Organic Phosphorus Forms And Effects Of Flooding And Dropping In The Soil Of Water-Level-Fluctuating Zone In Pengxi River,Three Gorges Reservoir

The Water-Level-Fluctuation Zone?WLFZ?in the Three Gorges Reservoir is the key area for material and energy transfer between the water body and the terrestrial ecosystem.The soil in WLFZ experienced periodic flooding and dropping under the fluctuation of water level in the reservoir area,and the environmental conditions of the soil were changed,which has an important impact on the phosphorus transfer and transformation between the soil and the reservoir water.Therefore,it is very important to understand the cycle of phosphorus between soil and water in the Three Gorges Reservoir by studying the soil phosphorus forms and the effects of flooding and drying on it.This paper takes the soil of Water-Level-Fluctuation Zone?WLFZ?in the Pengxi River of the Three Gorges Reservoir?TGR?as the research object.Soil samples were collected in September 2017,June 2018 and September 2018.The physical and chemical properties of soil,such as pH value,organic matter content and particle size distribution,were analyzed.The Hedley method and SMT method was used to characterize the organic phosphorus forms.The structure characteristics and bioavailability of soil organic phosphorus were analyzed by ultraviolet-visible scanning spectroscopy and enzymatic hydrolysis.The changes of soil phosphorus forms in different periods were also discussed.The results show:?1?The average contents of total phosphorus?TP?and inorganic phosphorus?IP?in in the soils at different altitudes of WLFZ were 582.01 mg/kg and 455.01 mg/kg respectively,which were significantly lower than those in sediments and riverside soil?p<0.05?.The average content of organic phosphorus?OP?in the soils at different altitudes of WLFZ was 128.63 mg/kg,which was significantly lower than that in the sediment?p<0.05?.The contents of H₂O-P_i?NaHCO₃-P_i?NaOH-P_i in the soil at altitude of 145m were significantly lower than those in sediments?p<0.05?,indicated that the adsorption capacity of sediments to inorganic phosphorus was higher than soils in WLFZ.The average content of NaHCO₃-P_o in the soils at altitude of 145m was lower than that in 165m.The content of NaOH-P_o in the soils at altitude of 145m was significantly lower than that in 165m?p<0.05?.Therefore,at altitude of 145m,the flooding time was longer and organic phosphorus released more.The contents of TP and IP in the soil of WLFZ decreased significantly after a flooding and drying cycle?p<0.05?,and the decreases were 67.58 mg/kg and 60.58mg/kg,respectively.There was no significant change in OP content.After flooding,the contents of H₂O-P_i,NaOH-P_i and HCl-P_i decreased by 3.10 mg/kg,3.30 mg/kg and10.80 mg/kg,respectively?p<0.05?.After drying,the contents of NaHCO₃-P_i?NaHCO₃-P_o?NaOH-P_o decreased by 5.97 mg/kg,2.02 mg/kg and 7.34 mg/kg,respectively?p<0.05?.Flooding and drying caused the release of phosphorus in soils of WLFZ.The content of organic matter and amorphous iron were important factor affecting the form of soil phosphorus in WLFZ.?2?The average content of NaOH-EDTA-P_o in the soils of WLFZ was 49.44mg/kg,accounting for 41.05%of the OP content.The NaOH-EDTA-Po content and the percentage of OP in the soils at altitude of 145m were significantly lower than those in riverside soil?p<0.05?.The average content of EHP in the soils of WLFZ was 23.83mg/kg,accounting for 21.04%of the OP.The bioavailability of organic phosphorus in soils of WLFZ was not low.The content of labile monoester P,diester P and phytate-like P in soils of WLFZ were 7.47 mg/kg,5.93 mg/kg and 10.43 mg/kg,respectively.The content of EHP was mainly affected by phytate-like phosphorus.The content and percentage of EHP in the soils at altitude of 145m were lower than those in165m.Soil flooding time is longer in 145m,and its EHP is easier to be decomposed and released.The content of NaOH-EDTA-P_o was positively correlated with organic matter content?p<0.05?and negatively correlated with soil pH value?p<0.05?.Organic matter content and pH were important factors affected organic phosphorus content.The content of NaOH-EDTA-P_o,labile monoester P,diester P and phytate-like P in soils of WLFZ were positively correlated with SUVA₂₅₄ and A₂₅₃/A₂₀₃?p<0.01?.The higher altitude soils organic matter has higher aromaticity and substituent group concentration,which can effectively prevent soil organic phosphorus from being interpreted and released.?3?After air-drying,the content of Olsen-P_t,Olsen-P_i and Olsen-P_o increased by6.24 mg/kg,4.21 mg/kg and 2.03 mg/kg respectively?p<0.05?.The pH and the variation of amorphous iron content were important factor affected the increase of Olsen-P_i after air-drying.After air-drying,the content of NaOH-P_i decreased by 4.12mg/kg?p<0.05?and the content of NaHCO₃-P_i increased by 4.75 mg/kg?p<0.05?.Air-drying promoted the transformation of part of NaOH-P_i to unstable NaHCO₃-P_i.After air-drying,the content of H₂O-P_o increased by 1.28 mg/kg?p<0.05?,the average content of NaOH-P_o decreased by 1.19 mg/kg,and the percentage of NaOH-P_o decreased by 3.21%?p<0.05?.So air-drying treatment made part of NaOH-P_o transform into H₂O-P_o.Air-drying treatment can improve phosphorus activity in soils of WLFZ.The decomposition of plant residues after flooding can lead to the decrease of water pH and DO concentration,and the maximum cumulative phosphorus release per plant is 0.37 mg/g.Most of DOPw released by plant decomposition during flooding is degraded to inorganic phosphorus.Soil in WLFZ can effectively adsorb inorganic phosphorus in overlying water and 58.78%of the adsorbed inorganic phosphorus is converted to H₂O-P_i,NaHCO₃-P_i and NaOH-P_i,which were easily released.After flooding,the contents of H₂O-P_o and NaHCO₃-P_o decreased by 1.89 mg/kg and 10.82mg/kg respectively,while the contents of NaOH-P_o increased by 3.23 mg/kg.The decomposition of plant residues during flooding can promote the release of organic phosphorus in soil of WLFZ.