Sequential Chemical Fractionation Research Articles

Organic/inorganic phosphorus partition and transformation in long-term paddy cultivation in the Pearl River Delta, China

Identification and quantification of different soil phosphorus (P) fractions level are important for improving agricultural productivity and developing sustainable management practices in these agricultural soils under long-term cultivation. However, few studies have been conducted to investigate P fractions level and their transformation in these soils. This study was conducted to characterize P fractions as affected by different paddy cultivation ages (200, 400-yr and 900-yr) among soils of the Pearl River Delta Plain in China. A sequential chemical fractionation scheme and 31P nuclear magnetic resonance spectroscopy (31P NMR) were employed to quantify various P fractions and speciation. Results showed soil easily-labile P, moderately-labile P and non-labile P had a positive relationship with total P (TP) and available P (AP). Analysis with 31P NMR spectroscopy revealed that inorganic P including orthophosphate (Ortho-P) and pyrophosphate (Pyro-P) increased with cultivation age, while organic species monoester phosphate (Mono-P) and diester phosphate (Diester-P) decreased. Moreover, acid phosphatase (AcP), neutral phosphatase (NeP), exchangeable Ca and sand contents are the main factors that affected the transformation of soil P composition, and non-labile P (Dil.HCl-Pi) and Pyro-P had significant contribution to soil P availability by affecting P activation coefficient. Therefore, long-term paddy cultivation, influenced by these soil parameters including NeP, AcP, exchangeable Ca and sand, accelerated the transformation of soil organic P/non-labile P to inorganic P.

PH: A core node of interaction networks among soil organo-mineral fractions

Mineral-associated organic matter (MAOM) is the largest soil organic carbon (OC) pool with the longest turnover. MAOM is expected to have relatively little sensitivity to climate change due to mineral protection, but its persistence involves several organo-mineral fractions. The uncertainty in the response of specific organo-mineral fractions to climate change hampers the reliability of predictions of MAOM preservation in the future. Here, we applied a sequential chemical fractionation method integrated with network analysis to investigate MAOM stabilization mechanisms across five alpine ecosystems: alpine desert, alpine steppe, alpine meadow, alpine wetland, and alpine forest. Hierarchical cluster analysis revealed grouping of seven extractable OM fractions in MAOM into three OM clusters: a cluster with weak bondings consisting of water-soluble OM (WSOM) and weakly adsorbed fractions (2.1–21.3% of total OC); a cluster with metal-bound complexes comprising Ca-OM complexes and Fe/Al-OM complexes (3.8–12.2% of total OC); and a cluster with strong bonding composed of Al oxyhydroxides, carbonates and Fe oxyhydroxides (12.2–33.5% of total OC). The relative percentages of OM from soils of the five ecosystems in the three clusters exhibited distinct pH dependence patterns. With the increase in pH, the cluster with weak bondings decreased, and that with strong bondings increased, while the one with metal-bound complexes showed a maximum at weakly acidic pH. Organo-mineral fractions and metal cations in MAOM constructed a complex network with pH as the central node. Results suggest that precipitation does not only alter vegetation type and microbial biomass but also regulate soil pH, which is balanced by specific metal cations, thus resulting in particular pH preference of specific OM clusters. These findings demonstrate that soil pH plays a central role in unveiling MAOM dynamics and can serve as a good predictor of soil organo-mineral fractions across alpine ecosystems.

Soil phosphorus forms and fertilizer use efficiency are affected by tillage and soil acidity management

Liming combined with no-tillage (NT) can increase the efficiency of P use by accumulating soil P in more available forms and improving root exploration due to the absence of toxic Al3+. This study aimed to evaluate the long-term effect of different combinations of soil tillage managements and liming in a subtropical Oxisol on the accumulation of P forms, legacy P index, and P use efficiency. A long-term experiment was used, established in 1987 testing (i) conventional tillage with lime (lime-CT) and control (control-CT), (ii) NT with lime (lime-NT) and control (control-NT), and (iii) occasional tillage with lime (lime-OT). The NPK fertilization was the same for all treatments. The accumulated grain yield and P use efficiency of 56 crops between 1987 and 2019 were evaluated. In 2019, soil samples were collected in the 0–5, 5–10, and 10–20 cm layers and submitted to a sequential chemical fractionation of P, P indexes, and soil acidity parameters. The results reveal that is necessary to correct acidity to increase crop yields and P use efficiency, regardless of the soil management used. This increase is enhanced when soil acidity correction by liming is combined with the use of conservation systems such as NT. Furthermore, after liming with incorporation, liming can be done on the soil surface, without the need to repeat the operation of lime incorporation, ensuring high efficiency of P use even after more than four decades.

An assessment of various pools of organic phosphorus distributed in soil aggregates as affected by long‐term P fertilization regimes

AbstractThe phosphorus (P) forms in long‐term fertilization determine the fate and transport of P in soil. However, the fate of various pools of organic P of added P in the long‐term measured with sequential chemical fractionation is not well‐understood. Four soil physical aggregates (>250, 125–250, 63–125 and <63 μm) from 0‐ to 20‐cm depth after 35 years of long‐term fertilization treatments including control (CK), nitrogen and phosphorus fertilizer (NP) and NP combined with farmyard manure (NPM) under continuous winter wheat were separated using settling tube apparatus. Results showed that the application of long‐term P fertilization had no apparent effects on promoting the mass proportion of soil aggregates except for >250 μm, where the NP and NPM treatments significantly increased the mass proportion by 60% and 70% over CK, respectively. Compared with CK, P fertilizer (NP and NPM) treatments significantly increased organic P (Po) contents in each size aggregate. In particular, mean labile Po increased by 35% and 246%, moderately labile Po by 125% and 161%, nonlabile Po by 105% and 170% and total Po (TPo) by 101% and 178%, respectively, under NP and NPM treatments, respectively. There was a significant correlation between soil organic carbon (SOC) and Po fractions. SOC was exponentially positively correlated with labile Po but linearly positively correlated with moderately labile Po, nonlabile Po and TPo fractions among soil aggregates. A reduced C:Po ratio (<100) in soil aggregates among treatment indicates a large amount of available P accumulated in soils, and soil P loss risk in the study site is still high. Our results show that the Po pool measured by sequential chemical fractionation may represent an important, yet often overlooked, source of P in agriculture ecosystems. According to the result, long‐term mineral P fertilizer combined with organic amendments better sustains soil structural stability in large aggregates, contributing more Po availability in the moderately labile P followed by labile P in soil aggregates.

Phosphorus species in sequentially extracted soil organic matter fractions

The majority of organic P (Porg) in soil is considered to be part of soil organic matter (SOM) associations, but its chemical nature is largely ‘unresolved’. In this study, we investigated the Porg composition in different SOM fractions of a Gleysol soil using the Humeomics sequential chemical fractionation (SCF) procedure combined with nuclear magnetic resonance (NMR) spectroscopy.In summary, SCF procedure with subsequent NaOH-EDTA extraction of the soil residue extracted a total of 1769 mgP/kgsoil compared to 1682 mgP/kgsoil of a single-step NaOH-EDTA extraction. Approximately 38 % of the extracted Porg was present in the form of the unresolved Porg pool, which was represented by one or two underlying broad signals in the phosphomonoester region of solution 31P NMR spectra. The SCF revealed that phosphomonoesters were recovered in each fraction: 47 % of the unresolved phosphomonoesters were associated with the SOM fraction released by breaking ester bonds (40 %) and ether bonds (7 %), whereas about 30 % of this unresolved Porg pool appeared in the SOM fraction closely associated with the soil mineral phase. Furthermore, the extractability of inositol phosphates (IP) was increased from 312 mgP/kgsoil to 534 mgP/kgsoil (factor 1.7) using the SCF procedure compared to a single-step NaOH-EDTA extraction. Previous studies have reported the presence of IP in molecular size fractions greater than 10 kDa. Our findings on the removal of IP with the fractionation of the SOM could explain the presence of IP in these large associations.We demonstrate that major pools of Porg are closely associated with SOM structures, comprising a diverse array of chemical species and bonding types. These results forward our understanding of Porg stabilisation, P transformation, and P cycling in terrestrial ecosystems towards an association point of view.

Chemical speciation and release kinetics of Ni in a Ni-contaminated calcareous soil as affected by organic waste biochars and soil moisture regime.

Biochars vary widely in properties and have been shown to have variable effects on potentially toxic element(s) stabilization in soil. This is the first study to examine the interaction effects of biochar and soil moisture regime on Ni stabilization in a Ni-contaminated calcareous soil. Three different organic waste (cow manure, municipal compost and licorice root pulp) biochars produced at two temperatures (300 and 600°C) were applied (3% wt.) to a Ni-contaminated calcareous soil and incubated at field capacity and saturated conditions for 70 d. Sequential chemical fractionation and Ni release kinetics were then performed. All applied biochars, especially the high-temperature biochars, were significantly able to enhance Ni stabilization in the studied soil. In particular, the biochars significantly decreased Ni content in the water-soluble and exchangeable fractions (10-42% decrease), while increasing the immobile residual fraction (13-38% increase). The biochars also significantly decreased the rate and cumulative amount of EDTA-extractable Ni from the calcareous soil. Among the studied biochars, the cow manure and municipal compost biochars produced at 600 °C were the most effective at reducing Ni mobility factor (27-28% decrease) and initial release rate (42-49% decrease), likely due to their high ash content and pH, which promotes Ni sorption in soil. Soil moisture regime was not found to significantly affect the Ni mobility factor or rates of Ni release from the calcareous soil but did, however, affect certain soil Ni chemical fractions. Soil water saturation significantly decreased Ni in the Mn (4%) and non-crystalline Fe oxides (17%) fractions, while increased the crystalline Fe oxide fraction (3%), attributed to reductive dissolution of Mn and Fe oxide crystallinity enhancement. Saturation also significantly enhanced Ni in the residual fraction (4%), attributed to the associated pH increase and potential sulfide formation. The results of this study demonstrate that high temperature, ash-rich, and alkaline biochars are most effective at Ni immobilization, and that soil water saturation can further enhance Ni in the residual fraction.

The impact of long-term field experiments under different cropping systems on the molecular dynamics and stability of the soil Humeome

Cultivation practices alter the molecular status of the soil Humeome, meant as the ensemble of all heterogeneous humic molecules, whose changes need to be understood and monitored in order to maintain the sustainability of agricultural soils. We applied the Humeomics sequential chemical fractionation, coupled to characterization of separated fractions by GC-MS and high-resolution Orbitrap LC-MS, on soils subjected for 20 consecutive years to the following treatments: i) non cultivated and untilled soil (Control); ii) maize monoculture (Maize); iii) maize-leguminous (Vicia faba) rotation (Mix). Humeomics fractions revealed a greater amount of organic carbon (OC) than for the traditional alkaline humus extraction (eSOM), double chromatographic visibility and one order of magnitude more detectable empirical formulae. Humeomics indicated that the ratio of organosoluble to hydrosoluble components decreased significantly passing from Control to Mix and Maize soils, thus unveiling that the loss of humic hydrophobic compounds, such as long-chain esters and fatty acids, rendered more physically and chemically fragile soils under long-term maize monoculture than under crop rotation. Saccharides undetectable in eSOM became instead visible after cleavage of esters weakly bound to the humic matrix, confirming a mechanism of protection of polar compounds by hydrophobic humic components. The same was observed for nitrogen-containing compounds, such as amides and heterocyclic nitrogen, which were significantly detected in Humeomics fractions of cropped soils only after the HI step disrupted ether linkages and organo-mineral complexes. Most of N-containing compounds in cropped soils were found to be bound to iron, thus implying that different forms of nitrogen entering soil by either synthetic fertilizer or nitrogen fixation are progressively sequestered into recalcitrant organic pools. Our findings highlight that a detailed knowledge on the molecular dynamics of the Humeome of soils under long-term field trials allowed a further understanding of the organic matter molecular distribution and the mechanisms of its stabilization.

Phosphorus fractions in Cerrado Oxisols fertilized with liquid swine wastewater

ABSTRACT Knowledge of the forms of phosphorus (P) accumulation in soils that received successive applications of swine waste is essential to prevent the potential contamination risk of shallow waters. This study aimed to evaluate the forms of P accumulation in Cerrado Oxisols subjected to successive applications of liquid swine wastewater. Soil samples were collected from areas with and without use history of swine waste in the 0-0.10 and 0.10-0.20 m soil layers in Campo Verde and Nova Mutum, MT, Brazil. A sequential chemical fractionation was performed, determining the total and available P concentrations. The main forms of P accumulated in the 0-0.20 m layer, despite the soil texture and usage or not of swine wastewater, were residual P and moderately labile P. There was no predominance of more available P forms concerning the unavailable forms when comparing areas with and without the use of swine waste.

Phosphorus speciation and distribution in a variable‐charge Oxisol under no‐till amended with lime and/or phosphogypsum for 18 years

AbstractLong‐term management with lime and/or phosphogypsum (PG) can change phosphorus (P) forms across the soil profile. In this study, we used synchrotron‐based spectroscopy and sequential chemical fractionation to investigate P chemical species after long‐term (18 years) application of amendments in a subtropical Oxisol. The availability of nutrients increased with the combined use of lime and PG and a stratification of attributes was identified due to long‐term broadcast applications. X‐ray absorption near edge structure (XANES) spectra profiles revealed the predominance of P bounded to haematite (Hm) (48% on average) and also P bound to kaolinite (32% on average) species. Low proportions (16%) of calcium‐phosphates in the uppermost soil layer (0–5 cm) were found in the treatment that received high calcium (Ca) inputs by lime and PG broadcast. Few correlations were identified between the proportions of P species obtained by XANES and P fractions operationally defined by sequential chemical fractionation (SF). Our results suggest that P‐species bound to kaolinite can store stable P forms, which correlated (p < 0.05) with the residual P fractions obtained by SF, highlighting the importance of kaolinite as a legacy P store in Oxisols.Highlights Lime and phosphogypsum broadcast under no‐till improve soil fertility and P availability. Labile and moderately labile P fractions were the only ones influenced by lime and/or phosphogypsum. Proportions among P bounded to Fe‐ and Al‐(hydr)oxides with different structural may be affected by long‐term Al‐suppressors. P‐species bounded to kaolinite may store stable P forms in Oxisols.

Closing global P cycles: The effect of dewatered fish sludge and manure solids as P fertiliser.

The aim of this study was to contribute to closing global phosphorus (P) cycles by investigating and explaining the effect of fish sludge (feed residues and faeces of farmed fish) and manure solids as P fertiliser. Phosphorus quality in 14 filtered and/or dried, composted, separated or pyrolysed products based on fish sludge or cattle or swine manure was studied by sequential chemical fractionation and in two two-year growth trials, a pot experiment with barley (Hordeum vulgare) and a field experiment with spring wheat (Triticum aestivum). In fish sludge, P was mainly solubilised in the HCl fraction (66±10%), commonly being associated with slowly soluble calcium phosphates, and mean relative agronomic efficiency (RAE) of fish sludge products during the first year of the pot experiment was only 47±24%. Low immediate P availability was not compensated for during the second year. Thus efforts are needed to optimise the P effects if fish sludge is to be transformed from a waste into a valuable fertiliser. In manure solids, P was mainly soluble in H2O and 0.5M NaHCO3 (72±14%), commonly being associated with plant-available P, and mean RAE during the first year of the pot experiment was 77±19%. Biochars based on fish sludge or manure had low concentrations of soluble P and low P fertilisation effects, confirming that treatment processes other than pyrolysis should be chosen for P-rich waste resources to allow efficient P recycling. The field experiment supported the results of the pot experiment, but provided little additional information.

Изменение фракционного состава тяжёлых металлов при моноэлементном загрязнении аллювиальных почв

The forms of Cd, Pb, and Zn compounds in alluvial soils were studied on the basis of sequential chemical fractionation according to the modified Tessier scheme. It is established that most of the studied elements are fixed in the composition of strongly bonded compounds. When lead is contaminated, the initial ratio of the forms of Cd, Zn elements changes slightly, and Pb - significantly. Their accumulation in the most mobile forms is noted. The different ability of the studied soils to fix lead is shown. It is revealed that the main mechanism of lead fixation in polluted soils is ion exchange absorption.

Organic acids promote phosphorus release from Mollisols with different organic matter contents

Organic acids could improve the phosphorus (P) availability through enhancing the release of inorganic phosphorus (P<sub>i</sub>) in the soil. However, the effects of organic acids on the P<sub>i</sub> release are still poorly understood, especially from soils with different organic matter contents. Here, a biochemically produced humic acid and P fertiliser were added to the soil to modify the content of the soil organic matter (SOM) and soil P, respectively. And then the soil samples were incubated at 25 °C for 30 days. The release of P<sub>i</sub> fractions (such as H<sub>2</sub>O-P<sub>i</sub>, NaHCO<sub>3</sub>-P<sub>i</sub>, NaOH-P<sub>i</sub>, HCl-P<sub>i</sub>, and Residual-P) from the soils with different organic matter contents in the presence of citric, oxalic, and malic acids was evaluated using a sequential chemical fractionation method. The results showed that the release of the NaHCO<sub>3</sub>-P<sub>i</sub>, NaOH-P<sub>i</sub>, and HCl-P<sub>i</sub> fractions also showed a decreasing trend with an increasing content of soil organic matter, and more NaOH-P<sub>i</sub> than the other P<sub>i</sub> fractions was generally released in the presence of organic acids. Considering the types of organic acids, oxalic acid and malic acid most effectively and least effectively released P<sub>i</sub>, respectively. The path analysis indicated that the NaOH-P<sub>i</sub> release had the highest direct and indirect effects on the total inorganic P (TP<sub>i</sub>) release. NaOH-P<sub>i</sub> was, therefore, the most effective source of P<sub>i</sub> in the Mollisols.

The effect of humic substances on metal migration at the border of sediment and water flow

The speciation of radionuclides in bottom sediments of the Yenisei River, sampled in the near-field region of the nuclear fuel cycle enterprise, was studied. It has been established that most of 152Eu and 241Am radionuclides isolated by sequential chemical fractionation are in a bound state with the organic matter of bottom sediments (up to 35% of the total amount). In this case, the content of 152Eu and 241Am in depth of the bottom sediment layers is directly dependent on the change in the organic matter content. Bottom sediments containing a high content of organic substances (humic acids – HA and fulvic acids – FA) were studied. By the HA: FA ratio, the studied bottom sediments are related to a fulvate-humate type. When fractionating the organic matter of bottom sediments, it was revealed that the distribution occurred as follows (% of the total content): in the form of fulvates 152Eu – 53, 241Am–16, 60Co – 10, 137Cs–3; 241Am–28% associated with humin; in the form of organomineral compounds ~ 2–5%; in the form of organometallic complexes ~ 2–3%. Model experiments with addition of HA to the aqueous medium of the bottom sediments-water system. HA was previously isolated from bottom sediments and identified by IR Fourier spectroscopy. The introduction of HA significantly influenced the release of radionuclides and some metals from bottom sediments into the liquid medium. So, the content of 152Eu and 241Am radionuclides are intensively released into the water: from 0 to 198 ± 8 Bq·L−1 and 167 ± 7 Bq·L−1, respectively. The content of 4 K, 60Co, 137Cs was not considerable. The content of some analysed metals in the aqueous medium also increased significantly: 9 times for U6+ and 32 times for Cu2+.

Phosphorus speciation and iron mineralogy in an oxisol after 11 years of pig slurry application

Application of phosphate fertilizers beyond plants needs favors phosphorus (P) accumulation in soils, which may alter its reactivity and chemical speciation. The objective of this study was to assess the changes in P speciation in a Brazilian oxisol that received consecutive applications of varying rates of pig slurry (PS) over 11 years. The soils were treated with PS at rates of 50, 100 and 200 m3 ha−1 year−1, whereas a control plot received P and potassium (K) to replenish the amounts removed by harvest. The soils were sampled and characterized for its P sorption capacity (PSC) as determined by Langmuir sorption isotherms, P partitioning by sequential chemical fractionation (SCF), P chemical speciation via P K-edge XANES and iron (Fe) mineralogy via Fe K-edge EXAFS spectroscopies. Increases in applied PS rates were accompanied by increases in PSC at the 0 to 2.5 and 0 to 10 cm soil layers. P accumulation was observed to be restricted up to the depth of 20 cm, regardless of the PS rate applied. The P K-edge XANES analysis indicated that P accumulation in the topmost soil layers, occurred predominantly associated with Fe-(hydr)oxide minerals. In this soil layer (0 to 2.5 cm), the organic P pool was of particular importance likely due to no-tillage. A dramatic change in Fe mineralogy in the topmost soil layer was observed across the studied soils, with the predominance of hematite in the reference soil and in the control plot, whereas the occurrence of goethite and ferrihydrite was followed by the application of PS.

Internal loading potential of phosphorus in reservoirs along a semiarid watershed

ABSTRACT Sediments are important to nutrient dynamics, especially due to phosphorus internal loading. Several studies have observed that internal loading could prevent water quality from improving in lakes, even when external phosphorus loading is significantly decreased. Therefore, there is an urgent need to assess the potential release of phosphorus contained in the sediment, and thus, its potential to impact water quality. In this study, the vertical and horizontal distributions of distinct phosphorus forms in the bottom sediments of artificial reservoirs, located at Forquilha watershed (Ceará, Brazil), were investigated through sequential chemical fractionation. The reservoirs Lagoa Cercada (R#1), Riacho do Algodão (R#2), Riacho Verde (R#3), Cachoeira (R#4), Chagas Manu (R#5), Quandu (R#6) and Balanças (R#7) were investigated. Reservoirs R#1 (most downstream reservoir of the watershed) and R#7 (most upstream reservoir of the watershed) had higher concentrations of total phosphorus (PT) and lower pH values, potentially exerting greater influence in phosphorus concentration in the water column. Reservoirs R#3 and R#4 presented a predominance of residual phosphorus (PRe), the least available fractions of phosphorus and thus, presented a lower potential for internal loading. Reservoirs R#5, R#1 and R#2 showed a tendency of decreasing total phosphorus (PT) as the sediment depth increased, probably indicating an increase of allochthonous phosphorus loading along time. Reservoir R#6 showed the predominance of PFeAl and PCa fractions on points A and B, respectively, showing that the characteristics of the sediments may vary in the same reservoir. Mobile (PM) and iron and aluminum-bound phosphorus (PFeAl) were the least and the most abundant fractions in most of the samples analyzed, respectively.

Phosphorus Availabilities Differ between Cropland and Forestland in Shelterbelt Systems

Shelterbelt systems play pivotal roles in providing goods and services to the rural community and the society at large, but phosphorus (P) cycling in shelterbelt systems is poorly studied, while P cycling and availability would be linked to the ecological function and services of shelterbelt systems. This study was conducted to understand how long-term (>30 years) land-use between cropland and forestland in shelterbelt systems affect soil P status. We investigated modified Kelowna (PKelowna) and Mehlich-3 (PMehlich) extractable P, P fractions (by sequential chemical fractionation), P sorption properties in the 0–10 and 10–30 cm soils and their relationship in six pairs of the cropland areas and adjacent forestland (each pair constitutes a shelterbelt system) in central Alberta. Both PKelowna and PMehlich in the 0–10 cm soil were greater in the cropland than in the forestland. The PKelowna ranged from 10 to 170 and 2 to 57 mg kg−1 within the cropland areas and forestland, respectively. The inorganic P fraction in the 0–30 cm depth was significantly related to PKelowna (R2 = 0.55) and PMehlich (R2 = 0.80) in cropland, but organic P fraction was not significantly related with neither PKelowna nor PMehlich. The iron (Fe) and aluminum (Al) associated P (Fe/Al-P) explained ~50% and ~45% of the variation of PKelowna in the 0–30 cm soil in the cropland and forestland, respectively. The Fe/Al-P and organic P fractions in the 0–10 cm soil were greater in the cropland than in the forestland. The differences in availability and P forms depending on the land use type in shelterbelts suggest that P management needs to be land-use type-specific for shelterbelt systems.

Effects of long-term paddy rice cultivation on soil arsenic speciation

Geochemical behavior of arsenic (As) in rice paddy soils determines the availability and mobility of As in the soils, but little is known about the long-term effects of paddy rice cultivation on As speciation in the soils. In this study, surface soil samples were collected from a rice paddy land and its adjacent dry land with similar soil properties and known cultivation histories. The soils of the paddy land and dry land contained 378 and 423 mg As kg−1, respectively. The predominant As species in the soils were investigated using As K-edge X-ray absorption spectroscopy (XAS) in combination with two sequential chemical fractionation methods. The XAS results showed that the predominant As species in the soils were As(III)- and As(V)-ferrihydrite, As(V)-goethite and scorodite. In comparison to the dry land soil, the paddy land soil contained a higher proportion of As(V)-ferrihydrite and a lower proportion of scorodite. The results of chemical fractionation revealed that As in the paddy land soil was more labile than that in the dry land soil. It is therefore suggested that long-term rice cultivation enhances the mobility and availability of As in paddy soils.

Geochemical phases of aluminum in the river sediments as indicators of its environmental availability

Natural fluvial sediments are predominantly composed of river transported debris resulting from weathering and erosion of the basin. Understanding both the concentration and the distribution of metals in the river bed sediments in headwaters is essential to provide a scientific reference for the protection of water resources and the control of water pollution downstream. Sequential chemical fractionation studies of elements in various phases in the environment are widely used in investigating the effects of a particular element in the ecosystem. In this study, determination of aluminum (Al) bound to six phases operationally defined (soluble/exchangeable/specifically adsorbed, bound to manganese oxides, associated with amorphous compounds, bound to oxidizable organic matter, associated with crystalline iron oxides, and residual fraction) in the river bed sediment samples was performed in order to evaluate its potential environmental availability in the monitoring area. Aluminum contents were determined by flame atomic absorption spectrophotometry. The pattern of fractionation of Al in the sediments that are studied suggests that this element does not pose a risk to the aquatic environment under the current environmental conditions since it is mainly part of the potentially less bioavailable geochemical fractions.

Modelling hydrolysis: Simultaneous versus sequential biodegradation of the hydrolysable fractions

Hydrolysis is considered the limiting step during solid waste anaerobic digestion (including co-digestion of sludge and biosolids). Mechanisms of hydrolysis are mechanistically not well understood with detrimental impact on model predictive capability. The common approach to multiple substrates is to consider simultaneous degradation of the substrates. This may not have the capacity to separate the different kinetics. Sequential degradation of substrates is theoretically supported by microbial capacity and the composite nature of substrates (bioaccessibility concept). However, this has not been experimentally assessed. Sequential chemical fractionation has been successfully used to define inputs for an anaerobic digestion model. In this paper, sequential extractions of organic substrates were evaluated in order to compare both models. By removing each fraction (from the most accessible to the least accessible fraction) from three different substrates, anaerobic incubation tests showed that for physically structured substrates, such as activated sludge and wheat straw, sequential approach could better describe experimental results, while this was less important for homogeneous materials such as pulped fruit. Following this, anaerobic incubation tests were performed on five substrates. Cumulative methane production was modelled by the simultaneous and sequential approaches. Results showed that the sequential model could fit the experimental data for all the substrates whereas simultaneous model did not work for some substrates.

Phosphorus Fractionation in Soils Fertilized with Recycled Phosphorus Products

Phosphorus (P) will be exhausted, and its recovery and cycling will play a pivotal role in agricultural systems. Different technologies have been developed to recover P from waste materials, such as mono-incineration, acid digestion, and coprecipitation of P from wastewaters. The aim of this study was to assess the plant availability of P from different recycled P products by using a sequential chemical P fractionation of soil P and to study the fate of P applied to the soil. We evaluated the plant P availability in soils fertilized with eight recycled P products in comparison with two commercial fertilizers (triple superphosphate [TSP] and rock phosphate [Rock-P]) by means of a sequential chemical extraction of soil P and combined this data with previously published information for soil isotopically exchangeable P (IEP) and P uptake by plants (PU). We sequentially extracted inorganic P with alkaline extractants and an acid solution and analyzed the increase in P fractions caused by the application of different fertilizers. P extracted by alkali solutions was positively correlated with the IEP and with PU, whereas P extracted by the acid solution was negatively correlated with these parameters. In soils fertilized with recycled P products obtained by a chemical procedure (magnesium-ammonium-phosphate), soil P was extracted mainly by alkali extractants, while in soils fertilized with P products obtained by thermal procedures (i.e. sewage sludge ashes), soil P was extracted by the acid solution and was of low plant availability. Sequential chemical fractionation of soil P is suitable for characterizing the plant availability of P from recycled P products. The increase in the alkali soluble fraction in soil characterizes the portion of P available to plants, while the acid soluble fraction characterizes the recalcitrant portion. Phosphorus from fertilizers obtained by chemical procedures, like magnesium-ammonium-phosphate, is mainly extracted by alkali solutions and is comparable to water-soluble P fertilizers, while P from products obtained by thermal processes is mainly extracted by acid solutions and remains mostly unavailable to plants.