Study On Nitrogen And Phosphorus Purification In Water By Typical Aquatic Plants

Eutrophication is one of the most serious ecological and environmental problems of the freshwater lakes.Aquatic plants represented by submerged and emergent plants play an important role in water purification and are the key factors to maintain the ecological balance of the shallow lakes.Scholars have carried out many studies on the removal effects of nitrogen（N）and phosphorus（P）by aquatic plants,but the research on the growth conditions for submerged plants to achieve the best purification effect are still incomplete,such as species selection,water depth regulation and density allocation.At present,the researches on water purification of emergent plants mainly focus on plant types,nutrient absorption capacity and other aspects,and little attention is paid to the changes in the whole life cycle of the emergent plants.Although emergent plants have a good capacity on absorption of N and P,the nutrients released by their litters are likely to cause secondary pollution to the water.In order to fully exert the water purification function of emergent plants,it is necessary to explore its reasonable harvest time from the viewpoint of N and P storage and release.In this study,we carried out two sets of experiments respectively.In the first set of experiments,we selected four common submerged plants（Vallisneria natans,Hydrilla verticillata,Ceratophyllum demersum,and Myriophyllum verticillatum）to carry out a two-factor field control experiment.Water quality and plant components were determined and analyzed regularly.The contribution of submerged plants to N and P removal in water and under different water depth and density combinations were investigated.In the second set of experiments,we selected four common emergent plants（Cyperus alternifolius,Phragmites australis,Typha orientalis and Canna indica）to investigate the N and P removal,the characteristics of the litter decomposition and the appropriate harvest time of the emergent plants through the seasonal investigation for one year.The main research results and conclusions were listed as follows:1.The purification characteristics for nitrogen and phosphorus of submerged plants1)Different plants need different water depth and density conditions to achieve the best nutrient removal effect,and different plants had different tolerance ranges for water depth and plant density.Myriophyllum verticillatum,Vallisneria natans and Ceratophyllum demersum had the best removal effect on total phosphorus（TP）and total nitrogen（TN）at 3.5 kg·m^-2 with the water depth of 0.5 m,while that of Hydrilla verticillata at 3.5 kg·m^-2 with 1 m was the best.Except for Ceratophyllum demersum,planting depth,density,and their interactions all had a significant effect on the total removal rate of TN and TP（P<0.05）.The results showed that the water depth and density of plant colonization had a synergistic effect on the TN and TP removal efficiency,and the inhibition of nutrients absorption of four plants at high density were affected by water depth,such as,the growth of Vallisneria natans was not inhibited by high density under a suitable water depth.The results showed that the optimal depth for Hydrilla verticillate was 1 m,and the Hydrilla verticillate nutrients absorption would be inhibited by high density at a water depth of 2 m.The absorption of TN and TP of Vallisneria natans might be affected by the lack of light.The Ceratophyllum demersum had a good removal effect on TN and TP at a depth of 2 m,but varied with the different densities.Myriophyllum verticillatum had a good removal effect in shallow water,and the best planting depth should be less than 1 m.Overall,the density of 3.5 kg·m^-2 is not the critical threshold of density at a suitable depth for these four plants.2)In this study,we found that the removal rate of TN and TP and the characteristics of water quality varied in the different sampling periods which reflecting the purification process more specifically.In shallow water,Hydrilla verticillata and Myriophyllum verticillatum all grew faster in the first half month,while with the increase of water depth,the absorption of N and P decreased.The rapid purification period of Vallisneria natansis was mainly within the beginning half month,after 20 days the variation was no longer significant.The rapid removal of TN by Ceratophyllum demersum lasted more than a half month,which was longer than the others.The removal rate of TN and TP by Vallisneria natans with 3.5 kg·m^-2 at 0.5 m water depth,Hydrilla verticillata with 3.5 kg·m^-2 at 1 m water depth,and Ceratophyllum demersum with 0.5 kg·m^-2 at 1 m water depth were still increasing before the end of the experiment,indicating that there was a possibility of continued increase in the later period.3)Under high-density conditions,it was relatively easier to occur the excessive absorption,but it did not mean that the absorption of N and P in a single plant or the removal rate was higher,and the removal rate should be the main factor in evaluating the effect of plants on water purification.The N and P absorption efficiency of plants might not be synchronized,and the removal paths of N and P were different.The contribution rate of plants would be restricted by factors such as environmental conditions and plant physiological growth,and the concentration of pollutants would also affect the N and P absorption efficiency and purification pathway of plants,indicating that the planting conditions of plants should be selected according to the characteristics of water pollutants.The differences in plant C/N and C/P at different depths were not obvious（P>0.05）,indicating that the light under 2 m might not be enough to exert a strong inhibitory effect on plant growth or change its metabolic substance content.2.The characteristics of nitrogen and phosphorus storage,decomposition and release of emergent plants1)Both species and seasonal differences would lead to extremely significant changes in TN and TP content of emergent plants（P<0.01）.Different species had different TN and TP content,among which Cyperus alternifolius was significantly higher than other species（P<0.05）.The TN and TP content of plants also differed between different seasons and the TN contents of most plants were higher in summer.Plant TN and TP reserves were not significantly related to TN and TP content,but had high correlations with biomass,indicating that species with higher biomass may have higher N and P accumulation and stronger removal capacity,while it was not directly related to its TN and TP content,therefore,it is more reasonable to determine the optimal harvest time based on plant biomass and nutrient reserves.Most plants had no significant difference in TN and TP storage in summer and autumn,but higher than winter and spring.2)The decomposition characteristics of litter under different environmental conditions were different,and the decomposition rates of stems and leaves were different.The decomposition rates of emergent plants under flooded conditions were much higher than in the air,indicating that if the plants are not harvested at the right time,the nutrients released by the litters will aggravate the eutrophication.The first month was the rapid decomposition stage of the flooded plant.As the temperature increased,the plant decomposition speeded up and entered the main decomposition stage after April,and the decomposition slowed down and tended to be steady after August.The plant stems decomposed slower than leaves,and the releases of P were faster than that of N.There may be more stable net N releases in leaves than in stems,and there may be N attachment in plant stems.Except for the reed stems,the cumulative release rate of the nutrients in the flooded parts exceeded 90%after September and gradually stabilized.The P release rates of flooded plants in the first month were more than 25%,and the cumulative release rates of P were higher than N before June.The N and P release amount of Canna indica were higher than other plants.The rate of the plant decomposition was significantly positively correlated with the initial N and P content（P<0.05）,but negatively correlated with C/N,C/P and N/P（P<0.05）,indicating that the plant tissues with higher initial N and P content and lower C/N and C/P had a faster decomposition rate,and the whole plant with lower stem and leaf ratio had a higher decomposition rate.3)The main time for dry matter loss of emergent plants was from April to August,however,the rapid release of N and P in the first month would result in the deterioration of water quality in reservoirs.Emergent plants had higher N and P reserves in summer and autumn than winter and spring generally,therefore,the harvest should be completed in the autumn to ensure the effect of N and P removal.In order to fully exert the N and P removal function of the plants and prevent secondary pollution,this study believes that it is relatively appropriate to harvest the emergent plants in October.