| Plant invasion has become a hot topic in ecology and environmental science for its serious impact on the global ecological environment.Alligator weed(Alternthera philoxeroides)is a worldwide malignant invasive plant,and is also one of the most important invasive plants in China.At present,it has formed a serious invasion in the Nansi Lake of Shandong Province.Through field investigation and combining with historical data,this study analyzed the present situation and trend of the invasion of alligator weed in main rivers flowing into the Nansi Lake(the Xinxue river,the Baima river,the Si river,the Fuxin river,the Dongyu river,the LaoWanfu river,and the Dashahe).The main types and contents of the organic acid of root exudates and the physicochemical properties of root sediments were studied by laboratory culture experiment.And this study determined the activities of key carbon sequestration enzyme RubisCO and CO dehydrogenase.The effects of root exudates and physicochemical properties on the two enzymes were analyzed.Finally,the abundance of cbbL gene and coxL genes of sediments microorganisms carbon cycling was studied.In this study,we comprehensively analyzed the effects of invasive plants on carbon sequestration key enzymes and functional genes in wetland sediments,which is of great significance for understanding of the effects of invasive plants on the carbon cycle of wetland ecosystem and its mechanism.The main conclusions are as follows:(1)Through field investigation,we found that the average coverage of alligator weed decreased in the studied rivers in 2019.This is inconsistent with the growth trend from 2013 to 2018.According to the hydrological data and water storage of Nansi Lake over the past years,the change of river water level caused by annual and seasonal precipitation would affect the spread of alligator weed.(2)The compositions and contents of root exudates of invasive plant alligator weed and native plant cattail(Typha angustata)were determined by the laboratory culture experiment.Seven organic acids,including total organic carbon of root exudates(P-TOC),oxalic acid,tartaric acid,formic acid,malic acid,acetic acid,citric acid and succinic acid were detected.The other physicochemical properties,including air temperature,water temperature,water depth,plant height,water dissolved oxygen(DO),water pH(W-pH),Oxidation-Reduction Potential(ORP),water content,pH(S-pH),dissolved organic carbon(S-DOC),total carbon(TN),total phosphorus(TP)and total organic carbon(TOC),were also determined.The P-TOC content of root exudates of the cattail was higher than that of alligator weed in the river wetland,while the P-TOC content of root exudates of alligator weed was higher than that of the cattail in the constructed wetland.We found:that the oxalic acid and citric acid were the main root exudates organic acid of alligator weed,but no tartaric acid and acetic acid were detected;the oxalic acid,malic acid,acetic acid and citric acid were the main composition of the root exudates organic acid of the cattail.The content of oxalic acid in root exudates of the two plants was the highest,and the content of oxalic acid in root exudates of the alligator weed was higher than that of the cattail.The TOC of the wetland sediments was significantly affected by habitat type and plant species identity,but the DOC was not.The TOC of wetland sediments was positively correlated with water content and TN,but negatively correlated with pH.In addition,TN was positively correlated with oxalic acid and malic acid.(3)By measuring the activities of RubisCO and CO dehydrogenase,the key enzymes of carbon cycle,in sediments of the river wetland and constructed wetland,it was found that the activity of RubisCO in the sediments of the constructed wetland was greater than that of the river wetland.The invasion of alligator weed had significant effects on the activity of RubisCO,and the role of alligator weed invasion on the RubisCO activity was more important in the constructed wetland than in the river wetland.The CO dehydrogenase activity in the sediments of the river wetland was higher than that of the constructed wetland.The CO dehydrogenase activity in the soil of the river wetland was significantly affected by the alligator weed,and the effect in the river wetland was greater than that in the constructed wetland.Two-way analysis of variance(ANOVA)of RubisCO and CO dehydrogenase showed that wetland type had significant influence on both of the two enzymes,while plant species identity had no significant influence on them.Wetland type and plant species identity had no significant interaction effects on RubisCO,while they had significant interaction influence on CO dehydrogenase.The correlation between the physicochemical properties of sediments and the activities of RubisCO,CO dehydrogenase was further studied.We found that the activities of RubisCO were mainly related to water environmental factors:it had a significant positive correlation with water temperature,but a significant negative correlation with water depth,SWC,and organic acids(malic acid and citric acid)of plant root exudates.However,the CO dehydrogenase activity was mainly related to the sediment physicochemical factors:it was positively correlated with S-pH and S-DOC,and negatively correlated with ORP,TN,STOC,and organic acids(tartaric acid,acetic acid and succinic acid)in root exudates.Therefore,the effects of alligator weed on RubisCO and CO dehydrogenase were not significant,and the enzyme activity was related to the wetland type and physicochemical properties.(4)Functional genes(cbbL gene and coxL gene)of RubisCO and CO dehydrogenase were further determined using quantitative polymerase chain reaction(qPCR).The abundance of cbbL gene and coxL gene in the rhizosphere sediments of the alligator weed and the cattail were significantly higher than that in the non-plant habitat.Habitat type and plant species identity had significant influence on the abundance of cbbL gene and coxL gene.Based on two-way ANOVA,we found that habitat type and plant species identity had significant interaction effects on the abundance of coxL gene,but not on the abundance of cbbL gene.While the interaction of habitat type and species had no significant influence on the abundance of cbbL gene,it had significant effect on the abundance of coxL gene.The abundances of the two genes were consistent with the activities of their corresponding enzymes.Both the abundances of the two genes and the activities of their corresponding enzymes were higher in the constructed wetland than in the river wetland.The abundance of cbbL gene was negatively correlated with water temperature,S-TP,tartaric acid,and acetic acid,while it was positively correlated with Rubisco and coxL gene abundance.The study of RubisCO activity and cbbL gene abundance is conducive to the comprehensive understanding of the effects of invasive plants on the Calvin Benson cycle(CBB)circulation in wetlands.In conclusion,through field investigation and laboratory culture experiment analysis,the study found that the coverage distribution of alligator weed in the studied main rivers entering the Nansi Lake showed a decreasing trend in 2019,which might be affected by hydrological fluctuations caused by drought in that year.The wetland TOC was significantly positively correlated with water content and TN,and negatively correlated with pH.RubisCO and CO dehydrogenase activities were correlated with water environment,soil environment and organic acids in root exudates.The abundance of cbbL gene and coxL gene in the root soil of alligator weed was significantly higher than that in the non-invasive habitat,and the habitat type and species had significant effects on the abundance of cbbL gene and coxLgene.The abundance of cbbL gene were consistent with the activity of RubisCO,and the abundance of coxL gene were consistent with the activity of CO dehydrogenase,and the abundance of all constructed wetlands was greater than that of river wetlands.This study is of great significance for understanding the effects of alligator weed on the carbon cycle and the potential mechanisms. |
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