Response of Robinia pseudoacacia L. rhizosphere microenvironment to Cd and Pb contamination and elevated temperature

Elevated air temperature and contamination of soil with heavy metals co-occur in natural ecosystems and have important effects on the soil microenvironment by influencing plant physiology. Effects of elevated temperature and cadmium (Cd) pollution on the rhizosphere microenvironment have been examined for wheat seedlings, but rhizosphere responses to elevated temperature might differ among plant species and for other heavy metals. This study examined the response of the black locust rhizosphere microenvironment to slightly elevated temperature in combination with Cd and lead (Pb) contamination. Elevated temperature combined with Cd and Pb exposure corresponded to an increase in soluble sugars, free amino acids, and organic acids and to a decrease in soluble phenolic acids in rhizosphere soils. These combined variables also corresponded to a decrease in microbial abundance, microbial biomass carbon, fluorescein diacetate hydrolysis, and the ratio of microbial biomass carbon (C) to total organic C compared to elevated temperature alone or to individual metals. The combination of elevated temperature plus exposure to heavy metals had varying effects on the different enzymes relative to the individual stress factors. The removal rate of Cd and Pb in rhizosphere soils increased significantly under elevated temperature and was greater for Cd than for Pb. Generally, elevated temperature led to increased microbial activity in rhizosphere soil of black locust seedlings exposed to heavy metals by stimulating microbial abundance, microbial biomass C, and enzyme activity. Variability in plant species and heavy metals might lead to different responses of phenolic acids to elevated temperature + heavy metals. Increased removal of heavy metals indicated that elevated temperature might improve the phytoremediation efficiency of heavy metal-contaminated soils by black locust seedlings. A capsule abstract: Elevated temperature led to increased microbial activity in rhizosphere soil of black locust seedlings exposed to heavy metals. (C) 2016 Elsevier B.V. All rights reserved.