structural properties of plant charred materials in andosols as revealed by x-ray diffraction...
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ORIGINAL ARTICLE
Structural properties of plant charred materials in Andosols asrevealed by X-ray diffraction profile analysis
Nargis SULTANA1, Kosuke IKEYA2, Haruo SHINDO3, Syusaku NISHIMURA3 andAkira WATANABE1
1Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya 464-8601, 2Graduate School of Engineering,
Nagoya Institute of Technology, Showa, Nagoya, 466-8555 and 3Faculty of Agriculture, Yamaguchi University, Yamaguchi 753-8515,
Japan
Abstract
Charred plant fragments are frequently observed in soil. However, their structure has not been elucidated. In the
present study, charred plant fragments detached from soil by HF treatment and collected by heavy liquid separa-
tion were characterized by X-ray diffraction (XRD) analysis. Three Andosols from the northeastern, central and
western districts of Japan were used. Supportive information was obtained by solid-state 13C nuclear magnetic
resonance (NMR) and d13C analyses. In the XRD analysis, the size of the carbon (C)-layer planes in the charred
plant fragments ranged from 0.96 to 1.92 nm, corresponding to 14–52 ring condensed aromatic structures. The
size distribution of the C-layer planes did not differ largely among the three soils. A minor effect of vegetation
on the composition of the condensed aromatic structures in the plant charred fragments was deduced from dif-
ferences in the content of the 1.92 nm C-layer plane and d13C. The relative content of condensed aromatic struc-
tures tended to be larger in the sample with more aromatic C content, which suggested that decomposition of
aliphatic moieties is a cause of enrichment of condensed aromatic structure.
Key words: black carbon, carbon-layer plane, char, condensed aromatic structure, X-ray diffraction.
INTRODUCTION
Global warming is the largest environmental problem to
be solved in the 21st century, and increasing atmospheric
CO2 is considered to be a major cause (Intergovernmental
Panel on Climate Change 2007). Sequestration of plant-
assimilated C in soil as refractory soil organic matter
(SOM) may be an effective strategy to reduce the rate of
increase in atmospheric CO2. For this purpose, informa-
tion about the properties, formation pathways and accu-
mulation mechanisms of refractory SOM are essential.
Type A humic acids (HAs), the greatest class with regard
to the degree of humification among HAs (Kumada
1987), are representative of refractory SOM.
Type A HAs are frequently observed in Andosols that
are developed on deposits of volcanic materials and are
widely distributed in Japan, a typical volcanic country.
The amount of SOM in the A horizon of Andosols reaches
a maximum 250 g kg)1 on a C basis (Wada 1986), and
HAs account for 20–30% of the total C (Watanabe and
Kuwatsuka 1991). Type A HAs are characterized by their
black color, high aromaticity (Maie et al. 2002), large
content of oxygen-containing functional groups (Tsutsuki
and Kuwatsuka 1978), small content of identifiable com-
ponents such as fatty acids and phenolic acids (Ikeya et al.
2004), and the presence of the 002 band at approximately
2h = 25� in an X-ray diffraction (XRD) profile, indicating
the occurrence of a graphite-like structure (Matsui et al.
1984). The presence of the 002 band and the large content
of non-phenolic aromatic C have been the basis of the
speculation that charred plant fragments are regarded as a
source of highly humified HAs (Haumaier and Zech
1995; Shindo et al. 2004a, b).
Charred plant fragments are produced by the incom-
plete combustion of vegetation, and a portion of these
fragments is carried away by rain and scattered over a
wide area as a result of Aeolian transport (Kuhlbusch
1998). However, >80% remains proximal to the site
Correspondence: N. SULTANA, Graduate School of Bioagricul-tural Sciences, Nagoya University, Chikusa, Nagoya 464-8601,Japan. Email: [email protected]
Received 28 March 2010.Accepted for publication 12 October 2010.
� 2010 Japanese Society of Soil Science and Plant Nutrition
Soil Science and Plant Nutrition (2010) 56, 793–799 doi: 10.1111/j.1747-0765.2010.00520.x
where it was formed and is incorporated into the soil (For-
bes et al. 2006). The reported proportions of charred C in
total soil organic C cover a wide range; for example, 10–
35% in the USA (Skjemstad et al. 2002), 2–9% in Central
Asia (Nishimura et al. 2009), undetectable to 45% in
Germany and the Netherlands (Schmidt et al. 1999) and
3–33% in Japan (Shindo et al. 2004a). Charred plant
fragments appear to be highly recalcitrant to decomposi-
tion (Baldock and Smernik 2002; Cheng et al. 2008), and
their relative increase with decreasing total SOM by crop
cultivation has been reported (Skjemstad et al. 2001).
Charred materials made by burning a grass plant,
Miscanthus sinensis, did not contain a considerable
amount of HAs (Shindo and Honma 1998), and the yield
of HAs increased with oxidization of the charred
materials with HNO3 or H2O2 (Shindo et al. 2004b;
Trompowsky et al. 2005). Although such a drastic reac-
tion does not occur in natural conditions, weathering of
charred plant fragments in soil oxidizes their surface and
increases the content of carbonyl C (Skjemstad et al.
1996). Thus, the formation of smaller molecules soluble
in alkali over a long period is expected. Charred plant
fragments separated from Andosols have been shown to
yield 300–400 mg HA C g)1 in an extraction with a hot
alkali solution (Nishimura et al. 2006).
Currently 13C nuclear magnetic resonance (NMR) spec-
troscopy is generally used to determine aromatic C con-
tent in humic substances (Dria et al. 2002) or charred
materials (Simpson and Hatcher 2004). Schnitzer et al.(1991) found a positive correlation between the aromatic
C content and the peak intensity of the 002 band in the
XRD profiles for various soil HAs. Watanabe and Takada
(2006) compared the aromatic C content in HAs esti-
mated from 13C cross polarization ⁄ magic angle spinning
(CPMAS) NMR spectra among the buried A horizons in
Andosol profiles, which increased with soil age up to ca
10,000 years. The peak intensity of the 002 band in the
XRD profiles of HAs was also greater in the older buried
A horizons (Kumada 1987). In a Mollisol profile, HAs in
a lower horizon showed a more intense 002 band (Xing
and Chen 1999). Condensed aromatic structure may be
the major skeleton of the HAs that survive in soil for a
long period, and charred plant fragments and polynuclear
pigments produced by microorganisms are their possible
precursor. However, the details of condensed aromatic
structures in HAs and plant charred fragments in soil are
unknown. In this context, an analysis of 11 bands at
approximately 2h = 80� in the XRD profile is more attrac-
tive than an analysis of the 002 band because an 11 band
analysis enables estimation of the size distribution of
C-layer planes in amorphous materials (Fujimoto 2003;
Fujimoto and Shiraishi 2001), whereas the 002 band
analysis provides information about the number of stack-
ing layers of C-layer planes and the distance between the
stacking layers.
The objective of the present study was to characterize
charred plant fragments in Andosols. For this purpose,
charred plant fragments obtained from three Andosols in
different geographic regions in Japan were used. The three
soils also varied in layer, depth and in the contents of total
C and charred materials. The size distribution of the
C-layer planes in the plant charred fragments was esti-
mated by analyzing 11 bands in the XRD profile. This is
the first application of the 11 band analysis for charred
materials in soil. In addition, solid-state 13C NMR spectra
and d13C were also measured to characterize the charred
plant fragments with regard to the C composition and C
sources, respectively.
MATERIALS AND METHODS
Isolation of charred plant fragments from soils
The three air-dried Andosols (<2 mm) used were collected
from the northeastern (Kanegasaki; 39�08¢N, 140�59¢E),
central (Fujinomiya; 35�22¢N, 138�39¢E) and western
(Ohda; 35�08¢N, 132�38¢E) districts of the main island
(Honshu) of Japan (Table 1). The average air temperature
over the past 30 years recorded at the nearest meteorolog-
ical observatories (Japan Meteorological Agency 2001)
was greater at Ohda (13.4�C) than at Kanegasaki and Fu-
jinomiya (8.9�C). Total organic C and charred material
contents were in the order: Fujinomiya > Ohda > Kanega-
saki (Table 1; Nishimura et al. 2006).
Plant debris was carefully removed and then charred
plant fragments were isolated using the heavy liquid
Table 1 Soil samples used and the yield and dl3C of the charred plant fragments
Sample name Sampling site in Japan Horizon Depth (cm) Total organic C (mg g)1)†
Charred plant fragment
Yield (mg g)1)† d13C (&)
Fujinomiya Fujinomiya, Shizouka Prefecture A12 15–27 181 90.9 )18.7
Ohda Ohda, Shimane Prefecture A12 22–47 146 77.1 )21.2
Kanegasaki Kanegasaki, Iwate Prefecture 3A11 35–63 115 25.2 )18.4
†Cited from Nishimura et al. (2006).
� 2010 Japanese Society of Soil Science and Plant Nutrition
794 N. Sultana et al.
separation method of Shindo et al. (2004a) after modifica-
tion (Nishimura et al. 2006). In brief, a suspension of the
soil samples in 12 mol L)1 HCl–16.75 mol L)1 HF was
heated at 200�C until it dried up. Then the residue was
washed onto a membrane filter (0.1 lm pore size; Advan-
tec, Tokyo, Japan) and dried in an oven. A 20 mL sodium
polytungstate solution (specific gravity, 1.6 g cm)3) was
added to 0.5–2 g of the residue and the suspension was
allowed to stand for 30 min. The suspension was then dis-
persed by sonication and centrifuged at 11,000 g for
20 min. Floating materials were collected and the same
treatment was applied to the precipitate repeatedly until
no floating materials were observed. The floating materi-
als were combined and further purified by shaking with
concentrated HCl–HF (1:2) for 24 h at 25�C twice, dia-
lyzing (Seamless Cellulose Tubing, Viskase, Darien, CT,
USA) against distilled water and freeze-drying.
Measurement and analysis of the X-raydiffraction profile
A 24.2 mg sample was set in a silicon holder (Overseas
X-Ray Service, Saitama, Japan) and the XRD profile was
measured using an X-ray diffractometer (XRD6100;
Shimadzu, Kyoto, Japan) under the following operating
conditions: target, cupper Ka; wavelength, 0.154 nm;
tube voltage ⁄ current, 40 kV ⁄ 30 mA; scan mode, set scan-
ning at 0.1�; scan range, 2h = 5–100� (counting time, 6 s)
and 60–100� (counting time, 12 s). The XRD profiles
were analyzed using the software Carbon Analyzer DiHi-
Ga Series 2007 (Ryoka Systems, Tokyo, Japan). This
method is based on the theory of Diamond (1957, 1958),
where the 11 bands in the XRD profile of amorphous C
material are derived from C-layer planes and can be
expressed as a combination of the 11 bands from C-layer
plane models with various sizes. The measured scattering
intensities were corrected for the coherent scattering
intensities derived from functional groups, incoherent
scattering intensities, polarization factor and the atomic
absorption factor (Fujimoto and Shiraishi 2004). Then,
the composition of the C-layer plane on a weight basis
was calculated by fitting the 11 band profile of the char
material samples to theoretical profiles of model com-
pounds, a mixture of two series of C-layer planes started
from benzene ⁄ coronene and pyrene (Fig. 1; Fujimoto
2003). In the fitting, the steepest-descent least-square cal-
culation method was used (Fujimoto and Shiraishi 2001).
The highest accuracy in fitting was obtained when the
2-D lattice constant (unit size of a C-layer plane) and
repeating number of least-square calculation were set at
0.240 nm and 280, respectively. The total number of
C-layer planes in arbitrary units (AU) per mg of sample
was evaluated from the theoretically calculated 11 band
profile (base-line method) and compared among the sam-
ples. The largest 11 band area among the three samples
was regarded as 100 AU mg)1. The experimental error
was <3 AU mg)1.
Polynuclear aromatic compounds originating from
the burning of fossil fuels or synthesized for industrial
use could be adsorbed to charred plant fragments,
which could affect the 11 band analysis. However,
such compounds are also expected to be strongly
adsorbed to the surface layer soil, and the soils used in
the present study were collected from lower layers, thus
no influence of organic contaminants on the XRD
result is expected.
Measurement of 13C NMR spectra by rampCPMAS and total side band suppression methods
A 15–30 mg sample was placed into a silicon nitride sam-
ple tube (4 mm diameter). 13C CPMAS NMR spectra
were recorded at 176 MHz on an ECA700 spectrometer
1.92 (52)
1.68 (37)
1.44 (30)
1.20(19)
0.96 (14)
0.72 (7)
0.48 (4)
0.24 (1)
Figure 1 Model structures of C-layer planes.
� 2010 Japanese Society of Soil Science and Plant Nutrition
Structure of plant charred materials in Andosols 795
(JEOL, Tokyo, Japan) using a ramp pulse sequence with
contact time of 1.0 ms, spinning rate at 12,500 Hz and
recycle delay of 1.0 s. Accumulation was repeated 5,000–
38,000 times. The chemical shift was relative to tetra-
methylsilane (0 p.p.m.) and adjusted with hexamethyl-
benzene (17.36 p.p.m.). The spectra were divided into five
regions; 0–45 (saturated alkyl C), 45–105 (alkyl C
substituted by hetero atom such as C-O in carbohydrates;
represented by O-alkyl C in the present study), 105–160
(aromatic C), 160–190 (carbonyl C, mainly carboxyl C)
and 190–210 p.p.m. Because the magnetic field of the
available spectrometer was too high to erase the spinning
side bands (SSBs), total side band suppression (TOSS) and
ramp CP at various spinning rates, 10,000–14,000 Hz,
were also applied to confirm the position and rough
intensity of the SSBs. The TOSS spectra were obtained
under similar conditions to the ramp CPMAS spectra,
except for the spinning rate (8000 Hz). The composition
of the C functional groups was estimated from the relative
area of each region in the total area of the CPMAS spec-
tra. A correction was conducted assuming that the signals
at 190–210 p.p.m. were almost derived from SSBs and
that the SSBs at higher and lower magnetic fields had the
same intensity; that is, the signal intensity at 190–
210 p.p.m. was subtracted from the signal intensity in the
O-alkyl C region and two of them were added to the
signal intensity in the aromatic C region (Watanabe and
Fujitake 2008).
Measurement of d13C
The d13C of charred plant fragments was measured using
a stable isotopic ratio mass spectrometer connected to an
elemental analyzer (Delta Plus ⁄ NC2500; Thermo-Finni-
gan, San Jose, CA, USA).
RESULTS AND DISCUSSION
The XRD spectra of the plant charred fragments (Fig. 2)
showed a large band at 2h = 25� (002 band). Other peaks
or shoulders observed at approximately 2h = 40–43� and
80� were assigned to 10 and 11 bands, respectively. These
three bands are typical in the XRD profile of turbostratic
C (Fujimoto 2003). Their intensities were greater in the
Kanegasaki sample than in the other two samples.
Although the 002 band has information about the degree
of stacking of C-layer planes and the distance between
C-layer planes, their estimation was difficult owing to the
presence of unknown bands at approximately 2h = 30–
40�. Although Warren–Bodenstein’s equation is also
applicable to the fitting of 11 bands, similar results to the
present method using Debye’s equation for amorphous
materials can be obtained (Fujimoto and Shiraishi 2004).
Furthermore, in the case of charred plant fragments, the
unknown bands at approximately 2h = 30–40� made
accurate fitting with Warren–Bodenstein’s equation,
which uses the whole profile range, impossible.
The accuracy of fitting the calculated 11 bands to the
observed one was 97.9–99.2%. The size of the C-layer
planes in the charred plant materials was distributed
between 0.96 and 1.92 nm (Fig. 3). The largest C-layer
plane corresponded to a condensed aromatic structure
0 20 40 60 80 100
Kanegasaki
Ohda
Fujinomiya
Kanegasaki
Ohda
Fujinomiya
2θ (o) 2θ (o)
(a) (b)
60 80 100
Figure 2 (a) X-ray diffraction profiles and (b) expanded (·6) 11bands with the least-square fitting of theoretical profiles (grayline) of charred plant fragments separated from buried Andosols.
50
40
30
Kanegasaki
20
10
0
50
40
30
Ohda
20
10
0
50
40
30
Wei
ght p
ropo
rtio
n (%
)
Fujinomiya
20
10
0
0.24
0
0.48
0
0.72
0
0.96
0
1.20
0
1.44
0
1.68
0
1.92
0
2.16
0
2.40
0
Size of C-layer plane (nm)
Figure 3 Weight proportion of the C-layer planes in charredplant fragments separated from buried Andosols.
� 2010 Japanese Society of Soil Science and Plant Nutrition
796 N. Sultana et al.
consisting of 52 rings (Fig. 1). The mean C-layer plane
size was 1.26, 1.37 and 1.28 nm in the Fujinomiya, Ohda
and Kanegasaki samples, respectively. The weight propor-
tion of the C-layer plane suggested that the most abundant
(34–44%) condensed aromatic structures in the charred
plant materials consisted of 19 rings. According to Ikeya
et al. (in press), the mean size of the C-layer planes in
Type A Has, including those from buried Andosols, was
0.82–0.96 nm. The 1.68 nm C-layer plane corresponding
to a 37-ring condensed aromatic structure was the largest
in Type A HAs, which occupied only <5% of total C-layer
planes on a weight basis. Thus, fragmentation is required
for charred plant fragments to change to or be incorpo-
rated into HAs. As a potential source of condensed aro-
matic structures in HAs, smaller condensed aromatic
components in charred plant fragments may be more
important than larger ones.
Variation in the size distribution of C-layer planes
among the three samples was not large. A few differences
included the relative abundance of the 1.92 nm C-layer
plane, which was greatest in the Ohda sample (12.6%)
(Fig. 3). Although the weight proportion of the 1.68 nm
C-layer plane, corresponding to a 37-ring condensed aro-
matic structure, in the Kanegasaki sample (14.7%) was
similar to that in the Ohda sample (14.6%), the 1.92 nm
C-layer plane was not detected in the Kanegasaki sample.
The intensity of the 11 bands was in the order: Kanega-
saki > Ohda > Fujinomiya, indicating that the total con-
tent of condensed aromatic structures was greater in the
Kanegasaki sample from a deeper soil layer than in the
other two samples (Table 2). Although we do not have
information about soil age, condensed aromatic structures
might be more refractory among charred plant compo-
nents.
The 13C CPMAS NMR spectra of the charred plant
fragments are shown in Fig. 4a. The peak of aromatic
C-C and C-H signals at 129–130 p.p.m. was always dom-
inant. Other peaks were observed at 29–30 (alkyl C),
169–172, (carboxy C = O) and 202–204 p.p.m. Signals
derived from alcohol C-OH with a maximum at
61–63 p.p.m. and phenolic C-O with a maximum at
152–154 p.p.m. were also observed as small peaks or
shoulders. Two peaks with maxima at 202–204 and 55–
58 p.p.m. shifted with changing spinning rates (data not
shown) and mostly disappeared in the TOSS spectra
(Fig. 4b) and were assigned to SSBs. Table 3 shows the
composition of C functional groups when regarding
the entire signal between 190 and 210 p.p.m. as SSB. The
proportion of aromatic C including phenolic C in total C
reached 61–74%. The smaller value in the Fujinomiya
Table 2 Relative contents of total carbon and the size of each C-layer plane in the charred plant fragments (AU mg)1)†
Carbon layer size (nm)
0.96 1.20 1.44 1.68 1.92
Fujinomiya 60 14 26 12 6 2
Ohda 80 14 27 17 12 10
Kanegasaki 100 22 39 25 15 N.D.
†Area of the 11 bands in the Kanegasaki sample was regarded as 100. AU, arbitrary units; N.D., not detected.
250 200 150 100 50 0 –50 (ppm)
250 200 150 100 50 0 –50 (ppm)
Kanegasaki
Ohda
Fujinomiya
(a) (b)
Figure 4 Solid state 13C nuclear magnetic resonance spectra ofcharred plant fragments separated from buried Andosols. (a)Ramp cross polarization ⁄ magic angle spinning and (b) total sideband suppression.
Table 3 Carbon composition of the charred plant fragments estimated from ramp cross polarization ⁄ magic angle spinning 13C nuclearmagnetic resonance spectra
Sample
% alkyl C
(0–45 p.p.m.)
% O-alkyl C
(45–105 p.p.m.)
% aromatic C
(105–160 p.p.m.)
% carboxyl C
(160–190 p.p.m.)
Fujinomiya 25 4 61 11
Ohda 15 5 68 11
Kanegasaki 11 1 74 14
� 2010 Japanese Society of Soil Science and Plant Nutrition
Structure of plant charred materials in Andosols 797
sample (61%) was consistent with the smaller intensity of
the 11 bands in the XRD profile. Variations in the propor-
tions of O-alkyl and carbonyl C in total C among the
samples were small, 1.1–5.4% and 10.5–14.1%, respec-
tively. The proportion of alkyl C in total C tended to be
larger in the order Fujinomiya (25%), Ohda (15%) and
Kanegasaki (11%). The greater loss of aliphatic compo-
nents might be related to the smaller yield of charred plant
fragments (Table 1) in the Kanegasaki soil. According to
Knicker et al. (2005) and Hockaday et al. (2007), aro-
matic C accounted for 68–85% of the total C in charred
particles picked up from the Ah horizon of a forest soil or
forest floor, whereas the relative abundance of aromatic C
and alkyl C in pieces (<1 cm) of charred plant was 57%
and 19%, respectively (Knicker et al. 2005). Thus, the C
composition of the Fujinomiya sample was similar to that
of a recently charred plant, whereas the Kanegasaki sam-
ple appeared to be closer to more completely charred
material.
According to Nishimura et al. (2006), the content and
degree of humification of HAs were larger in the Kane-
gasaki sample than in the Ohda and Fujinomiya samples.
Their observations may correspond to an advance in the
degradation of charred plant fragments in Kanegasaki
soil. Although the 4% difference in the carbonyl C con-
tent between the Kanegasaki and Fujinomiya samples
was critical, it may reflect a difference in the degree of
oxidation as was expected from the difference in HA
content.
Ketone C (including aldehyde C) was inferred to be a
minor C functional group in the charred plant frag-
ments from small resonances at 190–200 p.p.m. in the
TOSS spectra. When the shoulder at 190–197 p.p.m. in
the CPMAS spectra was assumed to be derived from
ketone C, the intensity was <3% of the total. This
manipulation decreased the relative content of aromatic
C to 56–69% and increased O-alkyl C to 4–8%. Thus,
if the influence of SSBs could be corrected completely,
the C composition would not change greatly. Schmidt
et al. (1999) estimated the composition of charred
organic C in soil by measuring 13C NMR spectra of the
<53 lm fraction after ultraviolet irradiation: alkyl C, 7–
23%; O-alkyl C, 6–34%; aromatic C, 29–74%; car-
bonyl C 11–18%. These ranges were much wider than
those recorded for our samples, suggesting difficulty in
the estimation of the structural properties of charred
materials in soil without separation from other soil
organic matter.
The d13C of the charred plant fragments is shown in
Table 1 as well as basic information about the samples.
The d13C values of the Fujinomiya and Kanegasaki sam-
ples were similar to each other, )18.7 and )18.4&, and
higher than that of the Ohda sample, )21.2&. Tree spe-
cies are generally C3 plants with lower d13C, whereas
there are C4 plants with higher d13C among grass species.
For example, Miscanthus sinensis a representative vegeta-
tion in grassland Andosols is a C4 plant ()12.5&; Hira-
date et al. 2004). Hence, there is a possibility that the
contribution of tree species to charred fragments was
greater in the Ohda sample than in the Fujinomiya and
Kanegasaki samples. Although the greater content in the
largest C-layer plane (1.92 nm) in the Ohda sample
(Table 3) might be related to a difference in vegetation,
further data accumulation is required to determine the
relationship between the vegetation and the size of C-layer
planes.
In conclusion, the composition of C-layer planes in
charred plant fragments attached to soil particles in buried
Andosols did not differ largely, notwithstanding the dif-
ferences in geographic regions (climatic conditions), soil
depth and contents of soil organic matter and charred
plant fragments in the soil. The enrichment of condensed
aromatic structures showed a similar trend to the aro-
matic C content among the three char samples, suggesting
the loss of aliphatic components as a cause of the enrich-
ment of condensed aromatic structure. Analysis of the
11 band profile in XRD spectra will be a powerful tool in
future studies evaluating changes in the amount and size
of condensed aromatic structures in charred plant frag-
ments in soil over time during the transformation to or
incorporation into HAs.
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