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Zhou J. The effects of organic fertilizer on loss risk of nitrogen and phosphorus in paddy ponded water. ENVIRONMENTAL SCIENCE AND POLLUTION RESEARCH INTERNATIONAL 2023; 30:55529-55540. [PMID: 36897445 DOI: 10.1007/s11356-023-26296-8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/21/2022] [Accepted: 03/01/2023] [Indexed: 06/18/2023]
Abstract
There is a great challenge globally for both achieving high crop yields via fertilization and minimizing environmental pollution from nutrient losses. Organic fertilizer (OF) application has been extensively reported to effectively improve arable soil fertility and mitigate nutrient losses. However, few studies are available that accurately quantified the substitution rates of OF for chemical fertilizers (CF) that affect rice yield, the nitrogen/phosphorus in ponded water, and its loss potential in paddy field. Here, an experiment with 5 levels of CF nitrogen substituted by OF nitrogen was performed during the early stage of rice growth in paddy field of Southern China. The results showed that the first 6 days and 3 days following fertilization generally were risky period for N losses and for P losses, respectively, due to corresponding high concentrations in ponded water. Compared to CF treatment, over 30% of OF substitution rates significantly decreased the daily mean TN concentrations by 24.5-32.4%, while TP concentrations unaffected and rice yield maintained comparative levels. OF substitution also improved acidic paddy soils, with the increment of 0.33-0.90 unit for ponded water pH compared to CF treatment. Conclusively, the 30-40% of CF substituted by OF based on N amounts can be considered an ecological fertilization practice for rice production to mitigate environmental pollution due to lower N losses and without significant effect of grain yield. However, the attention also must be paid concerning the rise of environmental pollution risk from NH3 volatilization and P runoff after long-term OF application.
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Affiliation(s)
- Jiangming Zhou
- The Agricultural Technique Popularization Centre of Jiangshan City, Jiangshan, 324100, Zhejiang, China.
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Lucas E, Mosesso L, Roswall T, Yang YY, Scheckel K, Shober A, Toor GS. X-ray absorption near edge structure spectroscopy reveals phosphate minerals at surface and agronomic sampling depths in agricultural Ultisols saturated with legacy phosphorus. CHEMOSPHERE 2022; 308:136288. [PMID: 36058369 PMCID: PMC9843306 DOI: 10.1016/j.chemosphere.2022.136288] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/22/2022] [Revised: 08/24/2022] [Accepted: 08/28/2022] [Indexed: 06/15/2023]
Abstract
Legacy phosphorus (P) soils have received excessive P inputs from historic manure and fertilizer applications and present unique management challenges for protecting water quality as soil P saturation leads to increased soluble P to waterways. We used P K-edge X-ray absorption near edge structure (XANES) spectroscopy to identify and quantify the dominant P minerals in four representative legacy P soils under conventional till and no-till management in Maryland, USA. Various measures of extractable soil P, including water-extractable P (20.6-54.1 mg kg-1 at 1:10 soil-to-water ratio; 52.7-132.2 mg kg-1 at 1:100 soil-to-water ratio), plant available P extracted with Mehlich 3 (692-1139 mg kg-1), and Mehlich 3P saturation ratio (0.54-1.37), were above the environmental threshold values, suggesting the accumulation of legacy P in soils. The quantification of dominant P minerals may provide insights into the potential of legacy P soils to contribute to P release for crop use and soluble P losses. Linear combination fits of XANES spectra identified the presence of four phosphate mineral groups, consisting of (i) calcium-phosphate minerals (11-59%) in the form of fluorapatite, β-tricalcium phosphate, and brushite, followed by (ii) iron-phosphate minerals (12-49%) in the form of ludlamite, heterosite, P sorbed to ferrihydrite, and amorphous iron phosphates, (iii) aluminum-phosphate minerals (15-33%) in the form of wavellite and P sorbed to aluminum hydroxide, and (iv) other phosphate minerals (5-35%) in the form of copper-phosphate (cornetite, 5-18%) and manganese-phosphate (hureaulite, 25-35%). Organic P consisting of phytic acid was found in most soils (13-24%) and was more pronounced in the surface layer of no-till (21-24%) than in tilled (16%) fields. Of the P forms identified with XANES, we conclude that P sorbed to Fe and Al, and Ca-P in the form of brushite and β-tricalcium phosphate will likely readily contribute to the soil WEP pool as the soil solution P is depleted by crop uptake and lost via runoff and leaching.
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Affiliation(s)
- Emileigh Lucas
- Nutrient Management and Water Quality Group, Department of Environmental Science and Technology, University of Maryland, College Park, MD, 20742, USA.
| | - Lauren Mosesso
- Department of Plant and Soil Sciences, University of Delaware, Newark, DE, 19716, USA
| | - Taylor Roswall
- Nutrient Management and Water Quality Group, Department of Environmental Science and Technology, University of Maryland, College Park, MD, 20742, USA
| | - Yun-Ya Yang
- Nutrient Management and Water Quality Group, Department of Environmental Science and Technology, University of Maryland, College Park, MD, 20742, USA
| | - Kirk Scheckel
- Center for Environmental Solutions & Emergency Response, Office of Research and Development, US Environmental Protection Agency, Cincinnati, OH, 45268, USA
| | - Amy Shober
- Department of Plant and Soil Sciences, University of Delaware, Newark, DE, 19716, USA
| | - Gurpal S Toor
- Nutrient Management and Water Quality Group, Department of Environmental Science and Technology, University of Maryland, College Park, MD, 20742, USA
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Yuan L, Guo P, Guo S, Wang J, Huang Y. Influence of electrical fields enhanced phytoremediation of multi-metal contaminated soil on soil parameters and plants uptake in different soil sections. ENVIRONMENTAL RESEARCH 2021; 198:111290. [PMID: 33965386 DOI: 10.1016/j.envres.2021.111290] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/14/2020] [Revised: 03/05/2021] [Accepted: 04/13/2021] [Indexed: 06/12/2023]
Abstract
The influence of electrical fields on phytoremediation of multi-metal (Cd, Cu, and Zn) naturally contaminated soils has been investigated based on different soil sections. After ryegrass and hybrid penisetum were sowed for 30 d, electrical fields were applied during 30 days with the switching polarity every 30 min and continuing for 16 h d-1. After electrokinetic (EK) assisted phytoremediation process, soil electrical conductivity (EC) in anode section and available soil potassium (K) in cathode section were obviously elevated. Plants biomass in middle and cathode sections were increased in both plants, especially in middle section the overall biomass of hybrid penisetum increased by 68.8%. The influence of electrical field on the contents of heavy metals in plants was different depending on the species of plants, kind of heavy metals and soil section. For Cd, Cu, and Zn co-contaminated soils, shoot metals accumulation in middle section in both plants were improved at least about 20% (with the exception of Zn in ryegrass). Electrical fields had the most significant effect on copper absorption by ryegrass and shoot Cu accumulation were elevated 32.5% in all the section. The soil EC maybe an important factor that affected electrical fields enhanced plants growth, plant metals concentrations and remediation efficiency.
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Affiliation(s)
- Lizhu Yuan
- Shandong Provincial Key Laboratory of Applied Microbiology, Ecology Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250103, China; National-Local Joint Engineering Laboratory of Contaminated Soil Remediation By Bio-physicochemical Synergistic Process, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, 110016, China
| | | | - Shuhai Guo
- Shandong Provincial Key Laboratory of Applied Microbiology, Ecology Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250103, China; National-Local Joint Engineering Laboratory of Contaminated Soil Remediation By Bio-physicochemical Synergistic Process, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, 110016, China.
| | - Jianing Wang
- Shandong Provincial Key Laboratory of Applied Microbiology, Ecology Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250103, China
| | - Yujie Huang
- Shandong Provincial Key Laboratory of Applied Microbiology, Ecology Institute, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250103, China
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Zhao Y, Li Y, Yang F. Critical review on soil phosphorus migration and transformation under freezing-thawing cycles and typical regulatory measurements. THE SCIENCE OF THE TOTAL ENVIRONMENT 2021; 751:141614. [PMID: 32889455 DOI: 10.1016/j.scitotenv.2020.141614] [Citation(s) in RCA: 17] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/19/2020] [Revised: 07/25/2020] [Accepted: 08/08/2020] [Indexed: 06/11/2023]
Abstract
Freezing-thawing period plays an important role in the soil nutrient cycling. The frequency of freezing-thawing cycles (FTCs) can directly affect the supply of effective soil nutrients, further influences the growth and development of crops. Phosphorus is one of the essential nutrients for crop growth, and almost no compounds in gas form in nature, which is non-renewable resources. In modern agricultural production, phosphorus required by plants is mainly from the soil, but the utilization rate of phosphorus fertilizer in soil is generally only 10%-25%. Therefore, it is of great significance to study phosphorus migration and transformation behavior of soil in the non-growth period and related interfacial processes for improving the utilization efficiency of phosphorus fertilizer, increasing crop yield, reducing excessive application of phosphorus fertilizer, and subsiding environmental pollution. This paper systematically concludes key interfacial process of soil phosphorus in freezing-thawing soil system and relative mechanisms describing migration and transformation behavior of soil phosphorus. Besides, it summarizes the mediating effects of widely used soil conditioner on phosphorus cycling. The results show that freezing- thawing will destroy the structure of the soil, causing phosphorus to migrate along with runoff, soil water and heat movement. It also affects the types of microorganisms, the activity of microbial communities and the oxidation-reduction reaction of related minerals, making the phosphorus in soil from an unstable form to an active form. Biochar and humic substances can improve the physical and chemical properties of the soil, and have favorable effects on soil during freezing-thawing period. This review has important significance for the rational utilization of existing phosphorus resources, the maintenance of soil phosphorus cycle balance and the sustainable development of agriculture, meanwhile, has guiding significance for the reasonable utilization of agricultural wastes.
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Affiliation(s)
- Ying Zhao
- Joint Laboratory of Northeast Agricultural University and Max Planck Institute of Colloids and Interfaces (NEAU-MPICI), Harbin 150030, China; School of Water Conservancy & Civil Engineering, Northeast Agricultural University, Harbin 150030, China
| | - Yuelei Li
- Joint Laboratory of Northeast Agricultural University and Max Planck Institute of Colloids and Interfaces (NEAU-MPICI), Harbin 150030, China; School of Water Conservancy & Civil Engineering, Northeast Agricultural University, Harbin 150030, China
| | - Fan Yang
- Joint Laboratory of Northeast Agricultural University and Max Planck Institute of Colloids and Interfaces (NEAU-MPICI), Harbin 150030, China; School of Water Conservancy & Civil Engineering, Northeast Agricultural University, Harbin 150030, China.
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Chen X, Gao M, Li Y, Zhang X, Zhang F, Hu B. Effects of freeze-thaw cycles on the physicochemical characteristics of animal manure and its phosphorus forms. WASTE MANAGEMENT (NEW YORK, N.Y.) 2019; 88:160-169. [PMID: 31079628 DOI: 10.1016/j.wasman.2019.03.039] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/24/2018] [Revised: 03/14/2019] [Accepted: 03/19/2019] [Indexed: 06/09/2023]
Abstract
The variations of phosphorus (P) in animal manure during freeze-thaw cycles (FTCs) profoundly influence on non-point source P loss in winter. Therefore, understanding how FTCs influence the physicochemical properties of animal manure and its P availability is crucial. In this study, the freeze-thaw treatment was performed by incubating the pig manure at -20 °C for 12 h and at 18 °C for 12 h. The freeze-only treatment was maintained at -20 °C as a control. In addition, the pig manure was kept at two moisture levels during the FTCs and sampled every five cycles. Six forms of P in the manure were extracted and analyzed. After 30 cycles, the dissolved organic carbon had increased from 10.49 to 13.56 g/kg, and the pH had decreased from 6.25 to 5.77. The particles originally >1000 μm were broken into particles <250 μm. The forms of P in manure shifted from Ca-P, occluded P, and residual P towards NH4Cl-P, Al-P and Fe-P, resulting in a 23% increase in bioavailable P. These variations were highly coincident with the increase in moisture content and FTC frequency. The proportion of particles <38 μm increased by more than 2% after the FTCs, and the manure P was mainly concentrated in these particles, which might be readily washed away by the melt water. Overall, the study indicated that FTCs could enhance the bioavailability of P in pig manure and the mobility of particle-associated P. These findings are significant for reducing animal manure pollution in freeze-thaw season.
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Affiliation(s)
- Xingcai Chen
- State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, 100875 Beijing, China
| | - Min Gao
- State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, 100875 Beijing, China
| | - Yanxia Li
- State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, 100875 Beijing, China.
| | - Xuelian Zhang
- Beijing Soil and Fertilizer Extension Service Station, Beijing 100029, China
| | - Fengsong Zhang
- Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, 100101 Beijing, China
| | - Baiyang Hu
- State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, 100875 Beijing, China
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Kloprogge JT, Wood BJ. X-ray Photoelectron Spectroscopic and Raman microscopic investigation of the variscite group minerals: Variscite, strengite, scorodite and mansfieldite. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2017; 185:163-172. [PMID: 28570987 DOI: 10.1016/j.saa.2017.05.042] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/16/2017] [Revised: 04/27/2017] [Accepted: 05/19/2017] [Indexed: 06/07/2023]
Abstract
Several structurally related AsO4 and PO4 minerals, were studied with Raman microscopy and X-ray Photoelectron Spectroscopy (XPS). XPS revealed only Fe, As and O for scorodite. The Fe 2p, As 3d, and O 1s indicated one position for Fe2+, while 2 different environments for O and As were observed. The O 1s at 530.3eV and the As 3d 5/2 at 43.7eV belonged to AsO4, while minor bands for O 1s at 531.3eV and As 3d 5/2 at 44.8eV were due to AsO4 groups exposed on the surface possibly forming OH-groups. Mansfieldite showed, besides Al, As and O, a trace of Co. The PO4 equivalent of mansfieldite is variscite. The change in crystal structure replacing As with P resulted in an increase in the binding energy (BE) of the Al 2p by 2.9eV. The substitution of Fe3+ for Al3+ in the structure of strengite resulted in a Fe 2p at 710.8eV. An increase in the Fe 2p BE of 4.8eV was found between mansfieldite and strengite. The scorodite Raman OH-stretching region showed a sharp band at 3513cm-1 and a broad band around 3082cm-1. The spectrum of mansfieldite was like that of scorodite with a sharp band at 3536cm-1 and broader maxima at 3100cm-1 and 2888cm-1. Substituting Al in the arsenate structure instead of Fe resulted in a shift of the metal-OH-stretching mode by 23cm-1 towards higher wavenumbers due to a slightly longer H-bonding in mansfieldite compared to scorodite. The intense band for scorodite at 805cm-1 was ascribed to the symmetric stretching mode of the AsO4. The medium intensity bands at 890, 869, and 830cm-1 were ascribed to the internal modes. A significant shift towards higher wavenumbers was observed for mansfieldite. The strengite Raman spectrum in the 900-1150cm-1 shows a strong band at 981cm-1 accompanied by a series of less intense bands. The 981cm-1 band was assigned to the PO4 symmetric stretching mode, while the weak band at 1116cm-1 was the corresponding antisymmetric stretching mode. The remaining bands at 1009, 1023 and 1035cm-1 were assigned to υ1(A1) internal modes in analogy to the interpretation of the AsO4 bands for scorodite and mansfieldite. The variscite spectrum showed a shift towards higher wavenumbers in comparison to the strengite spectrum with the strongest band observed at 1030cm-1 and was assigned to the symmetric stretching mode of the PO4, while the corresponding antisymmetric stretching mode was observed at 1080cm-1. Due to the band splitting component bands were observed at 1059, 1046, 1013 and 940cm-1. The AsO4 symmetric bending modes for scorodite were observed at 381 and 337cm-1, while corresponding antisymmetric bending modes occurred at 424, 449 and 484cm-1. Comparison with other arsenate and phosphate minerals showed that both XPS and Raman spectroscopy are fast and non-destructive techniques to identify these minerals based on their differences in chemistry and the arsenate/phosphate vibrational modes due to changes in the symmetry and the unique fingerprint region of the lattice modes.
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Affiliation(s)
- J Theo Kloprogge
- School of Earth and Environmental Sciences, The University of Queensland, Brisbane, QLD 4072, Australia; Department of Chemistry, College of Arts and Sciences, University of the Philippines Visayas, Miagao, Iloilo, Philippines.
| | - Barry J Wood
- Centre for Microscopy & Microanalysis, The University of Queensland, Brisbane, QLD 4072, Australia
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Liang Y, Cao X, Zhao L, Xu X, Harris W. Phosphorus release from dairy manure, the manure-derived biochar, and their amended soil: effects of phosphorus nature and soil property. JOURNAL OF ENVIRONMENTAL QUALITY 2014; 43:1504-1509. [PMID: 25603098 DOI: 10.2134/jeq2014.01.0021] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Land application of animal manure often risks excessive phosphorus (P) release into the surrounding water. The aim of this study was to convert the dairy manure into biochar, followed by their application into soil, and then to investigate P release from the manure and its derived biochar as well as from the manure- and biochar-amended soil. The results showed that P release was reduced when the manure was converted into biochar due to formation of less-soluble whitlockite [(Ca, Mg)(PO)]. The cumulative P released from biochar over 240 h was 0.26 g kg, a 76% reduction of that from the manure (1.07 g kg). The kinetic release of P from the manure was determined by the fast desorption process and was better fitted to Elovich equation, whereas P release from biochar was initially controlled by the diffusion process and then by slow but steady dissolution of (Ca,Mg)(PO), following the parabolic diffusion and linear models, respectively. When the manure or biochar was incorporated into the soil, P release in the CaCl and simulated acid rain water extraction from biochar-amended soil was consistently lower than that from the manure-amended soil during 210-d incubation. The lower P release in the biochar-amended soil was determined by stable P form (Ca, Mg)(PO) in the biochar itself, but less from the soil property effect. Results indicated that initial high P release from manure can be mitigated by converting the manure into biochar.
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Impact of electrokinetic-assisted phytoremediation of heavy metal contaminated soil on its physicochemical properties, enzymatic and microbial activities. Electrochim Acta 2012. [DOI: 10.1016/j.electacta.2012.04.112] [Citation(s) in RCA: 67] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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Doolette AL, Smernik RJ. Soil Organic Phosphorus Speciation Using Spectroscopic Techniques. SOIL BIOLOGY 2011. [DOI: 10.1007/978-3-642-15271-9_1] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
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Devau N, Le Cadre E, Hinsinger P, Gérard F. A mechanistic model for understanding root-induced chemical changes controlling phosphorus availability. ANNALS OF BOTANY 2010; 105:1183-97. [PMID: 20495198 PMCID: PMC2887072 DOI: 10.1093/aob/mcq098] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/21/2010] [Revised: 03/23/2010] [Accepted: 04/19/2010] [Indexed: 05/24/2023]
Abstract
BACKGROUND AND AIMS Plant nutrition models do not properly account for the effects of root-induced chemical changes in the rhizosphere, e.g. pH changes, on the availability of nutrients such as phosphorus (P). As a result, they underestimate the actual P uptake, i.e. P bioavailability to the plant, in low-P soils. The present study aims at simulating root-induced chemical mechanisms controlling P nutrition in a P-limited soil. METHODS In this work a mechanistic description for the adsorption of cations and anions by soil constituents (1pK-Triple Plane Model, ion-exchange and Nica-Donnan) was used to simulate changes induced by durum wheat (Triticum durum turgidum) in the P availability of the soil, as measured by water and CaCl2 extraction. Calcium (Ca) availability was also measured and simulated. KEY RESULTS The simulations were found to be in close agreement with experimental data. In the rhizosphere, the goodness-of-fit required to account for the measured uptake of Ca by plants, in addition to the measured uptake of P and root-induced alkalization, were satisfactory. Calcium uptake significantly increased P availability, as assessed through water extraction, by decreasing the promoting effect of Ca adsorption on P adsorption. The study thus enabled P and Ca availability to be related to their bioavailability for durum wheat under experimental conditions. It was also shown that P was primarily adsorbed onto Fe oxides and clay minerals (kaolinite and illite) depending on soil pH. The major source of P for durum wheat nutrition was P desorbed from goethite and kaolinite. CONCLUSIONS In addition to confirming the validity of our approach to model P availability, the present investigation suggested that in the studied soil, a novel root-induced chemical process was controlling P nutrition under P-deficient conditions, namely the uptake of Ca.
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Affiliation(s)
| | - Edith Le Cadre
- Supagro, UMR 1222 Eco&Sols – Ecologie Fonctionnelle & Biogéochimie des Sols (INRA-IRD-SupAgro), Place Viala, F-34060 Montpellier, France
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The chemical characteristics of soil which determine phosphorus partitioning in highly calcareous soils. JOURNAL OF THE SERBIAN CHEMICAL SOCIETY 2006. [DOI: 10.2298/jsc0611219t] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
Phosphorus fractions from three highly calcareous soils (average, 24.9 ? 4.8 %CO3 2-) from sampling sites with a Mediterranean climate were isolated by sequential extraction. In order to provide a more reliable basis for the definition of the obtained P-fractions, principal component analysis was applied and from the chemical characteristics of the 14 investigated soils, those characteristics which define the content and association features of the P-fractions were assessed. The soils are characterized by a relatively high pH (8.0 - 8.2) and by significantly differing contents of organic mater, acid-soluble Mg and total P. These differences affected the various association features of the P-fraction with the soil constituents. The NH4F-P fraction (isolated with 0.5 M NH4F, pH 8.2) is defined by the contents of the main metals of the oxide-hydroxide- clay associations (Al, Fe,Mn) or by the the redox potential (Eh) of Mn. The accumulation of NaOH-phosphorus (extractable with 0.1M NaOH) depended on the constituents of the oxide-hydroxide-clay association, the humic substances and Eh-related factors. In those soils in which NaOH-Pis defined by the oxide-hydroxide-clay association, the participation of Fe as a bridge-forming metal is proposed. The main part of total P, i.e., ?P = TP - (NH 4F-P + NaOH-P) is defined by the status of Mn- and Fe-humic complexes or by the concentration of hydroxyl-ions.
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