Effects of Cd~(2+) on the Biological Activities and the Enzyme Activities in Submerged Paddy Soil

As the important biological parameter, the biochemical activities are important indexes of heavy metal environmental safety assessment. The influence of Cd2+ on the dynamics of the biological activities and the enzyme activities in submerged red paddy soil in Jiangxi province was studied using traditional incubation method under laboratory conditions. The soil respiration was promoted by Cd2+ at lower concentrations, but was inhibited at higher concentrations. The effects of different concentrations of Cd2+ on other soil biochemical process in soils were also different significantly: the lower concentration of cadmium was helpful to increase most of the biochemical processes, but the higher concentrations of Cd2+ greatly inhibited microbial activities; the higher the concentration of cadmium applied, the more significant the inhibition to the observed activities. Compared with other aerobic biochemical activities, the methane emission activity was most sensitive to Cd2+ , followed by the sulfate reduction activity. The inhibition of Cd2+ for the denitrification activity was slightly less and very stable during the whole treated stage, and could be as an indicator of the Cd2+ pollution degree in paddy soil. The sensitive order to cadmium of four soil enzyme activities was: ureasecatalasephosphatasesinvertase. Cadium inhibited urease activities most with the maximum inhibition rate of 57.4% and had less influence on other soil enzyme activities with the maximum inhibition rates less than 25% . There were significant negative correlations between the cadmium concentration and microbial respiration, the sulfate reduction activity, the methane production activity, urease activity, and catalase activity, which suggested that these indexes might indicate the contamination degree of paddy soil by Cd2+ . According to these microbial indexes, about 1～ 5mg· kg- 1 was considered the critical level of cadmium in submerged paddy soil.