Impact of Cr3+ pollution on microbial characteristics in purple paddy soil

Impact of Cr(3+) pollution on soil microbial quantity, enzyme activity and biological activity in purple paddy soil were studied under incubation conditions. The results showed that amounts of all tested microbes and enzyme activities in soil were inhibited by low Cr(3+) concentration (200mg/kg). After 7-day incubation, sulfate-reducing activity, methanogen activity, denitrifying activity and anaerobic nitrogen-fixing activity in soil were reduced by 34%, 66%, 98% and 65% respectively. Amounts of soil microbes were remarkably inhibited with medium Cr(3+) concentration (400mg/kg), all with reduction of more than 50%; and all tested soil biological activity was almost recovered in the fourth week except soil denitrifying activity. Activities of urease, invertase, neutral phosphatase and catalase were decreased by 60%, 21%, 59% and 42%, respectively. With high Cr(3+) concentration (1600mg/kg), amounts and activities of tested microbes had only about 1% of that with control. As calculated from the regression equation, the ED50 (ecological dose) values of activities of soil urease, invertase and catalase were around 800mg/kg; the ED50 values of soil sulfate-reducing activity, methanogen activity and anaerobic nitrogen-fixing activity were also around 800mg/kg with an exception of soil denitrifying activity which ranged 35 to 39 mg/kg. According to the Standards of National Soil Environmental Quality in China and their sensitivities to 400mg/kg Cr(3+) concentration, quantity of denitrifying bacteria, urease activity and denitrifying activity could be selected as indicators of early warning for Cr(3+) pollution in purple paddy soil.