Edaphic characterization and soil ionic composition influencing plant zonation in a semiarid Mediterranean salt marsh

Soil characteristics and plant zonation were studied in a semiarid Mediterranean salt marsh in SE Spain. According to topographic sequences and plants distribution, two transects were established from the border of La Mata lagoon to the upland vegetation limit and soils were described and analysed. Regularly spaced plots were established in these transects in accordance with the stands of vegetation and surface soil samples were taken every 2 months for 2 years. The following edaphic factors were determined: soil moisture content, pH and concentration of Cl−, SO42−, Ca2+, Mg2+, Na+ and K+ in the saturation extract. In addition, the groundwater level was measured and the duration of the flooding periods established in each plot. Soil–plant relationships were studied by means of canonical correspondence analysis. Based on rainfall data for the study period, dry and wet seasons were separated and the habitats of the plant communities were compared for salt quantity and quality independently for each season. Soils were classified, according to FAO (1998), as Hypercalcic, Sodic and Mollic Solonchaks and Hypercalcic Sodic Calcisols. The most important variables which explained plant zonation were: the flooding period, total salinity, minimum Ca2+/Na+ ratio and the mean sodium adsorption ratio. When the habitats of the main plant communities were compared, differences in salt quantity, quality and seasonal variations were found. Two chenopod shrubs, Arthrocnemum macrostachyum and Sarcocornia fruticosa, predominated in the most saline areas. More pronounced seasonal variations in soil salinity were found in the A. macrostachyum zone than in the Sarcocornia fruticosa zone. The highest value for K+/Na+ and Ca2+/Na+ ratios were measured in the Suaeda vera stand. The Lygeum spartum zone was distinguished by the high Ca2+/Na+ and Ca2+/Mg2+ ratios in the wet period. Among the rushes, Schoenus nigricans predominated in the less saline areas, where the K+/Na+ ratio was higher and the Ca2+/Na+ ratio lower than where Juncus maritimus predominated. Limonium cossonianum communities occupied an intermediate position with respect to soil salinity, between the chenopod shrubs and the other communities. Our results suggest that salt marsh plant zonation is influenced by temporal and spatial edaphic gradients which must be jointly considered if soil–plant relationships in saline soils are to be fully understood.