Soil Phosphorus Status And The Risks Of Phosphorus Release In The Water-level Fluctuation Zone In A Tributary Of The Three Gorges Reservoir

Since the impoundment of the Three Gorges Reservoir(TGR),water blooms in the tributaries of the reservoir area have occurred.Phosphorus(P)is considered to be a limiting factor for the eutrophication of water bodies in the TGR area.At present,the external P has been effectively controlled,but the water quality in the TGR area has not been significantly improved.The internal release of P from the soil within the water-level fluctuation zone(WLFZ)was considered to be the important source of P in the overlying water in the TGR.During the periodical submergence-exposure alternation,erosion and deposition,drying and re-wetting as well as redox conditions transformations,etc would be observed,which are likely to change the P forms and affect the risk of P release in the WLFZ soil.However,due to limited research methods,the existing researches on the WLFZ soil P were confined to inorganic P.The fate of organic P and in-situ monitoring of P migration process are still lacking,resulting in the incomplete understanding of the mechanism of P migration and transformation in the WLFZ soil.In this paper,soil samples were collected at different altitudes and river sections in a tributary of the TGR(Pengxi River)and were determined for physicochemical properties as well as P forms.The main factors affecting the spatial distribution of soil P forms in the WLFZ were analyzed.By comparing the differences of active P as well as enzymatically hydrolyzable P fractions between field-moist and air-dried soil samples,the main sources of increased active P after soil drying and re-wetting were quantitatively determined.The in-situ passive sampling technique was used to monitor the distribution characteristics of soluble P in the soil-water profile of the WLFZ during different flooding periods,and was also used to explain the P migration process among soil particle,interstitial water and overlying water in different flooding periods.The aforementioned field monitoring investigations and simulation experiments were combined to estimate the P release risk from soil in the WLFZ at the present stage.The paper systematically elaborated the spatial variations of soil P forms and the risks of P release in the WLFZ in a tributary of the TGR.The results can provide a scientific basis for preventing eutrophication in the tributaries in the TGR.The major conclusions are listed as follows:(1)The field monitoring regarding the spatial distribution of P form in the WLFZ soil in the Pengxi River Basin suggested that the mean content of total P(TP)and bioavailable P(Bio-P)in the WLFZ soil were 613.9±100.7 mg/kg and 91.6±25.2mg/kg,respectively,which were lower than those of the sediments and the upland soils.The proportions of Bio-P and residual P(Re-P)in TP increased gradually as the altitude decreased,while Calcium-bound P(HCl-P)showed an opposite trend.Longitudinally,the contents of TP and Bio-P of the WLFZ soil gradually decreased from the estuary to the upstream of the tributary.The spatial distributions of P in the WLFZ soil in the tributary of the TGR were influenced primarily by the soil particle size distribution(PSD),which was jointly effected by the anti-seasonal hydrological regime,rainfall erosion and mainstream backwater.The coupled effects of fine particle deposition,mutual transformation of various P forms,and the material exchange between the soil and the overlying water in the WLFZ,led to the homogenization trend of soil P forms at the same altitude.(2)The comparison of the P forms and phychemical properties of the WLFZ field-moist soils among different altitudes revealed that,the more frequent and extreme drying and rewetting,the more promoted the decomposition of organic matter and the death of microorganisms in the surface soil of the low altitude.Therefore,the contents of TP,organic P(OP),Olsen-TP,Olsen-IP,Olsen-OP and the proportion of bioavailable P in Olsen-OP in field-moist soils at low altitude are significantly lower than those at high altitude.After drying and rewetting,OP content of the surface soil of the 145m and 165m WLFZ decreased by 29.52 mg/kg and 37.75 mg/kg,respectively,and IP content increased accordingly.Olsen-OP content of the surface soil of the 145m and 165m WLFZ increased by 1.33 mg/kg and 2.15 mg/kg,respectively,and Olsen-IP content increased by 6.50 mg/kg and 4.86 mg/kg,respectively.After drying and rewetting,the increase in active organic P such as Water-OP and Olsen-OP was due to the decomposition of organic matter and the death of microorganisms,while the increase in Olsen-IP was mainly due to the transformation of slowly cycling P.After drying and re-wetting,higher increase of the proportion of organic P in Olsen-TP in the soil of the165m WLFZ was observed than that of the 145m WLFZ soil.Inorganic P was the main contributor to the increase of P in bicarbonate extractable due to drying and rewetting,and the organic P components such as Diester P,Phytate-like P and non-enzymatic hydrolysis P also increased significantly.Transformation from slowly cycling P,death of microorganisms,decomposition of plant debris,and destruction of soil aggregates all contributed to the increase of soil active P after drying and rewetting.Among these contributions,transformation of slowly cycling P accounted for 78.22%and 50.64%in the increase of Olsen-TP in the145m and 165m WLFZ soils respectively,and the microbial deaths accounted for15.54%and 17.86%,respectively.Therefore,these two patterns were the major sources.(3)Diffusive gradients in thin films(DGT)and Diffusive equilibrium in thin-films(DET)technologies were used in the in-situ monitoring as well as the field simulation experiments to reveal the distribution characteristics of soluble P and iron in the soil-water profile of the WLFZ.P_DGT and Fe _DGT have a coupling relationship in the soil profile during the water level fluctuation period,storage period,and the falling period following the long-term flooding.During these periods,the replenishment of P in the interstitial water was mainly executed through the simultaneous desorption of P and iron from soil particles.During the flooding period when the previously air-dried soil was rewetted,the replenishment of P in the interstitial water was mainly executed by the rapidly increased soil active P entering the interstitial water.The averaged R(P_DGT/P_DET)values of soil vertical profile gradually decreased in the order of storage period(0.56±0.20)>falling period(0.37±0.19)>water level fluctuation period(0.17±0.11)>rising period(0.16±0.12).This result indicated the strongest ability of soil particle to replenish interstitial water P during the storage period,which was consistent with the response time(T_c)obtained according to the DGT-induced fluxes in sediments model(DIFS).A large T_c area was observed in the upper and lower part of the soil vertical profile in the WLFZ in each period,indicating that there were two P pools.The oxidation-reduction transition zone in the surface layer and the poor vertical void structure were responsible for the accumulation of phosphorus at the upper and bottom of the WLFZ soil profile,respectively.Based on Fick's diffusion law,P release flux at the WLFZ soil-water interface was between-3.3?17.1 ng/cm²/d,suggesting the WLFZ soil could be regarded as a P source to the water body of the TGR.After the gradual accumulation of desorbed P from soil particles in the interstitial water during the flooding period and the storage period,the soil P release flux became higher in the falling period and the water level fluctuation period when the concentration of P in the interstitial water was higher.The concentration of soluble P in the interstitial water is the main limiting factor that affects the P release flux from the WLFZ soil-water interface.(4)According to the field monitoring and simulation experiments,the water-P contents of all soil samples(except for the SJ point and some soils affected by farming)in the WLFZ is less than 8.5 mg/kg,molar Al:Fe ratios were greater than 3,and degrees of P saturation(DPS)were less than 25%.Overall,the risk of soil P release in the WLFZ of the Pengxi River Basin is small.Monitoring should be prioritized at the altitude of 165 m and in the estuary of the WLFZ considering the high DPS and/or low Al:Fe ratio in these areas.The WLFZ soil that is periodically submerged releases soluble P into the overlying water body,resulting in the decrease of DPS.Based on the lower DPS in the soil of the WLFZ relative to that of the upland soil,a formula estimating the total amount of soluble P loss in the WLFZ was established.The amounts of released soil P ranged between-45.41 mg/kg and 140.84 mg/kg,showing a gradual increasing trend with decreasing altitude.The release of soluble P from the WLFZ soil mainly occurred within the earlier e.g.,1-2 years of reservoir impoundment.After more than ten years of periodic submergence-exposure,the release of soluble P from the WLFZ soil at the current stage has a limited impact on the overlying water.