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DING ET AL.: TILLAGE MANAGEMENT AFFECTING SOIL ORGANIC MATTER 421
Wilkie, J.A., and J.G. Hering. 1996. Adsorption of arsenic onto hy- substance on the adsorption of As(V) on geologic materials. Water
Air Soil Pollut. 40:293–305.drous ferric oxide: Effects of adsorbate/adsorbent ratios and co-
occuring solutes. Colloids Surf. A: Physicochem. Eng. Aspects 107: Xu, W., C.T. Johnston, P. Parker, and S.F. Agnew. 2000. Infrared
study of water sorption on Na-, Li-, Ca- and Mg-exchanged (SWy-97–110.
Xu, H., B. Allard, and A. Grimvall. 1988. Influence of pH and organic 1 and SAz-1) montmorillonite. Clays Clay Miner. 48:120–131.
Soil Organic Matter Characteristics as Affected by Tillage Management
G. Ding, J. M. Novak, D. Amarasiriwardena, P. G. Hunt, and B. Xing*
ABSTRACT ganic C (SOC). Soils of the southeastern United States
of America, particularly sandy Coastal Plain soils, have
Soil organic matter (SOM) is of primary importance for maintaining inherently low SOC contents (typically below 1%, Hunt
soil productivity, and agricultural management practices may signifi-
et al., 1982). Consequently, small changes in the SOM
cantly influence SOM chemical properties. However, how SOM chem-
ical characteristics change with agricultural practices is poorly under- content are significant to the agricultural production of
stood. Therefore, in this study, we evaluated the impacts of tillage the region. An evaluation of tillage and crop residue
(conventional vs. conservation) management on the structural and management practices to rebuild SOC levels has been
compositional characteristics of SOM using cross-polarization magic- conducted by Hunt et al. (1996). These researchers mon-
angle-spinning (CPMAS) and total sideband suppression (TOSS) itored changes in SOC levels in numerous small tillage
solid-state
13
C nuclear magnetic resonance (NMR) and diffuse reflec- plots and found that after 9 yr of CnT, the SOC content
tance Fourier transform infrared (DRIFT) spectroscopy. We charac- in the top few centimeters was significantly higher than
terized both physically and chemically isolated SOM fractions from a the soil under CT management. Campbell et al. (1999)
Norfolk soil (fine-loamy, siliceous, thermic Typic Kandiudults) under reported that over an 11- to 12-yr period, increases in
long-term tillage management (20 yr). The solid-state
13
C NMR results
C storage in the 0- to 15-cm soil depth, because of
indicated that humic acid (HA) from conventional tillage (CT, 0–5
cm) was less aliphatic and more aromatic than HA from conservation adoption of no-tillage, were small (0–3 Mg ha
⫺
1
). Most
tillage (CnT). The aliphatic C content decreased with increasing depth of the differences were observed in the 0- to 7.5-cm soil
(0–15 cm) for both CT and CnT treatments. The reverse trend was depth, with little change in the 7.5 to 15 cm. However,
true for aromatic C content. Based on reactive/recalcitrant (O/R) the short and long-term influences of disturbance on C
peak ratio comparisons, HA was more reactive in the top soil (0–5 mineralization are complex and may vary depending
cm) under CnT than CT. Both soil organic C (SOC) and light fraction on types of soil and plant residues (Hu et al., 1995;
(LF) material were higher in the 0- to 5-cm soil of CnT than CT Franzleubbers and Arshad, 1996; Alvarez et al., 1998).
treatment. Our results show that long-term tillage management can The strong influence of soil management on the amount
significantly change the characteristics of both physical and chemical and quality of SOM was also reported by others (Janzen
fractions of SOM.
et al., 1992; Ismail et al., 1994; Campbell et al., 1996).
Another approach to evaluate the impact of agricul-
tural management on SOM dynamics is to separate
Soil organic matter strongly affects soil properties SOM into pools based on differences in decomposition
such as water infiltration rate, erodibility, water rates (Wander et al., 1994; Wander and Traina, 1996a).
holding capacity, nutrient cycling, and pesticide adsorp- Generally, those pools are conceptualized with one
tion (Stevenson, 1994; Campbell et al., 1996; Francioso small pool having a relatively quick decomposition rate
et al., 2000; Wander and Yang, 2000). It has been sug- (i.e., active pool LF) and pools that are more recalci-
gested that proper management of SOM is the heart of trant (i.e., humus) (Stevenson, 1994). The LF is sensitive
sustainable agriculture (Weil, 1992). Recent research to environmental and agricultural management factors
has also recognized SOM as a central indicator of soil and can be used as a functional description of organic
quality and health (Soil and Water Conservation Soci- materials (Wander and Traina, 1996a). Regardless of
ety, 1995). For example, a decline in SOM (biological active or recalcitrant SOM pools, structural chemistry
oxidation or erosion) significantly reduced the N supply is important for their chemical and biological activities.
and resulted in a deterioration of soil physical condi- Spectroscopic techniques can provide useful struc-
tions, leading to crop yield reduction (Greer et al., 1996). tural information of SOM. Diffuse reflectance Fourier
Therefore, it is important to maintain proper levels of transform infrared spectroscopy is considered to be one
SOM to sustain soil productivity. of the most sensitive infrared techniques for humic sub-
Intensive agricultural practices change SOM charac- stances analysis (Niemeyer et al., 1992; Ding et al., 2000).
teristics greatly, generally a substantial loss of soil or- According to Painter et al. (1985) and Niemeyer et al.
G. Ding and B. Xing, Dep. of Plant and Soil Sciences, University
Abbreviations: CnT, conservation tillage; CPMAS-NMR, cross-polar-
of Massachusetts, Amherst, MA 01003; J.M. Novak and P.G. Hunt,
ization magic-angle-spinning nuclear magnetic resonance; CT, con-
USDA-ARS-Coastal Plains Soil, Water, and Plant Research Center,
ventional tillage; DRIFT, diffuse reflectance Fourier transform infra-
Florence, SC 29501; D. Amarasiriwardena, School of Natural Science,
red spectroscopy; HA, humic acid; LF, light fraction; O/R, reactive/
Hampshire College, Amherst, MA 01002. Received 17 Jan. 2001.
recalcitrant functional group ratio calculated from peak heights of
*Corresponding author (bx@pssci.umass.edu).
DRIFT spectra; SOC, soil organic C; SOM, soil organic matter; TCN,
total combustible N; TOSS, total sideband suppression.Published in Soil Sci. Soc. Am. J. 66:421–429 (2002).
422 SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002
each soil layer was isolated using a modified density gradient
(1992), this technique offers several advantages over method of Wander and Traina (1996a). The LF was collected
transmission infrared spectroscopy: (i) a simple sample- by dispersion of 50 g soil of freshly sieved, field-moist soil
preparation procedure; (ii) insensitivity to water associ- sample in a NaBr solution (density 1.5 g mL
⫺
1
, 1:1 w/v). The
ated with the sample and enhanced resolution; (iii) high mixture was shaken for 30 min and centrifuged at 7500 ⫻g
resolution of the spectra because of reduction in the (8000 rpm) for 20 min. These rinses were then transferred
sensitivity towards light scattering; and (iv) a more reli- into a 250-ml separatory funnel and allowed to settle over-
able method for quantitative estimations of functional night. After three such separations, the composite supernatant
groups. Another spectroscopic technique is solid-state was filtered using a 0.45-m polycarbonate membrane filter.
13
C NMR spectroscopy that is probably the most useful The heavy SOM fraction which settled to the bottom of the
funnel was removed. The LF materials retained on the filter
tool for nondestructive characterization of SOM and its
were rinsed with a 0.5 MCaCl
2
and 0.5 MMgCl
2
solution
components (Preston, 1996; Xing and Chen, 1999; Mao followed by a final rinse in deionized water. This was done
et al., 2000). Studies by Capriel (1997) and Ding et al. to avoid any remnant biological toxicity because of Na
⫹
satura-
(2000) demonstrate that both DRIFT and
13
C NMR tion of the ion-exchange sites in the LF. The weight yield of
techniques are useful and suitable for examining the LF was measured and the light fraction organic C (LF-OC)
effects of agricultural management on SOM. and total combustible N content were determined using a
The goal of this research was to evaluate the changes LECO-CN 2000 analyzer (LECO Corp., Joseph, MI).
of SOM quantity and quality under CT and CnT systems
using both DRIFT and solid-state
13
C NMR. Spectro- Extraction, Fractionation, and Purification of HA
scopic investigations of SOM changes for the Norfolk and Elemental Composition Analysis
soil, located in the Southeastern Coastal Plain, have
Most of extraction techniques require the organic matter
never been conducted before. Furthermore, because or-
to be removed from soil (Stevenson, 1994). As a consequence,
ganic, sustainable agricultural systems depend increas- the OC constituents would be modified to some extent. There-
ingly on soil nutrient cycling mechanisms, it is necessary fore, we used neutral pyrophosphate (Na
4
P
2
O
7
) to extract
to understand the relationships between the LF, the SOM to minimize chemical modifications (Stevenson, 1994).
structural and compositional changes of HA, and nutri- Air-dry and sieved soil (50 g) was weighed into a 1000-ml
ent retention and supply characteristics. The specific ob- plastic bottle, and 500 ml of 0.1 MNa
4
P
2
O
7
were added. The
jectives were to: (i) characterize HA structural changes; air in the bottle and solution was displaced by N gas (N
2
) and
(ii) determine peak height O/R ratio for HA, which the system was shaken for 24 h at room temperature. The
reflects the biological activity; and (iii) compare the light samples were extracted three times. After separation from
fraction (LF) variations with soil depth under both CnT the Na
4
P
2
O
7
insoluble residues by centrifuging at 1100 ⫻g
(3000 rpm), the dark-colored supernatant solutions were com-
and CT systems. bined, acidified to pH 1 with 6 MHCl, and allowed to stand
for 24 h at room temperature for the precipitation of the HA
MATERIALS AND METHODS fraction. The HA was shaken for 24 h at room temperature
with 0.1 MHCl/0.3 MHF solution at least for three times.
Site Description and Sampling The insoluble residues (HA) was separated from the superna-
tant by centrifuging at 12 000 ⫻g(10 000 rpm), washed with
The study was conducted using soil samples collected from
deionized water until free of Cl
⫺
ions, and then freeze-dried.
the long-term CnT and CT research plots established in 1979
The C, H, and N contents of the isolated HAs were measured
at the Clemson University Pee Dee Research and Education
with a Fisons Model EA 1108 Elemental Analyzer (Mattson
Center (Darlington, SC). The soil at the research site is a
Instrument, Madison, WI).
Norfolk loamy sand. The coordinates are 34.3⬚N lat. and 79.7⬚
W long., and the elevation is 37 m above the mean sea level.
Treatments were arrayed in a randomized complete block Diffuse Reflectance Fourier Transform
design with split plots and five replications (Hunt et al., 1996). Infrared Analysis
The CT treatment within the plots consisted of multiple
disking (0–15 cm deep) and the use of field cultivators to The DRIFT spectra were collected using an Infrared Spec-
maintain a relatively weed free surface. Surface disking and trophotometer (Midac series M 2010, Midac Corp., Irvine,
field cultivation have been completely eliminated in soil under CA) with a DRIFT accessory (Spectros Instruments, Shrew-
CnT plots since 1979. Because of a root-restrictive E horizon bury, MA). All HA fractions were powdered with a agate and
which reforms annually in this soil (Busscher and Sojka, 1987), pestle and stored over P
2
O
5
in a drying box. Three-milligram
both tillage treatments received in-row subsoiling (30 cm solid HA samples were then mixed with KBr (total weight as
deep) at planting to fracture this horizon. Additional manage- to 100 mg) and reground to powder consistency. A sample
ment practices for the plots such as crop rotation, fertilization, holder was filled with the mixture (powder). A microscope
and pesticide application were described previously (Hunt et glass slide was used to smooth the sample surface. At the
al., 1996; Novak et al., 1996). In 1999, 苲50 soil cores were beginning of analysis, the diffuse-reflectance cell which con-
collected from the top 15 cm of soil using a 2.5-cm diam. soil tained the samples was flushed with N
2
for 10 min to reduce
probe at random locations from one plot under CnT and one the interference from CO
2
-C and water molecules. The sample
plot under CT treatment. The core samples were sectioned compartment was placed with anhydrous Mg(ClO
4
)
2
to further
(5-cm increments), composited, air-dried, and sieved (2 mm). reduce atmospheric moisture.
The DRIFT spectroscopy was acquired with a minimum of
100 scans collected at a resolution of 16 cm
⫺
1
. The spectroscopy
Density Gradient Separation of Light Fraction was calibrated with the background which consisted of pow-
Material and Analysis dered KBr and scanned under the same environmental condi-
tions as the sample-KBr mixtures. Absorption spectra wereThe LF has been recognized to be an important soil nutrient
reservoir and has been recommended as a fertility index (Wan- converted to a Kubelka-Munk function using Grams/32 soft-
ware package (Galactic Corp., Salem, NH). Peak assignmentsder et al., 1994). In this investigation, the LF material from
DING ET AL.: TILLAGE MANAGEMENT AFFECTING SOIL ORGANIC MATTER 423
Table 1. Soil organic C (SOC), total combustible N (TCN), and
and intensity (by height) ratio calculation were done following
light fraction (LF) in the soil under different tillage systems
the methods of Niemeyer et al. (1992), and Wander and Traina
(standard deviation in parentheses).†
(1996b). We used ratios of labile (O-containing) and recalci-
trant (C and H or N) functional groups to compare HA spectra
Soil depth CnT CT
with varying soil depth of different tillage treatments.
cm kg m
⫺
2
SOC 0–5 2.30 (0.02)a‡ 1.22 (0.01)a
5–10 0.89 (0.01)b 1.23 (0.01)a
Solid-State Carbon-13 Nuclear Magnetic
10–15 0.61 (0.01)b 0.81 (0.01)b
Resonance Spectroscopy
TCN 0–5 0.22 (0.01)a 0.11 (0.00)a
Spectra were obtained by using the CPMAS-TOSS tech-
5–10 0.08 (0.01)b 0.11 (0.00)a
10–15 0.05 (0.00)b 0.06 (0.00)b
niques. Xing et al. (1999), after examining several solid-state
LF 0–5 1.19 (0.02)a 0.63 (0.01)a
13
C NMR techniques including ramped CPMAS, reported that
5–10 0.17 (0.01)b 0.55 (0.01)a
CPMAS-TOSS has two advantages. The first is that an ade-
10–15 0.10 (0.00)b 0.12 (0.00)b
quate TOSS can eliminate the sidebands so that the spectrum
C/N ratio of Soil
shows only the true peaks for a given HA sample. The second
C/N 0–5 10.4 (0.02)a 11.1 (0.01)a
is that implementation of cross-polarization-TOSS can avoid
5–10 11.1 (0.02)a 11.2 (0.01)a
baseline distortion from the dead time. In addition, instrument
10–15 12.2 (0.02)a 13.5 (0.03)a
time required is about the same as the regular CPMAS. They
gkg
⫺
1
recommended that CPMAS-TOSS be used to analyze HA
LF-OC/SOC 0–5 160 (4.01)a 150 (3.48)a
samples when using a ⱖ300 MHz spectrometer. In this re-
5–10 40 (1.55)b 130 (2.78)a
10–15 42 (1.78)b 42 (1.29)b
search, HA samples were run at 75 MHz (
13
C) in a Bruker
LF-N/TCN 0–5 100 (2.45)a 90 (2.07)a
MSL-300 spectrometer (Bruker, Billerica, MA) with a 7-mm
5–10 30 (1.38)b 80 (2.17)a
CPMAS probe. The samples (300–350 mg) were packed in a
10–15 22 (1.29)b 22 (1.69)b
7-mm-diam. zirconia rotor with a Kel-F cap. The spinning
† Aerial mass of SOC, TCN, and LF was calculated from area and soil
speed was 4.5 kHz. A
1
H90⬚pulse was followed by a contact
bulk density.
time (t
c
) of 500 s, and then a TOSS sequence was used to
‡ Means with different letters are significantly different P⫽0.05.
remove sidebands (Schmidt-Rohr and Spiess, 1994; Xing et
al., 1999). Line broadening of 30 Hz was used. The 90⬚-pulse
length was 3.4 s and the 180⬚pulse was 6.4 s. The recycle management are shown in Table 1. It is clear that 20-
delay was 1 s with the number of scans 苲4096. The details yr different tillage management influenced the quantity
were reported elsewhere (Xing et al., 1999). In preliminary and distribution of C, LF, and N in the soil. Twenty-
experiments, we ran several samples at different contact times, year CnT treatment resulted in a significant increase in
and selected 500 s because this contact time gave the best the SOC, soil-TCN, and LF in the top 0- to 5-cm soil
signal/noise ratio and the spectra were similar to the ones gener- layer of the unfractionated soil, as compared with CT
ated by direct polarization magic-angle-spinning
13
C NMR. management. The quantities of SOC, TCN, and LF in
There was no signal observed for the rotor and Kel-F cap
(Mao et al., 2000), thus, no background correction was made the 0- to 10-cm layer were significantly higher than those
in this work. of 10- to 15-cm depth under CT treatment. The SOC
and TCN decreased with increasing soil depth under
both tillage treatments.
Statistical Analyses
The quantity of LF material in the 0- to 5-cm soil
All data presented were the mean of at least three replicate layer in the CnT system was approximately twice high
measurements, except for HA elemental composition and as that in the CT system. On the other hand, soil under
solid-state
13
C NMR data because of the high cost and low CT management at the 5- to 10-cm layer had signifi-
availability of the instrument. However, preliminary solid-
cantly higher LF than soil under CnT. The dependency
state NMR experiment with one HA sample indicated minimal
variations, which was consistent with the result of an extensive of tillage and depth on LF distribution was confirmed
NMR study in our lab (Mao et al., 2000). The HA elemental by the two-way ANOVA which showed that there was
composition and NMR measurements were performed on a significant tillage and depth interaction (P⬍0.01)
composite samples. when the quantity of LF was compared between tillages.
The fraction of total SOC and total combustible N (TCN) Additionally, regression analyses between the quantity
pools in the unfractionated soil and in LF was compared using of LF material and the quantity of SOC showed a signifi-
a two-way Analyses of Variance (ANOVA). Also, the O/R cant linear relationship (r
2
between 0.89 and 0.97, Pⱕ
ratios generated from DRIFT peak heights were examined 0.01) (data not shown). This relationship indicates that
between tillages and by depth using the ANOVA. Different
tillage treatments and soil depths were the experimental fac- 89 to 97% of the variation in quantity of LF material
tors and the interactions between tillage and depth were exam- isolated from soil under both tillages can be accounted
ined. SigmaStat software (SPSS Corp., Richmond, CA) was for by the SOC content.
used for each test at a 0.05 level of significance. The isolated LF material accounted for between 4
and 16% of the total SOC and 2 to 10% of soil TCN
from the Norfolk soil (Table 1). Only at the 5- to 10-
RESULTS cm soil depth was there a significant difference of LF-
Yields of Light Fraction Material and Elemental OC and LF-N percentages between tillages. Regression
Composition of Humic Acid analyses confirmed a significant relationship (r
2
be-
tween 0.79 to 0.95) between the LF-OC vs. the SOC
The total quantities of SOC, TCN, and LF found in
soils calculated from bulk density under CnT and CT content and the LF-TCN vs. the soil-TCN content. The
424 SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002
Table 2. Elemental composition of humic acids on an ash-free
generic chemical characteristics of the components pres-
basis and atomic H/C and C/N ratios.
ent in the samples. Unsubstituted aliphatic C is indicated
Sample Depth C H N H/C C/N
by signals in the 0- to 50-ppm region. Carbons in protein-
aceous materials (amino acids, peptides, and proteins)
cm g kg
⫺
1
have resonances between 40 and 60 ppm, and C in carbo-
CnT 0–5 527 41 39 0.94 15.7
CnT 5–10 479 36 31 0.92 18.1
hydrates gives signals between 60 to 108 ppm. Signals
CnT 10–15 516 35 31 0.81 19.4
between 108 to 162 ppm are because of aromatic C,
CT 0–5 546 40 37 0.88 17.2
while those near 155 ppm arise from phenolic C, indicat-
CT 5–10 548 40 35 0.88 18.3
ing the presence of O- and N-substituted aromatic
CT 10–15 538 40 31 0.89 20.3
groups (e.g., phenolic OH and aromatic NH
2
). The
strong signals between 170 and 180 ppm come from C
soil C/N ratio for both tillages increased slightly with
in carboxyl groups, with possibly some overlapping from
soil depth, but not significantly (Table 1).
The elemental compositions of the HAs from both phenolic, amide, and ester carbons (Stevenson, 1994;
CnT and CT systems are displayed in Table 2. Examina- Mao et al., 2000).
tion of the data showed that the HAs from two tillage It was difficult to directly compare the HA spectra of
systems were similar to each other. The C content of different treatments because visual comparison showed
HA under CnT was slightly lower in the middle layer no major differences in terms of presence or absence
(5–10 cm) than that of other two layers. The N content of specific peaks. However, we can obtain detailed infor-
was higher in the top soil than that of deeper layers for mation from these spectra by peak area integration.
both tillages. The HA atomic C/N ratio increased with The relative content of major C-types, calculated by
soil depth for both tillages, similar to the soil C/N ratio integrating the spectral profile according to standard
changes. The HA H/C ratio of CnT plot slightly declined chemical shift ranges (Xing et al., 1999) is shown in Fig.
with depth while this ratio was almost constant for 2. The most noticeable feature was at 60 to 96 ppm
CT soil. region (Fig. 1 and Fig. 2B), i.e., carbohydrate-C (ali-
phatic C bonded to OH groups, ether oxygens, or oc-
Solid-State Carbon-13 Nuclear Magnetic curring in saturated five or six-membered rings bonded
Resonance Spectroscopy of Humic Acid to oxygens). This C content for CnT in the top soil (0–5
cm) was 23.9%, and was 18.3% for CT (Fig. 3B). The
The HA
13
C CPMAS-TOSS NMR spectra of both CT
and CnT are shown in Fig. 1. The HA spectra revealed difference between the two treatments can be attributed
Fig. 1. Cross-polarization magic angle-spinning total sideband suppression
13
C NMR spectra of humic acids in a Norfolk soil under different
tillages: (1A) conservation tillage treatment (CnT1, 0–5 cm; CnT2, 5–10 cm; and CnT3, 10–15 cm); (1B) conventional tillage treatment (CT1,
0–5 cm; CT2, 5–10 cm; and CT3, 10–15 cm).
DING ET AL.: TILLAGE MANAGEMENT AFFECTING SOIL ORGANIC MATTER 425
to the accumulation of carbohydrate materials from Another interesting feature was revealed by the 108
to 162 ppm of NMR spectra and their integration resultsfresh residue input in the top soil of CnT treatment.
The reverse trend was true in the 10- to 15-cm layer, (Fig. 1A,B, and Fig. 2C). The two most pronounced
peaks in this region were recorded at 苲131 ppm (ringwhich showed carbohydrate-C content was higher in CT
than that of CnT system. There was not much difference carbons in which the ring is not substituted by strong
electron donors such as O and N) and at 155 ppm (phe-in the 5- to 10-cm layer between both tillages. The lowest
carbohydrate-C for both tillage managements occurred nols and aromatic amines). Aromatic-C (31.7%) in the
0- to 5-cm layer under CT was higher than that (28.1%)at 10- to 15-cm soil layer. The carbohydrate-C decreased
with soil depth for CnT. But for CT management, the of CnT treatment (Fig. 2C). Similarly, HA aromatic-C
content in 5 to 10 cm of CT system was greater thancarbohydrate-C content was almost the same in the first
two layers. that of CnT plot. However, the aromatic-C content was
almost the same in the 10 to 15 cm for both tillages,The total aliphatic C (0–108 ppm) of HA for CT
treatment (Fig. 2A) decreased from 52.5% in the top even though aromatic-C in both treatments increased
with soil depth. The aromaticity (expressed in terms ofsoil (0–5 cm) to 40.1% at the depth of 10 to 15 cm.
Similarly, the aliphatic C of HA for CnT treatment aromatic-C as a percentage of the aliphatic-C ⫹aro-
matic-C, according to Hatcher et al., 1981) increaseddecreased from 58.8% in the top soil (0–5 cm) to 40.8%
at the depth of 10 to 15 cm. Furthermore, when com- from 32.3% in the top soil of CnT to 50.7% at the 10-
to 15-cm layer, and from 31.7 to 50.8% for CT treat-paring the total aliphatic-C (0–108 ppm) and carbohy-
drate-C (60–96 ppm) of HA between the two treatments ment (Fig. 2D). Carboxyl groups were relatively en-
riched in CT treatment (data not shown). The value of(Fig. 2A and 2B), it was evident that both aliphatic-C
and carbohydrate-C were higher in the top (0–5 cm) carboxyl-C increased with soil depth for both treat-
ments, which was consistent withthereport by Stearmansoil of CnT than CT. The HA alkyl-C content (0–50
ppm, data not shown) at the 10- to 15-cm layer was et al. (1989). The chemical shift of carbonyl-C was dis-
tinct only for a few HA samples (e.g., CnT2 and CT3)higher in CnT than CT.
Fig. 2. Solid-state
13
C NMR data under different tillage systems: (A) aliphatic-C (0–108 ppm); (B) carbohydrate-C (60–96 ppm); (C) aromatic-C
(108–162 ppm); and (D) aromaticity (108–145 ppm)/(0–162 ppm).
426 SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002
Fig. 3. Diffuse reflectance Fourier transform infrared spectra of humic acids in a Norfolk soil under different tillage treatments: (3A) conservation
tillage treatment (CnT1, 0–5 cm; CnT2, 5–10 cm; and CnT3, 10–15 cm); (3B) conventional tillage treatment (CT1, 0–5 cm; CT2, 5–10 cm;
and CT3, 10–15 cm).
and its content was very low (Fig. 1). We did not observe and silicate vibrations (Francioso et al., 2000). The peak
any distribution pattern and change between tillage around 600 cm
⫺
1
of HA was associated with unknown
treatments. This may be because of the poorly resolved mineral compounds (e.g., silicate, oxides, or organo-
carbonyl-C peaks (Fig. 1). mineral fractions).
All DRIFT spectra of HA samples from CnT and CT
Diffuse Reflectance Fourier Transform Infrared systems were similar in their basic peak assignments.
Spectroscopy of Humic Acid However, to get a clear picture of tillage impacts on
spectral composition of HA, we examined ratios of reac-
The spectra of HA from CT and CnT are presented tive (O-containing) and recalcitrant (C, H, or N) func-
in Fig. 3. The resolution of the spectra exhibited a signifi- tional group peak heights (Table 3). Based on peak ratio
cant improvement, compared with previously published comparisons, the total O/R ratio (R
1
) of HA was the
spectra obtained using dispersive or FTIR spectropho- highest at 0 to 5 cm of CnT system, which was signifi-
tometers. Evidence for the presence of COOH groups cantly greater than that of CT system. The lowest O/R
was indicated by the peak at 苲1200 cm
⫺
1
, which was ratio appeared at the depth of 10 to 15 cm of CT treat-
attributed to C–O stretch and OH deformation of ment (Table 3). With increasing depth, the O/R ratio
COOH groups. The band around 1620 to 1600 cm
⫺
1
in declined for both tillages (Table 3). There were rela-
all of the HA spectra was assigned to aromatic C⫽Ctively little changes of R
1
between 5- to 10- and 10- to
and the asymmetric C⫽O stretching in COO
⫺
groups
15-cm layers for both tillage systems. The ratio of ke-
(Inbar et al., 1989; Gressel et al., 1995). But the fre-
tonic and carboxyl (1727 cm
⫺
1
) groups divided by CH
quency at 1660 cm
⫺
1
can also be attributed to internal
and aromatic peak heights (1457 ⫹1420 ⫹779 cm
⫺
1
)
H bonds of carbonyl groups (Bellamy, 1975). Peaks
(R
2
) was also the highest in the top soil (0–5 cm) of
苲1400 cm
⫺
1
were assigned to CH
2
and CH
3
bending,
CnT system (Table 3), suggesting relatively enrichment
C–OH deformation of COOH, and COO
⫺
symmetric
of O associated C (e.g., carbohydrates) over time in
stretch (Celi et al., 1997). The bands in the 1100 to 900
cm
⫺
1
region were usually attributed to polysaccharide CnT plots as compared with CT. However, there were
DING ET AL.: TILLAGE MANAGEMENT AFFECTING SOIL ORGANIC MATTER 427
Table 3. Ratios of selected peak heights from DRIFT spectra of humic acids (Wander and Traina, 1996b).
R
1
(O/R) R
2
1727 ⫹1650 ⫹1160 ⫹1127 ⫹1050 1727
HA Depth 2950 ⫹2924 ⫹2850 ⫹1530 ⫹1509 ⫹1457 ⫹1420 ⫹779 1457 ⫹1420 ⫹779
cm
CnT 0–5 0.87 (0.02)a† 0.51 (0.02)a
CnT 5–10 0.69 (0.01)b 0.44 (0.02)b
CnT 10–15 0.70 (0.01)b 0.43 (0.02)b
CT 0–5 0.74 (0.01)ab 0.44 (0.02)a
CT 5–10 0.69 (0.01)b 0.42 (0.01)a
CT 10–15 0.67 (0.01)b 0.43 (0.01)a
† Means with different letters are significantly different P⫽0.05.
Even though there might be a small fraction of HA
no significant differences of R
2
between soil depth for
still left in soil, we believe that the HA samples would
both tillages (Table 3).
represent the overall characteristics of total HA.
Microbial decomposition of plant residue has a large
DISCUSSION influence on the elemental composition of SOM pool.
Evaluating the effects of tillage on SOM dynamics In general, the compounds with narrow C/N ratios are
has been shown to take almost 10 yr of experiment in a more advanced stage of decomposition (Yakov-
before any significant change (Hunt et al., 1996). It has chenko et al., 1998). However, this was not the case for
been proposed that the LF be examined because it has our HAs. The C/N ratio of HA ranged from 15.7 to
been correlated with several procedurally defined soil 20.3 (for comparison, 10.4 to 13.5 for soil) (Tables 1 and
fractions (e.g., biological pools). The LF may act as an 2) and increased with soil depth for both tillages. This
indicator of organic matter status in soil (Wander and result suggests that some N-containing compounds of
Traina, 1996a). Thus, characterizing the size of this frac- SOM, particularly in the deeper soil layers, may be
tion, as well as its C and N contents, may show short- protected by physical encapsulation in three-dimen-
term changes because of management practices, which sional structures with soil minerals, which were resistant
may not be detected when measuring whole SOM pool. to chemical extraction. Thus, N contents in the Na
4
P
2
O
7
–
In both tillages, the quantities of LF materials, LF- extracted HA were relatively low (i.e., high C/N ratio).
OC and LF-TCN were highly dependent on the SOC This finding was consistent with the reports of Knicker
and soil-TCN contents. The CnT had higher yield of and Hatcher (1997), and Zang et al. (2000). By changing
LF material in the top soil (0–5 cm) while CT soil had the conformational structure of HA, Zang et al. (2000)
a higher amount of LF material in the 5- to 10-cm layer. created new refractory organic N in HA. The newly
The findings support the conclusions of Novak et al. formed proteinaceous materials in HA were physically
(1996) that long-term CnT management creates a SOC- encapsulated within the HA structure. They concluded
enriched surface zone. Conversely, SOC contents in the that physical protection is one of the important factors
CT managements were fairly similar between soil layers accounting for the preservation of organic N in soils
probably as a result of mixing by disking. However, for and sediments.
assessing the effects of tillage and fallow frequency on Examination of CPMAS-TOSS
13
C NMR data illus-
soil quality, Campbell et al. (1999) reported that total trated very interesting information. Compared with CT
organic C and N were surprisingly superior to the more system, HA in the top soil (0–5 cm) of CnT had a higher
labile attributes (e.g., microbial biomass). They had an- proportion of carbohydrate-C (Fig. 2B). The trends ob-
ticipated a significant influence of tillage on soil quality served were consistent with enhanced decomposition of
attributes, especially the labile ones, but they failed to plant inputs in the CT plot. This was supported by the
obtain the expected results. This is probably because of relatively higher O/R ratio (R
1
) of HA in the top (0–5
the difference in soil texture and environmental condi- cm) soil under CnT management. The high O/R ratio
tions. They used a silty loam (Typic Haploboroll) in a indicated that SOM was more biological active. From
cool-semiarid region (Saskatchewan, Canada), in com- this study, one may conclude that DRIFT and NMR
parison with a Norfolk loamy sand soil in a warm-humid can be used as complementary methods for the charac-
region (South Carolina, USA) in our study. terization of humic substances. This result also con-
Because HA is probably the largest single SOM pool curred with the LF data that CnT had a significantly
in mineral soils, which is representative of the stage of higher LF material in the 0 to 5 cm of soil than that
humification, decomposition pathways have been stud- of CT plot, indicating that 20 yr of CnT management
ied most extensively using HAs (Guggenberger et al., changed structures and compositions of SOM. Our re-
1994; Preston et al., 1994; Preston, 1996). In addition,
sults were in agreement with the observation that num-
without appropriate deashing treatments, low C con-
bers of microbes, microbial biomass and potentially-
tents and C/Fe ratios make humin extracted from soil
mineralizable N were greater for no-tillage than CT in
mineral horizons to be a poor candidate for NMR analy-
the 0- to 7.5-cm soil depth (Doran, 1980). These changes
sis (Preston and Newman, 1995; Ding et al., 2001).
can be sufficient to differences in the available soil water
Therefore, HA was used in this study. Soil samples were
extracted three times for HA recovery in this work. content between tillages (Hunt et al., 1996).
428 SOIL SCI. SOC. AM. J., VOL. 66, MARCH–APRIL 2002
Our observation of a substantial increase of aro- (0–5 cm). The CT treatment, on the other hand, mixed
residue within the 15-cm soil layer. As a result, SOCmatic-C in HAs with soil depth for both treatments
implied that the humification processes were more ad- and LF contents were higher at the deeper layers (5–15
cm) than CnT. The elemental composition of HA fromvanced in deeper soil layers. This result was in good
agreement with the report of Preston (1996) that as the two tillage systems was similar, but one cannot rely on
elemental composition only in evaluating managementaromatic rings of lignin are modified, a single broad
peak with its maximum around 131 ppm starts to domi- effect. Though single composite samples were used, the
solid-state
13
C NMR results showed that the aliphatic-Cnate in the aromatic region. With further humification,
HAs may become highly aromatic, with development of content of HA was higher in the top soil (0–5 cm) under
CnT than CT. Conversely, the aromatic C of HA waspolycondensed rings (Chen and Pawluk, 1995; Preston,
1996; Xing and Chen, 1999). Reduction of H/C ratio of higher in the top soil (0–5 cm) under CT than CnT.
Aliphatic C of HA declined with the increase of soilHA in CnT plot (Table 2) with soil depth supported the
increased aromaticity (Fig. 2D) as observed by NMR. depth under both tillages, whereas the aromatic C of
HA increased with soil depth. For DRIFT analysis, al-However, this was not the case for the CT plot as H/C
ratio was almost the same for all soil depths. The reason though the spectra of HA were similar in their basic
peak assignment, HA O/R ratio from the top soil (0–5was unknown.
Although single composite sample was used for NMR cm) of CnT treatment was higher than that from CT
treatment, indicating that HA contained more recalci-analysis (because of the high cost and low availability
of NMR instruments), the NMR results are consistent trant functional groups in CT tillage. In sum, tillage
management can substantially change the quantity andwith the DRIFT and LF data where at least three repli-
cates were used. Further, our NMR results are in agree- quality of SOM as reflected by relatively high contents
of LF materials and more biologically active SOM inment with that reported by other investigators for simi-
lar types of SOM research (Stearman et al., 1989; soils under CnT. Structural changes of SOM may change
the sorptive behavior of pesticides in soils and theirPreston et al., 1994; Francioso et al., 2000). Thus, the
HA structural and compositional differences observed fates, efficiency, and uses. Future research has to address
the relationship between spectroscopic characteristicsby NMR are most likely because of the tillage treatments.
From the data of LF, NMR, and DRIFT results of and their agricultural significance.
HA, the SOM in the top soil layer (0–5 cm) of CT plots
seems more chemically and physically stable than CnT. ACKNOWLEDGMENTS
This is consistent with the results of Stearman et al. This work was supported in part by the USDA, National
(1989), Wander et al. (1994), and Wander and Traina Research Initiative Competitive Grants Program (97-35102-
(1996) that the CT soil had a greater proportion of all 4201 and 98-35107-6319), the Federal Hatch Program (Project
SOM in the more stable fraction. From a more practical No. MAS00773), and a Faculty Research Grant from Univer-
point of view, CnT managements cannot only maintain sity of Massachusetts, Amherst. We thank Dr. H.B. Gunner
the levels of SOC, but also substantially improve soil and Dr. W.A. Torello for their useful comments and Dr. L.C.
quality as reflected by reactive HA and high content of Dickinson for his technical support. Two anonymous review-
LF materials. ers and Dr. Jon Chorover (associate editor) are gratefully
acknowledged for their constructive comments.
From the discussion above, it is clear that the tillage
treatments have changed the chemical composition and
structure of SOM. This can potentially affect pesticide REFERENCES
fate and efficacy in soil. Nanny and Maza (2001) re- Ahmad, R., R.S. Kookana, A.M. Alston, and J.O. Skjemstad. 2001.
ported that the HA aromaticity and the solution pH The nature of soil organic matter affects sorption of pesticides. 1.
influenced noncovalent interactions between HA and Relationships with carbon chemistry as determined by
13
C CPMAS
NMR spectroscopy. Environ. Sci. Technol. 35:878–884.
monoaromatic compounds. Celis et al. (1997) also re- Alvarez, R., M.E. Russo, P. Prystupa, J.D. Scheiner, and L. Blotta.
ported that sorption of atrazine and simazine was higher 1998. Soil carbon pools under conventional and no-tillage systems
on Fluka HA than on soil HA, even though the organic in the Argentine Rolling Pampa. Agron. J. 90:138–143.
C contents of the two HAs were nearly the same. More- Bellamy, I. 1975. The Infrared Spectra of Complex Molecules. 3rd
ed., Vol. 1. Chapman and Hall, London.
over, sorption of organic compounds was positively cor-
Busscher, W.J., and R.E. Sojka. 1987. Enhancement of subsoiling
related with aromaticity and negatively with polarity of effect on soil strength by conservation tillage. Trans. ASAE 30:
SOM (Xing et al., 1994a,b,c; Xing, 1997; Ahmad et al., 888–892.
2001). Sorption of several pesticides by the soils from Campbell, C.A., V.O. Biederbeck, B.G. McConkey, D. Curtin, and
both CnT and CT plots is currently under investigation. R.P. Zentner. 1999. Soil quality-effect of tillage and fallow fre-
quency. Soil organic matter as influenced by tillage and fallow
frequency in a silt loam in southwestern Saskatchewan. Soil Biol.
Biochem. 31:1–7.
CONCLUSIONS Campbell, C.A., B.G. McConkey, R.P. Zentner, F. Selles, and D.
Examinations of characteristics of LF material of a Curtin. 1996. Long-term effects of tillage and crop rotations on soil
organic C and total N in a clay soil in southwestern Saskatchewan.
Norfolk soil under long-term CnT and CT management Canadian J. Soil Sci. 76:395–401.
indicated that tillage can influence the distribution of Capriel, P. 1997. Hydrophobicity of organic matter in arable soils:
LF in soil profile. The CnT treatment favored the build- Influence of management. Eur. J. Soil Sci. 48:457–462.
up of surface plant residue which consequently in- Celi, L., M. Schnitzer, and M. Negre. 1997. Analysis of carboxyl groups
in soil humic acids by a wet chemical method, Fourier transform
creased the SOC and soil-TCN contents in the top soil
DING ET AL.: TILLAGE MANAGEMENT AFFECTING SOIL ORGANIC MATTER 429
infrared spectrophometry, and solution-state carbon-13 nuclear Niemeyer, J., Y. Chen, and J.M. Bollag. 1992. Characteristics of humic
acids, composts, and peat by diffuse reflectance Fourier transformmagnetic resonance. A comparative study. Soil Sci. 162:189–197.
Celis, R., J. Cornejo, M.C. Hermosin, and W.C. Koskinen, 1997. Sorp- infrared spectroscopy. Soil Sci. Soc. Am. J. 56:135–140.
Novak, J.M., D.W. Watts, and P.G. Hunt. 1996. Long term tillagetion–desorption of atrazine and simazine by model soil colloidal
components. Soil Sci. Soc. Am. J. 61:436–443. effects on atrazine and fluometuron sorption in Coastal Plain soils.
Agric. Ecosyst. Environ. 60:165–173.Chen, Z., and S. Pawluk. 1995. Structural variations of humic acids
in two sola of Alberta Mollisols. Geoderma 65:173–193. Painter, P., M. Starsinic, and M. Coleman. 1985. Determination of
functional groups in coal by Fourier transform interferometry. p.Ding, G., D. Amarasiriwardena, S. Herbert, J. Novak, and B. Xing.
2000. Effect of cover crop systems on the characteristics of soil 169–241. In J.R. Ferraro and L.J. Basile (ed.) Fourier transform
spectroscopy. Application to chemical systems. Vol. 4. Academichumic substances. p. 53–61. In E.A. Ghabbour and G. Davis (ed.)
Humic substances: Versatile components of plants, soil and water. Press, Orlando, FL.
Preston, C.M., R.H. Newman, and P. Rother. 1994. Using
13
C CPMASThe Royal Society of Chemistry, Cambridge.
Ding, G., J.-D. Mao, S. Herbert, D. Amarasiriwardena, and B. Xing. NMR to assess effects of cultivation on the organic matter of
particle size fractions in a grassland soil. Soil Sci. 157:26–35.2001. Spectroscopic evaluation of humin changes in response to
soil managements. p. 261–270. In E.A. Ghabbour and G. Davis Preston, C.M., R.H. Newman. 1995. A long-term effect of N fertiliza-
tion on the
13
C CPMAS NMR of de-ashed soil humin in a second-(ed.) Humic substances: Structures, models and functions. The
Royal Society of Chemistry, Cambridge. growth Douglas-fir stand of coastal British Columbia. Geoderma
68:229–241.Doran, J.W. 1980. Soil microbial and biochemical changes associated
with reduced tillage. Soil Sci. Soc. Am. J. 44:765–771. Preston, C.M. 1996. Application of NMR to soil organic matter analy-
sis: History and prospects. Soil Sci. 16: 829–852.Francioso, O., C. Ciavatta, S. Sanchez-Cortes, V. Tugnoli, L. Sitti,
and C. Gessa. 2000. Spectroscopic characterization of soil organic Schmidt-Rohr, K., and H.W. Spiess. 1994. Multidimensional solid-
state NMR and polymers. Academic Press, London.matter in long-term amendment trials. Soil Sci. 165:495–504.
Franzluebbers, A.J., and M.A. Arshad. 1996. Soil organic matter pools Soil and Water Conservation Society. 1995. Farming for a better
environment-A whitepaper. Soil Water Conserv. Soc.Ankeny, IA.with conventional and zero tillage in a cold, semiarid climate. Soil
Stearman, G.K., R.J. Lewis, L.J. Tortorelli, and D.D. Tyler. 1989.
Till. Res. 39:1–11.
Characterization of humic acid from no-tilled and tilled soils using
Greer, K.J., D.W. Anderson, and J.J. Schoenau. 1996. Soil erosion,
carbon-13 nuclear magnetic resonance. Soil Sci. Soc. Am. J. 53:744–
organic matter decline and soil quality indicators. p. 21–33. In G.M.
749.
Coen and H.S. Vanderpluym (ed.) Proc. of Soil Quality Assessment
Stevenson, F.J. 1994. Humus chemistry: Gensis, composition, reac-
for the Prairies Workshop, Edmonton, Alberta.
tions. 2nd ed. John Wiley and Sons, New York.
Gressel, N., Y. Inbar, A. Singer, and Y. Chen. 1995. Chemical and Wander, M.M., S.J. Traina, B.R. Stinner, and S.E. Peters. 1994. Or-
spectroscopic properties of leaf litter and decomposed organic mat- ganic and conventional management on biologically active organic
ter in the Carmel range, Israel. Soil Biol. Biochem. 27:23–31. matter pools. Soil Sci. Soc. Am. J. 58:1130–1139.
Guggenberger, G., B.T. Christensen, and W. Zech. 1994. Land-use Wander, M.M., and S.J. Traina. 1996a. Organic fractions from organi-
effects on the composition of organic matter in particle-size sepa- cally and conventionally managed soils: I. Carbon and nitrogen
rates of soil: I. Lignin and carbohydrate signature. Eur. J. Soil Sci. distribution. Soil Sci. Soc. Am. J. 60:1081–1087.
45:449–458. Wander, M.M., and S.J. Traina. 1996b. Organic fractions from organi-
Hatcher, P.G., M. Schnitzer, L.W. Dennis, and G.E. Maciel. 1981. cally and conventionally managed soils: II. Characterization. Soil
Aromaticity of humic substances in soils. Soil Sci. Soc. Am. J. 45: Sci. Soc. Am. J. 60:1087–1094.
1089–1094. Wander, M.M., and X. Yang. 2000. Influence of tillage on the dynamics
Hu, S., D.C. Coleman, M.H. Beare, and P.H. Hendrix. 1995. Soil of loose and occluded particulate and humified organic matter
carbohydrates in aggrading and degrading ecosystems: Influences fractions. Soil Biol. Biochem. 32:1151–1160.
of fungi and aggregates. Agric. Ecosyst. Environ. 45:77–88. Weil, R.R. 1992. Inside the heart of sustainable farming. The New
Hunt, P.G., T.A. Matheny, R.B. Campbell, and J.E. Parsons. 1982. Farm. 14 Jan. 43-48.
Ethylene accumulation in southeastern Coastal Plain soils: Soil Xing, B., W.B. McGill, and M.J. Dudas. 1994a. Cross-correlation of
characteristics and oxidative-reductive involvement. Comm. Soil polarity curves to predict partition coefficient of nonionic organic
Sci. Plant Anal. 13:267–278. contaminants. Environ. Sci. Technol. 28:1929–1933.
Hunt, P.G., D.L. Karlen, T.A. Matheny, and V.L. Quisenberry. 1996. Xing, B., W.B. McGill, and M.J. Dudas. 1994b. Sorption of ␣-naphthol
Changes in carbon content of a Norfolk loamy sand after 14 years onto organic sorbents varying in polarity and aromaticity. Chemo-
of conservation and conventional tillage. J. Soil Water Conserv. sphere 28:145–153.
51:255–258. Xing, B., W.B. McGill, M.J. Dudas, Y. Madam, and L.G. Hepler.
Inbar, Y., Y. Chen, and Y. Hadar. 1989. Solid state carbon-13 nuclear 1994c. Sorption of phenol by selected biopolymers: isotherms, ener-
magnetic resonance and infrared spectroscopy of composted or- getics, and polarity. Environ. Sci. Technol. 28:466–473.
ganic matter. Soil Sci. Soc. Am. J. 53:1695–1701. Xing, B. 1997. The effect of the quality of soil organic matter on
Ismail, I., R.L. Blevins, and W.W. Frye. 1994. Long-term no-tillage sorption of naphthalene. Chemosophere 35:633–642.
effects on soil properties and continues corn yields. Soil Sci. Soc. Xing, B., and Z. Chen. 1999. Spectroscopic evidence for condensed
Am. J. 58:193–198. domains in soil organic matter. Soil Sci. 164:40–47.
Janzen, H.H., C.A. Campbell, S.A. Brandt, G.P. Lafond, and L. Tow- Xing, B., J. Mao, W-G. Hu, K. Schmidt-Rohr, G. Davies, and E.A.
nley-Smith. 1992. Light fraction organic matter in soils from long- Ghabbour. 1999. Evaluation of different solid-state
13
C NMR tech-
term crop rotations. Soil Sci. Soc. Am. J. 56:1799–1806. niques for characterizing humic acids. p. 49–61. In G. Davies and
Knicker, H., and P.G. Hatcher. 1997. Survial of protein in an organic- E.A. Ghabbour (ed.) Understanding humic substances: Advanced
rich sediment. Possible protection by encapsulation in organic mat- Methods, properties and applications. The Royal Society of Chem-
ter. Naturwissenschaften. 84:231–234. istry, Cambridge.
Mao, J.D., W-G. Hu, K. Schmidt-Rohr, G. Davis, E.A. Ghabbour, and Yakovchenko, V.P., L.J. Sikora, and P.D. Millner. 1998. Carbon and
B. Xing. 2000. Quantitative characterization of humic substances by nitrogen mineralization of added particulate and macroorganic
solid-state carbon-13 nuclear magnetic resonance. Soil Sci. Soc. matter. Soil Biol. Biochem. 30:2139–2146.
Am. J. 64:873–884. Zang, X., J. Van Heemst, D.H. Jasper, K.J. Dria, and P.G. Hatcher.
Nanny, M.A., and J.P. Maza, 2001. Noncovalent interactions between 2000. Encapsulaation of protein in humic acid from Histosols as
monoaromatic compounds and dissolved humic acids: A deuterium an explanation for the occurrence of organic nitrogen in soil and
sediment. Org. Geochem. 31:679–695.NMR T
1
relaxation study. Environ. Sci. Technol. 35:379–384.
