Binding of Organic Ligands with Al(III) in Dissolved Organic Matter from Soil: Implications for Soil Organic Carbon Storage

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<ul><li><p>Binding of Organic Ligands with Al(III) in Dissolved Organic Matterfrom Soil: Implications for Soil Organic Carbon StorageGuang-Hui Yu, Min-Jie Wu, Guan-Ran Wei, Yi-Hong Luo, Wei Ran, Bo-Ren Wang,</p><p>Jianchao Zhang, and Qi-Rong Shen*,</p><p>Jiangsu Key Lab for Organic Solid Waste Utilization, Key Laboratory of Plant Nutrition and Fertilization in Low-Middle Reaches ofthe Yangtze River, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, People'sRepublic of ChinaSchool of Environmental Sciences and Technology, Shanghai Jiaotong University, Shanghai 200240, People's Republic of ChinaKey Laboratory of Plant Nutrition and Nutrient Cycling, Ministry of Agriculture of China and Institute of Agricultural Resources andRegional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, People's Republic of China</p><p>*S Supporting Information</p><p>ABSTRACT: The binding characteristics of organic ligandswith Al(III) in soil dissolved organic matter (DOM) isessential to understand soil organic carbon (SOC) storage. Inthis study, two-dimensional (2D) FTIR correlation spectros-copy was developed as a novel tool to explore the binding oforganic ligands with Al(III) in DOM present in soils as part ofa long-term (21-year) fertilization experiment. The resultsshowed that while it is a popular method for characterizing thebinding of organic ligands and metals, fluorescence excitationemission matrixparallel factor analysis can only characterizethe binding characteristics of fluorescent substances (i.e.,protein-, humic-, and fulvic-like substances) with Al(III).However, 2D FTIR correlation spectroscopy can characterizethe binding characteristics of both fluorescent and non-fluorescent (i.e., polysaccharides, lipids, and lignin) substanceswith Al(III). Meanwhile, 2D FTIR correlation spectroscopydemonstrated that the sequencing/ordering of organicsbinding with Al(III) could be modified by the use of long-term fertilization strategies. Furthermore, 2D FTIR correlationspectroscopy revealed that the high SOC content in thechemical plus manure (NPKM) treatment in the long termfertilization experiment can be attributed to the formation ofnoncrystalline microparticles (i.e., allophane and imogolite). In summary, 2D FTIR correlation spectroscopy is a promisingapproach for the characterization of metalorganic complexes.</p><p> INTRODUCTIONFluorescence excitationemission matrix (EEM) spectroscopycombined with parallel factor (PARAFAC) analysis is a popularmethod for investigating the complexes between Al(III) andorganic ligands in DOM from soil,13 water,4,5 and leachates.6</p><p>Dissolved organic matter (DOM) in soil is a small but reactivefraction of soil organic matter.7 However, the stability anddynamics of DOM are essential to soil organic carbon storage.Some investigators have shown that noncrystalline micro-particles such as allophane [Al2O3(SiO2)12(H2O)2.54] andimogolite [(OH)3Al2O3SiOH] in soil DOM control soilorganic carbon sequestration.7,9 Therefore, it is essential toinvestigate the formation of complexes between soil DOM andAl(III). Although the fluorescence EEM-PARAFAC method</p><p>can characterize complexes of protein-, humic-, and fulvic-likesubstances with Al(III), one limitation of this method is that itcannot characterize complexes of nonfluorescent substances(i.e., polysaccharides, lipids, and lignin) with Al(III), eventhough such substances are the predominant components insoil.Fourier transform infrared (FTIR) spectroscopy is a</p><p>commonly used technique that can distinguish between theprincipal organics in soil DOM, including both fluorescent and</p><p>Received: January 18, 2012Revised: April 19, 2012Accepted: May 2, 2012Published: May 2, 2012</p><p>Article</p><p>pubs.acs.org/est</p><p> 2012 American Chemical Society 6102 dx.doi.org/10.1021/es3002212 | Environ. Sci. Technol. 2012, 46, 61026109</p><p>pubs.acs.org/est</p></li><li><p>nonfluorescent substances.10,11 By determining the FTIRspectra of DOM after the titration of Al(III), it is possible toexplore the complexes of organic ligands in DOM and Al(III).However, the peaks in the conventional FTIR spectra oftenoverlap because of the extreme heterogeneity of DOM.12,13</p><p>Recent investigations have demonstrated that two-dimensional(2D) correlation spectroscopy can be applied to solve theoverlapped peaks problem by distributing the spectral intensitytrends within a data set, collected as a function of aperturbation sequence (e.g., time, temperature, or concen-tration change), over a second dimension.1214 Moreimportantly, 2D correlation spectroscopy can be used toprobe the specific sequencing/ordering (sometimes referred toas Nodas rule) of spectral intensity changes throughasynchronous analysis.14 To our knowledge, 2D FTIRcorrelation spectroscopy has not previously been applied toinvestigate the complexes of organic ligands in soil DOM andmetals.The objectives of the present study were (1) to provide more</p><p>information regarding the binding of organic ligands withAl(III) at the molecular level using 2D FTIR correlationspectroscopy combined with EEM-PARAFAC; (2) to give thesequencing of relative affinities of organic ligands for bindingwith Al(III) using 2D FTIR correlation spectroscopy; and (3)to explore the mechanisms of soil organic matter (SOM)sequestration by biogeochemical processes in the soil resultingfrom different fertilization treatments. For these purposes, theeffects of three treatments in a long-term (21-year) fertilizationexperiment were compared on the formation of complexesbetween organic ligands in soil DOM and Al(III): nofertilization (CK), chemical (NPK) fertilization, and chemicalplus pig manure (NPKM) fertilization.</p><p> MATERIALS AND METHODSSoil Sample and DOM Extraction. Soils were collected</p><p>from three treatments in a long-term fertilization experimentstation: CK-, NPK-, and NPKM-treatments. The long-termfertilization experiment was conducted initially in September1990 in fields that were double cropped with wheat and corn atthe experiment station of the Chinese Academy of AgriculturalSciences, Qiyang (26 45 N, 111 52 E, 120 m altitude),Hunan Province, southern China. The red soil was classified asFerralic Cambisol. A detailed description of the long-termfertilization experiment site can be found elsewhere.15 Soilsamples at depths of 020 cm were collected during September2011 using a 5-cm internal diameter auger. Each sample was acomposite of 10 random cores collected from a single plot. Thefresh soil was mixed thoroughly, air-dried, and sieved through2-mm and 0.25-mm screens for further analysis, respectively.The SOM contents of samples collected from plots receivingthe CK-, NPK-, and NPKM-treatments were 14.88 2.02 g/kg(mean standard deviation), 18.36 0.16 g/kg, and 25.13 2.02 g/kg, respectively (Table S1 of the SupportingInformation, SI).The DOM fraction of soil samples was extracted with</p><p>deionized water (solid to water ratio of 1: 2.5 w/v) by shakingfresh soil samples for 24 h on a horizontal shaker at roomtemperature. The DOM was filtered using 0.45 mpolytetrafluoroethylene (PTFE) filters and further diluteduntil the dissolved organic carbon (DOC) concentration was</p></li><li><p>A set of dynamic spectra is given by the following:</p><p> = I x t I x t I x( , ) ( , ) ( )j (3)</p><p>where I(x) denotes the reference spectrum, which is typicallythe average spectrum and is expressed as I(x) 1/mj = 1m I(x,tj).The synchronous correlation intensity can be directly calculatedfrom the following dynamic spectra:</p><p> =</p><p> =</p><p>x xm</p><p>I x I x( , )1</p><p>1( ) ( )</p><p>j</p><p>m</p><p>j j1 21</p><p>1 2(4)</p><p>Asynchronous correlation can be obtained by the following:</p><p> =</p><p> = =</p><p>x xm</p><p>I x N I x( , )1</p><p>1( ) ( )</p><p>j</p><p>m</p><p>jk</p><p>m</p><p>jk j1 21</p><p>11</p><p>2(5)</p><p>The term Njk corresponds to the jth column and the kth raw</p><p>element of the discrete Hilbert-Noda transformation matrix,which is defined as follows:</p><p>=</p><p>=</p><p>N</p><p>ifj k</p><p>k j</p><p>0</p><p>1( )</p><p>otherwisejk(6)</p><p>The intensity of a synchronous correlation spectrum ((x1,x2)) represents simultaneous changes in two spectral intensitiesmeasured at x1 and x2 during the interval between Tmin andTmax. In contrast, an asynchronous correlation spectrum ((x1,x2)) includes out-of-phase or sequential changes in spectralintensities measuredat x1 and x2.Prior to 2D analysis, the FTIR spectra were normalized by</p><p>summing the absorbance from 4000 to 400 cm1 andmultiplying by 1000. Subsequently, normalized FTIR spectrawere analyzed using principal component analysis (PCA) toreduce the level of noise.20 Finally, 2D correlation spectroscopywas conducted using 2Dshige software (Kwansei-GakuinUniversity, Japan). In this study, the FTIR region from 1800to 900 cm1 was focused upon because it contains the majorexcitation bands of the amide, carboxylic acid, ester, andcarbohydrate functional groups.20</p><p> RESULTSTOC and Metal Concentrations in DOM. DOM in soil</p><p>from the NPKM treatment had the highest TOC content (14.2mg/L), followed by the NPK- (5.23 mg/L) and CK- (2.31 mg/L) treatments (Table S2 of the SI). The trend of DOMconcentration was similar to that of SOM in the differentfertilization treatments (Table S1 of the SI). Furthermore,fluorescence EEM spectra demonstrated that two, three, andfour peaks were present in samples from the NPKM-, NPK-,and CK- treatments, respectively (Figure 1). Peaks A, B, C, andD were located at Ex/Em of 230/410, 330/410, 270/340, and220/340, respectively. For the NPKM treatment, soil DOMcontained only humic- and fulvic-like substances (Peaks A andB); however, for the NPK treatment, the DOM was composedof both humic- and fulvic-like substances and a small quantityof protein-like substances (Peak C). In contrast, the soil DOMfrom the CK treatment predominantly contained protein-likesubstances (Peaks C and D) and some humic- and fulvic-likesubstances. In combination with FTIR spectroscopy, it wasnoted that a small quantity of protein (i.e., amide I at 1650cm1) were also detected (Figure S3 of the SI). Therefore, it isconcluded that the high fluorescence intensity of the humic/</p><p>fulvic peaks is masking/hiding the protein peak presented inthe EEM contour of the NPKM DOM sample.It is notable that trivalent metals (Al and Fe) had a</p><p>significantly different distribution pattern from monovalent (Naand K) and divalent (Ca, Mg, Mn, and Zn) metals in thedifferent treatments (Table S2 of the SI). Specifically, trivalentmetals were mainly present in samples from the NPKMtreatment, and almost no trivalent metals were detected in theCK- and NPK-treatments; however, monovalent and divalentmetals were predominantly presented in the NPK treatmentwith concentrations higher than those observed for the othertwo treatments. The trend of trivalent metals concentrations isimportant for the sequestration of SOM, which will be</p><p>Figure 1. Fluorescence excitationemission matrix (EEM) spectra ofdissolved organic matter (DOM) for soils from the CK-, NPK-, andNPKM-treatments, respectively, in the long term experiment.Specifically, peaks A, B, C, and D were located at Ex/Em of 230/410, 330/410, 270/340, and 220/340, respectively.</p><p>Environmental Science &amp; Technology Article</p><p>dx.doi.org/10.1021/es3002212 | Environ. Sci. Technol. 2012, 46, 610261096104</p></li><li><p>discussed in the following sections. Meanwhile, the backgroundmetals will also cause the quenching of EEM contours at acertain extent (Figure S4 of the SI).Fluorescent Components-Derived from EEM-PARAF-</p><p>AC Analysis of Al(III) Binding with DOM. All of thefluorescence EEMs collected from 33 different concentrationsof Al(III)-DOM complexes in the CK-, NPK-, and NPKM-treatments could be successfully decomposed by DOMFluorPARAFAC analysis into a threecomponent model based onresidual analysis, split half analysis, and excitation and emissionloadings (Figure S1 of the SI). The excitation and emissionloadings of the three PARAFAC-derived components and thefluorescence intensity scores of the titration of the CK- andNPK- treatment-derived DOM components with Al(III) areplotted in Figure 2. Component 1 [Ex/Em = (240, 330)/410]belonged to humic- and fulvic-like substances, component 2[Ex/Em = (250, 340)/490] was mainly ascribed to humic-likesubstances, and component 3 [Ex/Em = (220, 270)/340] wasattributable to protein-like substances, according to theprotocol of Chen et al.21 The surface plots of the threecomponents are presented in Figure S2 of the SI. Thesecomponents had been previously identified in soil environ-ments.13</p><p>As for the CK treatment, component 3 had the highestfluorescence intensity scores, followed by components 1 and 2.However, in the case of the NPK treatment, component 1 hadthe highest fluorescence intensity scores, followed bycomponents 2 and 3. In contrast, the NPKM treatment onlycontained components 1 and 2 and lacked component 3. Theabove-mentioned results suggested that for the three treat-ments, humic- and fulvic-like substances were detected in the</p><p>soil DOM; however, a large amount of protein-like substanceswas present in the CK treatment, and no protein-likesubstances were found in the NPKM treatment.In the case of all three components, Al(III) titration led to</p><p>quenching, enhancement, and no change in the fluorescencesignals of samples from the CK-, NPK-, and NPKM-treatments,respectively, reflecting that the structures of organics in theCK-, NPK-, and NPKM-treatments were different (Figure 2).To measure the binding constants between organics andAl(III), the conditional stability constant (log KM) wascalculated using the modified SternVolmer equation. Thecalculated log KM was 4.19 (R</p><p>2 = 0.80) and 1.78 (R2 = 0.64) forcomponents 1 and 2 in the CK treatment, respectively, and3.52 (R2 = 0.75) for component 1 in the NPK treatment (TableS3 of the SI). Therefore, for the CK treatment, Al(III) had astronger binding capability with humic-like substancescompared with fulvic-like substances. However, in the NPKtreatment, Al(III) had a weak binding capability with humic-likesubstances when compared to the CK treatment.</p><p>FTIR Synchronous Maps. It can be observed from FigureS3 of the SI that the peaks in one-dimensional FTIR spectrastrongly overlap, and the spectrum of the CK treatment wassimilar with that of the NPK treatment but distinct from that ofthe NPKM treatment. It can be noted that the trends of theFTIR spectra for the three treatments are different from thoseof the EEM measurements, a difference that may be attributableto the contributions of polysaccharides, lipids, or lignindetected in FTIR spectra (Figure S4 of the SI), which hadno fluorescent and thus could not be detected by the EEMmeasurements. Following the titration of a series of organic</p><p>Figure 2. Excitation and emission loadings of three PARAFAC-derived components, and fluorescence intensity scores of the titration of the CK-,NPK-, and NPKM-treatment-derived dissolved organic matter components with Al(III).</p><p>Environmental Science &amp; Technology Article</p><p>dx.doi.org/10.1021/es3002212 | Environ. Sci. Technol. 2012, 46, 610261096105</p></li><li><p>fractions with Al(III), the intensities of only a few peaks werechangeable, and no further analysis was possible.The synchronous maps generated from the...</p></li></ul>

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