Seasonal variations in phytodesalination capacity of two perennial halophytes in their natural biotope

In Soliman sabkha (NE Tunisia), Tecticornia indica and Suaeda fruticosa tufts were divided into three size classes (small, medium, and big) in which the shoot sodium and potassium contents were determined in July 2007 and February 2008. Shoot dry weights per tuft and per hectare were estimated. Soil samples (20 upper centimeters) were taken from inside and outside the halophyte tufts and analyzed for electrical conductivity (EC1/10) and soluble sodium content. We found that these two parameters were significantly lower in the soil from inside the tufts than in the surrounding soil. This effect was more pronounced in winter (February 2008) when EC1/10 inside halophyte tufts was 63 to 72% lower than outside. In July 2007, the relevant reduction of EC1/10 inside the tufts was 55%. Soluble sodium content was reduced by 70.5% in winter and only 31 to 37% in summer. The ecosystem productivity was about 8.6 tonnes dry weight per hectare (t DW ha-1) with higher sodium than potassium contents (about 0.646 t Na+ ha-1 in summer and 0.752 tonnes Na+ ha-1 in winter). Regardless of the season, T. indica exhibited much higher phytodesalination capacity (77.7-94.4% of the whole shoot-removed sodium). For S. fruticosa, the decrease in soil salinity was due to roots that released sodium ions from the exchange sites and facilitated their leaching to the deeper horizons. From an ecological point of view, phytodesalination and sodium leaching enhancement are interesting processes since they provide glycophytes with a microhabitat suitable for their development, which maintains the biodiversity within the saline ecosystem.