Influencing Factors For The Purification Function Of Water Cress Floating-beds And Stoichiometry

In order to study the root characteristics of cress in river and tanks, the purification effect of cress floating-beds on eutrophicated water with different stubble height treatments, as well as the stoichiometry and homeostasis of cress, so as to provide theoretical support and reference for the research and application of cress floating-beds, we did some experimental research with focuses on different research questions. We analyzed the data using one way and two ways ANOVA analysis, Factor Analysis (FA), Grey Relational Analysis (GRA) and linear regression analysis. Based on our results, the following results were obtained:(1) With the comparative study of the cress root indicators and root characteristics, we found:the cress roots had exponential growth in the water tanks. More and more fibrous root were found which led to the smaller average root diameter with the growth of cress, and the average root diameter indicated the change of the number of cress fibrous roots. The total root length, root surface area, root volume, number of root tips and number of bifurcations in cress floating-bed in water tanks were greater than those in the river because the lower nutrient content in tanks resulted in better root growth. Average root diameter of cress in river is smaller than that in water tanks with floating-beds. The root length of cress in water tanks was greater than that in river, while total length, stem length, root diameter, total weight, stem weight and root weight in river were higher than those in water tanks. We found that the root grew better than the above water part of the cress in water tanks. Due to the greater root mass of cress in water tanks than that in river, we may overestimate the contribution of root in the purification and underestimated the performance of the aboveground part when studying the purification of floating-beds with controlled experiments in water tanks.(2) By continuously monitoring the influencing factors of eutrophication in river and water tanks, we found that:the biomass of cress floating-beds was small because of the river water. The temperature had the most important t impact on chlorophyll a according to Factor Analysis among all the impact factors we monitored; the redox potential decreased and the strong correlation between chlorophyll a content and temperature were found in our experiment. The pH remained unchanged because of the great cress biomass in water tanks. Redox potential fluctuated cyclically and dissolved oxygen content changed with temperature and light condition.(3) Based on the cress floating-bed purification experiments, we found that:In our experiments, compared with the zero control, the cress floating-beds could improve the oxidation reduction potential of water, reduce the water temperature, turbidity, chlorophyll a content and the conductivity of the water, and could accelerate the removal of nitrogen and phosphorus. But no difference was found in removing COD. No significant differences were found among different stubble heights of cress floating-beds not only in biomass but also the root. No significant difference was found in water quality improvement through nutrient removal among different stubble height treatments. Temperature had a good relationship with pH and dissolved oxygen. The removal rate of nitrogen and phosphorus was affected not only by initial concentration of nitrogen and phosphorus, but also affected by the temperature, pH and dissolved oxygen. Plant absorption played the most important role in phosphorus removal. Denitrification, nitrification and absorption of the plants played the most important role in the removal of ammonium nitrogen. The removal of ammonium nitrogen in ecological floating bed occurred mainly in the early and mid-term. It will need longer time in nitrate nitrogen removal. It is necessary to arrange different detention time for different target nutrient species in order to improve the operating efficiency of cress floating-bed system.(4) Based on the research of ecological stoichiometry and homeostasis in cress, we found that:The nitrogen, phosphorus contents in roots, stems and leaves of cress showed the same trend as the phosphorus concentration in water, nitrogen-phosphorus ratio and carbon-phosphorus ratio showed negative trend with nitrogen and phosphorus contents, while carbon content stayed stabile. The main reason could be the different pathways of absorption of nitrogen, phosphorus and carbon. Nitrogen and phosphorus were obtained from the environment, and the process was affected by nutrient contents in the water, while carbon was obtained from photosynthesis. In our experiment, nitrogen and phosphorus obtained from water were allocated to stems and leaves of cress for reproduction. The homeostasis index for phosphorus and for nitrogen phosphorus ratio in cress was small, and the homeostasis order was roots> stems> leaves. The small homeostasis index of phosphorus and nitrogen-phosphorus ratio in leaves of cress was due to the prior allocation of nitrogen and phosphorus in cress. On the contrary, the homeostasis index of phosphorus and nitrogen-phosphorus ratio in root of cress was greater than that in leaf of cress. The top-root ratio of cress in river and water tanks had a significant positive correlation with temperature. In our experiment, the top-root ratio of cress both in river and tanks decreased firstly and then increased, but the change rate of cress in river was larger than that in tanks because of the different biomass allocation strategy.The results from the studies on the ecological chemistry, homeostasis and influencing factors for the purification function of cress floating bed system will provide scientific basis for the chosen of floating bed plants, system detention time and purification function assessment in real application of floating bed systems.