Abstract: Polluted river water and Hoagland nutrient solution were taken as the culture media for detecting secretion characteristics of two plants, Cyperus alternifolius and Canna indica, under hydroponic culture and its relationships with pollutant removal in this study. Both plants with no significant intraspecific differences in size and weight were cultured to observe the growth, to compare the root exudation, radial oxygen loss as well as their relationships with water purification by the two plants in the two culture media. C. indica had a significantly higher growth rate in the nutrient solution than in the polluted river water, while C. alternifolius showed no significant difference in both media. The amounts of root exudation per unit weight of plant were significantly different between the two plants in the two media. C. alternifolius exuded more per unit weight than C. indica in both solutions. Both plants exhibited greater secretion in polluted water than in the nutrient solution. The maximums of root secretion in the nutritional solution and polluted river water for C. alternifolius and C. indica were 2.89 and 34.76, and 1.28 and 1.82 mg·g^-1·d^-1, respectively. Radial oxygen loss (ROL) rate of C. alternifolius was significantly higher than that of C. indica in both solutions. Both plants showed higher ROL rates in polluted water than in the nutrient solution. For C. alternifolius, the highest ROL rate was 0.92 μmol O₂·h^-1·g^-1 (plant) or 6.69 μmol O₂·h^-1·g^-1 (root) in the polluted water; while the lowest was 0.20 μmol O₂·h^-1·g^-1 (plant) or 1.15 μmol O₂·h^-1·g^-1 (root) in the nutrient solution. After the adaptation period, the removal efficiencies of ammonia, TN, TP reached more than 90%, while COD removal was lower and fluctuated within a certain range. The amounts of COD removal per unit weight of C. alternifolius and C. indica reached 6.95±1.78 and 1.35±0.52 mg·g^-1, respectively, while the amounts of TP removal were the smallest, being only 0.28±0.06 and 0.06±0.02 mg·g^-1, respectively. The higher efficiencies of removal of pollutants COD, ammonia, TN and TP by C. alternifolius per unit weight than by Canna indica, were significantly associated with root secretion and radial oxygen loss. The results indicated that using C. alternifolius as floating bed plant in treating polluted river water could achieve a higher efficiency.

Key words: biological soil crust, moss crust, soil enzyme activity, soil nutrient, Loess Plateau.