## 2 Materials and Methods
### Analytical Procedures  
The air-dried soil subsamples were used for the determination of grain size composition; pH; total contents of Organic Carbon (TOC), Nitrogen (TN), and Sulfur (TS); NH\({}_{4}\)\({}^{+}\); NO\({}_{3}\)\({}^{-}\) available P; cations exchange capacity (CEC); electrical conductivity (EC); exchangeable and soluble cations. The pH (in water; 1:2.5 solid:liquid ratio) was measured with a glass electrode calibrated using pH 4.0 and 7.0 standards. EC was measured with a conductivity meter (Crison Basic 30), whereas the soluble cations  
Figure 2: Studied plant species and soil sampling in the hypersaline tidal flat (HTF) of the Pacoti estuary.  
(Na\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\), K\({}^{+}\)) were determined by atomic absorption spectrophotometry (AAS; Perkin Elmer 1100B) in soil solution obtained by centrifugation (3.500 RPM during 15 min) immediately after soil sampling. TOC, TN, and TS were determined with a LECO CNS-2000 auto-analyzer (LECO Corp., St. Joseph, MI, USA).  
The grain size distribution was determined by using a combination of physical (overnight shaking) and chemical (0.015 M (NaPO\({}_{3}\))\({}_{6}\) + 1.0 M NaOH) dispersal methods [41]. To determinate exchangeable cations, the air-dried samples were washed with 60% (v/v) aqueous ethanol to remove soluble salts until the silver nitrate test (AgNO3 0.05N) indicated the absence of chloride before chemical and physical analyses [42]. Exchangeable K\({}^{+}\) and Na\({}^{+}\) were extracted with HCl 0.05 M and determined by flame photometry, whereas exchangeable Ca\({}^{2+}\) and Mg\({}^{2+}\) were extracted with KCl 1 M (1:5 soil/solution) and determined by AAS [43]. Cation exchange capacity (CEC) was calculated as the sum of exchangeable cations (K\({}^{+}\), Na\({}^{+}\), Ca\({}^{2+}\), and Mg\({}^{2+}\)) [42]. The equivalent sodium percentage (ESP) was calculated as the percentage of exchangeable sodium divided by the cation exchange capacity. The calcium carbonate equivalent (CCE) was obtained by the AOAC method [44]. Available P was extracted with Mehlich-3 [45], whereas NH\({}_{4}\)\({}^{+}\) and NO\({}_{3}\)\({}^{-}\) were extracted with KCl 2 M and determined by colorimetry [46].  
Additionally, a partitioning of the solid-phase Fe was performed using fresh samples based on a combination of methods [47; 48], which enables the identification of six operationally distinct fractions, defined as follow:  
1. Exchangeable and soluble Fe, extracted with MgCl\({}_{2}\) 1M (pH 7; 30 min shaking);
2. Iron associated with carbonate extracted with NaOAc 1M (pH 5; 5 h shaking);
3. Iron associated with poorly crystalline Fe (ferrihydrite) extracted with hydroxylamine 0.04 M + acetic acid 25% (6 h shaking at 30 \({}^{\circ}\)C);
4. Iron associated with poorly crystalline Fe (lepidocrocite) extracted with hydroxylamine 0.04 M + acetic acid 25% (6 h shaking at 96 \({}^{\circ}\)C);
5. Iron associated with crystalline oxyhydroxides extracted with sodium citrate 0.25 M + 0.25 M + sodium bicarbonate 0.11M + Sodium dithionite (30 min shaking at 75 \({}^{\circ}\)C);
6. Iron associated with pyrite extracted with HNO\({}_{3}\) (2 h shaking). This fraction was extracted following two pre-treatments (10 M HF and concentrated H\({}_{2}\)SO\({}_{4}\)), for the removal of Fe associated with silicate and organic matter.  
The degree of iron prioritization (DOP), calculated as DOP (%) = (F6 \(\times\) 100)/[(\(\sum\)F1\(\rightarrow\)F6)]. The DOP determines the percentage of Fe which is combined into the pyrite fraction, allowing the comparison of pyrite content in soils with different reactive iron contents [49].