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Fluoridic Acid in the Infrared Spectroscopy Analysis of Chemical Composition of Organic Matter

Abstract

Fourier transform infrared spectroscopy (FTIR) assists in investigating functional groups of soil organic matter (SOM). However, the use of this tool is impaired given the low organic carbon levels and high content of oxides in tropical soils, resulting in low quality spectra and in turn requiring the use of hydrofluoric acid (HF). The objective of this study was to verify the efficiency of using HF in removing the mineral fraction and to increase the C concentration to enable visualizing the bands related to SOM in infrared spectra in soil samples under forest system. The HF treatment was efficient in removing mineral components and proportionally increasing C. The FTIR with HF enabled identifying differences between coarse and fine fractions. The spectra of the HF samples showed that the coarse fractions presented bands related to aromatic material and the fine fractions presented more labile components, with the absence of more recalcitrant components.

Keywords:
FTIR; carbon stock; tropical soil; soil aggregates

Forest systems play an important role in soil C accumulation through the continuous deposition of plant residues. The stabilization of this C occurs through interaction with mineral particles (organomineral complex) and physical protection in the aggregates (Vicente et al., 2019Vicente LC, Gama-Rodrigue, EF, Gama-Rodrigues AC, Marciano R. Organic carbon within soil aggregates under forestry systems and pasture in Southeast region of Brazil. Catena, 2019, 182, 104139.). Physical fractionation allows separation into different organic matter fractions, which can function as a C source or sink, depending on its accessibility by microbiota and the physical location (Six and Paustian, 2014Six J, Paustian K. Aggregate-associated soil organic matter as an ecosystem property and a measurement tool. Soil Biology & Biochemistry, 2014, 68: A4-A9.). FTIR spectroscopy enables identifying the chemical composition and reactivity of C in these fractions. However, this technique is impaired by the low C levels in tropical soils. Thus, the removal of mineral components is essential for a proportional increase in the C concentration (Boeni et al. 2014Boeni M, Bayer C, Dieckow J, Conceição PC, Dick DP, Knicker H, Salton JC, Macedo MCM. Organic matter composition in density fractions of Cerrado Ferralsols as revealed by CPMAS 13C NMR: Influence of pastureland, cropland and integrated crop-livestock. Agriculture, Ecosystems and Environment, 2014, 190: 80-86.).

The objective of this study was to verify the efficiency of HF in removing the mineral fraction, increasing the concentration of C and N and visualizing the bands related to SOM in the FTIR spectra of different soil samples (fine earth fraction (FEF, 2,00 mm); macroaggregates; microaggregates; silt+clay fraction; particulate and occluded fraction of macroaggregates and microaggregates) under forest in the municipality of Uruçuca, Southern Bahia, Brazil, at a depth of 0-10 cm.

Four fixed plots of 900 m2 were delimited for soil sampling; the plots were separated by at least 100 m. Each composed soil sample was formed of 15 single soil samples taken at 0-10 cm depth. The soils were classified as Latossolo Amarelo according to the Brazilian System of Soil Classification (Embrapa, 2013Empresa Brasileira de Pesquisa Agropecuária. Sistema Brasileiro de Classificação de Solos. Rio de janeiro: CNPS, 2013. 353p.). To obtain the fine earth fraction (FEF) the soil samples were air-dried and passed through a 2 mm sieve. Physical fractionation in aggregates classes was carried out following the method of Elliot (1986Elliot ET. Aggregate strucuture and carbon, nitrogen and phosphorus in native and cultivated soils. Soil Science Society of America Journal , 1986, 50: 627- 633.) and adapted by Vicente et al. (2019Vicente LC, Gama-Rodrigue, EF, Gama-Rodrigues AC, Marciano R. Organic carbon within soil aggregates under forestry systems and pasture in Southeast region of Brazil. Catena, 2019, 182, 104139.). 100 g of soil sample was sunk into 500 ml of deionized water for 5 minutes and fractionated by wet-sieving (two sieve sizes: 250 µm and 53 µm) and three aggregates classes were obtained: 2000-250 µm, 250-53 µm and <53 µm. The particulate and occluded organic matter fractions within macro and microaggregates were obtained by the sonication method (Sarkhot et al., 2007Sarkhot D, Comerford N.B, Jokela E.J, Reeves III J.B, Harris W.G. Aggregation and aggregates carbon in a forested southeastern Coastal Plain Spodosol. Soil Science Society of America Journal , 2007, 71:1779-1787.). The fractions were submitted to treatment with hydrofluoric acid (HF) 10% with 2 hours of agitation on horizontal shaker (Tecnal TE-240), followed by 10 minutes of centrifugation (3000 rpm). In “Falcon” tubes, 10 g FEF, macro and microaggregates with 30 ml of HF, 8 times with HF and 3 times with water (Dick et al., 2008Dick DP, Martinazzo R, Dalmolin RSD, Jacques AVA, Mielniczuk J, Rosa AS. Impacto da queima nos atributos químicos e na composição química da matéria orgânica do solo e na vegetação. Pesquisa Agropecuária Brasileira, 2008, 43: 633-640.); 1 g of the silt+clay fraction with 40 ml of HF, 4 times with HF and 3 times with water; 0.5 g for particulate fraction and 0.3 g for occluded fraction, both with 20 ml of HF, 3 times with HF and 3 times with water (Boeni et al., 2014Boeni M, Bayer C, Dieckow J, Conceição PC, Dick DP, Knicker H, Salton JC, Macedo MCM. Organic matter composition in density fractions of Cerrado Ferralsols as revealed by CPMAS 13C NMR: Influence of pastureland, cropland and integrated crop-livestock. Agriculture, Ecosystems and Environment, 2014, 190: 80-86.). The following determinations were carried out (before and after HF): Total Fe; R index ((C/N)/(C/NHF)). The C (CE) and N (NE) enrichment and the C (CR) and N (NR) recoveries were calculated. Recoveries after HF were calculated using the initial mass and recovered mass (MR) values of the samples (Dick et al., 2008Dick DP, Martinazzo R, Dalmolin RSD, Jacques AVA, Mielniczuk J, Rosa AS. Impacto da queima nos atributos químicos e na composição química da matéria orgânica do solo e na vegetação. Pesquisa Agropecuária Brasileira, 2008, 43: 633-640.). C and N contents were determined by dry combustion using an automated analyzer. Fourier Transform Infrared Spectroscopy analyzes were conducted by Diffuse Reflectance using Shimadzu spectrometer (DRS-8000A). The fractions was mixed and ground with potassium bromide (KBr), after being dried in an oven at 60 °C, in the proportion 1 sample:100 KBr. The reading was performed with a diffuse reflectance accessory, using 40 scans and a resolution of 4 cm-1, in the spectral range of 4,000 to 400 cm-1. Spectra and band intensities data were obtained by the Shimadzu IR Solution 1.6 program (Gonçalves et al, 2003Gonçalves CN, Dalmolin RSD, Dick DP, Knicker H, Klamt E, Kogel-Knabner I. The effect of 10% HF treatment on the resolution of CPMAS 13C NMR spectra and on the quality of organic matter in Ferralsols. Geoderma, 2003, 116: 373-392.; Dick et al., 2008Dick DP, Martinazzo R, Dalmolin RSD, Jacques AVA, Mielniczuk J, Rosa AS. Impacto da queima nos atributos químicos e na composição química da matéria orgânica do solo e na vegetação. Pesquisa Agropecuária Brasileira, 2008, 43: 633-640.).

MR values were higher in particulate fractions and lower in silt+clay fraction. The lower MR values in silt+clay fraction are the result of the higher clay contents, resulting from the dissolution of the mineral fraction after HF (Table 1). The analysis of functional groups by DRIFT is hampered by the low levels of carbon and high concentrations of Fe oxides, as in tropical soils, and the presence of silicates (Gonçalves et al., 2003Gonçalves CN, Dalmolin RSD, Dick DP, Knicker H, Klamt E, Kogel-Knabner I. The effect of 10% HF treatment on the resolution of CPMAS 13C NMR spectra and on the quality of organic matter in Ferralsols. Geoderma, 2003, 116: 373-392.). For this reason, it is important to remove these mineral and increase the carbon concentration to obtain adequate spectra. The higher CE in the soil fractions (silt+clay, particulate and occluded fractions) compared to FEF is due to the soil fractionation which makes the mineral matrix more accessible to HF, increasing specific surface and the C protection (Jindaluang et al., 2003). The increase in C and N in the samples is also associated with the removal of Fe (FeR), one of the the clay fraction components. The samples presented R index that suggested negligible losses with HF (Table 1). According to Schimidt et al. (1997)Schmidt, M.W.I.; Knicker, H.; Hatcher, P.G.; Kogel-Knabner, K. Improvement of 13C and 15N CPMAS NMR spectra of bulk soils, particle size ftactions and organic material by treatment with 10% hydrofluoric acid. European Journul of Soil Science, 1997, 48: 319-328., R = 1.0 (± 0.2), there were no preferential losses of C and N.

Table 1
Means of recovered mass (%Mr), carbon enrichment (CE) and nitrogen (NE), recovered carbon and nitrogen (%CR e %NR), Iron removal (%FeR) and R index in soil samples (0-10cm)under forest, in southern Bahia.

The bands in samples without HF referring to SOM were not able to stand out from those of minerals: 3,694, 3,622 and 3,173 cm-1 (OH stretch in the Al-OH group of silicates) 1,097, 1,036 and 912 cm-1 (stretch of the Si-O in kaolinite and quartz) (Dick et al., 2008Dick DP, Martinazzo R, Dalmolin RSD, Jacques AVA, Mielniczuk J, Rosa AS. Impacto da queima nos atributos químicos e na composição química da matéria orgânica do solo e na vegetação. Pesquisa Agropecuária Brasileira, 2008, 43: 633-640.). The treatment efficiency with HF in the samples was evidenced by the absence of bands related to minerals and the presence of those related to SOM by the decrease of Fe and increase of CR. However, some bands related to the mineral fraction continued to be present, even after treatment: 3,300 cm-1 (angular deformation H-O-H in mineral fractions); 1,994, 1,870 and 1,794 cm-1 (Si-O stretch, quartz) and 799 and 694 cm-1 (clay and quartz minerals) (Haberhauer and Gerzabek, 1999Haberhauer G, Gerzabek MH. Drift and transmission FT-IR spectroscopy of forest soils: an approach to determine decomposition processes of forest litter. Vibrational Spectroscopy, 1999, 19: 413-417.). The presence of these bands, especially quartz, is due to their high resistance to dissolution with HF (Gonçalves et al., 2003Gonçalves CN, Dalmolin RSD, Dick DP, Knicker H, Klamt E, Kogel-Knabner I. The effect of 10% HF treatment on the resolution of CPMAS 13C NMR spectra and on the quality of organic matter in Ferralsols. Geoderma, 2003, 116: 373-392.; Dalmolin et al., 2006Dalmolin RSD, Gonçalves CN, Dick DP, Knicker H, Klamt E, Kogel-Knabner I. Organic matter characteristics and distribution in Ferralsol profiles of a climosequence in southern Brazil. European Journal of Soil Science, 2006, 57: 644-654.). Not to mention the influence of the mineralogical composition of this soil. According to Silva (2008) these soils showed kaolinite as the main phyllosilicate, and goethite as the most representative oxide. Djomgoue and Njopwouo (2013Djomgoue P, Njopwouo D. FT-IR Spectroscopy Applied for Surface Clays Characterization. Journal of Surface Engineered Materials and Advanced Technology, 2013, 3: 275-282. ) revealed that pure kaolinite presents four well resolved (-OH) bands in IR spectrum: the stretching vibrations of surface hydroxyl groups (3,652; 3,671, and 3,694 cm−1) and the vibrations of inner hydroxyl groups (3,620 cm−1). Furthermore, bands due to ν(AlFeOH) at 865-875 cm−1 and stretching at 3,607 cm−1 are typical of Fe bearing kaolinites.

Bands at 1,050, associated to the presence of cellulose (Calderón et al. 2011Calderón FJ, Reves III, JB, Collins HP, Paul EA. Chemical Differences in Soil Organic Matter Fractions determined by Diffuse-Reflectance Mid-Infrared Spectroscopy. Soil Science Society of America Journal, 2011, 75: 568-579.) and 1,160 cm-1 which represents aliphatic compounds linked to hydroxyl groups (C-OH) or can be assigned to C-O vibration of polysaccharides or other groups such as alcohols and esters (Janik et al. 2007Janik LJ, Merry RH, Forrester S, Lanyon D, Rawson A. Rapid prediction of soil water retention using mid infrared spectroscopy. Soil Sci Soc Am J , 2007, 71:507-514.), were observed in all samples. The 1,720, 1,680, 1,612 and 1,522 cm-1 bands represent aromatics (the first two refer to the C=O of esters or carboxylic acids, while 1,612 and 1,522 cm-1 refer to C=C in more condensed structures) (Calderón et al., 2011Calderón FJ, Reves III, JB, Collins HP, Paul EA. Chemical Differences in Soil Organic Matter Fractions determined by Diffuse-Reflectance Mid-Infrared Spectroscopy. Soil Science Society of America Journal, 2011, 75: 568-579.), and 1,250 cm-1 band (stretch in the C-O bond) indicating the presence of carboxylic acids and phenols functional groups (Bornemann et al., 2010Bornemann L, Welp G, Amelung W. Particulate organic matter at the field scale: rapid acquisition using mid-infrared spectroscopy. Soil Sci Soc Am J, 2010, 74:1147-1156.) were absent in the occluded fractions of macro and microaggregates and silt+clay fraction. However, were observed mainly in FEF, macro and microaggregates and particulate fraction samples (Figure 1). These coarser fractions present less protection of C to the action of the microbiota due to the lower complexation of organic matter to the mineral fraction (Lehmann and Kleber, 2015Lehmann J, Kleber M. The contentious nature of soil organic matter. Nature, 205, 528: 60-68.). Thus, labile forms of C were the first used by microorganisms, which led to the proportional enrichment of aromatic C from lignin (Calderón et al., 2011Calderón FJ, Reves III, JB, Collins HP, Paul EA. Chemical Differences in Soil Organic Matter Fractions determined by Diffuse-Reflectance Mid-Infrared Spectroscopy. Soil Science Society of America Journal, 2011, 75: 568-579.). The silt+clay and occluded (fine fractions) fractions showed a presence of aliphatic C (bands around 2,920 and 2,850 cm-1) (Figure 1) which is associated with the finer fraction (Pisani et al., 2014Pisani O, Hills KM, Courtier-Murias D, Haddix ML, Paul EA, Conant RT et al. Accumulation of aliphatic compounds in soil with increasing mean annual temperature. Organic Geochemistry, 2014, 76: 118-127.). In these fine fractions, even the most labile organic matter is protected from the action of microorganisms by the complexation with soil mineral fraction (Six et al., 2014Six J, Paustian K. Aggregate-associated soil organic matter as an ecosystem property and a measurement tool. Soil Biology & Biochemistry, 2014, 68: A4-A9.).

Figure 1
Spectra of Forest samples before HF treatment (A) and after HF treatment (B).(1) FEF fraction; (2) Macroaggregates; (3) Microaggregates; (4) Silt + Clay; (5) Particulate fraction of macroaggregates; (6) Particulate fraction of microaggregates; (7) Occluded fraction of macroaggregates; (8) Occluded fraction of microaggregates.

HF was efficient in the removal of less resistant mineral components, with expressive removal of Fe and a proportional increase of C and N. HF was also efficient for observing bands related to the organic components present in the studied samples. The FTIR with HF enabled identifying differences between coarse and fine fractions of organic matter under forest soils. There was a proportional increase in aromatic content during decomposition in the coarse fractions free from chemical or physical protection, as evidenced by bands referring to aromatic C and more condensed structures with the presence of double bonds in the molecules. On the other hand, the fine fractions (occluded fractions of macroaggregates and microaggregates and silt+clay) had more labile components, evidenced by the presence of bands referring to aliphatic structures of C and N suggesting less degree of transformation into the protected organic matter in soil aggregates. The use of physical fractionation and the chemical characterization of C by FTIR provided relevant information about the stabilization process of organic matter under forest soils.

ACKNOWLEDGEMENTS

We thank CAPES for granting a doctoral scholarship to the first author. This research was partially supported by a grant from FAPERJ (E-26/102.274/2013) and CNPq (475740/2010-6; 303844/2017-5). We thank the Agricola Cantagalo for allowing the soil sampling at their Farm. We are grateful to Kátia R. Nascimento Sales and Ederaldo Azeredo Silva of Soil Laboratory, North Fluminense State university for technical support in soil sample collection and analysis.

References

  • Boeni M, Bayer C, Dieckow J, Conceição PC, Dick DP, Knicker H, Salton JC, Macedo MCM. Organic matter composition in density fractions of Cerrado Ferralsols as revealed by CPMAS 13C NMR: Influence of pastureland, cropland and integrated crop-livestock. Agriculture, Ecosystems and Environment, 2014, 190: 80-86.
  • Bornemann L, Welp G, Amelung W. Particulate organic matter at the field scale: rapid acquisition using mid-infrared spectroscopy. Soil Sci Soc Am J, 2010, 74:1147-1156.
  • Calderón FJ, Reves III, JB, Collins HP, Paul EA. Chemical Differences in Soil Organic Matter Fractions determined by Diffuse-Reflectance Mid-Infrared Spectroscopy. Soil Science Society of America Journal, 2011, 75: 568-579.
  • Dalmolin RSD, Gonçalves CN, Dick DP, Knicker H, Klamt E, Kogel-Knabner I. Organic matter characteristics and distribution in Ferralsol profiles of a climosequence in southern Brazil. European Journal of Soil Science, 2006, 57: 644-654.
  • Dick DP, Martinazzo R, Dalmolin RSD, Jacques AVA, Mielniczuk J, Rosa AS. Impacto da queima nos atributos químicos e na composição química da matéria orgânica do solo e na vegetação. Pesquisa Agropecuária Brasileira, 2008, 43: 633-640.
  • Djomgoue P, Njopwouo D. FT-IR Spectroscopy Applied for Surface Clays Characterization. Journal of Surface Engineered Materials and Advanced Technology, 2013, 3: 275-282.
  • Elliot ET. Aggregate strucuture and carbon, nitrogen and phosphorus in native and cultivated soils. Soil Science Society of America Journal , 1986, 50: 627- 633.
  • Empresa Brasileira de Pesquisa Agropecuária. Sistema Brasileiro de Classificação de Solos. Rio de janeiro: CNPS, 2013. 353p.
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  • Haberhauer G, Gerzabek MH. Drift and transmission FT-IR spectroscopy of forest soils: an approach to determine decomposition processes of forest litter. Vibrational Spectroscopy, 1999, 19: 413-417.
  • Janik LJ, Merry RH, Forrester S, Lanyon D, Rawson A. Rapid prediction of soil water retention using mid infrared spectroscopy. Soil Sci Soc Am J , 2007, 71:507-514.
  • Jindaluang W, Kheoruenromne I, Suddhiprakarn A, Singh B.P, Singh, B. Influence of soil texture and mineralogy on organic matter content and composition in physically separated fractions soils of Thailand. Geoderma , 2013, 195-196: 207-219.
  • Lehmann J, Kleber M. The contentious nature of soil organic matter. Nature, 205, 528: 60-68.
  • Pisani O, Hills KM, Courtier-Murias D, Haddix ML, Paul EA, Conant RT et al. Accumulation of aliphatic compounds in soil with increasing mean annual temperature. Organic Geochemistry, 2014, 76: 118-127.
  • Sarkhot D, Comerford N.B, Jokela E.J, Reeves III J.B, Harris W.G. Aggregation and aggregates carbon in a forested southeastern Coastal Plain Spodosol. Soil Science Society of America Journal , 2007, 71:1779-1787.
  • Schmidt, M.W.I.; Knicker, H.; Hatcher, P.G.; Kogel-Knabner, K. Improvement of 13C and 15N CPMAS NMR spectra of bulk soils, particle size ftactions and organic material by treatment with 10% hydrofluoric acid. European Journul of Soil Science, 1997, 48: 319-328.
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  • Vicente LC, Gama-Rodrigue, EF, Gama-Rodrigues AC, Marciano R. Organic carbon within soil aggregates under forestry systems and pasture in Southeast region of Brazil. Catena, 2019, 182, 104139.

Edited by

Associate editor:

Marcos Gervásio Pereira http://orcid.org/0000-0002-1402-3612

Publication Dates

  • Publication in this collection
    15 Aug 2022
  • Date of issue
    2022

History

  • Received
    22 Dec 2021
  • Accepted
    15 July 2022
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