Soil microbial characteristics under long-term heavy metal stress: A case study in Zhangshi Wastewater Irrigation Area, Shenyang

Soil samples were collected from Zhangshi Wastewater Irrigation Area in the suburb of Shenyang City, China, an area with a 30-year irrigation history with heavy metal-containing wastewater. The chemical properties and microbial characteristics of the soils were examined to evaluate the present situation of heavy metal pollution and to assess the soil microbial characteristics under long-term heavy metal stress. In light of the National Environmental Quality Standards of China, the soil in the test area was heavily polluted by Cd and to a lesser degree by Zn and Cu, even though wastewater irrigation ceased in 1993. Soil metabolic quotient (qCO(2)) had a significant positive correlation, while soil microbial quotient (qM) had a negative correlation with content of soil heavy metals. Soil microbial biomass carbon (MBC) had significantly negative correlation with Cd, but soil substrate-induced respiration (SIR), dehydrogenase activity (DHA), cellulase activity, and culturable microbial populations had no persistent correlations with soil heavy metal content. Soil nutrients, except for phosphorous, showed positive effects on soil microbial characteristics, which to a certain degree obscured the adverse effects of soil heavy metals. Soil Cd contributed more to the soil microbial characteristics, but qM and qCO(2) were more sensitive and showed persistent responses to heavy metals stress. It could be concluded that qM and qCO(2) can be used as bioindicators of heavy metal pollution in soils.