[Change and Influencing Factors of Dissolved Carbon and Dissolved Nitrogen in Water of the Three Gorges Reservoir].

In order to understand the changes of dissolved carbon and dissolved nitrogen in the water of Three Gorges Reservoir,this research was carried out once a week by the bank of Yangtze River in Fuling beach from March 2011 to August 2012,and the variation characteristics of dissolved C,N composition and their source were analyzed.The results showed that the concentration of DOC ranged from 0.64 mg·L-1 to 9.07 mg·L-1,and had obvious seasonal change:summer >spring and autumn >winter.Annual total input of DOC was 1.78×109 kg,the seasonal change trend of the total input of DOC was similar to that of the concentration of DOC;The concentration of DTN ranged from 2.59 mg·L-1 to 4.35 mg·L-1:spring >winter >summer >autumn,annual total input was 1.32×109 kg,the seasonal input changed in the order of summer >autumn >spring >winter,among them DON,NO3--N accounted for 30.35%-63.45% and 35.87%-67.72%,respectively.DOC was affected by precipitation and air temperature,and mainly came from the exogenous input,in the spring and summer its exogenous input increased with the increase of rainfall runoff,but in the autumn and winter the endogenous contribution increased;DTN was relatively affected by human emissions and water dilution.Correlation analysis showed that there was a significant negative correlation between DOC and DON (P<0.05),DOC/DON ratio usually reflects the source of the DOM,the DOC/DON in the water of three gorges reservoir ranged from 0.35 to 7.28,the source of DOM had obvious seasonal characteristics.DOC/DON was the highest in summer,and the DOM mainly came from watershed erosion;DOC/DON was the lowest in winter,and the DOM mainly came from living sewage and endogenous field;the DOC/DON ratios in spring and autumn were higher than those in winter and lower than those in summer,and the DOM sources included watershed erosion,living sewage and endogenous field.

[Characteristics and Influencing Factors of CH4 Emissions from the Drawdown Area of the Three Gorges Reservoir].

Five levels (180 m, 175 m, 165 m, 155 m, and 140 m) in a typical drawdown area in Wangjiagou in the Three Gorges Reservoir were selected to study CH4 emissions from subtropical reservoirs. The experimental period lasted two years from September 2010 to August 2012. The methods of static opaque chambers during the drainage period and floating chambers during flooding period were used in this study. The elevations of 175 m, 165 m, and 155 m were all located in the drawdown area, whereas the 180 m elevation was located in the land and never flooded. The 140 m elevation was permanently flooded and used as a control area. The results showed that the CH4 fluxes showed no significant trends at 175 m and 165 m in the first year of the experiment, while the fluxes showed a single peak pattern with the climax in the summer at 155 m and 140 m. At 175 m, the CH4 emissions showed a single peak pattern with the climax during its flooding period, and then showed not regular CH4 emission sources or sinks alternately in the second year, whereas the CH4 fluxes at 165 m, 155 m, and 140 m presented a single-peak shape with winter climax. During the entire observation period, the CH4 emission fluxes at 180 m were stable and showed no obvious peaks. In addition, CH4 fluxes were higher during the flooding period than in the drainage period at 175 m, 165 m, and 155 m.The order of the annual CH4 cumulative emissions at the five elevations was 140 m (99.58 kg·hm-2) > 155 m (82.98 kg·hm-2) > 165 m (65.38 kg·hm-2) > 180 m (6.32 kg·hm-2) > 175 m (4.27kg·hm-2), suggesting that the soil was more conducive to CH4 production when the flooding period was longer. Correlation analysis indicated that there were no significant correlations between CH4 fluxes and the soil carbon component and pH on land and during the drainage period but CH4 fluxes increased with the increase in soil water content. There was a significant linear negative correlation between CH4 emissions from the gas-water interface at 140 m and in water. The soil moisture content was one of the key factors affecting the CH4 fluxes during the drainage period, while during flooding period, the CH4 fluxes were regulated by flooding depth.

[Features and Influencing Factors of N2O Emissions from Drawdown Area in the Three Gorges Reservoir].

Three heights (180 m, 175 m and 155 m) located in a typical drawdown area in Wangjiagou of the Three Gorges Reservoir were selected for studying the N2O emissions from subtropical reservoirs. The experimental period lasted two years from August 2010 to August 2012. The methods of static opaque chambers during the drainage period and floating chambers during flooding period were adopted in this study. The heights of 175 m and 155 m were both located in the drawdown area, whereas the 180 m height was located in the land as a control to 175 m and 155 m. N2O fluxes showed clear seasonal trends at each height and remarkable differences were observed between the two years at the 180 m height. N2O fluxes were lowest in spring at the 180 m height. N2O fluxes showed a single-peak pattern with climax in summer in the first year, whereas a double-peak pattern with climax in summer and after dry-wet alternating in the next year after the highest water level of 175 m was succeeded in the Three Gorges Reservoir. N2O fluxes presented a single-peak shape with summer climax at the 155 m height. Additionally, N2O fluxes were higher during the drainage period than in the flooding period at both the 175 m and 155 m heights. The order of the annual N2O cumulative emissions at the three heights was 175 m (853.92 mg·m-2) > 180 m (336.69 mg·m-2) > 155 m (324.69 mg·m-2), and there was a notable difference between 175 m and 155 m, indicating that short-term flooding could increase N2O emissions but long-term flooding could restrain N2O emissions. Correlation analysis showed that there were no obvious relativities between N2O fluxes and environmental factors in the land and during the drainage period. However, N2O fluxes were significantly negatively correlated with water temperature and wind speed during the flooding period. Principal component analysis found that soil nutrient conditions and physicochemical properties were the most important factors for N2O emissions in the land, the nitrogen distribution in water was a main determinant for N2O emissions during the flooding period, and soil physicochemical properties and microbial activity importantly affected N2O emissions during the drainage period in the drawdown area.

[Characteristics and Influencing Factors of Soil Microbial Biomass Carbon and Nitrogen in Drawdown Area in the Three Gorges Reservoir].

Taking a typical drawdown area located in Wangjiagou of the Three Gorges Reservoir as the study object, four elevations 180,175,165 and 155 m were selected to explore the effect of water level change on soil microbial biomass carbon (SMBC) and microbial biomass nitrogen (SMBN). Wherein, 175,165 and 155 m elevations located in the fluctuating zone, manifested as short, medium and long-term flooding, respectively; 180 m was used as the control, located on the land and never flooded. Sampling depth in soil samples was 0-20 cm, collected once a week. The results indicated that, soil organic carbon (SOC) and total nitrogen (TN) contents at 180 m had no obvious seasonal changes, while they showed remarkable seasonal trends at 175m, which in spring and summer were significantly higher than in autumn and winter; SMBC and SMBN contents and their allocation ratio at four elevations were similar and had significant seasonal fluctuation, which were highest in autumn and lowest in summer at each elevation, indicating that in drawdown area the microbial activity and turnover rate of soil organic carbon and nitrogen were limited by the high-temperature and low-humidity soil environment in summer. Data analysis showed that, compared with the 180 m elevation, contents of SOC, TN, SMBC and microbial quotient, SMBN and its allocation proportion showed varying degrees of increase, while contents of these indexes were significantly lower than control except SMBN and its allocation proportion, meaning that compared with 180 m short and medium-term flooding was conducive to improve soil carbon, nitrogen and their turnover rate and microbial biomass, however, contents of soil carbon and nitrogen and microbial biomass carbon were significantly restricted at 155 m as soil was subjected to flooding stress, meanwhile the turnover rate of SOC was reduced. Correlation analysis implied that SMBC and SMBN had very significant negative correlation with temperature at 5 cm soil depth and pH, meaning that the two environmental factors had a strong effect on soil microbial biomass.