Effects of long-term heavy metals stress on farmland soil microbial population, biomass and activity

An in situ investigation on the farmland soil heavy metals pollution caused by long-term irrigation with heavy metals containing wastewater was carried out in the Zhangshi Irrigation Area of Shenyang. The indices soil microbial population, biomass, and activity were used to evaluate the effects of long-term heavy metals pollution on farmland soil ecosystem. The results showed that in Zhangshi Irrigation Area, soils were heavily polluted by cadmium, with the cadmium content ranged from 1.75 to 3.89 mg · kg^-1, and parts of them were co-contaminated by cadmium, copper and zinc. At the present pollution level, the increased soil heavy metals content resulted in a substantial decrease in soil free-living nitrogen fixing bacteria, microbial biomass carbon (C_mic), microbial quotient (qM) and dehydrogenase activity, and a significant increase of metabolic quotient (qCO₂,). No significant changes were observed in soil bacteria, actinomyces, fungi, and substrate-induced respiration (SIR). Correlation analysis showed that the changes of soil microbial parameters were mainly caused by soil cadmium pollution. Comparing with other test microbial parameters, microbial quotient and metabolic quotient were more sensitive to soil heavy metals pollution.