg/L), whereas NO<sup>3</sup> − concentration in C1 and C2 decreased slightly to 91.25 and 94.2 mg/L, respectively. Except for day one, the daily measured NO<sup>3</sup> − concentration in C3 was lower than in C1 and C2 until day five, when concentrations were comparable at 70.61, 68.66, and 67.55 mg/L in C1, C2, and C3, respectively. The observed leachate concentrations dropped consistently until day eight, indicating that NO<sup>3</sup> − loading was roughly close to the leachate concentration in Soil 3 (55.73 mg/L). The NO<sup>3</sup> − leaching in C1 and C2 was close to the initial loaded concentration from D11 of the experiment. Afterward, the NO<sup>3</sup> − concentrations decreased slightly in the three columns and ranged between 56.08 and 49.47, 56.79 and 46.97, and 50.13 and 47.79 mg/L in C1, C2, and C3, respectively. The results of this experiment show that after fertigation of the raw soils with 51,56 mg/L of NO<sup>3</sup> <sup>−</sup>, the soils need more time to eliminate the NO<sup>3</sup> − excess already contained in the soil. The last days of the experiment showed that the loaded concentration of NO<sup>3</sup> − was moderately equal to the leached concentration. This experiment highlighted that after 17 days from the application of a constant NO<sup>3</sup> − concentration, the leaching rates do not decrease in the collected leachate. On the contrary, the raw sampled soils contribute, in turn, to the enrichment of NO<sup>3</sup> − in leachate. Nitrate-permissible level in groundwater was determined to not exceed 50 mg/L. This experiment showed that an excess of fertigation/irrigation rates in the R'mel area could cause the contamination of local groundwater by nitrate. *C* **2023**, *9*, x FOR PEER REVIEW 11 of 22

**Figure 4.** NO3<sup>−</sup> leaching from experiment 1 (**a**) and experiment 2 (**b**). **Figure 4.** NO<sup>3</sup> − leaching from experiment 1 (**a**) and experiment 2 (**b**).

Experiment 2, illustrated in Figure 4b, shows the effect of increased nitrate doses on leaching rates from the three sandy soils. Nitrate leaching on the first day was higher than the initially applied concentration of 0 mg/L (only water), reaching 15.1, 17.3, and 28.0 mg/L in columns C4, C5, and C6, respectively. These results indicate that although the soil columns were diluted several times with water to remove excess nitrate present in the soil, the three columns still contain nitrate, indicating residual or secondary leaching from soil samples. The following application (day two) shows that the measured NO3- concentration in C4, C5, and C6 decreased while the initially applied NO3<sup>−</sup> increased from 0 to 11.21 mg/L. After day three, all the measured concentrations in leachates were lower than the initially applied concentration until the end of the experiment. The leaching curve in-Experiment 2, illustrated in Figure 4b, shows the effect of increased nitrate doses on leaching rates from the three sandy soils. Nitrate leaching on the first day was higher than the initially applied concentration of 0 mg/L (only water), reaching 15.1, 17.3, and 28.0 mg/L in columns C4, C5, and C6, respectively. These results indicate that although the soil columns were diluted several times with water to remove excess nitrate present in the soil, the three columns still contain nitrate, indicating residual or secondary leaching from soil samples. The following application (day two) shows that the measured NO3 concentration in C4, C5, and C6 decreased while the initially applied NO<sup>3</sup> − increased from 0 to 11.21 mg/L. After day three, all the measured concentrations in leachates were lower than the initially applied concentration until the end of the experiment. The leaching

creased slightly from day three to day eight for an increasing loaded concentration ranging from 30.85 to 71.98 mg/L. These results show that the maximum adsorption capacity

In general, the nitrate leaching in columns C4, C5, and C6 seems to increase with respect to the applied doses of nitrate. However, experiment 2 demonstrated that after removing nitrogen excess from the raw soils, it appears that nitrate leaching decreases in the three columns. This indicates that R'mel soils have a low adsorption capacity which is

By comparing the collected leachate with the measured added concentrations in experiment 2, it is evident that, contrary to experiment 1, the NO3<sup>−</sup> leaching did not exceed the applied NO3<sup>−</sup> concentration. This suggests that the elevated applied N fertilizer affects the soil's adsorption capacity in the R'mel area. In this context, the following sections will be dedicated to discussing the main factors affecting the R'mel soil retention capacity

The adoption of a technique such as TGA-DSC could alleviate the problem of soil decomposition and provide an accurate description of soil composition by comparing the different temperature intervals of the soil. The simultaneous (TGA-DSC) measurements for both the fine and complete fractions of the sampled soils are presented in Figure 5. The observed effects of TGA variation include three intervals; the first ranges from 0 to 105 °C and represents the loss of interstitial water from the samples intra-pores, the second

quickly affected by higher nitrogen applications.

*3.3. Soil Thermal Characterization* 

through various physical and chemical characterizations.

the soils started to progressively lose their ability to adsorb nitrate.

curve increased slightly from day three to day eight for an increasing loaded concentration ranging from 30.85 to 71.98 mg/L. These results show that the maximum adsorption capacity of the soils was reached between days three and eight. Meanwhile, a rapid increase in NO<sup>3</sup> − leaching was observed in the three columns on day nine, potentially indicating that the soils started to progressively lose their ability to adsorb nitrate.

In general, the nitrate leaching in columns C4, C5, and C6 seems to increase with respect to the applied doses of nitrate. However, experiment 2 demonstrated that after removing nitrogen excess from the raw soils, it appears that nitrate leaching decreases in the three columns. This indicates that R'mel soils have a low adsorption capacity which is quickly affected by higher nitrogen applications.

By comparing the collected leachate with the measured added concentrations in experiment 2, it is evident that, contrary to experiment 1, the NO<sup>3</sup> − leaching did not exceed the applied NO<sup>3</sup> − concentration. This suggests that the elevated applied N fertilizer affects the soil's adsorption capacity in the R'mel area. In this context, the following sections will be dedicated to discussing the main factors affecting the R'mel soil retention capacity through various physical and chemical characterizations.
