Strategic Planting for Watershed Restoration in Coastal Urban Environment – Toward Carbon Sequestration by Stormwater Improvement

Abstract Organic compounds and nutrients in urban runoff create negative impacts to global warming. Riparian planting (RP) of urban watershed carried out for urban greening may induce the degradation of pollutants, while fixing the carbon and nitrogen in plant biomass. While a few previous publications have demonstrated the potential benefits of stormwater treatment by RP, the critical plant-specific indexes and corresponding contributions to the reduction of greenhouse gas (GHG) emission, in both the senses of water purification and carbon fixation, have never been elucidated quantitatively. This study investigated a total of 21 plant species to their capacities reducing carbonaceous pollutants and NH4+ in synthetic stormwater during a 30-day period and under different operational conditions. Water quality data were collected to analyze the half-life (t1/2) of pollutants degradation rates of each species. Carbon contents in the stem, leaf and root of each species were also measured and used to calculate the total carbon sequestration potential per planting area. Colocasia tonoimo (CT) and Thalia dealbata in freshwater; Crinum asiaticum and Phragmites australis in brackish water; and Kandelia obovateand Aegiceras corniculatum in seawater showed shortest average t1/2 for the degradation of all three pollutants. Closed negative correlation were found between the t1/2 of NH4+ and the increased biomass in leaves and shoot. The highest carbon sequestration density was the highest using plant CT, in both batch and continuous flow systems, i.e., 231.1 and 313.9 g/m2, respectively. Nitrogen sequestration density of CT in batch and continuous conditions were 16.7 and 22.6 g/m2, respectively, which was also the highest among all the tested species. Current GHG emission of the targeted watershed without planting (Tsui Ping River, Hong Kong) were 0.151 kg CO2-e/m3-water. When CT were planted in the simulated watershed at the maximum areas, the GHG emission can be reduced to 0.082 kg CO2-e/m3.