DECOMPOSITION OF TWO SUBMERGED MACROPHYTES AND THEIR MIXTURE: EFFECT OF SEDIMENT BURIAL

Decomposition of aquatic macrophytes makes the nutrients stocked in biomass release to the surroundings, and thereby can considerably affect nutrient cycling and energy flow in aquatic ecosystem. In most case, aquatic macrophytes may readily produce a considerable amount of stem and/or leaf fragments due to both natural and anthropogenic disturbances. The detached fragments will be deposited within the sediment. To date, the breakdown of aquatic macrophytes associated with sedimentation remains largely unexplored. Vallisneria natans and Potamogeton maackianus are two dominant submerged plants in shallow lakes of the Yangtze River basin of China. In the field, a substantial amount of the detached plant parts may be produced and deposited separately or coupled with each other at the substrate surface or buried within the sediment, contributing to the formation of an organic slime in the lakeshore areas. To investi-gate the effect of sediment burial on the decomposition of V. natans, P. maackianus and their mixture, three types of plant materials confined in litterbags were incubated and subjected to 0 cm and 5 cm buried treatments for 117 days. There were 5 g materials used for each species and mixture (2.5:2.5 of V. natans: P. maackianus, w/w basis) in the litterbags. The decomposition rates, nitrogen and phosphorus content of the remaining materials, and microbial respiration and biomass were measured at days 7, 19, 35, 56, 82 and 117 respectively. Decomposition rates of both V. natans and P. maackianus were significantly and positively correlated with initial N contents (P0.05, r=0.896), and nega-tively correlated with C/N ratio (P0.05, r=-0.963). The decomposition rate was the highest for V. natans, the lo-west for P. maackianus, and intermediate for their mixture. The decomposition rate and nutrient release of V. natans, P. maackianus and their mixture were greatly reduced in 5 cm burial treatments compared to 0 cm treatments. After 35 days experiment, the dry mass remaining percentage of V. natans was 0 and 43.51% under 0 cm and 5 cm burial treatments, respectively. After 82 days experiment, the dry mass remaining percentage of V. natans was close to 0 in 5 cm burial treatments. Dry mass remaining of P. maackianus and V. natansP. maackianus was significantly higher in 5 cm burial treatments compared to 0 cm treatments, with an increase of 31.09% and 37.44%, respectively. The nutrient release of V. natans, P. maackianus and their mixture were significantly inhibited when they were buried at 5 cm depth. After 117 days experiment, the N and P remaining percentage of P. maackianus was significantly higher in 5 cm burial treatments compared to 0 treatments, with an increase of 19.45% and 14.73%, respectively. For the mixture, the N and P remaining percentage was higher in 5 cm burial treatments than 0 treatments, with an increase of 41.57% and 22.82%, respectively. After 35 days experiment, the N remaining percentage of V. natans was close to 0 and 31.28%, and P remaining percentage was close to 0 and 24.45% under 0 and 5 cm burial treatments, respectively. For the mixture, the observed mass remaining did not significantly differ from the expected under two treatments, indicating that an addi-tive effect existed for the decomposition rate and the mixing effect was irrelevant to the incubated locations (i.e., at the surface or buried within the sediment). In addition, additive effect of N and P release of the mixed material also occurred at the early stage of decomposition. In the subsequent time, the observed N remaining of the mixed material were lower than the expected (P0.05) while the observed P remaining did not differ from the expected (P0.05) in 0 cm burial treatment, indicating a synergistic effect on N release and an additive effect on P release occurred. In contrast, the observed N and P remaining were higher than the expected in 5 cm burial treatment, indicating antagonistic effects on N and P release occurred. The microbial respiration rate of both V. natans and P. maackianus and the microbial biomass of the mixed material were much lower in 5 cm burial treatments compared to 0 treatments. This implied that limitation of microbial activities to plant materials induced from the sediment burial would greatly reduce the rate of decomposition and nutrient release of aquatic macrophytes. We concluded that sediment burial may lead to a decrease of decomposition rates and nutrient release of both single submerged plants and their mixture, which is closely linked to substantial decline of microbial activities by sedimentation. We further suggested that the observed N and/or P-stimulated increases of the mixed plant material in response to sedimentation would contribute to the changes in nutrient availability in the lakeshore area.