Impacts of Environmental Conditions on the Soaking Release of Nitrogen and Phosphorus from Cynodon dactylon (Linn.) Pers. in the Water-level Fluctuation Zone of the Three Gorges Reservoir Region

Abstract

The flooding-drying cycles in the water-level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) result in an abundance of plants that are submerged and decomposed to release nutrients. This has the potential to lead to water quality deterioration of the TGR. Cynodon dactylon (Linn.) Pers., one of the typical plants in the WLFZ, was collected and inundated in the laboratory under different conditions under temperatures of 15℃ and 25℃, a pH 5.0 to 9.0, and under light and dark conditions, respectively, to measure the decomposition rate and nitrogen and phosphorus release mechanisms. The dry weight lost with the contents of nitrogen and phosphorus was found to decrease after Cynodon dactylon (Linn.) Pers. was soaked for decomposition. The amount released of total nitrogen (TN) and total phosphorus (TP) at 25℃ for 200-days inundation was(2.66±0.29)g·kg-1 or(3.76±0.04)g·kg-1, respectively and at 15℃ was(0.79±0.03) g·kg-1 or (1.40±0.02) g·kg-1, respectively. When the pH of the water submerging the grass was 5.0, 7.0 and 9.0, the amount of TN released was (3.76±0.08) g·kg-1, (2.66±0.29) g·kg-1, and (2.55±0.12) g·kg-1, respectively while the amount of TP released was (1.53±0.04) g·kg-1, (0.79±0.03) g·kg-1, and (1.70±0.07) g·kg-1, respectively. The TN and TP released was (3.87±0.14) g·kg-1 and (1.78±0.08) g·kg-1 under dark condition. The lower the temperature, the higher the amount of TN and TP will be released for inundation from Cynodon dactylon (Linn.) Pers. When the overlying water is acidic or alkaline, more TN and TP is released. Dark conditions are beneficial to nitrogen and phosphorus release into the overlying water. Thus, the water environment changes in each flooding season in winter. Sewage discharge will also accelerate the nutrients released from soaked plants through their decomposition in the WLFZ, and then will aggravate the deterioration of water quality in TGR.