Evaluation of different agroforestry tree species for their suitability in the phytoremediation of seleniferous soils

Seven agroforestry tree species were grown in a clay loam soil treated with different levels of selenate-Se, viz. 0, 1.25, 2.5 and 5.0 mg/kg supplied through sodium selenate. After 1 year of growth, a progressive decrease in dry matter of leaves, stem and roots was observed with increasing levels of applied Se. However, a significant decrease in dry matter yield was observed only at or above 2.5 mg Se per kg soil and shisham (Dalbergia sissoo) proved to be highly sensitive to the presence of selenate-Se in the soil. On average, the largest above-ground and below-ground biomass was accumulated by arjun (Terminalia arjuna) and the lowest by the acacia tree (Acacia tortillas). The selenium content of leaves, stem and roots of all the tree species increased significantly with increasing levels of applied Se, although a large variation within species was observed. In the stem portion of different trees, the highest concentration of Se was found in dek (Melia azedarach) (5.1 mg/kg) and the lowest in mulberry (Morus alba) (2.6 mg/kg). The efficiency of selenium removal (including leaves, stem and roots) was the highest in arjun followed by eucalyptus (Eucalyptus hybrid) - Clone 10, mulberry, jambolin (Syzygium cumini), dek, shisham and acacia. Effective removal of Se takes place through the stem portion of different trees where it constitutes 30-50% of total Se. Large variation in Se uptake by different tree species suggests that trees vary in their potential for phytoremediation of seleniferous soils. In one growing season, shisham aged 24 years, poplar (Populus deltoides)- Clone G 48 (10 years old) and eucalyptus - Clone 10 (10 years old) could remove 2385, 1845 and 1407 g Se per hectare respectively. Corresponding reductions in Se capital of the soil varied between 24 and 37, 19 and 29 and 14 and 32%, respectively, in the surface layer (0-15 cm) alone or 7-11, 6-9 and 4-7% for the whole soil profile (0-120 cm). Removal further increased to 4207 g Se per hectare under an agroforestry farming system of poplar-mentha/wheat with Se being reduced from 43 to 65% for the surface layer and from 13 to 20% for the whole profile.