Influencing Factors And Flux Estimation Of Greenhouse Gas Emissions From Typical Ditch Ponds In The Hilly Area Of The Upper Reaches Of Taihu Lake

Ditch ponds are typical small and micro wetlands.Although their area is small,they are numerous and widely distributed.They are an important part of the global inland water bodies and play an important role in the global carbon and nitrogen cycle.In hilly areas with undulating terrain,ditch and pond are mainly distributed in low-lying gully areas,which are formed by the collection of slope runoff.They are an important hydrological channel to carry the runoff of catchment slope and transport materials to the downstream.At the same time,they are also a hot area for biogeochemical reaction of carbon and nitrogen.However,the greenhouse gas emission process of gullies and ponds in hilly areas has been neglected for a long time,and its emission potential needs to be revealed.In this study,four types of typical ditch ponds（tea garden pond,village pond,forest pond and aquaculture pond）in the hilly area of the upper reaches of Taihu Lake are selected as the research object.Through the observation of the physical and chemical properties of ditch pond water and sediment and the concentration of dissolved greenhouse gases（N₂O,CH₄ and CO₂）in the water,combined with static box sampling,the emission potential of greenhouse gases from different types of ponds is accurately estimated,and the main influencing factors affecting their emission are revealed.The main conclusions are as follows:（1）There are significant temporal and spatial differences in the physical and chemical indicators of water bodies and sediments in different ditch ponds.In general,affected by different land use compositions of the catchment sloping land,the annual average concentrations of total nitrogen and nitrate nitrogen in the water body of the tea garden pond are the highest,the annual average concentration of COD and p H in the aquaculture ponds are the highest,and the conductivity SPC of the village pond is relatively high;Among them,the index concentration of nitrogen in the sediment of the tea garden pond was the highest,while the average concentration of the related indexes of total phosphorus and carbon in the aquaculture pond was the highest.From the perspective of seasonal changes,the seasonal fluctuations of physical and chemical indicators of different types of ditch ponds water bodies are obvious.Among them,the seasonal variation trends of physical indicators in different types of ditch ponds are basically the same,showing seasonal variation characteristics of low in autumn and winter and high in spring and summer;while chemical indicators of nitrogen and carbon source concentrations show different trends with seasons,such as tea garden pond nitrogen concentration in winter.It is high in spring,indicating that it is seriously affected by fertilization,while the carbon source concentration in the aquaculture pond is high in summer and autumn,which is related to the feeding of breeding feed.（2）By observing the concentrations of dissolved N₂O,CH₄ and CO₂ in the pond water,the temporal and spatial characteristics of various greenhouse gas concentrations in the ponds were further revealed,and the main controlling factors driving greenhouse gas emissions were analyzed.In general,there are large differences in the concentrations of greenhouse gases in different types of ponds.For example,the concentration of N₂O in the tea garden pond is relatively high,exceeding its saturation concentration by 366.7-1555.0%,reaching 2-3 times that of other ponds,indicating that the N₂O in the tea garden pond The emission potential is the largest;the CO₂ emission potential of the village pond is the largest,and the CO₂ saturation of the village pond is 1.4-4.3 times that of other ponds;and the CH₄ emission potential of the aquaculture pond is the largest,which is 3-5 times that of other ponds respectively;It can be seen that there are obvious seasonal differences between different types of greenhouse gases.The dissolved concentrations of N₂O and CH₄ in most ponds are much higher in summer and autumn than in winter and spring,and the CO₂ concentration shows a trend that is much higher in winter and spring than in summer and autumn.The dissolved concentrations of N₂O and CH₄ in the pond gradually increased at night,indicating that the emission potential at night was greater;while the concentration of CO₂ gradually increased in the morning during the day,indicating that the respiration was enhanced.The study further analyzed the main factors driving the temporal and spatial changes of different greenhouse gas concentrations.It is found that the emission of N₂O is mainly affected by（NO₃-N,NO₂-N）and DO,in which the saturation of DO and N₂O are negatively correlated,and nitrogen will promote the denitrification process;the emission of CO₂ is mainly affected by SPC,water temperature,TN and f DOM,SPC was positively correlated with CO₂saturation;CH₄ emissions were mainly affected by temperature and nitrogen（NO₂-N,NO₃-N）,and NO₂-N,NO₃-N were negatively correlated with CH₄ saturation.（3）The research adopts the flux estimated by the static box to correct the gas diffusion model,and estimates the emission fluxes of different greenhouse gases in typical ponds in the hilly area of the upper reaches of Taihu Lake.The model significantly reduces the uncertainty of flux estimation,and it is concluded that the Eq3（Crusius and Wanninkhof,2003）model is suitable for estimating discharge fluxes from ponds and ponds in hilly areas;further applying the revised diffusion model,it is found that for N₂O,tea garden ponds The emission flux of the ponds was up to 12.02μmol·m^-2·d^-1,which was 3-4 times that of the village ponds,forest ponds and aquaculture ponds,respectively;for CH₄,the emission fluxes of the aquaculture ponds were up to 1.11 mmol·m^-2·d^-1,the lowest in forest pond is 0.24 mmol·m^-2·d^-1;for CO₂,the emission flux of village and pond is up to 87.44 mmol·m^-2·d^-1,which is 5.3 times that of forest pond.However,due to the influence of water in sloping land,the GHG emissions of ditch and pond are variable,and the estimation uncertainty is relatively large;in the future work,the sampling frequency can be increased,the monitoring method can be improved,or the gas diffusion model can be corrected in situ,etc.