Soil salinity and sodicity in the Camargue (Rhône river delta, France) are strongly controlled by elevation, land use, soil depth

Soil salinization threatens agricultural production and wetland functioning in coastal deltas. This threat is expected to intensify with climate change because increasing evapotranspiration, decreasing fresh water supply from rivers, and sea-level rise will expand salt influence into low-lying areas. In such settings, shallow brackish groundwater, evapotranspiration, and land-use–specific hydrology interacts across subtle topographic gradients, with yet unconstrained consequences for both salinity levels and sodicity risk. In this study, we quantified the combined effects of elevation, land use, and soil depth in soils of the Camargue (southern France), a multifunctional delta dominated by paddy rice, dry agriculture (e.g., wheat, clover) and pastureland.

At three elevation classes (low = 0.2–0.6 m a.s.l.; mid = 0.6–1.0 m a.s.l.; high = 1.0–1.4 m a.s.l.), we collected 362 soil cores by manual drilling (using a 1-m auger), which were subdivided into five 20-cm soil-depth increments (0–20, 20–40, 40–60, 60–80, 80–100 cm). We used 1:5 soil:water extracts to measure electrical conductivity (EC) and a targeted ion suite [mg L⁻¹; meq L⁻¹]. We derived dissolved salts (DS = sum of quantified ions), Na-dominance-ratio (Na⁺/√[(Ca²⁺+Mg²⁺)/2]), and a Na⁺–Cl⁻ imbalance metric (ΔNa⁺ = Na⁺ − Cl⁻ [meq L⁻¹]) to distinguish Na⁺–Cl⁻ dominance from Na⁺ enrichment decoupled from Cl⁻.

EC and DS generally increased towards the lower elevations and with soil depth, indicating salt accumulation where drainage is constrained and groundwater influence is strongest. These elevation trends were most pronounced in soils under paddy rice and pastureland (rice median EC = 0.44–0.97 mS cm⁻¹; DS = 306–642 mg L⁻¹; pasture median EC = 0.90–1.97 mS cm⁻¹; DS = 502–942 mg L⁻¹). Soils under dry agriculture showed a different pattern (EC = 0.27–0.49 mS cm⁻¹; DS = 236–314 mg L⁻¹) toward lower elevations. Ion composition was dominated by Na⁺ (20%) > Cl⁻ (18%) > K⁺ (17.7%) > SO₄²⁻ (16%) > NO₃⁻ (10.6%) > Ca²⁺ (4.7%) > Mg²⁺ (0.6%). ΔNa was predominantly positive, especially in soils under paddy rice, coinciding with elevated Na-dominance-ratio (3.4–12.7), indicating widespread Na⁺ excess relative to Cl⁻ and suggesting potential sodicity risk. Negative ΔNa⁺ values occurred mainly in some pasturelands (−0.15 to −8.7 meq L⁻¹), consistent with Cl⁻-dominant inputs (e.g., sea salts, fertilizers).

Projected increases in evapotranspiration and sea-level rise under global warming are likely to reduce arable land availability in the Camargue, suggesting a heightened vulnerability to combined salinity–sodicity pressures. Specifically, to maintain rice cultivation along with all its cultural heritage for the people in the Camargue, a sustained effort for freshwater irrigation and effective drainage needs to be prioritized.