Plant exposure to cadmium in Moroccan calcareous salty soils treated with sewage sludges and wastewaters

The aim of this study was to determine cadmium availability in soils collec ted from the future sewage spreading field in Marrakesh city (Morocco). The se agricultural soils are characterised by significant salinity due to the proximity of the Tensift river. The investigation was conducted under arid climatic conditions using a pot procedure. The effect of various cadmium or igins (raw wastewaters (EUB), treated wastewaters (EUT), sludge (B) and Cd( NO3)(2) on the plant availability according to a salinity gradient, was det ermined, using lettuce (Lactuca sativa L var pommee d'ete) and rye-grass (L olium perenne L var Maprima). The dry weight yield of plant leaves and root s produced on salt soil treated with B, EUB and EUT was relatively stable. However, it decreased by about 50 % with the addition of Cd(NO3)(2) and Cd( NO3)(2)+EUB except for rye-grass. The lettuce cultivated on salty soils sho wed a yield decrease of 38.9 to 79 % and 38 to 69.6 A (leaves) respectively in proportion to non and little salted soils. Addition of cadmium salt to the soils induced a more important bioaccumulation than for other forms. Th e Cd(NO3)(2) supply was taken up much more than the sludge-borne cadmium. T his tendency increases further after culture irrigation with EUB (Cd(NO3)(2 ) + EUB, treatment). The concentrations ranged between 3.15 to 8.23; 2.16 t o 2.88 and 3.83 to 8.88 mg Cd kg(-1) in the lettuce leaves for the treatmen ts and from 2.49 to 7.83; 1.54 to 2 and 3.09 to 8.61 mg Cd kg(-1)in its roo ts, according to the gradient of salinity. Furthermore, the concentration o f Cd accumulated in plants on the sludge-treated soils could be due to incr eased growth of plant resulting from the fertilising power of sludge. Incre ase in plant metal uptake for all treatments coincided with increase in soi l salinity. Thus, the highest concentrations recorded in leaves increased a pproximately two fold with an increase of about 1.6 times of the salt conce ntration estimated in terms of soil chloride (Cl) amounts. A positive and s ignificant correlation was found between the cadmium concentration in the t est plants and the amounts of chlorides in the soils (threshold 0.1 %). For cadmium uptake by plants, the extraction of cadmium by CaCl2 (0.1 M) was m ore important in soils supplied with cadmium salt. Likewise the extracting quantities were substantially higher in soils with elevated chlorides conce ntrations. It was in the order of 1.2 compared with 0.7 mg exchangeable Cd kg(-1) in the non-salted soils. These data suggest that the possible format ion of CdClnn-2 compounds can facilitate the plant exposure to cadmium and so its accumulation into edible plant parts.