## 3 Results and Discussions
### \(\text{NO}_{3}\text{${}^{-}$}\) and DOC Release
Graphene batches showed a slight increase of DOC in CS compared to the Control, but statistically relevant only for L/S = 1 and L/S = 5; while a significative increase of DOC respect to the Control was recorded in SS for all L/S ratios except for L/S = 5. The higher DOC in SS was probably due to the low amount of carbonates in this soil (Table 1), which often promote complexation and flocculation of organic substances [42]. Compost batches showed a significant increase of DOC both in CS and SS soils except for CS L/S = 5, due to the dissolution of humic and fulvic acids. Biochar batches showed a significative increase of DOC in CS at L/S = 5 and L/S = 10; while it showed significant differences in SS respect to the Control only for L/S = 2. Zeolites showed a slightly but not significant increase of DOC both in CS and SS, except for SS L/S = 1 and L/S = 2, which were affected by limited exchange among solid and liquid phases. The increase of DOC carried on by the application of the soil improvers represented a very important result. Especially in a scenario where the rise of global temperatures leads to a greater degradation of soil organic matter [43]. In addition, [PERSON] et al. [44] pointed out that the application of nanomaterials to soils corresponds to an increase in soil organic matter and microbial activity, this could lead to potential positive and negative impacts on the agricultural fields [45]. So, it is essential to investigate how the application of a new nanomaterial can impact the environment. In general, there are two different leaching control mechanism: (i) the release of a chemical species can be controlled by its availability; or (ii) the release can be controlled by the solubility of the chemical species and/or by sorption-desorption processes [16]. NO3\({}^{-}\) salts have elevated solubility, indeed NO3\({}^{-}\) release in all of the batches was controlled by NO3\({}^{-}\) availability since its concentration is strongly dependent on the L/S ratio. In fact, an increase in L/S ratio corresponds to a decrease in NO3\({}^{-}\) concentration for both CS and SS soils (Table 3) [16]. The only exception is the high concentration of NO3\({}^{-}\) for L/S = 2 compared to L/S = 1. This was possibly due to the fact that in L/S = 1 the solid-liquid contact was not optimal and NO3\({}^{-}\) cannot be easily transferred into the liquid phase. Despite the fact that there were no significant differences between NO3\({}^{-}\) concentration in L/S = 5 and L/S = 10 for all batches, the values in batches with L/S = 10 were always smaller than the ones for L/S = 5, highlighting that NO3\({}^{-}\) release was controlled by its availability and not by its solubility [16]. As regards DOC, its solubility also depends on the nature of the organic carbon contained in the different soil improvers. In fact, for example, it is possible to notice large differences of DOC among CS and SS Biochar batches. This is due to the very refractory nature of the biochar organic carbon, which gives it a low solubility and a great sensitivity even to small pH changes (pH in CS is higher than in SS due to CaCO3 dissolution) [42]. When the leaching process is limited by the solubility of the chemical species, the L/S ratio has no influence on the concentration. But in field conditions is rather common to find a mix of solubility and availability control mechanisms [16]. In general, DOC concentrations tend to increase with the increase of the L/S ratio for all of the batches, or as in the case of graphene there are no statistically significant differences in DOC values for the different L/S ratios. Likewise, the other soil improvers show non-significant differences for some L/S ratios in DOC concentrations. This means that DOC release was controlled by a mix between solubility and availability for Compost, Zeolites, and Biochar, since the increase of the liquid phase led to an increase in DOC solubility [15].