## 4 Discussion
### Plant Effects on Biogeochemical Soil Properties in HTF  
Our results demonstrated distinct edaphic processes ruled by the three dominant plant species in the studied HTF (Bm, Bp, and Sv), indicating changes in both bulk and rhizosphere soils. The first noteworthy rhizospheric process is the increase in nutrient retention capacity, which was also observed in other studies [56]. Another one is a clear decrease in pH within the rhizosphere, as a physicochemical process occurring at the soil-root interface [38; 57; 58]. The decrease in pH for the rhizopheric soils (when compared to bulk soils) is primarily promoted by the root's release of H\({}^{+}\), root exudation of organic acids, respiration, redox-coupled reactions, and by environmental constraints [57]. Indeed, Bm and Sv are plants capable of growing within a large range of pH (from 5.2 to 8.8), due to their ability to alter in-situ soil conditions [36]. The organic matter incorporation and nutrient bioavailability at the rhizosphere [3; 51] is another remarkable process observed in our study (Figure 3). These processes were specific to each plant species and reflected on the TOC and NH\({}_{4}\)\({}^{+}\) differences in rhizosphere when compared to bulk soils. The strong Bm-TOC association (Figure 6A) suggests that Bm is the plant contributing the most to build up soil organic matter contents, compared to Sv and Bp. Halophytes plants are capable of using inorganic nitrogen forms (NH4\({}^{+}\) and NO3\({}^{-}\)) to overcome highly salt environments, as typically observed by the greater contents of NH4\({}^{+}\) and NO3\({}^{-}\) in rhizospheric soils [59]. The NH4\({}^{+}\) contents in our study for the rhizospheric soils were up to three times greater than the bulk ones, while NO3\({}^{-}\) did not present significant differences between areas (Figure 3). Moreover, the NH4\({}^{+}\) content in the studied plants was related to rhizosphere activity, due organic matter incorporation, corresponding to Bm soils, which contained the highest values of TOC and NH4\({}^{+}\) among species in both seasons (Figure 3). Plants may favor their growth conditions in a hypersaline habitat by altering soil conditions, and thus increasing the pool of NH4\({}^{+}\) and other nutrients [56]. The topmost benefit of using NH4\({}^{+}\) rather than NO3\({}^{-}\) is the decrease in water consumption by plants in this water-stressful hypersaline environment [60; 61]. Clonal plants supplied with NH4\({}^{+}\) were able to reduce water consumption, buffering the negative effect of water deprivation and causing an increase in plant performance [62]. Thus, the selective use of NH4\({}^{+}\), instead of NO3\({}^{-}\), by the studied species can ameliorate water use-efficiency and mitigate the negative effects of hypersaline conditions. Together with plant activity and exudates, microbial communities are also greatly important for the biogeochemical cycling of N and other elements in humid coastal areas [21; 26; 63]. Microbial communities in HTF are predominantly halotolerant to halophilic microorganisms that may vary along the tidal flat in biodiversity, structure, and richness, as well as in association with different plants [64]. Indeed, investigations regarding microbial functioning and biodiversity in saline and hypersaline soils are rare [65]. The major microbial N pathways in humid coastal areas include nitrification and denitrification. Nitrification refers to the oxidation of ammonium (NH4\({}^{+}\)) to nitrite (NO2\({}^{-}\)) and nitrate (NO3\({}^{-}\)) under aerobic conditions, carried out by chemoautotrophs that uses NH4\({}^{+}\) as energy source [66; 67]. Denitrification comprises the reduction of NO2\({}^{-}\) and NO3\({}^{-}\) to gaseous N forms (such as N2O), which is mediated by heterotrophic bacteria under anaerobic conditions [68]. Some microorganisms also provide important ecosystem services to coastal hypersaline areas by actively fixing nitrogen [69].