Relation between various soil phosphorus extraction methods and sorption parameters in calcareous soils with different texture

Low intensity magnetic separation of vivianite induced by iron reduction on the surface layer of Fe(III)[Fe(0)] iron scrap

Phosphorus (P) recovery through vivianite, which can be found in activated sludge, surplus sludge and digested sludge in the wastewater treatment plants (WWTPs), is a cutting-edge and efficient technology in recent years. However, how to generate and separate vivianite in an effective and economical way with natural iron oxide mineral was still the bottleneck to limit its application. Therefore, in this study, the P recovery efficiency (EP) and vivianite recovery efficiency (EV) of three kinds of iron oxides were investigated. We found that the EP of Akaganeite was 1.83 times and 4.88 times higher than that of Geothite and Hematite. Simultaneously, EV of Akaganeite was 1.64 times and 2.88 times higher than that of Geothite and Hematite. As Akaganeite is main component of rust on the surface of iron scrap, we used Fe(III)[Fe(0)] iron scrap with Fe(0) inside and Akaganeite outside as iron source and electron acceptor for vivianite production and magnetic separation. At the terminal stage (60 day), the P recovery efficiency with 20 g/L Fe(III)[Fe(0)] iron scrap was 36%. Applying a magnetical separator with magnetic field intensity of 0.3 T, vivianite was separated from the solution efficiently and immediately. Low intensity magnetic separation with iron scrap would recover P resources economically with the total cost to be $2.23/kg P, which was much lower than recovery via iron salts. Besides, it provided a significant insights into the P recovery and vivianite separation by reusing Fe waste during wastewater treatment.

Leaching and fractionation of phosphorus in intensive greenhouse vegetable production soils

Greenhouse vegetable production systems use excessive phosphorus (P) fertilizer. This study is set out to look into the P fractionation, mobility, and risk of P leaching in ten greenhouse soils. The mean P concentrations in leachates varied from 0.4 to 1.6 mg l-1 (mean of 30 days of soil leaching). Between 5.7 and 31.0 mg kg-1 of P was leached from soils during 30 days of column leaching. Organic matter (OM) and Olsen-extractable P (Olsen P) correlated strongly with cumulative P leached after 5, 10, 15, 20, 25, and 30 days of leaching. The high correlation between OM and Olsen P with cumulative P leached at 5 days of leaching suggests that in future leaching experiments, the leaching period should be extended to 5 days of leaching. The first two P fractions correlated significantly with the total P leached in the primary days of leaching. The pH had little effect on P leaching but had a significant impact on soluble and exchangeable P fraction, suggesting that P mobility would increase in these calcareous greenhouse vegetable soils as pH rose. The calculated change point (194 mg kg-1) was high, indicating that a high percentage (40%) of the studied greenhouse soils had exceeded the change point. In conclusion, due to the high degree of P saturation and change point in greenhouse vegetable soils, P mobilization is a significant risk, and the findings can be used to provide future direction for fertilizing greenhouse vegetable soils.

The status of phosphorus levels in Iranian agricultural soils - a systematic review and meta-analysis

Phosphorus (P) inputs are essential for maximizing agronomic potential, yet high P inputs and subsequent P losses can cause eutrophication of water bodies. There is a need to evaluate P contents in agricultural soils globally both from an agronomic and environmental perspective. This systematic review and meta-analysis estimated the pooled mean levels of P contents of Iran. In this study, data on available and total P contents of Iran's calcareous soils was compiled (main focus on Olsen P) and compared to (i) estimated Iranian background and global agricultural soil P contents, and (ii) agronomic and (iii) environmentally critical Olsen P values. The pooled mean estimate from the meta-analysis indicates that the levels of Olsen P across 425 soil samples (27 studies) were 21.3 mg kg^-1 and total P across 190 soil samples (12 studies) 805.5 mg kg^-1. Using 26 mg kg^-1 as the agronomic critical Olsen P value above which no increase in crop yield occurs, crops grown on 61% of the soil samples in the investigated region would respond to P fertilizer and 20% of soils are currently in the optimum category (26-45 mg kg^-1 Olsen P). The environmentally critical Olsen P value (~ 63 mg kg^-1), defined as the amount above which P leaches from soil rapidly, was exceeded by 11% of soils with a further 4% of soils with elevated eutrophication risk. To maximize crop yields while maintaining a minimal risk of P leaching in Iran's calcareous soils, we suggest an ideal Olsen P of 26 mg kg^-1. The outcomes from this study inform about the P status of Iranian soils and could help update recommendations for P fertilizer applications in calcareous soils globally. The framework presented here could further be adopted to evaluate the P status in other soil types.

Long-term soil warming decreases microbial phosphorus utilization by increasing abiotic phosphorus sorption and phosphorus losses

Phosphorus (P) is an essential and often limiting element that could play a crucial role in terrestrial ecosystem responses to climate warming. However, it has yet remained unclear how different P cycling processes are affected by warming. Here we investigate the response of soil P pools and P cycling processes in a mountain forest after 14 years of soil warming (+4 °C). Long-term warming decreased soil total P pools, likely due to higher outputs of P from soils by increasing net plant P uptake and downward transportation of colloidal and particulate P. Warming increased the sorption strength to more recalcitrant soil P fractions (absorbed to iron oxyhydroxides and clays), thereby further reducing bioavailable P in soil solution. As a response, soil microbes enhanced the production of acid phosphatase, though this was not sufficient to avoid decreases of soil bioavailable P and microbial biomass P (and biotic phosphate immobilization). This study therefore highlights how long-term soil warming triggers changes in biotic and abiotic soil P pools and processes, which can potentially aggravate the P constraints of the trees and soil microbes and thereby negatively affect the C sequestration potential of these forests.

A meta-analysis of arable soil phosphorus pools response to manure application as influenced by manure types, soil properties, and climate

Manure amendments to agricultural soils is an excellent opportunity for sustainable utilization of agricultural waste while providing multiple benefits to improve soil quality and increase the availability of nutrients to plants, including phosphorus (P). In this study, a meta-analysis of published data from 411 independent observations based on 133 peer-reviewed papers was performed for an in depth understanding of various factors affecting the transformation of soil P pools with manure application. Manure application increased all soil inorganic P (P_i) by 58.0%-282% and organic P (P_o) by 65.0%-105%, while decreasing P_o/total P (TP), compared to those in unamended soils. Manure types, soil TP, and manure application rates were the important factors that influenced soil P fractions. Elevation of soil labile P_i was more pronounced with compost application, while poultry and pig manure were more beneficial for promoting soil P_i fractions and stable P_o contents compared with other manure types. The manure application rate had pronounced effect on increasing the stable P_o fractions. The effects of manure application on increasing soil P fractions were greater in soils with lower TP contents as compared to that in high TP soils. Manure effects on enhancing soil labile P_i and moderately labile P_i were greater in acidic soil than that in neutral and alkaline soils. In addition, soil P fractions showed significant correlation with latitude and mean annual precipitation (MAP). By integrating the impacts of manure types, soil properties, and climate, this meta-analysis would help to develop the management of manure application in a specific region of agriculture as well as promote the interpretation of the interfering factors on the soil P fractions changes in the manure-amended soils.

Earthworm inoculation and straw return decrease the phosphorus adsorption capacity of soils in the Loess region, China

Loess Plateau is important for maize production in China. Therefore, a good understanding of soil phosphorus (P) behavior in the Loess region is crucial for optimizing fertilization in its agriculture systems. To date, research on factors influencing P adsorption/desorption has mainly focused on fertilization. Widespread application of straw return and increasing soil fauna in agricultural croplands inevitably affect soil P behavior either directly or indirectly in this area. However, less attention has been focused on these effects and their interactions. Here, a field plot experiment was performed based on a completely randomized design to investigate the response of P adsorption-desorption characteristics to the presence/absence of earthworms and straw return. Treatments included: (1) control without earthworms and straw (E0S0); (2) treatment with only earthworms (E1S0); (3) treatment with only straw (E0S1); (4) treatment with both earthworms and straw. The Langmuir model was superior to the Freundlich model in interpreting the P adsorption data and allowed better evaluation of the maximum P adsorption values. The maximal P adsorption, P adsorption affinity constant, and maximum buffer capacity in the earthworm and straw treatments were 2.4-8.3%, 8.3-13.9%, and 2.2-26.3% lower than those in E0S0. The readily desorbable P, standard P requirement, and degree of P saturation increased by 15.6-44.3%, 13.1-23.1%, and 4.4-16.5%, respectively, in earthworm and straw treatments. Additionally, earthworm inoculation and straw return treatments significantly increased total soil P, Olsen P, soil organic carbon, free Fe₂O₃, and CaCO₃ contents and specific surface area of the soil. Redundancy analysis showed that soil organic carbon explained most (14.7%) of the total variation in P adsorption and desorption. These results show that combining earthworm inoculation with straw return can effectively reduce soil P adsorption capacity, increase its P desorption capacity, and thus, increase its available P content. These results provide a scientific basis for improving the utilization efficiency of soil P.

Soil Microbial Communities Affect the Growth and Secondary Metabolite Accumulation in Bletilla striata (Thunb.) Rchb. f

Bletilla striata (Thunb.) Rchb.f. is a perennial herb belonging to the Orchidaceae family. Its tubers are used in traditional Chinese medicine to treat gastric ulcers, inflammation, silicosis tuberculosis, and pneumogastric hemorrhage. It has been reported that different soil types can affect the growth of B. striata and the accumulation of secondary metabolites in its tubers, but the biological mechanisms underlying these effects remain unclear. In this study, we compared agronomic traits and the accumulation of secondary metabolites (extractum, polysaccharide, total phenol, militarine) in B. striata grown in sandy loam or sandy clay soil. In addition, we compared physicochemical properties and microbial communities between the two soil types. In pot experiments, we tested how irradiating soil or transplanting microbiota from clay or loam into soil affected B. striata growth and accumulation of secondary metabolites. The results showed that sandy loam and sandy clay soils differed significantly in their physicochemical properties as well as in the structure and composition of their microbial communities. Sandy loam soil had higher pH, SOM, SOC, T-Ca, T-N, T-Mg, T-Mn, T-Zn, A-Ca, A-Mn, and A-Cu than sandy clay soil, but significantly lower T-P, T-K, T-Fe, and A-P content. Sandy loam soil showed 7.32% less bacterial diversity based on the Shannon index, 19.59% less based on the Ace index, and 24.55% less based on the Chao index. The first two components of the PCoA explained 74.43% of the variation in the bacterial community (PC1 = 64.92%, PC2 = 9.51%). Similarly, the first two components of the PCoA explained 58.48% of the variation in the fungal community (PC1 = 43.67%, PC2 = 14.81%). The microbiome associated with sandy clay soil can promote the accumulation of militarine in B. striata tubers, but it inhibits the growth of B. striata. The accumulation of secondary metabolites such as militarine in B. striata was significantly higher in sandy clay than in sandy loam soil. Conversely, B. striata grew better in sandy loam soil. The microbiome associated with sandy loam soil can promote the growth of B. striata, but it reduces the accumulation of militarine in B. striata tubers. Pot experiment results further confirmed that the accumulation of secondary metabolites such as militarine was higher in soil transplanted with loam microbiota than in soil transplanted with clay microbiota. These results may help guide efforts to improve B. striata yield and its accumulation of specific secondary metabolites.

Sorption and Environmental Risks of Phosphorus in Subtropical Forest Soils

Phosphorus (P) is one of the key limiting factors for the growth of forests and their net primary productivity in subtropical forest ecosystems. Phosphorus leaching of the forest soil to the catchment and groundwater in karst region is the main source of water eutrophication. Strong P sorption capacity of minerals is generally assumed to be a key driver of P leaching in subtropical ecosystems which varies among different soil types. Here, we estimated P adsorption capacity of the O/A and AB horizon in both limestone soil and red soil of subtropical forests by fitting the Langmuir and Freundlich isotherm to investigate the potential environmental risks of P. The maximum P sorption capacity ( $Q_{\text{m}}$ ), P sorption constant ( $K_{\text{L}}$ ), P sorption index (PSI), degree of P saturation (DPS), and maximum buffer capacity (MBC) were calculated. The results indicate that $Q_{\text{m}}$ of the O/A horizon in both soils were similar. Comparing these two soils, the red soil had a higher $K_{\text{L}}$ and MBC in the AB horizon; $Q_{\text{m}}$ of limestone soil was larger but $K_{\text{L}}$ was lower, indicating that the adsorption capacity of limestone soil was weaker and MBC was lower. There was no significant difference in PSI between the two soils. The DPS values of both soils were below 1.1%, indicating that P saturation is low in both subtropical forest soils due to the lack of marked anthropogenic disturbance. In the O/A horizon, P saturation associated with available P (DPSM3 and DPSOlsen) and that associated with P in the Fe-Al bound state (DPScitrate) were higher in the red soil than in the limestone soil. DPS did not differ significantly in the AB horizon, except for higher DPSM3 and DPScitrate in the red soil. The findings highlight the influence of the soil types on P adsorption. The P adsorption and buffering of red soils were higher than those of limestone soils, indicating a lower risk of P leaching in red subtropical forest soils.

Assessment of trace element pollution in northern and western Iranian agricultural soils: a review

The pollution of Iranian agricultural soils with trace elements (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) was assessed and compared with other agricultural soils around the world. Experimental data on trace element content in Iranian agricultural areas in the west and north were collected from the literature: 39 studies covered a total of 46 agricultural sites of 17 provinces in Iran, in order to characterize their patterns of accumulation of trace elements. Two pollution indices, namely, the pollution index (PI) and the integrated pollution index (IPI), were used to evaluate trace element accumulation. The data revealed a remarkable variation in trace element content among soils in different areas. Exploratory data analyses (EDAs) showed that a number of trace elements (Pb, Cu, and Zn) are asymmetrically distributed and scattered. Surveys indicated that 45.5% of the studied samples had elevated PI values for Cd, 13.0% for Cu, and 16.7% for Pb, clearly indicating an anthropogenic contribution of these three elements. The IPI of the agricultural soils also indicated that most areas are classified as having moderate and high pollution. Higher contents of trace elements (except for Mn) were found in some cities of the Isfahan, Hamadan, and Tehran provinces. Excessive application of conventional and organic fertilizers, pesticides, animal manure, and sewage sludge for enhancing crop production is responsible for high trace element content in Iran's agricultural soils. This in turn, through the food chain, is a threat to human health. Analysis of the correlation between trace elements exhibited that Cu, Pb, and Zn (Cd, Pb and Zn) were very closely associated with each other, showing that their prevalent sources are common and the efforts to regulate them linked in common actions. We consider this evaluation as a viable approach to other similar areas in the Middle East and beyond, which could be used by environmental scientists for risk assessment and decision making.

Phosphorus recovery from waste activated sludge by sponge iron seeded crystallization of vivianite and process optimization with response surface methodology

As a novel phosphorus recovery product, vivianite (Fe₃(PO₄)₂·8H₂O) has attracted much attention due to its enormous recycling potential and foreseeable economic value. Taking sponge iron as seed material, the effect of different reaction conditions on the recovery of phosphorus in waste activated sludge by vivianite crystallization was studied. Through single factor tests, the optimal conditions for vivianite formation were in the pH range of 5.5-6.0 with Fe/P molar ratio of 1.5. Scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS) were used to analyze the components of the crystals. The results showed that the vivianite produced by sponge iron as the seed crystal were larger and thicker (300-700 μm) than other seed (200-300 μm) and without seed (50-100 μm). Moreover, vivianite, which was synthesized with sponge iron as seed, was obviously magnetic and could be separated from the sludge by rubidium magnet. The Box-Behnken design of the response surface methodology was used to optimize the phosphorus-recovery process with sponge iron (maximum phosphorus recovery rate was 83.17%), and the interaction effect of parameters was also examined, pH had a significant effect on the formation of vivianite. In summary, this research verifies the feasibility of using sponge iron as the seed crystal to recover phosphorus in the form of vivianite from waste activated sludge, which is conducive to the subsequent separation and utilization of vivianite.

Effect of organic and inorganic phosphorus fertilizers on phosphorus availability and its leaching over incubation time

The use of organic and inorganic phosphorus (P) fertilizers in agricultural soils is very common, and few studies have been conducted to study the effect of different P sources on relative P extractability (RPE) and leaching using different P extractants and degree of P saturation (DPS), over a long period of time. Thus, this study was conducted to investigate the effect of incubation time and different P sources on RPE, DPS, and to predict the concentration of P leached from soil using different P extractants. In order to achieve these goals, nine sewage sludges (SSs), two biochars, animal manure (AM), poultry manure (PM), wheat residue (WR), diammonium phosphate (DAP), and triple superphosphate (TSP) were added to the soil as much as 100 mg P kg^-1 in a 163 days incubation experiment. On average across all amendments and incubation periods, Mehlich-3 extractable P (M3EP) gave the highest mean RPE (42.9%, SE = 7.1%), with water-extractable P (WEP) the lowest (4.6%, SE = 0.93%), and Olsen-extractable P (OEP) (38.3%, SE = 6.3%) in between. Among SSs and based on average across of all incubation periods, soils treated with Shiraz and Takestan SSs were the least soluble source of P, while the highest soluble source of P were soils treated with Kermanshah and Tehran SSs. The results indicated that soil samples taken 16 days following the addition of amendments should reflect agronomic and environmental purposes aiming to assess available and the potential P loss from agricultural soils. The split line model perfectly fitted to the relation between OEP and M3EP (r = 0.93). The DPSs were calculated and the P leaching rate was estimated. Based on OEP, the soils treated with TSP and DAP were at high risk, the medium risk was for soils treated with Kermanshah, Saveh, Tehran, Rasht, Sanandaj, and Isfahan SSs, and PM. Control soil, and soils treated with WR were at no risk, and the soils treated with Arak, Shiraz, and Takestan SSs, ABC, WBC, and AM were classified as low risk.

A novel approach of synchronously recovering phosphorus as vivianite and volatile fatty acids during waste activated sludge and food waste co-fermentation: Performance and mechanisms

This research proposed an innovative approach to synchronously enhance the recovery of phosphorus (P) as vivianite and volatile fatty acids (VFAs) during waste activated sludge (WAS) and food waste (FW) co-fermentation. A high performance was achieved under 30% FW addition and pH uncontrolled, which gained 83.09% of TP recovery as high-purity vivianite (93.90%), together with efficient VFAs production (7671 mg COD/L). The FW supplement could enhance VFAs production and subsequently lower pH to contribute to the release of Fe2+ and PO43-. Also, it could dampen disrupting effects of strong acidic pH on microbial cells (lowering LDH release). Moreover, the flexible pH variation caused by biological acidification could maintain relatively higher microbial activities (increasing enzymes' activities), which was advantageous to the biological effects involved in Fe2+ and PO43 release and VFAs generation. Therefore, this research provide a promising and economic alternative to dispose of WAS and FW simultaneously for valuable resource recovery.

Assessment of the health risks of heavy metals in soils and vegetables from greenhouse production systems in Iran

Overuse of chemical and organic fertilizers in greenhouse (GH) crop production may cause the accumulation of heavy metals in soils and risks to human health. The aims of this study were to compare physical and chemical properties of GH with open-field (OF) soils, to clarify the buildup of heavy metals and phosphorus (P) in soils, and to assess the risks of selected heavy metals in soils and cucumber (Cucumis sativus L.) and tomato (Lycopersicon esculentum Mill.) from GH vegetables in Hamedan, western Iran. The average total and Olsen P of GH soils were significantly higher than the OF soils for both vegetables. The order of total and available heavy metal content in tomato GH soils has been set as zinc (Zn) > nickel (Ni) > chromium (Cr) > lead (Pb) > copper (Cu) > cadmium (Cd) and Zn > Cr > Cu > Pb > Ni > Cd, respectively. The same order was found for cucumber GH soils, except that the position of Pb and Cu was changed. The results indicated that in both GH cucumber and tomato soils, the mean content of total and available Zn, available Cu, Ni, and Pb, was extra than in OF soils. There were no significant differences between average total Cr, Cu, Ni, and Pb in GH and OF soils. Tomato vegetables had higher heavy metal content and transfer factors, particularly for Cr than cucumber vegetables. According to the health risk indices, Cr and Pb represented a high potential risk for health through cucumber and tomato consumption. There were limited Cd, Cu, Pb, and Zn inputs from the irrigation waters, while the input of Cr and Ni may be important. However, the amount of manure application and heavy metal content of the manures was significant.

Phosphorus recovery as vivianite from waste activated sludge via optimizing iron source and pH value during anaerobic fermentation

This study presented an innovative method for phosphorus (P) recovery as vivianite from waste activated sludge (WAS) via optimizing iron dosing and pH value during anaerobic fermentation (AF). The optimal conditions for vivianite formation were in the pH range of 6.0-9.0 with initial PO₄^3- >5 mg/L and Fe/P molar ratio of 1.5. Notably, FeCl₃ showed advantages over ZVI for the simultaneous release of Fe²⁺ and PO₄^3- during WAS fermentation, especially in acidic conditions. The FeCl₃ dosing at pH 3.0 could contribute to 78.81% Fe²⁺ release and 85.69% of total PO₄^3- release from WAS. They were ultimately recovered in the form of high-purity vivianite (93.67%). Clostridiaceae (40.25%) was the predominant bacteria in FeCl₃-pH3 reactors, which played key roles in inducing dissimilatory iron reduction for Fe²⁺ formation. Therefore, P recovery as vivianite from WAS fermentation might be a promising and highly valuable approach to relieve the P crisis.

Potentials and challenges of phosphorus recovery as vivianite from wastewater: A review

Due to the shortage of phosphorus resources and the limitations of existing phosphorus recovery methods, phosphorus recovery in the form of vivianite has attracted considerable attention with its natural ubiquity, easy accessibility and foreseeable economic value. This review systematically summarizes the chemistry of vivianite, including the characteristics, formation process and influencing factors of the material. Additionally, the potential of phosphorus recovery as vivianite from wastewater has also been comprehensively examined from the prospects of economic value and engineering feasibility. In general, this method is theoretically and practically feasible, and brings some extra benefits in WWTPs. However, the insufficient understanding on vivianite recovery in wastewater/sludge decelerate the development and exploration of such advanced approach. Further researches and cross-field supports would facilitate the improvement of this technique in the future.

Investigating the behavior of binding properties between dissolved organic matter (DOM) and Pb(II) during the soil sorption process using parallel factor analysis (PARAFAC) and two-dimensional correlation spectroscopy (2D-COS)

Dissolved organic matter (DOM) is the most active component in an environmental system. It can influence the chemical and structural characteristics of soil. In this work, three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy, parallel factor analysis (PARAFAC), and two-dimensional correlation spectroscopy (2D-COS) integrated with synchronous fluorescence were used to explore the interaction between soil-derived DOM and Pb(II) during the soil sorption process. According to the data of batch sorption experiments, the adsorbing capacities of soil, soil + 5 mL DOM, and soil + 10 mL DOM were 16.96, 18.29, and 19.32 mg g^-1, respectively, which indicated that DOM significantly enhanced the adsorption efficiency of Pb(II). The pseudo-second-order kinetic equation could well explain the adsorption process. The adsorbing data conformed to the isotherm of Langmuir adsorption. According to EEM-PARAFAC results, there are two major components from DOM. Protein-like substances were represented by component 1, and humic-like and fulvic-like substances were represented by component 2. Based on 3D-EEM, the results further showed that the intensities of component 1 and component 2 were obviously quenched with the increase of Pb(II) concentrations. The combined interpretations of the 2D-COS map for the DOM revealed that Pb(II) binding might occur sequentially in the order of humic-like fraction > protein-like fraction (346 > 282 nm). According to synchronous fluorescence spectra, static fluorescence quenching was the major process of quenching. Graphical abstract ᅟ.

Assessment risk of phosphorus leaching from calcareous soils using soil test phosphorus

Accurate estimation of phosphorus (P) leaching is important because excess P may reduce surface and ground water quality. Little attention has been paid to estimate P leaching from soil tests in calcareous soils. The relation between different soil tests P (STP), P sorption index (PSI) and degree of P saturation (DPS) and leaching of P were examined for assessing the risk of P loss from calcareous soils. Columns leaching repacked with native soils were leached with either distilled water or 10 mM CaCl₂ solutions, separately. Four leaching events were performed at four days, and 28.7 mm of distilled water or 10 mM CaCl₂ solutions was applied at each leaching events. Compared with distilled water, CaCl₂ had a small ability to solubilize P from soils. Concentration of P in leachate in both leaching solutions was exceeding 0.1 mg l^-1 associated with eutrophication. Cumulative P leached P was ranged from 0.17 to 18.59 mg P kg^-1 and 0.21-8.16 mg P kg^-1, when distilled water and 10 mM CaCl₂ solutions were applied, respectively and it was higher in sandy clay loam soils compared with clay soils. Among evaluated environmental soil P tests, P_CaCl2-3h (P extracted by 10 mM CaCl₂ for 3 h), P_CaCl2-1h (P extracted by 10 mM CaCl₂ for 1 h) were more accurate than other soil P tests for predicting P concentration in the leachates in both leaching solutions and accounting for 83% and 72% of variation of P concentration, respectively. The water extractable P (WEP) (r = 0.771) and Olsen-P (P_Ols)(r = 0.739) were significantly related to the leached P concentration using distilled water solution in a split line model, with a change point of 27.4 mg P kg^-1 and 61.5 mg P kg^-1, respectively. Various DPS were calculated and related to the leached P concentration. Based on P extracted by Mehlich-3 (P_M3) and HCl (P_HCl) and PSI, the change point of the relationship between leached P concentration and DPS_M3-3 (P_M3(P_M3+PSI)×100) and DPS_HCl-2 (P_HCl(P_HCl+PSI)×100) for both leaching solutions was approximately the same, thus a mean value of 49% for DPS_M3-3 and 73% for DPS_HCl-2 was obtained. Soils were grouped into four categories of increasing P leaching potential based on WEP, P_Ols, and DPS_M3-3. The results indicated that 8.00%-25.50% of the soil grouped in no risk category whereas 8.00%-13.70% of the soils fell into the high risk category.

Insight into chemical phosphate recovery from municipal wastewater

Phosphate plays an irreplaceable role in the production of fertilizers. However, its finite availability may not be enough to satisfy increasing demands for the fertilizer production worldwide. In this scenario, phosphate recovery can effectively alleviate this problem. Municipal wastewater has received high priority to recover phosphate because its quantity is considerable. Therefore, phosphate recovery from municipal wastewater can bring many benefits such as relieving the burden of increasing production of fertilizers and reduction in occurrence of eutrophication caused by the excessive concentration of phosphate in the released effluent. The chemical processes are the most widely applied in phosphate recovery in municipal wastewater treatment because they are highly stable and efficient, and simple to operate. This paper compares chemical technologies for phosphate recovery from municipal wastewater. As phosphate in the influent is transferred to the liquid and sludge phases, a technical overview of chemical phosphate recovery in both phases is presented with reference to mechanism, efficiency and the main governing parameters. Moreover, an analysis on their applications at plant-scale is also presented. The properties of recovered phosphate and its impact on crops and plants are also assessed with a discussion on the economic feasibility of the technologies.