Effect Of Sediment Arsenic Pollution On Phosphorus Uptake And Distribution In Wetland Macrophyte Typha Angustifolia And Their Ecological Mechanisms

Arsenic has become a widely concerned metalloid because of its toxicity and carcinogenicity.Wetland,especially for the sediment of wetland,is vulnerable to arsenic pollution as it's a sink area of a runoff.The treatment of arsenic pollution has become one of the hot issues in the field of water environment.Typha angustifolia,one of cattail species,as a wetland macrophyte,has great potential in ecological remediation due to its large biomass,rapid growth ability and strong adaptability.Although there have been a lot of reports on the purification by T.angustifolia in eutrophication or heavy metal polluted water,the process and influence mechanism of phosphorus uptake in arsenic polluted environment are still unclear,and the further distribution of phosphorus in plants and its ecological significance have not been reported.Therefore,this paper adopted the method of indoor simulation and field test,and carried out a systematic research on the aspects of T.angustifolia under arsenic stress including the growth and physiological response,the effectiveness of phosphorus,the uptake and distribution of phosphorus,root morphology and rhizosphere microbes,etc.The main research contents and key results are listed as follows.(1)The growth,accumulation and distribution of arsenic,physiological response,and arsenic tolerance of T.angustifolia in different arsenic concentrations and different growth stage including seedling,fast-growing and breeding stage were studied.The results showed that low-medium concentrations of arsenic treatments(<150 mg/kg)promoted plant growth and biomass accumulation.The growth pattern of T.angustifolia could be adjusted to improve the adaptability for different arsenic levels,which could be noted that the rapid growth exhibited in the early stage of life cycle(i.e.the seedling stage)at lower arsenic concentrations,whereas that appeared in the breeding stage at higher arsenic concentrations.Arsenic was mainly accumulated in the underground parts,and the total amount of arsenic extracted by plants increased with the extension of growth and development period.The tolerance mechanism of T.angustifolia to arsenic stress varied in different phenological stages.The arsenic tolerance was mainly dominated by the biomass accumulation in the early stage of growth,whereas the physiological metabolic process(e.g.the synthesis of glutathione)gradually became the dominant factor for arsenic tolerance in the middle and later growth stages.(2)The inorganic phosphorus speciation and phosphorus availability in sediment under arsenic contamination were studied.The results showed that the proportion of inorganic phosphorus was affected by both the concentration of arsenic and the presence or absence of plant.The concentrations of Ca₂-P exhibited the highest values in the seedling stage,and increased with the increasing of arsenic concentrations.The proportions of Fe-P were the highest in the seedling stage and the fast-growing stage under all arsenic treatments.Furthermore,the proportions of Ca₁₀-P and Occluded-P increased whereas that of Ca₂-P decreased at breeding stage.In the treatment with absence of plant,there was no significant change in phosphorus speciation among arsenic gradients except that the proportion of Occluded-P increased with the increasing of arsenic concentrations.The concentration of available phosphorus increased with the increasing of arsenic concentrations in the treatment with presence of plant,whereas that did not change significantly in the treatment with absence of plant.The abovementioned result was confirmed by the experiment data from field test with multiple sites.These conclusions indicated that in addition to the total phosphorus concentration,the planting of plants(especially in the seedling stage)activated parts of the insoluble phosphorus under arsenic contamination.(3)The impacts of sediment arsenic on phosphorus uptake,resorption in dead leaves and the distribution in the plant modules at different growth stages were investigated.The results showed that under the arsenic treatment of low to medium concentrations,uptake of phosphorus and arsenic showed the synergistic effect,whereas the uptake of phosphorus decreased under high arsenic treatments.The distribution of phosphorus among plant modules varied with arsenic concentrations.In general,the phosphorus content in the infructescence of the sexual reproductive organ was the highest(Distribution Index,DI;DI values varied from 3.09 to 4.69),followed by the ramet of the asexual reproductive organs(DI?1.39 to 2.35).The phosphorus content in the leaves(DI?1.42 to 1.60)was higher than that in the roots(DI=1),and the phosphorus content in the dead leaves was the lowest(DI?0.26 to 0.43).It was found that with the increase of arsenic concentration,the phosphorus content in the infructescence decreased significantly,and that in ramet did not change significantly.Furthermore,the phosphorus content in roots and leaves increased first and then decreased,and the distribution index of phosphorus in dead leaves increased.The above conclusions indicated that the high arsenic stress inhibited phosphorus uptake,significantly reduced the phosphorus content in infructescence(33.2%lower than that in control),and prevented the resorption and reuse of phosphorus from dead leaves.(4)The relationship between root morphology and plant phosphorus uptake under arsenic contamination was studied.The results showed that the root surface area(Surf A)and root bulk density(RBD)were significantly higher than that of the control under low to medium arsenic treatments,whereas the total root length(RL)and root average diameter(Avg D)were not significantly different compared to the controls.Under higher arsenic treatment,Surf A,RBD,RL and Avg D were all decreased,whereas the proportion of fine roots with diameter varying from 0.1 to 0.5 mm was increased(from15.8%to 16.5%higher than that of control).The results of correlation analysis between the root morphological indexes and the phosphorus in root/leaf indicated that the root morphology occurred adaptational change under arsenic stress,which affected the P uptake.(5)The relationship between the bacterial community structure,species composition abundance,?-diversity,the dominant species(or genus)and the environmental factors(especially the P content in root and leaf)in the rhizosphere under arsenic treatment was studied.The results showed that the bacterial community composition and abundance were changed under low to medium As treatment,and the bacterial community structure was simplified under high As treatment.Thus,the arsenic-tolerant bacteria became the absolute dominant species,and the level of?-diversity of bacteria community decreased.The structure of bacteria flora and the abundance of endemic species under different arsenic treatments were significantly different.Significant positive correlation was found between phosphorus content in root/leaf and the?-diversity index of rhizosphere bacteria flora.Through further consulting the function of dominant bacteria species(or genus)which were significantly related to P content of root/leaf,it is found that the bacteria with high abundance may mediate the processes including the degradation of organic phosphorous,P solution and hormone production,which could adjust the P uptake by T.angustifolia.These results can provide supplemental information for biogeochemical cycle of phosphorus in the environment of arsenic pollution,and can also provide technical support for the wetland ecological restoration which facing the dual pollution of both arsenic and phosphorus.