Phosphorus (P) release risk in lake sediment evaluated by DIFS model and sediment properties: A new sediment P release risk index (SPRRI)

Environmental Pollution 255 (2019) 113279

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Phosphorus (P) release risk in lake sediment evaluated by DIFS model and sediment properties: A new sediment P release risk index (SPRRI)* Zhihao Wu a, b, c, Shengrui Wang a, b, c, *, Ningning Ji a, b a National Engineering Laboratory for Lake Pollution Control and Ecological Restoration, Institute of Lake Environmental, Chinese Research Academy of Environmental Sciences (CRAES), Beijing 100012, China b College of Water Sciences, Beijing Normal University, Beijing, 100875, China c Yunnan Key Laboratory of Pollution Process and Management of Plateau Lake-Watershed, Kunming, Yunnan Province, 650034, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 January 2019 Received in revised form 3 August 2019 Accepted 18 September 2019 Available online 19 September 2019

A new sediment P release risk index (SPRRI) for “in-situ” phosphorus (P) release risk in lake sediment, is developed based on diffusive gradients in thin films (DGT) technique, DGT induced flux in sediments (DIFS) model and sediment properties. SPRRI includes three sub-indexes, which contain (1) the labile P pool size, (2) resupply constant (r) and desorption rate (Dspt rate) for P transfer and (3) the molar ratio between iron (Fe) in sequential extraction for sediment P by bicarbonate-dithionite (BD) and aluminum (Al) by NaOH (at 25
C), i.e. BD(Fe)/Al[NaOH25] in sediment solid. The first sub-index considers P release from (i) sediment with NH4Cl-PþBD-P pool, i.e. the loosely sorbed P (NH4Cl-P) plus iron associated P (BDP), or (ii) sediment with NH4Cl-P pool, respectively. The second and third sub-indexes reflect kinetic P desorption and resupply ability of solid phase, and the effect of P sequestration by Al hydroxide on P release, in turn. The inner relationship between SPRRI and sub-indexes, and their effects on P release risk are elucidated. SPRRI can be used to evaluate sediment P reactivity by five release risk ranks. For Lake Dianchi (China), P transfer dynamics, labile P pool, resupply ability and Al-P in sediment, and “external Ploading” control and affect P release risk in different regions, which is reflected by the spatial distribution map for SPRRI. The present SPRRI can be applied for lakes with (1) pH range varying from moderate acidity to weak alkalinity in waterbody and (2) NH4Cl-P or NH4Cl-PþBD-P pool in sediment solid. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Phosphorus DGT induced flux in sediments Sediment P release risk index Diffusive gradients in thin films

1. Introduction Sediment is a large phosphorus (P) sink in lake bottom, which can be remobilized back into water through biogeochemical and physical processes (Haggard et al., 2012; Kraal et al., 2015). P release from lake sediment engenders “internal P-loading”, which impedes lake restoration regardless of the reduction of “external P-loading” (Pei et al., 2013; Powers et al., 2014). Sediment P release to water through ion exchange, adsorption-release balance and diffusion (Gardolinski et al., 2004; Xu et al., 2012) leads to “internal P cycle” at sediment/water interface (SWI). In summer, lake stratification

* This paper has been recommended for acceptance by Bernd Nowack. * Corresponding author. National Engineering Laboratory for Lake Pollution Control and Ecological Restoration, Institute of Lake Environmental, Chinese Research Academy of Environmental Sciences (CRAES), Beijing 100012, China. E-mail address: [email protected] (S. Wang).

https://doi.org/10.1016/j.envpol.2019.113279 0269-7491/© 2019 Elsevier Ltd. All rights reserved.

forms hypolimnetic anoxia at SWI and P is mainly remobilized from iron (Fe)-bound P due to the reductive dissolution of Fe(III) in sediment (Norton et al., 2008). The other mechanisms for P release are the breakdown of algae biomass (Palmer-Felgate et al., 2011) and the coupled P-Fe-sulfur reactions (Ding et al., 2012). P sequestration by aluminum (Al) hydroxide [Al(OH)3(s)] under circumneutral pH in sediment is unaffected by redox condition; and
 Al-P is normally a permanent sink (Kopa cek et al., 2005). The precipitation of hydroxylapatite or fluorapatite (Ca5(PO4)3(OH,F)) (Miot et al., 2009; Palmer-Felgate et al., 2011) also leads to P sequestration. The sequential extraction scheme (Hieltjes and Lijklema, 1980; Psenner and Pucsko, 1988) and phosphorus-31 nuclear magnetic resonance spectroscopy (Carman et al., 2000; Ahlgren et al., 2006) have been used to determine the fractions or species of inorganic and organic P. P can also be released from organic P in sediment, providing a nutrient source for phytoplankton uptake followed by

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phosphatase hydrolysis, bacterial decomposition and photolysis of organic P compounds (Jansson et al., 1988; Ahlgren et al., 2005). Organic P is partly available and treated as the residue fraction (Ruttenberg, 1992; Golterman et al., 1998; Ruban et al., 1999). The present normal methods for P mobility and release risk at SWI include (i) the sequential extraction scheme for P (Zhang, 2000; Wilson et al., 2008) and the associated Al/Fe in extraction fractions (Kop
a cek et al., 2005) to reveal P mobility affected by sediment properties; (ii) the simulation experiment using oscillating grid and annular tank (flume) (Orlins and Gulliver, 2003; Zhang et al., 2012) for dynamic P transfer under hydrodynamic conditions at SWI; (iii) the sorption-desorption ability revealed by isotherm sorption (Freundlich or Langmuir), equilibrium P concentration (EPC0) and native adsorbed phosphorus (NAP) (An and Li, 2009; Lin et al., 2009; Chen et al., 2014) and (iv) P release risk index (ERI) based on P sorption index (PSI) and degree of phosphorus saturation (DPS) (Huang et al., 2004). However, the “in-situ” dynamic P transfer or P release risk in sediment can't be revealed by these methods. Diffusion Gradients in Thin films (DGT), an “in-situ” passive sampling technique, has been developed (Davison et al., 1994) for the concentration and flux of pollutants in sediment porewater. DGT operates by inducing a controlled perturbation into sediment, and the measurement result reflects the response of sediment to perturbation. To quantify influence factors (the reaction/transport parameters at sediment/porewater interface, DGT parameters and sediment properties) on DGT response, Harper et al. (1998) developed a numerical model of DGT induced fluxes in sediments and soils (DIFS). The present DGT researches include biogeochemistry responsible for P release (Ding et al., 2012, 2016), “internal Ploading” (Wu and Wang, 2017) and DIFS simulation for P transfer in lake sediment (Monbet et al., 2008; Wu et al., 2016). There are only a few studies for dynamic P release in lake sediment using DIFS (Monbet et al., 2008; Wu et al., 2016). The diffusion and release process of P at DGT/sediment interface can be reflected by DIFS parameters and curves, such as r (resupply constant) for the resupply ability of sediment solid; and the dissolved or sorbed concentration in sediment solid or sediment solution. However, P release risk in lake sediment can't be reflected by DIFS model directly. Until now, there are two references for “in-situ” P release in sediments in Lakes Poyang (Yang et al., 2016) and Dianchi (Wu and Wang, 2017) in China based on DGT/Peeper and DGT/DIFS, respectively. However, in the first paper, the diffusion fluxes of soluble labile P at SWI and resupply kinetics (r) for the buffering ability of sediment solid can't reveal P release risk related to some influence factors (sediment properties) on P release comprehensively. In the second paper, it only considers dynamic P process in sediment by DIFS model. In fact, the labile P pool in solid phase and the effect of Al-P on P release are also key factors influencing P release. As mentioned above, the present “in-situ” methods are only focused on kinetic P exchange across DGT/sediment interface, or geochemical reactions. In this paper, a new evaluation method of SPRRI (sediment P release risk index) has been developed for lake sediment; the inner relationship between (i) P release risk and (ii) dynamic P transfer, biogeochemical reactions and sediment solid (P, Fe and Al), is revealed by SPRRI. SPRRI is based on DIFS model and physicochemical properties in sediment, which fully consider all the key factors controlling sediment P reactivity. In this paper, DGT and DIFS are designed for P transfer in lake sediment to (1) reveal kinetic P exchange at DGT/sediment interface and ascertain the labile P pool in sediment solid accounting for P release; and (2) develop SPRRI index for P release risk in lake regions based on (a) P fractions, (b) P transfer dynamics from solid

phase to sediment solution, and (c) the effects of chemical properties in sediment on P mobility. 2. Materials and methods 2.1. Research area and sediment sampling Lake Dianchi (Fig. 1) in Yunnan-Guizhou Plateau, with the surface area of 306.3 km2 and the average water depth of 5.0 m, is the sixth largest freshwater lake in China. Cyanobacterial blooms have frequently occurred in this eutrophic lake (Hu et al., 2006) due to high N and P levels and serious “internal P-loading”. Lake Dianchi with a length of 39 km (N~S) and a width of 13 km (E~W), includes two large regions (Caohai and Waihai) by a manmade dike. Caohai is situated in the north; while Waihai is main waterbody, accounting for 96.7% of whole lake area (Fig. 1). Twenty-six sites in six regions in lake (Fig. 1 and Table S1) are chosen for P release risk by SPRRI. Six regions include the north area (sites 1e4) in Caohai (I), and the middle-north area (II) (sites 5e8), the east area (III) (sites 9e13), the middle-south area (IV) (sites 14e17), the west area (V) (sites 18e21) and the south area (VI) (sites 22e26) in Waihai. The classification of land use in Dianchi river basin is demonstrated in Fig. S1e1 (Wang, 2014) in supplementary material (SM). The emission intensity of TP and the key pollution source in the sub-watersheds are demonstrated in Fig. S1e2 (Gao et al., 2013). “External P-loading” means all supply sources for P excluding waterbody, which consist of the point and non-point P sources. In Lake Dianchi, the intense “external P-loading” includes municipal effluent into Caohai (I) and the middle-north (II), agriculture nonpoint input into the east (III) and the west (V), and agriculture non-point source and phosphorite mine in the south (VI) (Wu et al., 2016). The agriculture non-point input includes pesticide, fertilizer and farm waste, for example, organophosphorus pesticide (parathion and disulfoton), phosphatic fertilizer, domestic waste, rural solid waste, farming solid waste and livestock breeding waste. In Sep., 2013, Orion 4-star meter (Thermo, USA) was used to measure Eh, pH and T for three times at each site; then, sediment and overlying water with depths of 60 and 10 cm in turn, were collected by a PVC tube (70 cm in length) fixed to a Beeker corer (Holland); then, the open end of each PVC tube with a diameter of 12 cm was sealed by a rubber stopper; these sediment cores were brought into the lab at lake shore in 1 h; then, they were immediately kept in incubators (MIR-262, Panasonic Company) at same temperature (T) (21 OC) as in-situ sites. DGT test was conducted for 24 h in surface sediment layer at once. The disturbance of the microenvironment in each sediment core was controlled to minimum by method in Sect. 2.2 and SM A (supplementary material). Eh, pH and T were measured for five times during 24 h DGT deployment. The overlying water was collected in acid-cleaned polyethylene (PE) bottle and kept at 4 OC in a refrigerator. 2.2. DGT deployment and measurement DGT pistons and materials were manufactured by DGT Research Ltd. (Lancaster University, U.K.). DGT piston includes a ferrihydrite gel (0.40 mm in thickness) for P or chelex-100 gel (0.40 mm in thickness) for Fe, a diffusive gel (APA2; 0.78 mm in thickness), a filter (0.14 mm in thickness), a base and a cap with a window (1.80 cm in diameter). After deoxygenation (Wu and Wang, 2017), one ferrihydrite and one chelex-100 DGT pistons were pushed back-to-back into sediment layer along the length of each sediment core by method in SM A; the lower edge of each DGT piston was 4.0 cm away from SWI. Then, two DGT pistons began to absorb soluble labile P and Fe in sediment porewater. After 24 h, DGT pistons were retrieved and dismantled. Each gel disc of ferrihydrite

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Fig. 1. Locations of sampling sites in Lake Dianchi. They includes six regions: I (Caohai), II (Middle-North), III (East), IV (Middle-South), V (West) and VI (South). þ or D stands for sampling site or Kunming city, in turn.

or chelex-100 was eluted using 0.25 mol L1 H2SO4 for P or 1.0 mol L1 HNO3 for Fe for 24 h, respectively. Soluble labile P in elution solution was determined using the molybdenum blue (ascorbic acid reduction) spectrophotometric method (Murphy and Riley, 1962). The elution solution for Fe was diluted and measured using the high resolution inductively coupled plasma mass spectrometer (HR-ICP-MS) (ELEMENT XR, Thermo Scientific) with internal standard (103Rh) (Wu and Wang, 2017). The certified reference materials (CRMs) of GBW07311 and GSD9 (National Sharing Platform for Reference Material) were used as standard solutions for Fe and P (DGT test), respectively. The measurement of DGT pistons (n ¼ 5) in each standard solution (Fe or P) accords to methods in references (Zhang et al., 1998).

2.3. Procedure for sediment and overlying water Eh, pH and T at depth of 4.0 cm in each sediment core were measured for five times during DGT test using Orion 4-star meter (Thermo, USA). Then, the surface sediments (0~ 8 cm at depth) were retrieved from sediment cores; they were lyophilized at 80
C, sieved to pass through a 63-mesh screen and kept at 4 OC. A sequential extraction scheme (Hieltjes and Lijklema, 1980; Psenner and Pucsko, 1988) and a molybdenum blue (ascorbic acid reduction) spectrophotometric method (Murphy and Riley, 1962) were used for P fractions in sediment. Five P fractions include (i) NH4Cl extractable (NH4Cl-P); this fraction is loosely bound or adsorbed P; (ii) iron associated P (BD-P), i.e. NaHCO3þNa2S2O4 extractable (40
C); the reductant soluble phosphorus forms are extracted, mainly from iron hydroxide surfaces; (iii) NaOH extractable (25
C) P (NaOH25-TP); it includes (a) NaOH25-P (Al-P), representing P adsorbed to metal oxides (Al2O3) and other surfaces, and (b) the minor fraction of NaOH25-nrP; (iv) calcium associated

P-HCl extractable (HCl-P); it represents P bound to calcite (CaCO3) and apatite (Ca5(PO4)3(OH)); and (v) NaOH extractable (85
C) (RP); R-P includes organic and refractory P compounds. Total P (TP) content is the sum of five fractions (iev). NH4Cl(Fe), BD(Fe) and Al(NaOH25) represent Fe and Al contents in P extraction fractions of NH4Cl-P, BD-P and Al-P, respectively. The extraction solutions were measured using HR-ICP-MS for Fe and Al. The total organic carbon (TOC) stands for the content of organic matter (OM) in sediment; TOC was determined by potassium bichromatedilution heat colorimetric method (Bao, 1999). The contents of total Fe (TFe), aluminum (TAl) and calcium (TCa) in sediment were measured through HNO3eHFeHClO4 digestion and subsequent HR-ICP-MS analysis (Lin et al., 2008). GSD9 and GBW07435 (National Sharing Platform for Reference Material) were used as CRMs for TFe, TAl, TCa and TOC. The procedure for sediment porewater was conducted in a specially designed glove box purged with nitrogen (O2 concentration <1%) (Monbet et al., 2008). Sediment porewater (C0 (P)) was extracted by centrifugation (50 g sediment) for 30 min at 5000 rpm at room temperature (Yang et al., 2010); after filtration through a 0.45 mm polysulfone filter, the soluble reactive P (SRP) in sediment porewater in water sample was determined by the molybdenum blue (ascorbic acid reduction) spectrophotometric method (Murphy and Riley, 1962). P content in the centrifugation extract after filtration and digestion is the dissolved total P (DTP), which was also measured. DTP includes the dissolved organic (DOP) and SRP (Glæsner et al., 2012). According to research results in references (Van Moorleghem et al., 2011; Glæsner et al., 2012), DGT measured P fraction is mainly SRP, but it may include a small portion of DOP in porewater. The soluble Fe concentration in sediment porewater was measured using HR-ICP-MS. GBW07311 and GSD9 (National Sharing Platform

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for Reference Material) were used as CRMs for the measurement of Fe and P in porewater. 2.4. DGT calculation and DIFS simulation According to DGT theory and Fick's 1st law (Zhang et al., 1998), the mass of solute (M) accumulated on binding gel, induced DGT flux (FDGT) and DGT concentration (CDGT) are calculated by SM Eq. s (1e3) in SM A. r can be calculated by the ratio between CDGT and the initial sediment solution concentration (C0(P)). r is an indication of the depletion extent of sediment solution concentration at DGT/ sediment interface. DIFS software was provided by DGT Research Ltd. (Lancaster University, U.K.). DIFS model (Harper et al., 2000) reveals the dependence of r on P resupply from solid phase to sediment porewater coupled with diffusive supply to DGT/sediment interface and from diffusive layer into the resin (Fig. S2). kd is distribution ratio of the labile P fraction in solid phase (Cs) to the initial sediment solution concentration (C0(P)). The characteristic time for the system to approach equilibrium is indicated as Tc (response time). TC and kd are controlled by kinetic constants of adsorption (k1) and desorption (k-1) (SM B and SM Eq. s (4e7) in SM C). The input parameters of C0(P), sediment porosity (fs), diffusive layer thickness (Dg), the effective diffusive parameter in sediment (Ds) (Table S2 and SM Eq. (8) in SM C), the diffusive parameter in diffusive layer (Dd) and particle concentration (Pc) are required for DIFS. kd and r are also input into DIFS software; while, DIFS outputs Tc; and solution and solid phase concentrations at DGT/sediment interface are output for any chosen time. Moreover, r against time (t), and Csolu(P) (soluble labile P concentration in porewater) against t and distance from DGT/sediment interface, can be derived by DIFS model. The distance from DGT surface to which sediment porewater (Xsolu) in sediment solid is reduced at 24 h, is derived by DIFS output data and the related method (SM C). The output DIFS curve for r against t is used to derive the maximum r (rmax) and the time at which rmax is attained (Trmax) (SM C). The desorption rate parameter (Dspt rate) (nmol ml1 d1) is derived by SM Eq. (9) in SM C (Menezes-Blackburn et al., 2016).

Table 1e1 The grade standard for P release risk. SPRRI Range [lg (nmol cm3 d1)]

Grade for P release risk

0～5 5～15 15～30 30～45 >45

light moderate relative high high very high

(Fig. S3e1) were chosen for the classification of five ranks of P release risk. The SPRRI indexes for these 100 sites are indicated in Table S3, which are calculated by Eq. (1). The introduction of the classification method is demonstrated in SM D. Gaussian distribution curve for SPRRI indexes (n ¼ 100) is indicated in Fig. S3e2. 2.6. Quality control (QC) and statistical methods All reagents were analytical grade. The CRMs and reagent blank were analyzed during the measurements of every batch of samples. CRMs included GSD9, GBW07435 and GBW07311 (National Sharing Platform for Reference Material), which were used for (1) TFe, TAl, TCa, TOC or TP in sediment and (2) the soluble labile P or Fe in sediment porewater, water and DGT elution. The recoveries, limits of detection, repeatability and reproducibility (QC) for analytical methods and the reference materials are demonstrated in Table S4. The good QC results indicated DGT, HR-ICP-MS and colorimetry were the reliable measurement methods (sediment, porewater and water). The linear correlation analysis between independent variables and the standard T test for the measurement results were conducted through Origin 9.0 (Origin Lab). CDGT(P), CDGT(Fe), C0(P), TP, NH4Cl-PþBD-P, BD-P, NH4Cl(Fe)þBD(Fe), BD(Fe) or Al[NaOH25] for sediment samples are estimated using the two-tailed T test (significance level at p ¼ 0.01). Google Earth 7.1.5.1557 (Google Earth Enterprise) and Surfer 11.0 (Golden Software) were used for the map of Lake Dianchi.

2.5. SPRRI (sediment P release risk index) 3. Results In order to assess P release risk, a SPRRI index based on DIFS parameters, P fractions and Fe/Al molar ratio in lake sediment was developed. The evaluation equation is indicated as Eq. (1). SPRRI ¼ 10  lg (1000  LAP/TP)  [r  lg(Dspt rate)]  BD(Fe)/Al [NaOH25]. (1) where, SPRRI [lg (nmol cm3 d1)] is the assessment index of “sediment P release risk index”; SPRRI consists of three parts (subindexes); in part 1 (lg (1000  LAP/TP)), LAP (mmol g1) is the labile P pool in sediment solid, which is NH4Cl-PþBD-P or NH4Cl-P, respectively; TP (mmol g1) is total P content in sediment; part 1 demonstrates the relative reactive P pool size in solid phase; in part 2 (r  lg(Dspt rate)), r is resupply constant and Dspt rate (nmol ml d1) is desorption rate; part 2 reflects P exchange between porewater and sediment solid and desorption kinetics; part 3 reflects the effect of Al on P release based on the ratio of BD(Fe) to Al [NaOH25], which can be used as a criteria to evaluate the relative abundance of Al-P (Al[NaOH25]) and the effect of Al(OH)3 precip
 itation on P remobilization. In references (Kopa cek et al., 2005; Norton et al., 2008), the reciprocal of BD(Fe)/Al[NaOH25] has been used as the norm (>3) for the distinct effect of Al on preventing P release. P release risk grades with five ranks ranging from “light” to “very high” are indicated in Table 1e1. The intensive sampling sites (n ¼ 100) in the whole Lake Dianchi

3.1. Labile P pool in sediment solid TP (674e3110 mg kg1, dw), TFe (17432e28742 mg kg1, dw), TAl (5543e8642 mg kg1, dw) and TCa (8013e12041 mg kg1, dw) in sediment and other physicochemical properties at 26 sites (Table S5) represented eutrophic status in lake. There was significant difference between CDGT(P) and C0(P), TP, kd, Al[NaOH25], Eh or pH in sediment sites (n ¼ 26) by the paired-samples T test (significant level set at: p ¼ 0.01), in turn. The relative standard deviations (RSDs) for Eh, pH or T (n ¼ 8) for both in-situ site and sediment core in incubator were less than 8.8%, 7.1% or 6.2%. It reflected the microenvironment in sediment core was similar to insitu site. pH can influence P sorption-desorption and P release at sediment/porewater interface. At high pH, OH can displace PO3 4 sorbed to iron complexes and promote ion exchange of PO3 4 with € m, 1984). The relationship between OH on mineral surface (Bostro pH and P sorption to FeOOH has been revealed by Lijklema (1980). The weak alkalinity (pH ¼ 7.84e8.37) in sediment favored P release from Fe bound P in sediment in Lake Dianchi. The sediment sites were classified to two types of (1) twenty sites (1e8, 11e19 and 22e26) with Eh range (54~ 137 mV) and (2) six sites (9, 10, 14, 15, 20 and 21) with Eh scope (60e120 mV), respectively (Table S5e1). It is based on the rough classification of

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redox status by Eh. In fact, the measurement of oxidants (O2 and others) is the only accurate method to define oxic and anoxic sediments (Sigg, 2000; Canfield and Thamdrup, 2009). The detailed inform about this content is demonstrated in Sect.4.2. The linear correlation relationships of CDGT (P) against CDGT(Fe), BD-P, NH4Cl-PþBD-P, BD(Fe) or NH4Cl(Fe)þBD(Fe) for twenty sites (1e8, 11e19 and 22e26) with Eh scope (54~ 137 mV) indicated the good correlation coefficients (R2)  0.71 (Table S6). It demonstrated the loosely bound and adsorbed phosphorus (NH4Cl-P), and especially BD-P (P associated with amorphous Fe(OH)3), were responsible for P release. Due to more abundant BD-P than NH4Cl-P (Table S5e2), BD-P was the main labile P pool when hypolimnetic anoxia at SWI in Sep. was formed (Norton et al., 2008; Wu and Wang, 2017). The high pH, T (21OC) and the abundant TFe (Table S5e1) favored P release from Fe-bound P in sediment (Ding et al., 2016). On the contrary, at six sites (9, 10, 14, 15, 20 and 21) with Eh range (60e120 mV), the linear correlation relationships of CDGT (P) against (i) CDGT (Fe) with poor R2 of 0.60; (ii) NH4Cl-P with R2 of 0.94 or (iii) BD-P, BD(Fe) or NH4Cl(Fe)þBD(Fe) with poor R2 of 0.08, 0.32 or 0.53, respectively (Table S6), demonstrated NH4Cl-P was the labile P pool at these six sites. CDGT(P) values (<1.0 nmol ml1) at sites (9, 10, 20 and 21) with NH4Cl-P pool were lower than twenty sites with NH4Cl-PþBD-P pool. CDGT(P) values (<5.5 nmol ml1) at sites (14 and 15) with NH4Cl-P pool were lower than the average value of 6.55 nmol ml1 for twenty sites with NH4Cl-PþBD-P pool. It is mainly due to NH4ClPþBD-P pool at twenty sites larger than NH4Cl-P pool at six sites. However, the linear correlation relationship of CDGT(P) against R-P (organic and refractory P compounds) was poor with R2 of 0.40 (P < 0.01) or 0.37 (p < 0.05) for twenty sediment sites (54 < Eh < 137 mV) or six sites (60 < Eh < 120 mV), respectively. It means that R-P is not the labile P pool. According to research result mentioned above, kd can be rightly derived for two types of sediments with the reactive P pools in solid phase, i.e. NH4Cl-P and NH4Cl-PþBD-P pool, respectively. 3.2. P diffusion and resupply at DGT/sediment interface According to DIFS model, P transfer across DGT/sediment interface includes (1) P diffusion in porewater induced by DGT sink, and (2) kinetic desorption from solid phase into porewater to offset porewater concentration depletion (Fig. 2e1). The research methods for dynamic P transfer and SPRRI are indicated in Fig. 2e2. DIFS parameters for six lake regions are indicated in Table 2. The introduction to DIFS parameters is demonstrated in SM B. The sediments at sites (1 and 2) in Caohai (I), site 6 in middle-north (II), site 13 in east (III), site 19 in west (V) and sites (22 and 24e26) in south (VI) with the large kd > 300 (cm3 g1) and NH4Cl-PþBD-P pool; while, those at sites (9 and 10) in east (III) and site 21 in west (V) with low kd < 100 (cm3 g1) and NH4Cl-P pool. The range of 95.9e529.6 (cm3 g1) for kd in this lake was similar to 104e602 cm3 g1 in the other Plateau Lake Erhai in China (Wu et al., 2016). The kd distribution in each region, the distribution of “external P-loading” in the sub-watersheds in Lake Dianchi, and the effect of OM on P release are demonstrated in SM C. According to the criterion in references (Harper et al., 1998; Wu et al., 2016), r curve character can be classified to three groups: “fast resupply” (sites 1, 2, 6, 13, 24 and 26) with r > 0.70, “slow rate of resupply” (sites 9 and 10) with r < 0.20 and “intermediate rate of resupply” (the other sites) with 0.20 < r < 0.70. Sites (1 and 2) in Caohai (I), site 6 in middle-north (II), site 13 in east (III) and sites (24 and 26) in south (VI) with r > 0.70 demonstrated the resupply ability larger than that at sites (9 and 10) in east (III) with r < 0.20. For example, kd, r, k-1 and Dspt rate at sites (1 and 2) with anoxic

5

status belonged to “fast resupply” for P transfer; and they were larger than those at sites (9 and 10) with oxic status and “slow resupply” (Wu et al., 2016). In Fig. S4, Csolu(P) curve for “fast resupply” at site 2 corresponded to short depletion distance (Xsolu ¼ 0.061 cm); on the contrary, Csolu(P) curve for “slow rate of resupply” at site 10 demonstrated the long depletion distance (Xsolu ¼ 1.294 cm); r curve for “fast resupply” indicated a large Trmax (2.77 h) and then a slow depletion shape; while, r curve for “slow rate of resupply” was characterized by a low Trmax (0.28 h), i.e. a maximal value in the first hour (rmax ¼ 0.408) with a peak and then a distinct decline. kd and r controlled change character of Csolu(P) curve, reflecting the depletion of porewater concentration and P desorption kinetics at DGT/sediment interface (Wu et al., 2016). 3.3. Effects of DIFS parameters and sediment property on P mobility The linear correlation relationships of FDGT(P) against C0(P), CDGT(Fe), P fractions, DIFS parameters, TOC (OM), Al[NaOH25] or TCa at twenty sites with NH4Cl-PþBD-P and six sites with NH4Cl-P pool are indicated in Table S7~S9. At twenty sites with NH4Cl-PþBD-P pool, FDGT(P) standing for P diffusive flux at DGT/sediment interface, was significantly dependent on (1) C0(P) or FDGT(Fe) with R2 of 0.81 or 0.71, respectively; (2) NH4Cl-PþBD-P or BD-P with R2 of 0.86; (3) r, rmax or kd with R2 of 0.66, 0.68 or 0.63, in turn; and (4) k-1, k1 or Dspt rate with R2 of 0.58, 0.62 or 0.56. The multivariable regression relationships of FDGT(P) against (5) r and TOC with R2 ¼ 0.77; (6) NH4Cl(Fe)þBD(Fe) and TCa with R2 ¼ 0.94, and (7) NH4Cl(Fe)þBD(Fe), Al[NaOH25] and TCa with R2 ¼ 0.96 (Table S9). FDGT(P) was mainly controlled by desorption parameters, resupply ratio, labile P pool, Fe/Al/Ca/OM in solid phase. The effect of Al or Ca on prevention of P release from sediment, has been reported in references (Kop
a cek et al., 2005; Palmer-Felgate et al., 2011). OM sorbed to sediment mineral can increase P solubility through the competition with P for sorption sites (Easterwood and Sartain, 1990). While, NH4Cl-PþBD-P and NH4Cl(Fe)þBD(Fe) responsible for the co-release of P and Fe, influenced P remobilization from sediment solid. Resupply ratio was mainly controlled by kd and Tc (Table S7). The desorption/ sorption rate constants (k-1 and k1) affected Dspt rate; FDGT(P) and C0(P) were impacted by Dspt rate (Table S7). Dspt rate reflected P desorption rate from solid phase into porewater. At six sites with NH4Cl-P pool, kd did not influence FDGT(P) distinctly; but, kd affected Dspt rate, k-1 and k1 evidently (Table S8); r was mainly controlled by NH4Cl-P, OM, FDGT(P), Trmax, Tc and k-1; NH4Cl-P, FDGT(Fe), FDGT(P), kd, rmax, k1 and k-1 controlled Dspt rate (Table S8). Moreover, NH4Cl-P, C0(P), FDGT(Fe), r, kd, rmax, k-1, k1, Dspt rate, OM, Al[NaOH25] and TCa in solid phase influenced FDGT(P) (Table S8~S9), which was similar to twenty sites with NH4Cl-PþBD-P pool. Above all, P transfer at DGT/sediment interface at 26 sediment sites was controlled by porewater concentration, labile P pool, Fe/ Al/OM/Ca in solid phase, resupply ability, desorption/sorption constants and Dspt rate. 3.4. P release risk in Lake Dianchi SPRRI indexes (Fig. 3 and Table 1e2) at twenty (NH4Cl-PþBD-P pool) and six (NH4Cl-P pool) sites in lake were in ranges of 3.20e49.73 and 0.18e10.37 [lg (nmol cm3 d1)], respectively. P release risks belonged to “moderate” ~ “high” in Caohai (I), “moderate” or “high” in middle-north (II), “light” ~ “relative high” in east (III), “light” or “moderate” in middle-south (IV) and west (V), and “moderate” ~ “very high” in south (VI). The average SPRRI indexes [lg (nmol cm3 d1)] in six regions were ranked in following order:

6

Z. Wu et al. / Environmental Pollution 255 (2019) 113279

Fig. 2. 2e1: The schematic graphs for P transfer and biogeochemistry at DGT/sediment interface; 2e2: The research methods for dynamic P transfer and SPRRI index.

south(VI)(28.37)>Caohai(I)(25.87)>middle-north(II)(12.94)>east(III)(10.72)>middle-south(IV)(7.72)>west(V)(4.20). Basically, the sequence of SPRRI indexes accorded to that of each sub-indexes at 26 sites in lake (Table S10). However, there were still some inconsistencies among them (SM E). At twenty sites (1e8, 11e19 and 22e26) with NH4Cl-PþBD-P pool and six sites (9, 10, 14, 15, 20 and 21) with BD-P pool, (i) the good linear correlation relationships (0.58 < R2 < 0.95) of SPRRI index against LAP/TP, BD(Fe)/Al[NaOH25], r, lg(Dspt rate), C0(P) or CDGT(P), and (ii) the good multivariable regression relationships of SPRRI against BD(Fe) and Al[NaOH25] are demonstrated in Table S11. For two sediment types, NH4Cl-PþBD-P or NH4Cl-P, r and Dspt rate played key roles in P mobility; while, Al[NaOH25] prevented P release from sediment P pool. The biogeochemical reactions influenced P release risk obviously. At the end of algae blooms in Sep., 2013, the algae biomass accumulated in top sediment layer, the hypolimnetic anoxia at SWI and weak alkaline (7.80e8.37) favored P release from NH4Cl-P or NH4Cl-PþBD-P pool in sediment. For example, SPRRI rank of “very high” at sites (24 and 26) was corresponded to the intense Fe-redox controlled P release from BD-P; while, SPRRI rank of “light” at sites

(9 and 10) was corresponded to the low NH4Cl-P pool. Basically, SPRRI indexes at twenty sites with NH4Cl-PþBD-P pool were larger than those at sites (9, 10, 20 and 21) with NH4Cl-P pool except for SPRRI of “moderate” at sites (14 and 15). The most serious release risk at sites (24 and 26) in south (VI) with “very high” level (46.52 and 49.73 [lg (nmol cm3 d1)]) was corresponded to the large labile P pool size in solid (kd ¼ 350.8 and 449.9 cm3 g1), respectively. It was due to phosphate ore mine and agriculture non-point effluent for two sites (Sect. 2.1). The “high” SPRRI level at sites (1 and 2) in Caohai (I) (38.62 and 32.03 [lg (nmol cm3 d1)]) and site 6 in middle-north (II) (31.97 [lg (nmol cm3 d1)]) was corresponded to the large kd of 409.0, 529.6 and 470.8 cm3 g1 at these sites, respectively. It was mainly due to the intense municipal effluent into Caohai (I) and middle-north (II) (Sect. 2.1). On the contrary, the “light” release risk at sites (9 and 10) in east (III) and sites (20 and 21) in west (V) was corresponded to SPRRI values of 0.20, 0.18, 1.53 and 4.18 [lg (nmol cm3 d1)], respectively. It was due to (1) the lowest relative labile P pool size in solid, i.e. lg(1000  LAP/TP) < 1.0 at these four sites with NH4Cl-P pool; (2) the lowest r  lg(Dspt rate)0.10 [lg (nmol cm3 d1)] for resupply capacity and desorption kinetics between porewater and

Z. Wu et al. / Environmental Pollution 255 (2019) 113279

7

Fig. 3. The distribution of SPRRI index and sub-indexes in Lake Dianchi., including (Fig. 3e1): SPRRI [lg (nmol cm3 d1)], (Fig. 3e2): lg(1000  LAP/TP), (Fig. 3e3): r  lg(Dspt rate) [lg (nmol cm3 d1)] and (Fig. 3e4) BD(Fe)/Al[NaOH25].

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Z. Wu et al. / Environmental Pollution 255 (2019) 113279

Table 1e2 The assessment result of P release risk in sediment sites with NH4Cl-PþBD-P or NH4Cl-P pool in Lake Dianchi. Region

Site

lg(1000  LAP/TP)

r  lg(Dspt rate)

BD(Fe)/Al[NaOH25]

[lg (nmol cm3 d1)] Caohai I

Middle-North II

East III

Middle-South IV

West V

South VI

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

1.98 2.03 1.70 1.57 1.39 1.88 1.58 1.54 0.93 0.85 1.47 1.39 1.84 1.38 1.19 1.51 1.53 1.57 1.65 0.97 0.82 1.84 1.55 1.82 1.92 1.94

4.47 3.82 2.86 2.24 1.37 3.03 1.04 0.99 0.10 0.09 1.92 1.24 3.76 2.69 1.99 1.02 1.11 1.46 0.58 0.71 1.30 1.29 1.76 3.46 1.95 4.11

sediment solid at sites (9 and 10), and (3) the lowest BD(Fe)/Al [NaOH25]0.23 with the most distinct effect of Al-P at sites (9, 10 and 20) of all 26 sites (Table 1 and Table S5). While, the “light” level at site 16 in middle-south (IV) and site 19 in west (V) with NH4ClPþBD-P pool was corresponded to SPRRI values of 4.84 and 3.20, respectively. It was mainly due to the low r  lg(Dspt rate) and BD(Fe)/Al[NaOH25] (Table 1 and Table S5). SPRRI for some sediments with NH4Cl-PþBD-P pool belonged to “moderate” or “relative high” at sites (3, 4, 5, 7, 8, 12, 14, 15, 17, 18, 22, 23 and 25) because of the medium labile P pool size, r, Dspt rate and BD(Fe)/Al [NaOH25] (Table 1 and Table S5). Kinetic P exchange between porewater and solid phase influenced SPRRI distinctly besides the labile P pool and BD(Fe)/Al [NaOH25]. SPRRI levels of “very high” at sites (24 and 26) and “high” at sites (1 and 2) with NH4Cl-PþBD-P pool (Table 1) were corresponded to r, NH4Cl-PþBD-P, k-1, Dspt rate and CDGT (Table 2), which were much larger than sites (9, 10, 20 and 21) with “light” level and NH4Cl-P pool (Table 2). k-1 (19.01 d1) and r (0.624) at site 14 (Table 2) demonstrated the fastest desorption and largest resupply ability of six sites (9, 10, 14, 15, 20 and 21) with NH4Cl-P pool. Above all, the labile P pool size, resupply ability, desorption parameters and Al-P controlled P release risk.

3.5. The comparison to other research results The dynamic P release for sediments in Lakes Victoria and Wellington (Australia) were researched by DGT probe and DIFS model (Monbet et al., 2008). At each CDGT(P) profile in sediment with the length of 13 cm, there was a thin “anoxic” sediment layer above an oxic brown layer. NH4Cl-P pool in two Australian lakes was used for kd in DIFS model for sediment profiles with a vertical spatial resolution of 2e2.5 mm. Sediment properties (redox status, P fractions, Fe, Al and OM) associated with P release were researched. The research method for DIFS model with NH4Cl-P pool for sediment layers in two Australian lakes is similar to SPRRI with NH4Cl-P or NH4Cl-PþBD-P pool in surface sediment in Lake

SPRRI

Release risk grade

[lg (nmol cm3 d1)] 0.44 0.41 0.38 0.41 0.43 0.56 0.4 0.33 0.21 0.23 0.55 0.51 0.42 0.28 0.35 0.31 0.43 0.35 0.33 0.22 0.39 0.49 0.56 0.74 0.50 0.62

38.62 32.03 18.35 14.46 8.12 31.97 6.58 5.07 0.20 0.18 15.58 8.72 28.90 10.37 8.36 4.84 7.29 7.89 3.20 1.53 4.18 11.57 15.26 46.52 18.79 49.73

high high relative high moderate moderate high moderate moderate light light relative high moderate relative high moderate moderate light moderate moderate light light light moderate relative high very high relative high very high

Dianchi. So, sediment reactivity related to DIFS parameters for two lakes in Australia can be compared to Lake Dianchi. DIFS parameters related to the second sub-index in SPRRI for Lake Dianchi were compared to two lakes (Australia). At six sites with NH4Cl-P pool in Lake Dianchi, Tc (36e42660 s) was close to 55e149400 s in Lake Wellington, but lower than 4128e183400 s in Lake Victoria; k-1 (0.02e19.01 d1) was close to Lake Victoria (0.3e21 d1), but lower than Lake Wellington (0.6e1558 d1); r (0.164e0.624) varied between Lakes Victoria (0e0.40) and Wellington (0.40e1.00); while, at twenty sites with NH4Cl-PþBD-P pool in Lake Dianchi, Tc (1.4e11690 s) was distinctly lower than Lakes Victoria (4128e183400 s) and Wellington (55e149400 s); k-1 range (0.02e179.39 d1) was larger than Lake Victoria (0.3e21 d1) and lower than Lake Wellington (0.6e1558 d1); r range (0.226e0.774) was larger than Lake Victoria (0e0.40) and close to Lake Wellington (0.40e1.00). It reflected the high sediment P reactivity at 26 sites in Lake Dianchi. It was mainly due to (i) the larger TAl in sediment in lakes Wellington and Victoria compared to Lake Dianchi, and (ii) the difference between NH4Cl-PþBD-P pool at most sites in Lake Dianchi and NH4Cl-P pool in sediment in Lakes Victoria and Wellington. P releaser risk evaluated by SPRRI index can be compared to the other evaluation methods of (i) the equilibrium phosphorus concentration (EPC0) by Elovich kinetic equation of P adsorption (Chen et al., 2014), (ii) P mobility evaluated by the labile P fractions and
 the effect of Al-P (Kopa cek et al., 2005; Wilson et al., 2008), and (iii) the estimated diffusive fluxes across SWI in Lake Dianchi (Wu and Wang, 2017). In 2013, EPC0 and SRP in overlying water in Lake Dianchi changed in the range of 0.0049e0.3644 mg L1 with the average of 0.0320 mg L1, and 0.0149e0.0980 mg L1 with the average of 0.0304 mg L1, respectively (Chen et al., 2014). According to the comparison between EPC0 and SRP (Fig. S6), P release risk was assessed. The main areas of Caohai (I), East (III), Middle South (IV), West (V) and South (VI) indicated the low release risk (EPC0
Z. Wu et al. / Environmental Pollution 255 (2019) 113279 Table 2 The input and output DIFS parameters for 26 sites in Lake Dianchi. Symbols (# and þ) stand for (i) NH4Cl-PþBD-P pool for twenty sediment sites and (ii) NH4Cl-P pool for six sediment sites, in turn. Region

Site

kd

C0 (P) 3

1

(cm g Caohai (I)

Middle-North (II)

East (III)

Middle-South (IV)

West (V)

South (VI)

Region

Caohai (I)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

409.0 529.6 247.7 214.8 174.4 470.8 272.8 252.7 95.9 98.2 216.5 175.3 400.8 160.9 102.6 218.0 224.0 288.6 304.2 110.3 93.1 375.1 198.8 350.8 367.2 449.9

)

(nmol ml 23.28 17.87 11.64 9.28 8.10 10.52 6.74 7.89 2.12 1.56 7.78 8.94 9.56 8.56 9.93 9.06 7.95 6.51 7.83 2.19 1.79 14.09 12.27 11.92 15.18 16.21

Site Labile P pool Tc

1 2 3 4 Middle-North 5 (II) 6 7 8 East 9 (III) 10 11 12 13 Middle-South 14 (IV) 15 16 17 West 18 (V) 19 20 21 South 22 (VI) 23 24 25 26

CDGT (P) 1

)

(nmol ml

1

rmax

0.751 0.774 0.668 0.619 0.475 0.726 0.411 0.385 0.164 0.179 0.582 0.435 0.757 0.624 0.547 0.387 0.419 0.226 0.284 0.374 0.504 0.435 0.529 0.741 0.569 0.772

0.908 0.882 0.807 0.698 0.495 0.799 0.418 0.392 0.404 0.4082 0.627 0.448 0.894 0.8093 0.6588 0.396 0.429 0.514 0.365 0.397 0.581 0.442 0.555 0.857 0.585 0.903

)

17.48 13.84 7.78 5.75 3.85 7.65 2.77 3.04 0.35 0.28 4.53 3.89 7.24 5.34 5.44 3.50 3.33 1.47 2.23 0.82 0.90 6.12 6.48 8.83 8.64 12.52 Trmax k-1

r

k1

Dspt rate

(mmol g1)

(s)

(h)

(d1)

(d1)

(nmol ml d1)

9.52 # 9.46 # 2.88 # 1.99 # 1.41 # 4.95 # 1.84 # 1.99 # 0.20 þ 0.15 þ 1.68 # 1.57 # 3.83 # 1.38þ 1.02þ 1.97 # 1.78 # 1.88 # 2.38 # 0.24þ 0.17þ 5.28 # 2.44 # 4.18 # 5.57 # 7.29 #

4.8 1.4 57 195 926 80 1702 1799 42660 39650 343 1109 10.8 36 199 2101 1557 11690 6265 2306 400 1376 497 23.2 492 7.9

2.08 2.77 2.40 3.70 4.93 3.70 3.70 3.20 0.37 0.28 4.27 5.69 2.40 2.08 2.77 3.20 4.27 6.57 0.66 1.17 3.20 6.57 5.69 2.77 6.57 2.40

58.38 179.39 9.17 2.72 0.55 2.13 0.18 0.16 0.02 0.03 1.34 0.35 20.93 19.01 4.00 0.16 0.24 0.02 0.05 0.31 2.28 0.15 0.72 8.49 0.51 24.73

17793 60666 1517 440 93 1083 50.6 47.9 2.0 2.1 251 77.6 7979 2394 431 41.0 55.2 7.4 13.7 37.2 214 62.1 171 3714 175 10912

910190 84610 19149 4206 762 15062 346 383 4 3 1989 703 93094 20499 4278 436 446 782 113 82 382 910 2156 46755 2700 210017

the distinct release risk (EPC0>SRP). It seemed that “internal Ploading” was not serious because the distinct P release only existed in two areas mentioned above. According to one DGT research paper for “internal P-loading” in Lake Dianchi in 2013 (Wu and Wang, 2017), the “apparent” upward diffusive fluxes across SWI were 28.7 (Caohai), 242.9 (Middle South), 124.9 (South), 260.2 (Middle, West and East) and 96.8 (Middle North) mg m2 d1, respectively. So, the “internal P-loading” was estimated to be 19.23 t a1. SPRRI index indicated “relative high” and “high” release risk in

9

Caohai (I) and South (VI) and “light” risk in west (V) and East (III), which was basically consistent with the research result in the last paragraph (Wu and Wang, 2017). However, P release risk estimated by EPC0 (Chen et al., 2014) was not consistent with both SPRRI and the diffusive fluxes across SWI (Wu and Wang, 2017). The “ex-situ” adsorption/desorption method can't be applied for P exchange dynamics for P release risk for the undisturbed sediment. P fraction (NH4Cl-PþBD-P or BD-P pool) (Zhang, 2000; Wu et al., 2016), DIFS parameter (P desorption and resupply ability) (Monbet et al., 2008) or the effect of Al on P release (BD(Fe)/Al[NaOH25])
cek et al., 2005; Dong et al., 2011) can partially reflect P (Kopa release risk. For SPRRI evaluation, each sub-index can also be solely used to evaluate P release risk. However, there is the minor inconsistence among the evaluation results of three sub-indexes (SM E). Only when they are combined into SPRRI index, can the real “in-situ” P release risk in lake sediment be reflected. 4. Discussion 4.1. Advantages of SPRRI index The present “ex-situ” methods (P fraction, Al-P or sorptiondesorption parameter) only consider one factor influencing P release and lack the understanding of “in-situ” P transfer in sediment. DIFS model can't be directly used to estimate P release risk. SPRRI index, an “in-situ” estimation method, is a unique opportunity to discuss methodological issues mentioned above, along with the inner relationship between SPRRI and sub-indexes and the drivers of variation of the evaluated parameters. Three sub-indexes contributing to SPRRI index and their interrelationship are demonstrated in Fig. S5. The labile P pool, P desorption, resupply ability and Al-P are the main factors influencing P release risk. Firstly, the accurate identification method for the labile P pool by DGT measurement result and chemical properties in sediment is superior to the previous methods (Zhang, 2000; Wilson et al., 2008). Secondly, the “in-situ” kinetic parameters of Dspt rate and r reflect desorption process and resupply ability in sediment more real than the “ex-situ” isotherm sorption (An and Li, 2009), EPC0 (Lin et al., 2009) or P release risk index (ERI) (Huang et al., 2004). Thirdly, SPRRI considers a permanent P sink (Al-P); however, the present methods (Huang et al., 2004; Lin et al., 2009), don't consider it in evaluation equation. Fourthly, the present DIFS model (Monbet et al., 2008; Wu et al., 2016) for kinetic parameters and curves, can't be directly used for “in-situ” P release risk. SPRRI index is based on both DIFS model and chemical properties (P, Fe and Al) in sediment, which can be applied to directly evaluate the “in-situ” P release risk. The eutrophic lake (Dianchi) has been used for a case study. Two types of P release mechanisms were related to NH4Cl-PþBD-P or NH4Cl-P pool in sediment solid. NH4Cl-PþBD-P pool at twenty sites is mainly distributed at Caohai (I), Middle-North (II), South (VI) and several sites in other three regions; NH4Cl-P pool is only distributed at six sites in East (III), Middle-South (IV) and West (V). The high SPRRI level is corresponded to the large labile P pool, the high resupply capability (r) and desorption rate, the low effect of Al-P on P sequestration and the large “external P-loading” (Sect. 3.4). 4.2. Problems and improvements of SPRRI index SPRRI can be applied to lakes with the inorganic P pool (NH4ClPþBD-P or NH4Cl-P) in sediment solid (Sect. 3.1). Fe-coupled mobilization of P in sediment has been found in many lakes, for example, Lake Dianchi (Wu and Wang, 2017); Lake Taihu (Ding et al., 2016); Lake Dongting (Gao et al., 2016) in China; and

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Z. Wu et al. / Environmental Pollution 255 (2019) 113279

Salmon Lake, Sebasticook Lake, China Lake, Togus Pond and Webber Pond (Amirbahman et al., 2003) in Marine (USA). NH4Cl-PþBDP can be used as labile P pool for this kind of lake and the most sites in Lake Dianchi. For six sites in Lake Dianchi, NH4Cl-P should be the labile P pool. However, if organic P (Po) is the labile P pool, it should be included in LAP (Sect. 2.5). For example, in Lakes Taihu, Dongting and Pongyang with the mesotrophic or hypereutrophic condition in Eastern Plain lake region (China), Po played an important role in P release (Zhang et al., 2013). In this case, Po should be treated as the labile P pool for LAP (Sect. 2.5). The five-step sequential extraction scheme (Psenner and Pucsko, 1988) for P fractions can't be used to accurately measure Po because it includes both organic and refractory P compounds. Po fractions include labile Po, moderately labile Po, moderately resistant Po and highly resistant Po (Bowman and Cole, 1978). The accurate identification method for Po pool accounting for P release should be developed. According to some references (Cooke et al., 1993; Gomez et al., 1999; Emil, 2000; Peng et al., 2007), pH in lake water determines the main fractions of aluminum hydrolysis products, and solubility. At pH ¼ 6e8, 4e6 or <4, the insoluble polymeric Al(OH)3, the various soluble intermediate forms or the hydrated and soluble A13þ dominates, in turn. At high pH level (>8.0), the amphoteric nature of aluminum hydroxide forms aluminate ion. At pH ¼ 8, amorphous Al-P is formed through adsorption and co-precipitation between Al(OH)x3x floc and PO3 4 , which is transferred gradually to crystal AlPO4. The maximum adsorption content of Al-P occurs at pH ¼ 8. Altogether, under strong acidic or basic condition, Al(OH)3 is hydrolyzed to dissoluble Al3þ or Al(OH)4 ions, which induces adsorbed P to liberate from Al(OH)3.
 In two papers (Kopa cek et al., 2005; Norton et al., 2008), the molar ratio of Al[NaOH25]/BD(Fe) has been developed for the effect of Al on P mobility in sediment in circumneutral and moderately acidic lakes. Al[NaOH25]/BD(Fe) has also been successfully applied for Lake Baiyangdian (China) with the weak alkaline condition (pH ¼ 8.05e8.31) (Dong et al., 2011). pH range (7.81e8.23) in Lake Dianchi was a little lower than Lake Baiyangdian. According to Al(OH)3 solubility mentioned in last paragraph, at pH about 7.81e8.23, Al(OH)3 is completely insoluble (7.81e8.0) at most sites or partially soluble (8.0e8.23) at several sites, which leads to the distinct effect of Al on P mobility. Above all, SPRRI index with BD(Fe)/Al[NaOH25] can be used for lakes with pH range (moderate acidity~ weak alkalinity). According to references (Sigg, 2000; Canfield and Thamdrup, 2009), the accurate method to define oxic or anoxic sediment is 4þ 3þ the measurement of oxidants (mainly O2 and then NO 3 , Mn , Fe , 2 and SO4 ) in sediment; moreover, oxic status refers to oxygenated environment with O2 respiration (Canfield and Thamdrup, 2009). However, it is almost not the case in lake sediment below top few mm and particularly in the eutrophic lake sediment with abundant OM. Eh is not an accurate norm for redox status existing in sediment layer (Sigg, 2000) and it can't be used for the judgement of the labile P pool in sediment. In this paper, sediment sites can be roughly divided into two types with twenty or six sites according to Eh scope (Sect. 3.1). While, the linear correlation relationship (R2) between CDGT (P) and CDGT (Fe), NH4Cl-P, BD-P, NH4Cl-PþBD-P, BD(Fe) or NH4Cl(Fe)þ BD(Fe) was used to identify the labile P pools and biogeochemical reactions for two kinds of sediments. The labile P pool lays the foundation for the derivation of kd (DIFS model) and SPRRI index. The present DGT test method can't avoid a slight difference of microenvironment parameters between sediment core in incubator and lake sediment (Sect. 3.1). It can lead to the minor deviation for DGT data and SPRRI. Further work should be done to develop an “in-field” equipment for DGT test to avoid the experimental

deviation. Moreover, a new multi-parameter analyzer should be fixed to this equipment for pH, T and especially O2 standing for redox status in sediment. 5. Conclusions In this paper, a new evaluation method (SPRRI index) for P release risk in lake sediment is developed. Compared to the present “ex-situ” methods, SPRRI with five risk grades for sediments with NH4Cl-PþBD-P or NH4Cl-P pool reflects P release risk more comprehensively. All influence factors (labile P pool, Al-P, resupply ability and dynamic P transfer) on “in-situ” P mobility are included in SPRRI. P release risk in six regions in Lake Dianchi related to point or non-point pollution source, is evaluated by SPRRI ranging from “light” to “very high” level. The sediments with different biogeochemical processes demonstrate the different diffusion-resupply characters in r and Csolu (P) curves derived by DIFS model mainly because of two types of NH4Cl-PþBD-P and NH4Cl-P pools. Sediment P in lake regions demonstrates the serious or light release risk, because of the distinct difference of labile P pool size, the resupply-desorption, or the effect of Al-P at DGT/sediment interface. SPPRI is suitable to assess P release risk with inorganic P pool in sediment solid and pH range (moderate acidity ~ weak alkalinity) in lakes. Further research should be conducted to (1) develop DGT test for SPRRI indexes in different sediment layers; (2) improve SPRRI equation and extraction method for lake sediment with organic P responsible for P release; and (3) measure O2 in sediment layer for the “real” redox status, which aids the identification of labile P pool in sediment solid. Conflicts of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Phosphorus (P) Release Risk in Lake Sediment Evaluated by DIFS Model and Sediment Properties: A New Sediment P Release Risk Index (SPRRI)” (Manuscript number ENVPOL_2019_5_R1)”. Acknowledgments This research was supported by National Major Science and Technology Program for Water Pollution Control and Treatment, China (2012ZX07102-004); Open fund project of Yunnan Key Laboratory of Pollution Process and Management of Plateau LakeWatershed, China (No. 230200069) and the talent project of Beijing Normal University, China (No. 312232102). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2019.113279. References An,

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