Published in Soil Sci. Soc. Am. J. 69:136-140 (2005).
© 2005 Soil Science Society of America
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Division S-2—Notes
USE OF THERMOGRAVIMETRY–DIFFERENTIAL SCANNING CALORIMETRY TO CHARACTERIZE MODELABLE SOIL ORGANIC MATTER FRACTIONS
Elisa Lopez-Capela,*,
Saran P. Sohib,
John L. Gauntb and
David A. C. Manninga
a School of Civil Engineering and Geosciences, Univ. of Newcastle, Newcastle on Tyne, NE1 7RU UK
b Agriculture and Environment Division, Rothamsted Research, Harpenden, Herts, AL5 2JQ, UK
* Corresponding author (elisa.lopez-capel{at}ncl.ac.uk).
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ABSTRACT
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We used thermal analysis to compare the proportions of active and more stable components in soil organic matter (SOM) fractions and whole soil under contrasting agricultural land-uses. The fractions (free light, intra-aggregate, and organomineral fractions) were isolated using density fractionation. Exothermic weight loss between 300 and 350°C was attributed to a relatively labile portion comprising carboxyl and aliphatic C, and that between 400 and 450°C to the decomposition of material rich in aromatic components. Under arable cultivation, free light SOM showed much greater weight loss in the first exothermic range than intra-aggregate SOM. In soil receiving very small inputs of organic matter (a long-term bare fallow) the free light and intra-aggregate fractions displayed similar characteristics and resembled the intra-aggregate fraction from the arable soil. The difference between the free light and intra-aggregate fractions was also small for the grassland soil but the fractions resembled the free light fraction from the arable soil. Small total weight loss for whole soil and organomineral fractions demonstrated the value of physical fractionation techniques in establishing the effect of land-use on SOM with greater precision than is possible whole (unfractionated) soil.
Abbreviations: DSC, differential scanning calorimetry • FLF–SOM, free light fraction–soil organic matter • IALF–SOM, intra-aggregate light fraction–soil organic matter • M–SOM, organomineral–soil organic matter • NMR, nuclear magnetic resonance • SOM, soil organic matter • TG, thermogravimetry • TG–DSC, thermogravimetry–differential scanning calorimetry • WS, whole soil
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INTRODUCTION
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ATWO-STAGE DENSITY fractionation procedure described by Sohi et al. (2001) allows the quantification and characterization of different pools of SOM required by new models of SOM turnover (Christensen, 1996). Organic matter in free light fraction (FLF–SOM) and in intra-aggregate light fraction (IALF–SOM) is separated by density (<1.80 g cm–3 in both cases) before and after ultrasonic dispersion of stable aggregates (Sohi et al., 2001). Visual inspection suggests FLF–SOM comprises recognizable plant debris while IALF–SOM is finely divided, more decomposed, and darker in color. Chemical characterization of these fractions using noninvasive techniques (solid-state 13C nuclear magnetic resonance and diffuse reflectance Fourier transform infrared spectroscopy) has allowed the decomposition state of these fractions to be inferred (Sohi et al., 2001).
Thermal analysis by thermogravimetry–differential scanning calorimetry (TG–DSC) is widely applied to coal, charcoal, peat, and lignite (Jones et al., 1995; Varey et al., 1996) and has also been used to assess humification of organic matter in whole soils, chemically extracted humic substances, and composts (Grisi et al., 1998; Dell'Abate et al., 2000, 2002; Siewert, 2001). It has not, however, been previously used to characterize physically separated SOM fractions.
A TG–DSC experiment involves continuous and simultaneous measurement of weight loss (TG) and energy change (DSC) during heating. Initial weight loss is dominated by the exothermic decomposition of labile aliphatic and carboxyl groups (
300°C) while exothermic loss of more refractory aromatic C occurs at higher temperatures (450°C) (Flaig et al., 1975; Leinweber and Schulten, 1999; Schulten and Leinweber, 1999; Czimczik et al., 2002). The weight loss that occurs in these two parts of the heating cycle can be used to compare the relative abundance of more and less labile C whilst the position of DSC peaks reflects structure and chemical composition (Brown, 1988). If calcium carbonate minerals are present they decompose at higher temperatures (exceeding 600°C) and can be quantified.
In this short note we report the application of TG–DSC to SOM fractions isolated from one soil under contrasting long-term agricultural land-use. Our objective was to test TG–DSC as a direct and relatively rapid technique for comparing the abundance of more and less stable components in these fractions, and hence how land-use may impact their reactivity and turnover in the soil (Arah and Gaunt, 2001).
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Materials and Methods
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Composite soil samples were collected from the 0- to 23-cm horizon (conventional plow layer) of the Highfield long-term experiment at Rothamsted, southeast England, using an auger. The sampled plots were under conventional arable rotation ("arable"), permanently bare soil with small weed growth plowed in annually ("bare fallow"), and adjacent unmanaged grass ("grassland"). The soil is a fine silty clay loam and classified (according to the USDA system) as an Aquic Paleudalf (USDA, 1999). Each sample was fractionated according to the procedure described by Sohi et al. (2001). Briefly, an initial density separation yields free light organic matter (FLF–SOM), and a second separation after ultrasonic disruption gives light intra-aggregate organic matter (IALF–SOM) plus a residual organomineral fraction (M–SOM). Samples of unfractionated whole soil (WS) were also analyzed. Alpha cellulose (Sigma, Dorset, UK) and sugarcane lignin were used as reference materials to compare against labile and more stable components in the SOM fractions.
Thermal analysis was conducted using a Netzsch Simultaneous Thermal Analyzer STA 449 C Jupiter equipped with a TG–DSC sample carrier type S supporting a PtRh10-Pt thermocouple (Netzsch-Gerätebau GmbH, Selb, Germany), with the samples contained in alumina crucibles. Samples were gently ground in an agate mortar. For IALF–SOM and FLF–SOM 10 mg of sample was analyzed and 40 mg of WS and M–SOM. Analysis was conducted under air as previously reported (Leinweber et al., 1992b; Grisi et al., 1998; Dell'Abate et al., 2000, 2002; Siewert 2001). The experimental conditions were 10°C min–1 heating rate from 20 to 1000°C, and an air flow of 30 mL min–1. Analyses of the TG traces and DSC peaks (exotherms represented as positive displacement) were performed using the instrument software. Using the nomenclature of Dell'Abate et al. (2000)(2002) results were expressed as total weight loss associated with thermal decomposition of organic constituents; that is, between 200 and 600°C (Exotot), the weight loss associated with the first exotherm as a proportion of Exotot (Exo1), and the proportion of Exotot associated with the second exotherm (Exo2). The stability indices Exo1 and Exo2 are considered to correspond to predominantly aliphatic and carboxyl C and predominantly aromatic C content, respectively. Such interpretation is based on information obtained from pyrolysis–field ionization mass spectrometry (Leinweber and Schulten, 1999; Schulten and Leinweber 1999) and 13C NMR analysis of both intact and thermally oxidized soils (Almendros et al., 2003; Gonzalez-Perez et al., 2004). To compare Exo1 and Exo2 against established measures of lability and recalcitrance, we also obtained spectroscopic data using solid-state 13C cross-polarization magic-angle spinning (CPMAS) NMR in this study. Nuclear magnetic resonance spectra were obtained for FLF–SOM and IALF–SOM fractions only, using a MSL 300 spectrometer (Bruker, Coventry, UK) and the parameters described in Sohi et al. (2001): spectrometer frequency 75.5 MHz, contact time 1 ms, relaxation time 500 ms, spinning speed approx. 4.5 kHz (with elimination of spinning side-bands using the total suppression of sidebands sequence; Dixon, 1982), and line broadening 100 Hz. The average accumulation for the samples analyzed in this experiment was 34600 scans (4.7 h). Peak areas were calculated using Bruker software and limits defined by Randall et al. (1995).
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Results and Discussion
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The DSC and TG traces for all samples are given in Fig. 1
. FLF–SOM and IALF–SOM fractions indicated an endothermic reaction between 62 and 91°C and two exothermic reactions between 300 to 310°C and 422 to 455°C (Fig. 1d–1f). The M–SOM fraction and the WS (unfractionated soil) samples displayed an endothermic reaction between 77 and 91°C and a single, defined exotherm peaking at 320°C (Fig. 1g). The endothermic reactions result from dehydration, and the two exothermic reactions from the decomposition and combustion of two organic matter components, with distinct and contrasting thermal stability (Grisi et al., 1998). In analysis of organomineral size fractions and whole soils amended with organic matter, Leinweber et al. (1992a)( 1992b) attributed the first exothermic peak to the liberation of aliphatic compounds and decarboxylation and the second exotherm to the decomposition of aromatic moietes such as lignin dimers. The position of the corresponding peaks for the FLF–SOM and IALF–SOM samples analyzed in this study were close to those reported by Dell'Abate et al. (2000)(2002) for composts and chemically extracted humic substances. The exothermic peaks also approximately correspond with those seen for cellulose, lignin, and the cellulose–lignin mixture (300–340°C for cellulose decarboxylation and a single broad peak between 410 and 430°C for lignin decomposition; Fig. 1a–1c). The DSC trace for the cellulose sample showed an additional exothermic peak between 460 and 480°C (Fig. 1a), identical to a feature observed in an analysis of fresh plant material by Kaloustian et al. (2001) and attributable to an artifact of char formation during cellulose decomposition. This artifact was much less prominent in the samples considered in this study, possibly due to reactions between cellulosic and mineral constituents. However, it may be the explanation for the shoulder in FLF–SOM and IALF–SOM from arable and grassland soils (Fig. 1d–1e). If so, it is not surprising that it is not apparent in the fractions isolated from the fallow soil, which will contain much less labile material (Fig. 1f).
The TG traces indicated a defined and rapid weight loss associated with the first exotherm ("cellulose") as shown by DSC and a more gradual weight loss over a wider temperature interval for "lignin" decomposition. The TG traces indicated that the relative proportion of the more labile ("cellulose") and more recalcitrant ("lignin") components in FLF–SOM and IALF–SOM (i) differed between fractions, and (ii) were affected by land-use (Fig. 1d–1f). For the arable soil, the weight loss associated with the first exothermic peak (300–310°C) was a larger proportion of the total weight loss (Exotot) for FLF–SOM than IALF–SOM (Fig. 1d; Table 1). This was reflected in the ratio Exo1/Exo2 (Table 1) being 0.70 for FLF–SOM compared to 0.43 for IALF–SOM and was consistent with the suggestion (Sohi et al., 2001) that FLF–SOM is relatively enriched in more labile material. However, the trend was not apparent for the grassland or bare fallow soils (Fig. 1e–1f; Table 1). For both of these treatments Exo1 ("cellulose") and Exo2 ("lignin") differed little between FLF–SOM and IALF–SOM (Table 1), though Exo2 had a lower value in the grassland fractions reflecting the constant input of fresh organic matter. The small weight loss associated with Exo1 in the bare fallow soil for both FLF–SOM and IALF–SOM presumably reflected the very small inputs of fresh plant material to this soil. These results suggest that FLF–SOM is substantially more reactive than IALF–SOM in arable soil, and both fractions are less reactive than the same fractions under grass. The reactivity of IALF–SOM from the grassland was closer to that of the arable FLF–SOM, and conversely the reactivity of FLF–SOM from the bare fallow closer to that of arable IALF–SOM.
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Table 1. Thermogravimetry (TG) and differential scanning calorimetry (DSC) parameters for soil organic matter (SOM) fractions, whole soils, and reference materials.
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The results from the TG were compared with peak area data from 13C NMR analysis of FLF–SOM and IALF–SOM samples. For these six samples, Exo1 was closely correlated with O-alkyl C (r2 = 0.89, P = 0.005), and Exo2 with aromatic C (r2 = 0.87, P = 0.007) (Fig. 2a–2b)
. In NMR studies of SOM decomposition, the peak area ratio of O-alkyl to alkyl C is considered a measure of degradative state (Preston, 1996). There was a fairly close relationship between this indicator and Exo1/Exo2 for the FLF–SOM and IALF–SOM samples analyzed in this study (r2 = 0.75, P = 0.025; Fig. 2c). We therefore propose that thermal analysis can provide an index of reactivity for these fractions, comparable to that obtained using spectroscopic methods.
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Fig. 2. Correlation of NMR and thermogravimetry (TG) results for free light fraction–soil organic matter (FLF–SOM) and intra-aggregate light fraction–soil organic matter (IALF–SOM) samples under all three land uses (a) Exo1 and O-alkyl C (b) Exo2 and aromatic C, and (c) Exo1 and O-alkyl/alkyl ratio.
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The traces for unfractionated whole soils (WS) and heavy fraction (M–SOM) (Fig. 1h) suggested that organic matter was qualitatively similar in WS and M–SOM and was distinct from FLF–SOM and IALF–SOM in that its DSC trace was dominated by a single exothermic peak. Also, the quantitative effect of removing light fractions from WS (leaving residual M–SOM) was discernible. It seems that the dominance of a mineral matrix made the qualitative effects of land-use on the organic matter in these fractions hard to establish, partly due to thermal decomposition of minerals and partly by simple "dilution" of the organic component. The parameters Exo1 and Exo2 could not be determined and therefore it seems unlikely that thermal analysis can reliably detect subtle effects of land-use on the composition of organic matter using samples of whole soil or particle-size fractions analogous to M–SOM. It is possible that the organic components of such samples could be chemically extracted before thermal analysis to improve the sensitivity of the analysis (Satoh, 1984a, 1984b; Leinweber et al., 1992a, 1992b; Grisi et al., 1998; Dell'Abate et al., 2003). However, since such extractions may themselves alter the labile components that they seek to characterize, analysis of organic fractions separated by physical methods as demonstrated in this paper is preferable.
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Conclusions
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The results of our analysis demonstrated the sensitivity of TG–DSC in comparing the labile and more recalcitrant components of physically separated SOM fractions, and were generally correlated with those from solid-state 13C NMR analysis of the same samples. By comparing fractions from soil under an extreme range of agricultural managements, we found the approach sufficiently sensitive to establish limits to the assumption of consistent reactivity within fractions between land-uses. The technique offers a relatively rapid analysis that which could be used to trace variations in the reactivity of SOM fractions represented in models, over space and time, and thus verify or refine the parameters needed to predict the turnover of C in soil.
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ACKNOWLEDGMENTS
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The NMR data used in the validation of the TG–DSC results was provided by Dr N. Mahieu (Queen Mary, University of London) with the assistance of Dr. A. Aliev, and a NMR machine provided by the University of London Intercollegiate Research scheme. We thank H. Yates (Rothamsted Research) for performing the soil organic matter fractionations, and Prof. H. Gilbert and Dr. R.M. Wilkins (University of Newcastle) for supplying cellulose and lignin reference materials. We also appreciate the helpful comments provided on this text by Prof. D.S. Powlson and two anonymous reviewers. The research was supported by the Engineering and Physical Sciences Research Council (GR/R34332), and the Biotechnology and Biological Sciences Research Council who provide Rothamsted Research grant-aided support.
Received for publication January 6, 2004.
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ABSTRACT
INTRODUCTION
