Influence Of Different Spartina Alterniflora Control Measures On Soil Microbial Diversity

Spartina alterniflora has well-developed root system and excellent adaptability to salinity,which promotes its rapid spread in tidal flat wetlands.S.alterniflora has become an invasive species in the estuarine and coastal ecosystems in China,which has caused serious impacts on the ecological environment.At present,the most effective physical and chemical methods for controlling S.alterniflora are mowing +flooding and spraying herbicide haloxyfop-R-methyl(haloxyfop).However,the effects of these measures on the soil ecosystem is not known.Soil microorganism is an important part of wetland soil ecosystems,which is sensitive to environmental changes and can be used as an indicator organism to characterize the health of ecosystems.In this paper,the impact of different concentrations haloxyfop on rhizosphere soil bacterial communities of S.alterniflora over the dissipation process of the herbicide has been studied in a coastal wetland.The response of the bacterial community in the rhizoplane of S.alterniflora subjected to haloxyfop treatment was also investigated.High-throughput sequencing was used to detect the DNA of soil bacteria so as to obtain the composition structure of the soil bacterial communities.Biolog-ECO technology was used to analyze the soil microbial community functional diversity and utilization of different types of carbon sources.Effects of mowing+different heights of flooding on bacterial community in the rhizoplane of S.alterniflora were also be studied by using field control experiments.The main results are summarized as follows:(1)The persistence of haloxyfop in the rhizosphere soil followed an exponential decay with a half-life of 2.6-4.9 days,and almost all of the haloxyfop dissipated on Day 30.The diversity of rhizosphere soil bacteria was decreased at the early stages(Days 1,3 and 7)and recovered at late stages(Days 15 and 30)of the haloxyfop treatment.Application of haloxyfop treatment increased the relative abundance of the genera Pseudomonas,Acinetobacter,Pontibacter,Shewanella and Aeromonas.Strains isolated from these genera can degrade herbicides efficiently,which possibly played a role in the degradation of haloxyfop.The rhizoplane bacterial diversity was reduced on Day 15 while being vastly enhanced on Day 30.Soil variables,including the electric conductivity,redox potential,and soil moisture,along with the soil haloxyfop residue,jointly shape the bacterial community in rhizosphere soil.(2)The activity and community functional diversity of rhizosphere soil microorganisms were changed at the early stages(Days 1,3 and 7)and recovered at late stages(Days 15 and 30)of the haloxyfop treatment.The most significant difference among the utilization of six types of carbon sources by soil microorganisms in different treatments is the utilization of carboxylic acids and amines/amides carbon sources.Seven days after treatment,the soil microorganism's ability of different carbon sources utilization all decreased.In both the treatment groups and the control groups,the rhizosphere soil microbial utilization rate of carbohydrates and polymers carbon sources were the highest.Haloxyfop did not change the dominant carbon source metabolic community of soil microorganisms.(3)The effects of low concentration haloxyfop treatment on soil urease,sucrase and peroxidase activities were first decreased and then increased.The soil urease activity decreased significantly at day 1,while the soil sucrase and peroxidase activity decreased at day 3.The effect of high concentration haloxyfop treatment on soil urease and sucrase was first increased and then decreased,while the effect on soil peroxidase was first decreased and then increased.At day 30 after treatment,the peroxidase activity of rhizosphere soil treated with high concentration haloxyfop was significantly enhanced,which might be caused by the increase of soil organic matter caused by the decomposition and death of plant roots and frequent microbial oxidation activities.(4)Mowing + flooding treatments of S.alterniflora increased the alpha diversity(Shannon diversity increased)of rhizosphere soil bacterial communities.The flooding treatment increased the relative abundance of anaerobic bacteria such as Bacillus,Desulfuromonas,and reduced the relative abundance of aerobic bacteria such as Deinococcus-Thermus and Paracoccus.Flooding treatment resulted in a decrease in soil oxidation-reduction potential(Eh)and increased soil electrical conductivity(EC).The differences of soil Eh and EC were the main environmental factors that caused differences in the soil bacterial community structure between the treatment and control groups.To sum up,when used to control S.alterniflora,haloxyfop can quickly dissipate in wetland soil within a reasonable concentration range,with little and short-term impact on the soil environment.The anaerobic environment caused by mowing +flooding is the main reason for the change of soil bacterial community structure.Under the same time scale,the effect of mowing and flooding on soil environment lasted longer than that of haloxyfop.When selecting the treatment methods of S.alterniflora,the haloxyfop with reasonable concentration should be used to reduce the impact on the wetland soil environment.