Decomposition of Myriophyllum aquaticum and the Associated Release of Nitrogen and Phosphorus

Abstract

Decomposition of wetland plants could release pollutants, which may affect the removal efficiency and effluent quality of constructed wetlands. The experimental decomposition test of Myriophyllum aquaticum was carried out for 60 d using nylon bags, and release characteristics of nitrogen and phosphorus during the decomposition process were studied. The results showed that the decomposition rate of M. aquaticum was fastest during the first 0-4 d, with a weight loss of 30%, while the degradation rate slowed gradually during the period 4-60 d, with weight loss of 31%. The fitting first-order kinetic decomposition rate constant was 0.0142 d-1, and the calculated time to degrade 50% of dry matter was 48.8 d. The water pH decreased rapidly from 7.60 to 5.63 during 0-4 d, stabilized during 4-32 d, and finally increased to 7.03 (which was close to the control sample without M. aquaticum). The dissolved oxygen concentration decreased rapidly from 6.30 mg·L-1 to 0.61 mg·L-1 during 0-4 d, and remained in an anaerobic state. The total nitrogen concentration in the water increased rapidly to 12.7 mg·L-1 within 2 h, gradually decreased to 5.80 mg·L-1 during 2 h-32 d, and then finally increased slightly. The phosphorus concentration increased rapidly to 18.4 mg·L-1 at the beginning of the experiment, and then gradually stabilized. The main forms of nitrogen and phosphorus released by M. aquaticum were organic nitrogen (accounting for 65.7%-94.7% of total nitrogen) and inorganic phosphorus (accounting for 61%-89% of total phosphorus), respectively. The total nitrogen content of M. aquaticum increased from 24.3 mg·g-1 to 60.5 mg·g-1 with increasing degradation time; the total phosphorus decreased initially from 6.09 mg·g-1 to 2.94 mg·g-1 and then remained constant. These trends may have been related to the fixation of nitrogen by attached microorganisms. Therefore, suitable harvesting and management strategies should be adopted for wetland plants to reduce secondary pollution.