EFFECT OF MIXED FLOATING AND SUBMERGED MACROPHYTES ON DECOMPOSITION RATE AND NUTRIENT DYNAMICS

Decomposition of aquatic macrophytes can considerably influence nutrient cycling and energy flow in aquatic ecosystems, and may therefore alter aquatic ecosystem structure and functioning. Most studies of decomposition processes have focused on single species of aquatic macrophytes; however, most aquatic ecosystems consist of a mixture of aquatic plant species and fragments they produce which intermingle during decomposition. To explore the effect of species mixtures of aquatic macrophytes with different life forms, decomposition rate and nutrient dynamics were quantified in mixed-species litterbags (containing Nymphoides peltatum, a floating-leaved plant, and Potamogeton crispus, a submerged plant) and in litterbags containing fragments of a single species in a laboratory experiment. There were 10 g of materials used for each species in the litter bags, and for the mixture experiment, also 10 g of the dried macrophyte fragments were used at a mixtures rate of 5:5, Nymphoides:Potamogeton, w/w basis). The decomposition rates, nitrogen and phosphorus content of the remaining materials were determined after 10, 20, 30, 40, 60, and 90 days. Data from single-species litterbags were used to generate expected decomposition rates, nitrogen and phosphorus dynamics for mixed-species litterbags experiments. The result showed that the decomposition rates of N. peltatum (0.032/d) was pronounced higher than that of P. crispus (0.017/d), 24.74% and 44.91% dry mass remaining after 90 days, respectively. The decomposition rates of both N. peltatum and P. crispus were significantly and positively correlated with initial N content (P 0.05, r = 0.862). The decomposition rate of the mixture was 0.023/d which was intermediate between N. peltatum and P. crispus. The observed remaining mass of the mixture at the early stages of decomposition in ten days was 6.63% (P 0.05) higher than the expected, indicating the occurrence of negative, non-addtivie effects of mixed species early on. In contrast, there was no significant mixing effect after ten days in subsequent samplings. After 90 days, the remaining dry mass of the mixture was 30.39%. The N and P contents of both N. peltatum and P. crispus released rapidly at the early stages and then slowed down. The remaining percentage of N and P of N. peltatum were lower than that of P. crispus. During the early stages of decomposition of mixed material in ten days, the observed N and P remaining were 14.36% and 12.88% (P 0.05) higher than the expected, indicating the occurrence of antagonistic effects on N and P release in the mixture. However, there were no significant antagonistic mixing effects in subsequent times for N. After 90 days, the observed P remaining was 4.26% (P 0.05) lower than expected, indicating a synergistic effect on P release occurred. The remaining percentage of N and P were 43.60% and 15.88%, respectively. Nutrients and polyphenol concentrations in the mixture decreased rapidly at the early stages and then decreased slowly through the end of the study, in a manner similar to that of the single species. Our results indicated that there were negative, non-additive effects on decomposition rate, N and P releases when two species were mixed together at the early stages, while there was a synergistic effect on P release in the final stage of the decomposition. This suggests that neither decomposition nor nutrient release patterns can be assessed on basis of single species dynamics. In addition, there was a significant time-independent non-additive effect of species interactions. We further suggest that different aquatic macrophytes of contrasting life forms such as floating-leaved plants and submerged plants may differ in initial chemical quality and may exhibit major determinants for decomposition of mixed aquatic macrophytes.