Strategic planting for watershed restoration in coastal urban environment – Toward carbon sequestration by stormwater improvement

Organic compounds and nutrients in urban runoff create negative impacts to global warming. Riparian planting (RP) of urban watershed can enhance the degradation of pollutants, while fixing the carbon and nitrogen in plant biomass. Although 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 both carbonaceous pollutants and ammonia in synthetic stormwater during a 30-day period and under different operational conditions. Water quality data were collected to analyze the half-life (t_1/2) of pollutants degradation rates of each plant species. Carbon contents in the stem, leaf, and root of each species were 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 obovate and Aegiceras corniculatum in seawater showed shortest average t_1/2 for the degradation of all three pollutants. Negative linear relationships were found between the t_1/2 of ammonia and the increased biomass in leaves and shoot. The highest carbon sequestration densities calculated using plant CT in the batch and continuous flow systems were 231.1 and 313.9 g/m², respectively. Nitrogen sequestration densities of CT in batch and continuous flow conditions were 16.7 and 22.6 g/m², respectively, which were also the highest among all the tested plant species. Current GHG emission of the targeted watershed without planting (Tsui Ping River, Hong Kong) were 0.151 kg CO₂-e/m³-water treated. When CT were planted in the simulated watershed at the maximum areas the GHG emission can be reduced to lower than 0.082 kg CO₂-e/m³.