Effects Of Macrophytes On Phosphorus Transfer And Speciation In Intertiadal Marsh

Eutrophication of surface water body is a worldwide concern. Phosphorus (P) is one of the key nutrients that can cause algal blooms and other water quality problems in lakes, rivers and estuaries. Wetlands are called the "Kidney of the Earth". Coastal wetland is one type of wetlands composed of a complex and assemblage of swamps, marshes, mudflats and etc. As a transitional zone between terrestrial and marine ecosystems, salt marshes are one of the most biologically productive habitats that drive P cycles. The important ecosystem functions particularly nutrient recycling of coastal wetlands has attracted great attention. However, the driving processes and mechanisms of biological factor in P retention of the tidal flat coastal wetland system are still lack of extensive research. In this thesis, the South Bank of Hangzhou Bay, a typical tidal wetland was taken as the study area. By field investigation and litterbag field experiment, the P retention by marsh plants and sediments were evaluated. Plant litter decomposition and release of P from sediment were studied by litterbag field experiment. We investigated typical intertidal plants on P absorption by batch incubation experiments. The influences of sediment properties and macrophytes on phosphorous speciation in the intertidal marsh were investigated. We also analyze the relationship between sediment microbial biomass, enzyme activity and P forms distribution and transformation. The followings were the main results.(1) The plant biomass, concentration and pools of P were measured seasonally in three marsh species Phragmites australis, Spartina alterniflora and Scirpus mariqueter. Results showed that plant aboveground biomass displayed a unimodal curve with nutrient concentration generally decreased from spring to winter. The belowground biomass was relatively low during the rapid growth period with nutrient concentration tended to decrease and then increase during this period. Plant total P (TP) pools showed significant seasonal variations and were significantly correlated to plant biomass. The pools among plant species were under S. mariqueter<P. australis<S. alterniflora. Considering the purification capacity of P, July would be the best harvest time of the study area for three plants. Wetlands litter decomposition affects wetlands nutrient cycling. There are distinctive stages of the plant litter decomposition in litterbag simulation experiments. The loss rate is faster during0-15d than that of later days. The loss rate in root decomposition of three plants are under S. mariqueter> P. australis> S. alterniflora, while the trend is opposite for that of aboveground tissues. The time needed for95%of dry mass decomposition in the plant tissues is between 1.2-8.3a. Pearson’s correlation coefficient shows that there is no significant correlation between the litter decomposition rate and C/N ratio. However, the litter C/P ratio effects greatly on plant decomposition rate. Environmental factors in the atmospheric temperature also have an impact on the decomposition rate of leaves.(2) The influences of sediment properties and plant community types on P speciation in sediments under four plant community types in the tidal flat and offshore sandbar were investigated. The rank order of P species in sediment based on concentration was exchangeable P (Exch-P)<Fe/Al bound P (Fe/Al-P)<organic P (Org-P)<Ca bound P (Ca-P). Sediment TP and Fe/Al-P concentrations were lower in offshore sandbar than that of tidal flat, reflecting effects of anthropogenic contamination in the latter. Sediment particle size distribution strongly affected P speciation, as indicated by significant correlation between them. TP and Org-P concentrations in vegetated sediments were higher than that of bare mudflat. Additionally, there was significant negative correlation between Ca-P and Org-P, and Fe/Al-P, indicating the presence of plant may result in P speciation by converting Ca-P to soluble and active P, and higher Org-P. Overall, sediment particle size distribution is the most fundamental physical property that affects P speciation, and plant community types are important factors that influence Org-P concentration. The bioavailable P in sediments generally included Exch-P, Fe/Al-P and part of Org-P in study area varied from32.9-134.2mg/kg, with measured sedimentary P ranging from5.78～21.3%.(3) Phosphate adsorption kinetics and isotherms on sediment under different plant community types in Hangzhou Bay wetland were studied, and the influence of the sediment physicochemical properties on P sorption characteristics were analyzed. The results showed that there were three stages during sediment adsorption process with rapid adsorption (0-1h), slow adsorption (1～16h) and balance (16～72h). The trend was not affected significantly by different plants. Indexes fitted from improved Langmuir model showed that sediment Qmax is between154.5～436.3mg·kg-1, and is significantly higher in sediment with plant growth than that of bare mudflat. The Qmaxis also higher than the results of adjacent areas. Sediment NAP is between1.84～4.78mg·kg-1, indicating a low native adsorbed exchangeable P. NAP trends between different types of sediments are similar to that of Qmax values. Sediments EPC0under different plant community types are lower than soluble reactive P concentration in the overlying water. So the sediment acts as the P "sink" from tidal water. The variation of EPCo between different sediments is minor. Correlation analysis showed that the sediment Qmax and NAP in Hangzhou Bay wetland were affected by organic matter, particle composition and total inorganic P concentration, and Qmax is also affected by the electrical conductivity value. However, there are no significant correlations between sediments EPCo and physicochemical properties. In conclusion of that, plants can affect the physical and chemical parameters of sediments, thus affecting the adsorption kinetics and isotherms of P.(4) The influences of sediment biochemical properties and plant community types on P speciation in sediments were investigated. Surveys carried out in spring and autumn showed that alkaline phosphatase activity (APA) varied among seasons and plant community types. There were significant positive correlation between APA and Fe/Al-P concentrations, and significant native correlation between APA and Ca-P. This indicates that APA affects the transformation of P forms in sediments. Sediment microbial community functional diversity analysis using Biolog Eco-plate showed that the sediment microbial carbon metabolic function diversity increased with the succession of plant community. Analysis of PCA showed that carbon sources of sugars and amino acids and their derivatives indicate strong differentiation for the microbial communities under different plant community types. Microbial total PLFAs were under S. alterniflora> P. australis> S. mariqueter> bare mudflut. The plant growth increased the microbial biomass than that of bare mudflut. There was significant positive correlation between bacteria PLFAs and APA, while no significant positive correlation between different types of microbial biomass and different P species. As a result, further attention on key functional bacteria in P cycling was needed.(5) The distribution patter of P pools in the sediments and plants were analyzed. Phosphours retention capacity during the process of plants uptake and sedimentation in the marshes were calculated. More than98%of the P pools distributed in sediment while only little percent in plant system. The water purification coefficient (WPC) of P by plant assimilation was6.04～24.0t·(m-2yr-1). According to the sedimentation rate, retention of P in sediments were25.4～31.5g·(m-2yr-1). Sediment has a higher retention capacity than that of plants. Overall, these results suggest that higher annual plant biomass and nutrient assimilation contribute to greater nutrient retention capacity and accumulation in sediments, thereby enabling reduced eutrophication in the transitional waters.