Effects Of Lead And Bensulfuron-methyl/Metsulfuron-methyl Complex Pollution On Soil Microbial Ecology

In the soil and water environment, it is common that the pollution is caused by many pollutants rather than by a single one. Because it is not realistic to estimate the environmental carrying capacity or set the environmental assessment standard based on a single pollutant, studies on the complex pollution have become popular and attracted many researchers internationally. It is theoretically significant to study soil complex pollution in order to understand the physical, chemical, and biological fate and/or ecological effects of various pollutants in soils under complex conditions. The studies are also theoretically and practically important for improving soil and water quality, guaranteeing food safety of agricultural products, and making feasible environmental standards. Three paddy soils, Quaternary red clay soil, Blue clayed soil and Desalting muddy polder soil, were used to study decomposition and residual characteristics of metsulfuron-methyl (MSM) under lead (Pb) stress in this study. A commonly used herbicide in paddy fields, bensulfuron-methyl (BSM), was also used as an organic pollutant to study the ecological effects of Pb-BSM on soil microbes and the effects on changes of bio-chemical processes. Interaction effects between Pb and BSM on soil quality and the significance were explored, too. Major results are reported as follows:(1) Decomposition and residual characteristics of ¹⁴C-MSM in 3 different soils were systematically studied under controlled conditions. The results showed that the first order kinetics equation fitted well (r>0.923) to decomposition of the extractable ¹⁴C-MSM parent compound in soils. The decomposition of MSM in soils was inhibited by Pb, and the decomposition half life of extractable MSM parent compound was prolonged for 1 to 5 days. However, the inhibition was not the strongest with the highest Pb (1000 mg kg^-1) treatment. The effect of Pb on ¹⁴C-MSM mineralization differed with soil properties. The inhibition effect on mineralization of low concentration ¹⁴C-MSM (1 mg kg^-1) treatment increased with concentration of Pb in the Blue clayed soil and Quaternary red clay soil. On the contrary, mineralization of high concentration ¹⁴C-MSM (10 mg kg^-1) treatment was stimulated by concentration of Pb in the soils. In the Desalting muddy polder soil, mineralization of low concentration ¹⁴C-MSM (1 mg kg^-1) treatment was stimulated by Pb, whereas mineralization of high concentration ¹⁴C-MSM (10 mg kg^-1) treatment was inhibited. The bound residual rate of ¹⁴C-MSM parent compound and its decomposition derivatives increased quickly with time at the beginning, and then decreased slowly. The bound residual rate of low concentration treatment of ¹⁴C-MSM residues decreased to some extent with Pb treatments, whereas the effect of Pb on the bound residual rate of high concentration treatment of ¹⁴C-MSM residueswas not obvious.(2) The response of soil microbial carbon to Pb-BSM complex pollution was explored and characterized by biological indicators such as soil microbial biomass, microbial metabolism entropy, and soil basic respiration. Results showed that the soil microbial carbon at day 7 decreased by 1-65% and 1-71% under Pb and BSM treatment, respectively. Generally, the inhibition effect increased with concentrations of the pollutants, and this was especially obvious in the soil of Quaternary red clay soil. The soil microbial carbon under Pb or BSM treatment decreased with time at the beginning of incubation, being lowest during day 7 to 30. It was then increased and stabilized afterwards. Similar results were found in the complex treatments of Pb-BSM, and the interaction effect of the pollutants was significant. Generally, the combined toxic effect of Pb and BSM on microbial carbon was smaller compared with the simple effect of Pb plus BSM under the corresponding concentration. Therefore, the interaction decreased the effect of Pb and BSM on microbial carbon, indicating an antagonistic effect. Soil microbial metabolism entropy increased with Pb or BSM concentrations during initial incubation period. This was also true for the Pb-BSM complex treatment. These results indicated that soil microbes were stressed by the pollutants. Afterwards, soil microbial metabolism entropy recovered gradually to the level of the control as toxicity of the pollutants reduced and microbial tolerance increased with time. Because the change of metabolism entropy was more sensitive than that of soil respiration, metabolism entropy should be a good indicator for assessing the biological effect in polluted soils.(3) The change of soil enzyme activity can reflect, to some extent, the single or complex pollution effect. Our results showed that the activity of soil hydrogen peroxidase, acid phosphatase and urease was stimulated by low Pb concentration, while high Pb concentration inhibited the enzymes. During initial incubation period, BSM inhibited the activity of soil hydrogen peroxidase, acid phosphatase and urease. The inhibition to the enzyme activity was alleviated during later incubation period, or the enzyme activity was slightly stimulated. The interaction types between Pb and BSM and the action intensity of Pb-BSM complex pollution were closely related to the retention time and concentrations of the pollutants and to soil properties. Therefore, it was still difficult to use soil enzyme activity for quantitatively assessing Pb-BSM complex pollution.(4) The characteristics of the soil microbial community structure change and its major affecting factors under the Pb-BSM complex pollution condition was investigated with the method of PLFA profile. Results showed that the soil microbial community structure changed obviously under Pb, BSM, or Pb-BSM complex pollution conditions. Soil microbial community structure changes could mainly be attributed to the changes of soil G⁺ bacteria (i15:0, a15:0 and i17:0), G^- bacteria (cy17:0 and 18:1ω7c), fungi (16:1ω5c and 18:2ω6,9c), and actinomyce (10Me16:0, 10Me17:0 and 10Me18:0). Pollutant concentrations were the major factors influencing soil microbial community structure changes in Blue clayed soil. No matter it was single pollutant or combined pollutants, the concentrations could be classified into two types, high concentration type and low concentration type. The concentration effect dominated the property effect. However, BSM was the major factor influencing soil microbial community structure changes in Quaternary red clay soil and Desalting muddy polder soil. The effect of BSM dominated the effect of Pb, and this was especially obvious in the Desalting muddy polder soil.(5) Decomposition characteristics of exogenous ¹⁴C-glucose in Pb-BSM complex polluted soils was investigated using orthogonal rotatable central composite method for experimental design. Results indicated that Pb and BSM significantly (p<0.05) affected ¹⁴C-glucose mineralization. The effect of Pb was greater than that of BSM. The pollutants inhibited ¹⁴C-glucose mineralization during the initial period of incubation. Antagonistic interaction between Pb and BSM was found in the period, and the interaction enhanced with pollutant concentrations. The antagonistic interaction was then changed into synergistic interaction to stimulate ¹⁴C-glucose mineralization afterwards. Concentrations were the important factors influencing the interactions between the pollutants.