| Decomposition is the process that dead organic material stepwise degrade. In the process, inorganic nutrients release from organic matter and the decomposition can considerably affect nutrient cycling and energy flow in ecological ecosystem, and then the process drives the change of ecosystem structure and function. As primary producers in nature, the primary production of manufacturing plants is very huge and organic synthesis and decomposition could directly affect the elements cycling in ecological ecosystem. The factors associated with litter decomposition are primarily driven by litter quality, the activities of decomposers and conditions factors (i.e., temperature, oxygen level, pH and nutrient availability). Hydrological characteristics is one of the important environmental factors affecting decomposition and sediment deposition is considered to have important effect on plant decomposition. The previous research has focused on sediment deposition on the decomposition of riparian vegetation, wetland and marsh plant litter and the decomposition of macrophyte response to sediment burial is unclear. In addition, many researches have found that elevation altitude has an important influence on plant decomposition. The previous research has focused on terrestrial ecosystem research and the decomposition of the organic matter from the surrounding vegetation in the river with different altitude is unclear. River hydropower development, especially, the cascade hydropower station construction in different elevation of the river form the hydrostatic habitat (reservoir) and the reservoir become the detritus pool intercepting the plant litter. At present, the research about litter decomposition of riparian zone vegetation in different reservoir with the increasing elevation is less. So we investigated:(1) The decomposition of Vallisneria natans, Potamogeton maackianus and their mixturte response to sediment burial; (2) The decomposition of Alnus cremastogyne, Cryptomeria japonica and their mixturte response to elevational gradient.1. 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 investigate 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 (P<0.05, r= 0.896), and negatively correlated with C/N ratio (P<0.05, r=-0.963). The decomposition rate was the highest for V. natans, the lowest 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. natans×P. maackianus was significantly higher in 5 cmburial 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 a additive 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 (P<0.05) while the observed P remaining did not differ from the expected (P>0.05) in 0cm burial treatment, indicating a synergistic effect on N release and a 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 a 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.2.Based on the litterbag method, we explore the decomposition dynamics of the plant litter from riparian vegetation. Alnus cremastogyne and Cryptomeria japonica are two dominant vegetation along an altitude gradient in Baoxing river basin, Sichaun, China. In the field, a substantial amount of the detritus produced from riparian vegetation were input to streams and the litter decomposition allows the incorporation of carbon and nutrients into secondary production. To investigate the effect of altitude gradient on the decomposition of A. cremastogyne, C. japonica and their mixture, three types of plant materials confined in litterbags were incubated and subjected to Alt.593 m,760 m,990 m, 1352 m in four reservoirs for 220 days. There were 10 g materials used for each species and mixture (5:5 of A. cremastogyne:C. japonica, w/w basis) in the litterbags. The decomposition rates, nitrogen and phosphorus content of the remaining materials, and microbial biomass and colony forming unit (CFU) were measured at days 40,100,160 and 220 respectively. The decomposition rate was highest for A. cremastogyne, the lowest for C. japonica, and intermediate for their mixture. Along the altitudinal gradient the decomposition rate of A. cremastogyne, C. japonica and their mixture were highest in Yucheng reservoir (Alt.593 m) and lowest in Baoxing Reservoir (Alt.1352 m). The nutrient release of A. cremastogyne and the mixture were significantly accelerated when they were incubated in Yucheng reservoir (Alt.593 m) compared to the other three reservoir. After 220 days experiment, in Yucheng reservoir, the N and P remaining percentage of A. cremastogyne and the mixture was 35.71%,57.37% and 37.83%, 63.57%, respectively. In other three reservoirs, the N and P remaining percentage of A. cremastogyne and the mixture had no obviously difference. For the C. japonica, the nutrient release irregularly. For the mixture, the observed mass remaining were lower than the expected (P<0.05) in Yucheng reservoir (Alt.593 m), indicating a synergistic effect existed for the decomposition rate, in other three reservoirs, the observed mass remaining did not significantly differ from the expected, indicating that a additive effect existed for the decomposition rate. After 220 days experiment, a synergistic effect on N and P release occurred in Yucheng reservoir and a additive effect existed for N and P release in Xiaoguanzi and Tongtou reservoir, While a antagonistic effect occurred on N and P release in Baoxing reservoir. As times go on, the microbial biomass of C. japonica and the mixture were much higher in Yucheng reservoir than three other sites, impling that limitation of microbial activities to plant materials induced from the increasing elevation would greatly reduce the rate of decomposition and nutrient release. Simultaneously, we found the data for the CFU indicated an antagonistic relationship between fungi and bacteria. We concluded that increasing elevation may lead to a decrease of decomposition rates and nutrient release of riparian vegetation plant litter, which is close to the decline of microbial activities. In addition, the role of bacteria and fungi on the plant litter decomposition was closely related to different litter decomposition stage.Overall, our studies indicated that the change of conditions factors could have a important effects on the plant litter decomposition. Both sediment burial and increase elevation could suppression the microbial activities, and then reduce the decomposition rate and N and P release. Our research could provide reference for understaning the influence of sediment on the decomposition of aquatic plants and nutrient dynamic, at the same time our studies could provides a theoretical basis in order to understand the impact of cascade hydropower development on river ecological environment. |
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