Study On The Distribution And Flux Of Soil Nitrogen And Phosphorus In The Fluctuating Zone Of The Reservoir Bay Of The Pengxi River Backwater Area In The Three Gorges Reservoir Area

As a cohesion zone between the water and land area,the water-level-fluctuating zone（WLFZ）is the key area of nutrient‘sorce-sink’.The anti-seasonal dry and wet alternation leads active and complex material exchange between water and soil,which include many processes,such as migration,transformation,release,and so on.At present,spatio-temporal distribution and the relationship between source and sink of the nitrogen and phosphorus fractions under dtying-wetting alternation conditions have not been clear.Therefore,in this thesis,the nitrogen and phosphorus forms and changes of source and sink were studied to lay the theoretical basis for the research of nitrogen and phosphorus in the soils of the WLFZ of the Three Gorges Reservoir,and to provide scientific and technological support for water security of the Three Gorges reservoir,and provide reference for prevention and treatment of eutrophication of tributaries water of the Three Gorges reservoir.The soils of the WLFZ of bay of the backwater area of typical tributary（i.e.Pengxi River）were selected as the object to analyze the distribution of different nitrogen and phosphorus fractions under drying-wetting conditions and different elevations.Meanwhile,in order to analyze the variability of distribution of concentrations of nitrogen and phosphorus fractions,the data of concentrations of nitrogen and phosphorus fractions in the soils of the WLFZ of Bay of Pengxi River backwater area during the inundated and non-inundated periods of this paper’s research period（2018-2019）were compared with that surveyed by our group from 2010 to 2011.Additionally,the exchange fluxes of the soils under inundated or non-inundated condition were estimated by the simplified model.And the effect of different influencing factors on the exchange flux of nitrogen and phosphorus at the water-soil interface of the WLFZ was analysed by an indoor simulation experiment of column samples of the soils.The main results are as follows:（1）The pH in the soils of the research area was neutrality and alkalescency.The concentration of soil organic matter（SOM）exceeded 40 mg/g,and the concentration of SOM at the altitude of 145-155 m was higher than that at any other altitude,indicating that flooding is beneficial to the accumulation of SOM.The value of C/N which was higher than 25 was considered that there was low degree of soil organic carbon humification,nitrogen mineralization and SOM might derive from terrestrial plant residues.（2）In this study,organic nitrogen（ON）occupied the main form of total nitrogen,and ranks of the percentage of inorganic nitrogen（IN）in 2018 and 2019 respectively were nitrate nitrogen（NO₃^--N）>ammonium nitrogen（NH₄⁺-N）>nitrite nitrogen（NO₂^--N）,and NO₂^--N>NH₄⁺-N>NO₃^--N.During inundated period form 2018 to 2019,the concentrations of TN and NO₂^--N had a significant increase whose statistic significance was p<0.01,but the concentrations of NH₄⁺-N and NO₃^--N both had significantly decrease whose statistic significance was p<0.01.During non-inundated period in 2019,the downward trend of the concentration of TN was statistically very significant,p<0.01;the upward trend of concentration of NO₃^--N was statistically very significant,p<0.01;the concentrations of NH₄⁺-N and NO₂^--N both had little change.Compared with data from 10years ago,it was all found that the concentration of TN increased first and then decreased,but the degree of increase and decrease between years from 2010 to 2011 and from 2018 to2019 compared was different.As the altitude ascended,the concentration of TN in the soils increased,NO₃^--N and NH₄⁺-N both decreased,and NO₂^--N had a little change.Compared with the soils above 175 m,the concentrations of NH₄⁺-N and NO₃^--N in the soils of the WLFZ were significantly less than those of the riverside at the altitude of 175-185 m,which may be related to the changes of soils environmental factors easily affected by drying-wetting conditions.The results of correlation analysis indicates that SOM,pH and C/N,may,to some extent,affect the distribution of nitrogen fractions.（3）The inorganic phosphorus（IP）accounted for the largest proportion of TP in soils of the WLFZ,and calcium bound phosphorus（Ca-P）was the main form of IP in soils of the WLFZ.The trends of concentrations of TP,IP,Ca-P decreased during non-inundated period in 2018.While during inundated period from 2018 to 2019 and non-inundated period in 2019,those trends were just opposite.During the non-inundated period in 2019,the concentrations of Organic phosphorus（OP）,iron/aluminum bound phosphorus（Fe/Al-P）and bioavailable phosphorus（BAP）decreased but had little change during the investigated periods.Compaired with the data investigated from ten years ago,there were obviously different trends of concentrations of TP and phosphorus fractions in the soils of the WLFZ,which may result from the frequent drying-wetting alternation,soils background values,the vegetation growth states,and so on.The rank of concentrations of phosphorus fractions of the soils at different altitudes of the WLFZ was all as follows,Ca-P>OP>Fe/Al-P.As the altitude descended,the concentrations of TP,IP,Ca-P of the soils decreased,while Fe/Al-P and BAP increased,and OP had a little change.In addition there weren’t obvious differences of TP and Ca-P of soils at different altitudes.It indicated that frequent periods of drying and wetting may increase the concentrations of active phosphorus,and risk of release of active phosphorus during the inundated period.The results of correlation analysis shows that TP derives from Ca-P,and SOM and C/N can,to some extent,affect the concentration of Fe/Al-P.（4）During the non-inundated period,the results of the flux of TN,TP in the soils of the WLFZ were-2.92 t,2.15 t in 2018,and 5.85 t,-1.68 t in 2019,respectively.During the inundated period from 2018 to 2019,the results of the flux of TN,TP of the soils in WLFZ were-2.2 t and-6.75 t,respectively.After undergoing a cycle（two non-inundated periods and a inundated period）,the decrease of content of TN in the soils was 0.73 t,and the increase of content of TP in the soils was 6.28 t,respectively.（5）Based on analytical results of influential factors on exchange flux,as the temperature increased,the exchange fluxes value of NO₃^--N and IN ranged from source to sink and then to source;the exchange fluxes value of NH₄⁺-N always appeared as sink;and the exchange fluxes value of phosphate always appeared as sink.As the pH value increased,the exchange fluxes value of NO₃^--N and IN always appeared as source;the exchange fluxes value of NH₄⁺-N varied from source to sink;and the exchange fluxes value of phosphate appeared as sink.The exchange fluxes value of NO₃^--N,NH₄⁺-N,IN and phosphate all varied from source to sink under the different concentrations of NO₃^--N,NH₄⁺-N,IN and phosphate in the overlying water,respectively.