Soil symbionts in alleviation of heavy metals-laden sewage sludge-impacts in legumes: implications for phytoremediation

Abstract Increasing urbanization and industrialization are releasing a considerable proportion of solid wastes into the environment globally. A large number of addition of solid wastes including sewage sludge contributes to the agricultural soils with organic matter and several nutrients. However, sewage sludge has been the major source of large amounts of heavy metals in agricultural soils. Therefore, the impacts of the heavy metals-laden sewage sludge on the health of agricultural soil and plant/crops are obvious. Notably, soil microorganisms particularly arbuscular mycorrhizal (AM) fungi and root-nodule bacterium Rhizobium are two root symbionts widely reported to alleviate the impact of heavy metal-laden sewage sludge and promote plant growth and productivity. Integration of AM fungi with Rhizobium has been found promising especially in leguminous crops, where the net influence of this combination is supposed to be very high. This paper aims to: (i) briefly introduce the representative leguminous crops chickpea ( Cicer arietinum L.) and lentil ( Lens culinaris Medik.); (ii) overview the microbial symbiosis in legumes; (iii) highlight the major sources of the heavy metal-contamination; (iv) enlighten major constituents of sewage sludge, status of heavy metals in sewage sludge, and the main impacts of sewage sludge on the health of both soil and plants; (v) discuss the major insights into the interaction of microbes with heavy metals, with implications of the outcomes for the phytoremediation; and also to (vi) mention the major aspects, to date least explored in the current context.