Short-Term Influence of Herbicide Quinclorac on Enzyme Activities in Flooded Paddy Soils

The influence of quinclorac (3,7-dichloroquinoline-8-carboxylic acid) on enzyme activities in flooded paddy soils was assessed under laboratory conditions. The enzymes differed markedly in their response to quinclorac. Quinclorac inhibited proteinase, hydrogen peroxidase, phosphorylase, and urease activities.The higher the concentration of quinclorac applied, the more significant the inhibition to these observed activities with a longer time required to recover to the level of the control. However, soils supplemented with quinclorac were nonpersistent for proteinase, phosphorylase and urease as opposed to soils without quinclorac. Dehydrogenase activity was also sensitive to quinclorac. Three soil samples with concentrations of quinclorac higher than 1 μg g-1 soil declined to less than 20% of that in the control. However, the highest dehydrogenase activity (up to 3.28-fold) was detected in soils with 2 μg g-1 soil quinclorac on the 25th day after treatment. Quinclorac had a relatively mild effect on saccharase activity at the concentrations used in this experiment and a stimulatory one on soil respiration when added to soil at normal field concentrations.Nonetheless it was inhibited at higher concentrations in paddy soils. Quinclorac is still relatively safe to the soil ecosystem when applied at a normal concentration (0.67 μg g-1 dried soil) but may have some effects on soil enzymes at higher concentrations.

Influences of Quinclorac on Culturable Microorganisms and Soil Respiration in Flooded Paddy Soil

Objective To investigate the potential effects of herbicide quinclorac (3,7-dichloro-8-quinoline-carboxylic) on the culturable microorganisms in flooded paddy soil. Methods Total soil aerobic bacteria, actinomycetes and fungi were counted by a 10-fold serial dilution plate technique. Numbers of anaerobic fermentative bacteria (AFB), denitrifying bacteria (DNB) and hydrogen-producing acetogenic bacteria (HPAB) were numerated by three-tube anaerobic most-probable-number (MPN) methods with anaerobic liquid enrichment media. The number of methanogenic bacteria (MB) and nitrogen-fixing bacteria (NFB) was determined by the rolling tube method in triplicate. Soil respiration was monitored by a 102G-type gas chromatography with a stainless steel column filled with GDX-104 and a thermal conductivity detector. Results Quinclorac concentration was an important factor affecting the populations of various culturable microorganisms. There were some significant differences in the aerobic heterotrophic bacteria. AFB and DNB between soils were supplemented with quinclorac and non-quinclorac at the early stage of incubation, but none of them was persistent. The number of fungi and DNB was increased in soil samples treated by lower than 1.33μg·g-1 dried soil, while the CFU of fungi and HPAB was inhibited in soil samples treated by higher than 1.33μg·g-1 dried soil. The population of actinomycete declined in negative proportion to the concentrations of quinclorac applied after 4 days. However, application of quinclorac greatly stimulated the growth of AFB and NFB. MB was more sensitive to quinclorac than the others, and the three soil samples with concentrations higher than 1 μg·g-1 dried soil declined significantly to less than 40% of that in the control, but the number of samples with lower concentrations of quinclorac was nearly equal to that in the control at the end of experiments. Conclusion Quinclorac is safe to the soil microorganisms when applied at normal concentrations (0.67μg·g-1).

Arsenic mobilization and nitrous oxide emission modulation by different nitrogen management strategies in a flooded ammonia-enriched paddy soil

Elevated arsenic(As)mobilization and increased nitrous oxide(N_(2)O)emission are two primary environmental concerns existing in flooded paddy soils.In this study,dissolved As(Ⅲ),N isotope-labeled Na^(15)NO_(3),and/or^(14)NH_(4)Cl were incorporated into a microcosm incubation to determine the effects of N fertilization regimes on As mobilization and N_(2)O emission in a flooded paddy soil.Because nitrate had a higher redox potential and comprised a higher proportion of N substrate,As(Ⅴ)was preferentially formed due to enhanced nitrate-dependent microbial As(Ⅲ)oxidation.Thus,As availability was correspondingly attenuated due to the improved production of less mobile and toxic As(Ⅴ).After 2-d incubation,more than 90%and 98%of soluble As(Ⅲ)were immobilized in the As+NH_(4)^(+)-N+NO_(3)^(-)-N and As+NO_(3)^(-)-N treatments,respectively.Following nitrate depletion(after 2 d),microbial As(Ⅴ)and Fe(Ⅲ)reductions were gradually enhanced,which was attributed to stimulation of anaerobic ammonium oxidation(anammox)coupled to Fe(Ⅲ)reduction,known as Feammox,by the abundance of NH_(4)^(+).By the end of the incubation period(10 d),the As+NO_(3)^(-)-N treatment led to higher As immobilization of originally added As(Ⅲ)(ca.61%)than the As+NH_(4)^(+)-N+NO_(3)^(-)-N treatment(42%).The As+NH_(4)^(+)-N+NO_(3)^(-)-N treatment prominently mitigated N_(2)O emission compared to the As+NO_(3)^(-)-N treatment,which was ascribed to anammox,inducing accumulation of byproducts from incomplete denitrification.High-throughput sequencing indicated that the relative abundances of denitrifiers(e.g.,Azoarcus,Ochrobactrum,and Thiobacillus denitrificans)increased in the As+NO_(3)^(-)treatment,whereas quantitative polymerase chain reaction results indicated higher 16S rRNA gene copy numbers for anammox and Feammox(Acidimicrobiaceae bacterium A6)bacteria in the As+NH_(4)^(+)+NO_(3)^(-)treatment.Collectively,the experimental results demonstrated that N fertilization can be a feasible As remediation strategy while providing an effective strategy for mitigating N_(2)O emission from paddy soils at the same time.