The Release Amount Of Carbon And Nitrogen From Soil And Plant Under Inundation In The Water-Level-Fluctuation Zone Of The Wujiang Hongjiadu Reservoir

The Wujiang River,which is the largest tributary of Yangtze River in the upper reaches,mainly flows through Guizhou Province and part of Chongqing municipality.The Wujiang River Basin's terrain is very undulated and water resources are abundant here.In order to make full use of water energy,11 reservoirs have built in the upper and lower reaches of the Wujiang River.Through the join operation mode of hydropower stations jointly developed by cascades,the maximum benefits can be exerted to ensure the normal operation of the “West-toEast Power Transmission” project.Hongjiadu(HJD)Reservoir,which is the only annual regulating reservoir,is the leading reservoir of the cascade reservoirs.The water level of the reservoir fluctuates resulting in formation of the water-level-fluctuation zone(WLFZ).The WLFZ is the ecotone of aquatic ecosystems and terrestrial ecosystems,and the biogeochemical cycle in this area is very active.The rising water level will affect the content of carbon and nitrogen in the soil.And vegetation decomposing after flooding will release carbon and nitrogen into the water.In order to clarify whether the contribution of carbon and nitrogen released by soil and vegetation flooding to the water threatens water quality safety.This study collected soil samples from the drydown period of the HJD Reservoir in June 2017,April 2018,and April 2019,analyzed the dynamic changes of carbon and nitrogen content after soil flooding.Through the in-situ litterbag technique and simulated experiment indoors to analyze the characteristics of carbon and nitrogen release after vegetation flooding.Finally,we estimated carbon and nitrogen releases amount from soil and vegetation in WLFZ.The main conclusions as followings:1.The soil TC,SOC and TN content in WLFZ were not significantly different during the year of 2017-2019.The interannual differences in TC,SOC and TN content in each soil layers are also not significant,which indicate that the soil carbon and nitrogen contents have reached a stable state.2.There are two types of landforms in WLFZ of HJD Reservoir,the abandoned farmland and the degraded forest land.The results show that the soil carbon and nitrogen content in WLFZ of the abandoned farmland is significantly higher than in WLFZ of the degraded forest land.And the soil TC,SOC and TN contents in abandoned farmland were not significant differences between different elevations.However,the contents of soil TC,SOC and TN in degraded forest land were significantly higher in the higher elevations.This may be due to the abandoned farmland soil was protected by the side foundation rock,and the soil nutrients are not easy to be lost.3.The vegetation decomposition process had obvious stages.In the early stage,the decomposition rate was fast and the dry weight loss was large.As the decomposition time was extended,the decomposition rate was slow and the dry weight loss was not obvious.The herbaceous plants released many carbon and nitrogen substances in the process of dry weight loss.The difference in carbon and nitrogen release between species may be related to the initial substrate of plants.The higher initial N content of plant,or the lower initial C/N of plant,the release amount of TC and TN were higher.4.The DOC and DN of plants released quickly in the early days,and slowly in the late period under immersion experiment condition.The release amount of DOC and DN is characterized by the cumulative release of plant in overlying water.The difference in carbon and nitrogen release between species is related to the initial substrate of soaking plants.The higher initial N content of plant,or the lower initial C/N of plant,the cumulative release amount of TC and TN were higher in the overlying water.5.Combined with the carbon and nitrogen release,plant biomass and importance value for each plant,we calculated the carbon and nitrogen loading from soaking plants.After 120 days of soaking plant decomposition,TC and TN loading of the WLFZ were 533.27kg/ha,40.06kg/ha,respectively.We also suggested that 96.68kg/ha of DOC and 26.01kg/ha of DN had been released into overlying water in the WLFZ once the plants were decomposed for 120 d.6.The carbon and nitrogen release load multiplied by the area of WLFZ,we can know how much carbon and nitrogen is released to water after plant soaking.The loading of TC and TN were 1260 t,95t,respectively,for an area of 23.64km2.The TN loading of herbaceous plants in WLFZ is much smaller than the TN retention in the HJD Reservoir,so we inferred that the nitrogen release after the plants soaking has not threatened the water quality.