The initial mass of residue can regulate the impact of Phragmites australis decomposition on water quality: a case study of a mesocosm experiment in a wetland of North China

Plant residue decomposition can significantly affect the water environment of wetland ecosystems. However, little is known about the trajectory and the key drivers of the different initial mass of residue (IMR) that regulate water quality. Here, we conducted a study of 210 days with an in situ experiment to examine the impact of the change in IMR and the key decomposition parameters of Phragmites australis on the water quality in the growing season of 2019 in Baiyangdian wetland in North China. Five IMR of the Phragmites were applied, ranging from 0 to 1000 g with 250 g increments. The chemical oxygen demand (COD_W), total organic carbon (TOC_W), total nitrogen (TN_W) and total phosphorus (TP_W) of the water, and the total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), cellulose, lignin, decomposition rate (k) and the remaining mass of the Phragmites in the bag (RM) were monitored. Our results showed that the IMR of the Phragmites only marginally influenced the decomposition of itself and; the variations in the level of the water indicators were ranked as TN > TP > COD > TOC and; the impacts of IMR were longer on TOC and COD than on TP and TN and; the change in water quality was significantly influenced by the Phragmites decomposition and highly correlated with the change in TOC, cellulose and k of decomposing Phragmites residue. These results indicated that the impact of 250 g of IMR on the water was negligible after 120 days, suggesting that the optimal harvest rate of the Phragmites should be above 75% in the Baiyangdian wetland, despite the strong impact at the beginning and; the impact of change in IMR on water quality can be explained by the change in the decomposition parameters of the decomposing Phragmites residue, where such a decomposition-based explanation might be a promising tool to link wetland plant management and water quality.

Automated summary

The impact of plant residue decomposition on wetland water environments was studied in Baiyangdian wetland in North China. Results showed that the initial mass of residue (IMR) of Phragmites only marginally influenced its own decomposition, with TN, TP, COD, and TOC being the most affected water indicators. Changes in water quality were significantly influenced by the decomposition of Phragmites residue, particularly correlated with TOC, cellulose and decomposition rate. Harvest rate of Phragmites above 75% was recommended for optimal water quality management.