| In recent decades, most freshwater bodies are affected by consequences of global change resulting from human activities, such as the increased global temperature, increasing heavy metal pollution and eutrophication. All these factors affect the functional organization of freshwater ecosystems and lead to fracture of the food chain, a simplification and impoverishment of the biota within these ecosystems. As one of the most important components in aquatic system, litter decomposition is more vulnerable to environmental stress such as heavy metal pollution and eutrophication. To date, a lot of studies have emphasized the effects of single factor stress of heavy metals (HM) or eutrophication on litter decomposition in freshwater ecosystems. As the co-occurrence of these factors may vary in response to environmental changes, exposed to several stressors simultaneously may not always respond as predicted from studies that used only single metal and temperature or nutrients (nitrogen and/or phosphorus) stressors. Therefore, studies on environment exposure, heavy metal stress under a warming scenario or eutrophication are necessary to determine the combined effects on litter decomposition and associated biological activity.These studies were conducted in a constructed wetland located at the foot of Zijin Mountain or the Qinhuai River in East China. At first, a field experiment was conducted to test whether the pre-exposure of plant litter to ambient atmospheric environment increases its degradability and microbial activities associated litter decomposition in aquatic environment. In the experiment, the litter samples of Typha angustifolia were placed in the field and exposed to ambient atmospheric environment for a length of 0-month,3-month and 6-month, separately, then placed 40 mm below the water surface to decomposition for 7 months. Second, based on the fact that the present situation of heavy metals (Cu and Zn) pollution, eutrophication (nitrogen and phosphorus) in the Huaihe River and a simulation of the effects of increased water temperature (future temperature increase, i.e., from 10 to 15 ℃ for upper streams and from 15 to 20 ℃ for lower streams, in springtime at the Huaihe River Basin), two microcosm experiments were carried out to assess the impacts of multiple stressors on plant-litter decomposition under the co-occurrence of heavy metals (Cu and Zn) and temperature or eutrophication (nitrogen and phosphorus) in freshwater ecosystems. In the experiment, the litter samples were incubated in microcosms with their original stream water. The Typha litter samples were supplemented with heavy metals (two levels) under three temperature (10,15, and 20 ℃), and the Pterocarya stenoptera litter samples supplemented with heavy metals (two levels) and/or nutrients (two levels). During the incubation, chemical composition (i.e., lignin, total carbohydrate, and nitrogen), litter mass losses, microbial biomass, the activity and diversity of aquatic fungus, and the activities of degradative enzymes were determined.The results of this study are as follows:(1) Dry mass loss of Typha litter during ambient atmospheric exposure was 6% in 3-month treatment and 21% in 6-month treatment. And the pre-exposure treatments of Typha litter also increased its nitrogen content which was 0.60,0.76 and 0.92% in the treatments of 0-,3- and 6-month, separately, but reduced its lignin content and carbon-to-nitrogen ratios. The results from the decomposition of pre-exposed litter in aquatic environments showed that the decomposition of Typha litter was significantly affected by the different pre-exposure treatments, for example the decomposition rates were 0.069,0.090 and 0.102 month-1 in the treatments of 0-,3- and 6-month, separately. The duration of pre-exposure also changed the dominant species and their contributions, for example, Heliscella stellata was the dominant species in both 6-month and 3-month treatments but not in 0-month treatment, and contributed 30%, 27%, and 19%, respectively, to the total conidial production. In addition, the duration of pre-exposure also changed the relationships of enzyme activities with litter mass loss. These results suggest that the duration of pre-exposure of Typha litter can increase its digestibility by changed its chemical characteristics, which altered litter materials exhibited shifts in fungal community composition and changes in extracellular enzyme activities.(2) Results showed that the decomposition rates of Typha litter were varied from 0.0016 to 0.0061 day-1 across treatments, and increased with the increased temperature under the same low HM treatments. With the exception of low HM levels at 20℃ treatment (control vs low HM level=0.0051 vs 0.0061 day-1), the decomposition rates of Typha litter were decreased with the increased HM levels under the same temperature treatment. And the sensitivity of litter decomposition to temperature was regulated by the HM levels, for example, the rate of decomposition with high HM levels was higher than with low HM levels with increased temperature from 10 to 15 ℃ (Q10-q=4.01 vs 2.43). The increase of the lignin/litter weight loss ratio and lignin/carbon weight loss ratio at 20 ℃ and low HM levels suggested that increase in water temperature may enhance the attack of lignin by microbial communities at low HM levels. The enhanced activities of cellulose and P-Glucosidase at 20 ℃ and low HM levels showed that stimulation of carbon mineralization would probably occur under a warming scenario at low HM pollution streams. These resulted suggested that if water quality of presently heavy metal polluted streams is worsen, the potential stimulatory effects of future increases in water temperature on leaf decomposition by microbes may be strengthened in the lower reaches (from 15 to 20 ℃) but attenuated in the upper reaches (from 10 to 15 ℃) of the Huaihe River.(3) The decomposition rates of P. stenoptera were slower in the polluted stream waters than those in the unpolluted ones (polluted vs unpolluted=0.0089 vs 0.0112 day-1), which corresponded to lower microbial biomass and integrated enzyme activities of cellulose and β-glucosidase. The decomposition rates were accelerated at low HM level (control vs low HM level=0.0089 vs 0.0104 day-1), which was associated with the stimulated enzyme activities of hydrolytic enzymes or was stimulated by both NP levels in polluted stream waters. In particular, the hydrolase enzyme activities of microbial communities in polluted stream waters were stimulated by low HM level, suggesting that low HM level-stimulated litter decomposition may be due to the increased enzymatic activities. When microbial communities were exposed to HM and NP simultaneously, the inhibitory effect (in unpolluted stream waters) or the stimulated effect (in polluted stream waters) of low HM concentration was enhanced and attenuated, respectively, which suggests that the NP antagonistic effect against HM toxicity on litter decomposition may contribute to the litter-associated extracellular enzyme activities. These results suggest that the co-occurrence of HM and NP may have antagonistic effects on stream ecosystem functioning. |
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