| Arsenic(As) is one of the five soil pollution elements in soil with strong toxicity. Arsenic changes its speciation and valence with soil moisture conditions, which influence the bioavailability and environmental risk of arsenic. Soil enzymes involve in all biochemical reactions, promote the transformation of nutrients, energy metabolism and pollutants purification, which to some extent could be a good bioindicator for the extent of soil arsenic pollution. At present, researchers have carried out a large number of studies on soil enzyme activities under arsenic pollution. However, these researches were mainly focused on the effect of long-term arsenic contamination and limited on a single factor like arsenic concentration, but ignored the effect of environmental factors, including soil moisture, on arsenic toxicity and soil enzyme activities. So it is necessary to investigate the effect of short-term arsenic contamination on soil enzyme activities under different soil moisture conditions.In this study, we investigated the correlation between available arsenic and soil FDA hydrolytic, dehydrogenase, alkaline phosphatase and arylsulfatase activities at three soil moistures 35%, 65% and 110% maximum water holding capacity(WHC).We also explored the effects of soil moisture on arsenic inhibition mechanism of alkaline phosphatase activity by enzyme kinetics method. The main results were as follows:1. The adsorption amount of arsenic increased and absorption rate decreased in soil with increasing of arsenic concentrations. Arsenic desorption amount and desorption rate increased with the amount of arsenic adsorption. Langmuir model and Freundlich model could be used to describe the arsenic adsorption characteristics on tested soils. The maximum arsenic adsorption amount of paddy soil in Jiangsu and lou soil in Shanxi were 658.88、514.11 mg·kg-1, respectively.2. Soil moisture condition, total arsenic concentration and soil type were factors affecting available arsenic in soil. Available arsenic concentration was declined with aging which can be described using Elovich equation and converted into other forms rapidly as increased with soil moisture content.3. Arsenic pollution activated soil FDA hydrolase and dehydrogenase activity in the beginning(1~15d), and reduced the activated degree or even inhibited enzymes activities later(15~30d). Flooding(110%) increased FDA hydrolase activity, but reduced dehydrogenase activity to a certain extent. Soil FDA hydrolase activity in paddy soil was significantly higher than in Lou soil.4. Arsenic significantly inhibited soil arylsulfatase activity, but the inhibition degree changed with soil types and moisture condition. Flooding significantly reduced soil arylsulfatase activity. Soil arylsulfatase activity in paddy soil was significantly higher than in Lou soil.5. Arsenic in soil significantly inhibited soil alkaline phosphatase activity. The model U=A/(1+B×C) was fitted well in the relationship between soil enzymatic activities(U) and arsenic concentration(C). Soil alkaline phosphatase activity could be a good bioindicator for the extent of soil arsenic pollution. Soil alkaline phosphatase activity was reduced in both drought and flooding. Alkaline phosphatase activity and the beginning inhibition concentration of arsenic in paddy soil were significantly greater than in Lou soil, indicated that paddy soil had strong buffering ability that Lou soil.6. Soil total enzyme index(TEI) summarized all soil enzymes information, and the result using TEI was consistent with the principal component analysis(PCA). Arsenic in soil significantly decreased the value of soil total enzyme index(TEI), and the model TEI=A/(1+B×C) could be used to fit the relationship between arsenic concentration and TEI. The value of TEI in Lou soil changed with soil moistures as 110% WHC<35% WHC <65% WHC, while in paddy soil as 65% WHC<110% WHC <35% WHC. The value of TEI in paddy soil was significantly greater than in Lou soil. Based on the analysis above, ecological dose10%(ED10) of total and available arsenic concentration were 37 mg·kg-1 and 15 mg·kg-1 in Lou soil, respectively; while 66 mg·kg-1 and 11 mg·kg-1 in paddy soil, respectively.7. Enzyme kinetics method was used to illuminate the inhibitory mechanisms of arsenic to alkaline phosphatase. The results showed that arsenic decreased Michaelis constant(Km), the maximum reaction rate(Vmax), the initial velocity of enzymatic reaction(Vmax/Km), and reaction rate constant(k), which indicated arsenic inhibited the enzymatic reactions essentially. The high soil moisture increased substrate affinity(Km), while weakened the speed of enzymatic reaction(Vmax) and inhibited the enzymatic reactions essentially. The model Y=A/(1+B×C) and Y=A+B×C were fitted well in the relationship between kinetic parameters(Y) and arsenic concentration(C). ED10 of arsenic concentration using kinetic parameters was 20.52 mg·kg-1 in paddy soil. Soil moisture conditions affected inhibition mechanism of arsenic on alkaline phosphatase. The inhibition mechanism was complete-competitive inhibition under 35%WHC and 65%WHC, while complete-linear mixed and mainly completely non-competitive inhibition under 110%WHC.The main conclusions are summarized based on the analysis above: soil moisture condition, total arsenic concentration and soil type were main factors affecting available arsenic in soil. Flooding(110%) increased soil FDA hydrolase and dehydrogenase activity, while decreased arylsulfatase and alkaline phosphatase activity. Soil alkaline phosphatase and kinetic parameter(Vmax/Km) could be good bioindicators for the extent of soil arsenic pollution... |
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