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Soil Biology & Biochemistry 32 (2000) 1151±1160
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In¯uence of tillage on the dynamics of loose- and occludedparticulate and humi®ed organic matter fractions
Michelle M. Wander a,*, Xueming Yang b
a

Department of Natural Resources and Environmental Sciences, College of Agriculture, University of Illinois-Urbana, 1102 South Goodwin Avenue,
Urbana, IL 61801, USA
b
Department of Land Resource Science, University of Guelph, Richards Building, Guelph, Ont., Canada, N1G 2W1
Received 19 May 1999; received in revised form 11 October 1999; accepted 3 March 2000

Abstract
This study was carried out at a site (®ne silty mixed mesic Argiaquic Argialboll) where use of no-tillage (NT) practices, for
over a decade, had not increased soil organic carbon (SOC) sequestration relative to plots that had been moldboard plowed
(MP). Even though the total SOC contents of these soils were known to be similar, we expected input and decay rates of
residue-derived C, conservation of root-derived C, and the importance of aggregate protection of particulate organic matter
(POM) to di€er among these tillage treatments. Corn (Zea maize L.) was pulse-labeled with 13CO2 repeatedly during the 1995
growing season to allow the fate of residue-derived C retained in loose-POM (LPOM), aggregate-occluded POM (OPOM) and
in mineral associated humi®ed (HF) fractions, to be tracked through April 1997. Tillage practices were related to fundamental

di€erences in the depth and rate at which residues decayed and the distribution of those residues among SOC fractions. In
December 1995, approximately 50% of the C derived from labeled residues was recovered in the LPOM, OPOM, and HF
fractions of the NT plots, while only 22% was recovered in those fractions from MP plots. After initial rapid losses of labelderived C, C turnover rates were relatively slow in the MP plots compared to C turnover rates observed at the surface of the
NT plots. As a result, after 1.5 years the MP and the NT plots retained similar amounts (110%) of label-derived C in the 0±20
cm depth. Shifts in the percent label recovery suggest that newly assimilated C was rapidly lost from the LPOM fraction as it
accumulated in the OPOM and HF fractions. Increases in the fractional abundance of label-derived C in the OPOM and HF
fractions accounted for approximately half of the label lost from LPOM. Trends in both the fractional abundance and percent
label recovery in the OPOM and HF fractions indicated that C derived from 1995-residues was concentrated at 0±5 cm depth in
NT plots and was more evenly distributed in the MP plots. In December 1995, the fractional abundance of OPOM and HF was
greater in the root than shoot labeled plots, indicating that root-derived C was incorporated into SOC more rapidly than shootderived materials. By spring, the fractional abundance of OPOM and HF had increased in tilled plots amended with labeled
shoots. Our fractionation scheme revealed the in¯uence of aggregation on the decay dynamics of C introduced by newly
incorporated residues and identi®ed fundamental di€erences in the depth, decay dynamics and distribution of C, newly
assimilated into the SOC fractions of NT and MP soils. 7 2000 Elsevier Science Ltd. All rights reserved.
Keywords: C sequestration; Tillage; Physical protection; Particulate organic matter;

1. Introduction
In situations where erosion is not a major factor,

* Corresponding author. Tel.: +1-217-333-9471; fax: +1-217-2443219.
E-mail address: mwander@uiuc.edu (M.M. Wander).

13

Carbon

the use of no-tillage (NT) practices for a decade or
more has not always increased soil organic carbon
(SOC) contents relative to conventionally tilled soils
(Havlin et al., 1990; Franzluebbers and Arshad, 1996;
Angers et al., 1997; Alvarez et al., 1998; Wander et al.,
1998). This atypical e€ect of NT practices on SOC
sequestration indicates that either C inputs are reduced

0038-0717/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 8 - 0 7 1 7 ( 0 0 ) 0 0 0 3 1 - 6

1152

M.M. Wander, X. Yang / Soil Biology & Biochemistry 32 (2000) 1151±1160

and/or net C mineralization rates are similar to those
occurring in tilled soils. Cold and wet soil conditions,
soil compaction, and root diseases can reduce crop
productivity in soils under NT management (Grith et
al., 1988) and therefore reduce the quantity of C
returned to the soil. Additionally, the manner in which
residues are incorporated into the soil can in¯uence
fate of C. Frequently, SOC concentrations increase at
the surface of NT soils as a result of residue concentration and erosion abatement. In some cases, decay
rates of residues placed at the soil surface are slow
compared to residues that are incorporated (Beare et
al., 1993; Parton et al., 1994). However, surface residues appear to decay rapidly at the surface of soils
where moisture and nutrient status are non-limiting
(Scott et al. 1996; Alvarez et al., 1998). Yang and
Wander (1999) found that in the spring after soybean
production in a corn soybean rotation, the quantity of
residues did not di€er signi®cantly among plots that
had been under MP and NT management for over a
decade. Disappearance of surface residues might have
resulted from decay or their incorporation by earthworm activity.

According to Linden and Clapp (1998), enhanced
earthworm activity in NT soils can increase SOC as a
result of increased casting, or decrease SOC as a consequence of enhanced mineralization of residues within
earthworm gut and casts. In tilled soils, residues introduced from above ground are mixed into the plow
depth, while the soil typically is simultaneously loosened and oxygenated. It is well known that this practice stimulates the mineralization of both residues and
native SOC. However, the short- and long-term in¯uences of disturbance on C mineralization are complex
and may vary among soils. Hu et al. (1995), Franzleubbers and Arshad (1996) and Alvarez et al. (1998)
found net C mineralization rates to be greater in NT
than in tilled soils. Tillage's ability to accelerate organic matter mineralization may be tempered by
edaphic factors. The stimulatory e€ects of tillage on C
mineralization can be minimized when residues were
buried in fall in cool or moist regimes (Franzleubbers
and Arshad, 1996; Angers et al., 1997). Greater knowledge of the C dynamics occurring after tillage are
needed to understand and accurately predict its e€ects
on SOC sequestration.
Balesdent and Balabane (1996) concluded that roots
were major contributors to soil carbon storage in
maize cultivated soils. They argued that root contributions to SOC were greater than shoot contributions
because root matter decayed more slowly than shoot
matter. The slower decay of root-derived SOC was

attributed to its composition and greater physical protection by soil minerals. Roots may be greater contributors to SOC in NT than in tilled soils receiving large
inputs of C as incorporated residues. W.J. Gale and

C.A. Cambardella (National Soil Tilth Lab, Iowa, personal communication) have shown that root inputs
play a disproportionately large role in the formation
and stabilization of aggregates in non-disturbed soil
systems such as NT. The increased physical protection
of SOC by aggregates in NT may explain how NT
soils frequently maintain SOC contents equal to or
greater than amounts maintained in tilled soils (Beare
et al., 1994). According to Angers (1998), aggregate
protection of particulate or macro organic matter
plays a particularly important role in the C equilibrium levels achieved by ®ne-textured soils with inherently high SOC contents.
Our purpose was to identify factors controlling C
turnover in a site where use of NT practices for over a
decade had not increased SOC sequestration (Yang
and Wander, 1999). Findings from that and other studies (Wander et al., 1998; Needelman et al., 1999)
suggest that application of NT practices to Illinois
soils frequently increases SOC concentrations in the
top few cm of the soil while depleting the SOC contents of the lower rooting zone. To gain insight into

the C dynamics of NT and MP plots, we labeled corn
shoot and root residues in situ by pulse-labeling growing corn with 13CO2. We anticipated that: the inputs
and decay rates of crop-derived C would be lesser in
the NT than in MP plots, that root-derived C would
be a greater contributor to SOC in the NT than in MP
plots, and that aggregate protection of macroorganic
matter would be greater in the NT than in MP plots.

2. Materials and methods
2.1. Labeling and soil sampling
This experiment was conducted at a long-term tillage
experiment established in 1986 that is located on the
Agricultural Engineering Research Farm of the University of Illinois at Urbana, IL. The soil is a Thorp
silt loam (US Taxonomy: ®ne-silty, mixed, mesic
Argiaquic Argialboll; FAO classi®cation: Orthic Greyzem). This work was carried out in NT and MP plots
randomized in four experimental blocks in plots producing corn (Zea mays L.) in 1995 and soybean [Glycine max (L) Merr.] in 1996. The only soil disturbance
in the NT treatment occurred during planting operations. After corn harvest, the MP plots were moldboard-plowed (20±25 cm deep) followed by spring
disking (7.5±10 cm deep). After soybean harvest, the
MP plots were fall chisel-plowed (30±35 cm) followed
by spring disking.

Corn was 13C pulse-labeled by enclosing sections of
a plant-row (microplots) with a portable chamber and
introducing 13CO2 during the 1995 growing season.
Weekly during the early spring and twice monthly

M.M. Wander, X. Yang / Soil Biology & Biochemistry 32 (2000) 1151±1160

during the summer, crops in microplots were exposed
to 99-atom% 13CO2 (Cambridge Isotopes, Cambridge,
MA) using a portable chamber. The chamber was
loosely modeled after the design of Berg et al. (1991).
A 1 m2 PVC frame, adjustable in height, was covered
with a tedlar bag equipped with a gas-tight septum
(US Plastics). The frame was attached to a metal base
that was forced into the soil (5 cm). The base was
equipped with ports that attached to a recirculating
cooling system. The entire system was constructed to
®t between 75 cm crop rows. The quantity of 13CO2
introduced on each date was based on estimates of
canopy leaf area and maximum photosynthetic rate.

Plots for ``reciprocal transfer'' of unlabeled surface
residues were established adjacent to labeled plots.
These reciprocal transfer plots were treated identically,
except that non-labeled CO2 was vented through the
chamber. Labeling was carried out between 11:00 h
and 14:00 h on all dates; the labeling sequence was
varied to avoid bias. Four replicates were established
for each tillage and labeling treatment (16 plots).
Corn residues were harvested in November from
labeled and unlabeled plots before tillage. Composites
of the eight labeled and unlabeled residues were
weighed, clipped into 5±15 cm lengths, divided into
eight equal parts (425 g plot), and transferred reciprocally to establish `shoot' and `root' labeled plots. We
assumed the mass of shoot and root-derived residues
were equal (Buyanovsky and Wagner, 1986) and,
based upon the work of Gregorich et al. (1995) and
Wander (unpublished data), that the isotopic composition of shoots and roots were equal. Soil samples
were collected after fall ®eld operations on 7 December
1995, before seeding soybean on 20 May 1996, and
again before corn planting on 21 April 1997, using a

splitable soil sampler of 4.9 cm diameter. Sampling
intensity increased from one sample collected from
each plot in 1995 to three random replicates per plot
in 1996 and, further to nine replicates in 1997. All
samples were divided into 0±5 and 5±20 cm increments, weighed, sieved (8 mm) and air-dried. Subsamples were dried at 1058C to determine soil moisture
for bulk density determination.
2.2. Soil organic matter fractionation
Particulate organic matter (POM), not protected by
aggregates (LPOM), was separated from samples using
a modi®ed version of the method described by Golchin
et al. (1994). The method was adapted to maximize
recovery of macroaggregates. A 20 g soil sample was
placed in a 250 ml Oakridge tube, to which 50 ml
sodium polytungstate (1.6 g cmÿ3; Geoliquids, Chicago, IL) was added. The tube was orbitally shaken at
low speed (200 oscillations minÿ1) for 30 min. Particles
adhering to the side of the tube were rinsed free with

1153

10 ml of sodium polytungstate, and the solution was

allowed to stand overnight. Tubes were then centrifuged. Materials recovered from the supernatant on
0.5 mm polycarbonate ®lters were rinsed with 50 ml of
deionized water and dried at 808C. To obtain the
aggregate occluded POM (OPOM) and the relatively
humi®ed, mineral associated fraction (HF), the soil
remaining in the tube was further shaken at high speed
(350 oscillations minÿ1) for 60 min. The suspension
was ®ltered though a 53 mm polyester mesh (Gilson,
Columbus, OH). The OPOM captured on the mesh
and the HF fraction passing through the 53 mm sieve
were collected, dried in an oven at 808C, weighed and
ground with a disk mill.
2.3. Analysis
Carbon and carbon isotope ratios of whole soil and
SOC fractions were determined at the University of
Saskatchewan, Canada with a Continuous Flow Isotope Ratio Mass Spectrometer (CF-IRMS) using a
TracerMass mass spectrometer interfaced with a
RoboPrep combustion system (Europa Scienti®c,
Crewe, UK). The isotopic composition of whole soil
and SOC fractions were expressed in delta `d' units

where d=[(RSample/RPDBÿ1)  1000], when RSample is
the sample ratio of 13C/12C, and RPDB=0.0112372
(based on the Pee Dee Bee standard) (Wolf et al.,
1994). Soil samples that were collected from unlabeled
plots between residue harvest and amendment were
fractionated as described previously. Those fractions,
which included the 1994 inputs of non-labeled corn
roots, were used to establish control values to determine the fractional abundance of C derived from
labeled residues …dLRES-95ˆ 159:36-† in SOC fractions.
The fractional abundance in the LPOM, OPOM, and
HF fractions …fLPOM , fOPOM and fHF † was taken to be
equal to …dSample ÿ dControl )/(dLRES-95 ÿ dControl † (Wolf et
al., 1994). The 1995 d values of individual unlabeled
fractions were used as the controls for 1995 and 1996
samples. During that period, there were no other organic matter inputs. For the 1997 controls, we subtracted 1.4- from all SOC fraction values to re¯ect
the inputs of soybean. The magnitude of this correction factor was based upon the d of SOC fractions collected in Spring of 1994, before corn was seeded, and
on the magnitude of change observed in the isotopic
content of the whole soil. The percent recovery of
labeled residues in LPOM, OPOM and HF fractions
was computed by multiplying fractional abundance by
the C concentration in the fraction, fraction concentration in soil, soil bulk density and the volume of soil
amended with residue: 30,000 cm3 in the 0±5 and
90,000 cm3 in the 5±20 cm depth. This sum was then
divided by the quantity of C applied in labeled residues and then expressed as a percentage. Tillage, date,

1154

Factor

Tillage
Depth
Tillage  depth
Date
Date  depth
Date  tillage
Date  depth  tillage
Origina
Tillage  origin
Date  origin
Depth  origin
Date  origin  tillage
Tillage  depth  origin
a

LPOM

OPOM

HF

d13 C

mg C g fractionÿ1

fLPOM

RECLPOM

d13 C

mg C g fractionÿ1

fOPOM

RECOPOM

d13 C

mg C g fractionÿ1

fHF

RECHF

P value
0.841
0.336
0.093
0.001
0.805
0.459
0.116
D
D
D
D
D
D

0.284
0.001
0.001
0.020
0.770
0.961
0.026
0.712
D
D
D
D
D

0.840
0.340
0.097
0.001
D
D
D
0.390
D
D
D
D
D

0.460
0.307
D
0.006
0.403
0.342
D
0.211
D
D
0.077
D
D

0.467
0.335
0.180
0.001
0.613
0.009
0.012
0.244
0.466
0.007
D
0.026
D

0.284
0.001
0.0001
0.025
0.770
0.961
0.062
D
D
D
D
D
D

0.450
0.335
0.183
0.009
0.612
0.009
0.012
0.320
0.615
0.001
D
0.046
D

0.032
0.0001
0.448
0.007
0.085
0.224
0.005
0.291
0.369
D
D
D
D

0.911
0.175
0.026
0.001
0.713
0.027
0.008
0.049
0.043
0.438
0.713
0.038
0.028

0.956
0.153
0.137
0.008
0.089
0.064
D
0.827
0.661
D
0.432
D
0.087

0.824
0.183
0.028
0.0001
0.722
0.028
0.009
0.052
0.033
0.448
D
0.052
D

0.633
0.0001
0.077
0.0001
D
0.097
0.008
0.447
D
D
D
D
D

Label was of corn shoot or root origin.

M.M. Wander, X. Yang / Soil Biology & Biochemistry 32 (2000) 1151±1160

Table 1
Probability values summarizing the in¯uences of tillage, depth, date, and label origin (shoot versus root) on the isotopic composition …d13 C), carbon contained in (mg C g fraction), and fractional
abundance ( fLPOM) and percent recovery (REC) of C derived from label in loose particulate organic matter (LPOM), occluded-particulate organic matter (OPOM), and humi®ed material (HF)
MP 5±20 cm > MP 0±5 cm > NT 5±20,
re¯ected residue placement patterns (Table 3, Fig. 3a,
inset). Heterogeneity in the isotopic composition and
relatively limited sample numbers in 1995 prevented
observation of any treatment-based di€erences in the
LPOM that might have existed (n = 32, mean=26.5-

Table 2
The isotopic composition of unlabeled and 13C-labeled corn residues,
bulk soils, and loose-particulate organic matter (LPOM), occludedparticulate organic matter (OPOM), and the humi®ed fraction (HF)

Fig. 2. Temporal change in (a) percent recovery and (b) the fractional abundance of label-derived C in loose- and occluded-particulate organic matter (LPOM and OPOM), and in mineral associated
fractions (HF). For individual fractions, di€erences between letters
identify means that di€er at P < 0.05.

Table 3
Tillage and depth e€ects on the loose particulate organic matter
(LPOM) isotopic composition and fractional proportion of C derived
from labeled residues. Values are means from all dates
SOC fraction

Material

MP

NT

Isotopic composition
0±5 (cm)
Control-unlabeled

13

1995

1995

5±20 (cm)

0±5 (cm)

5±20 (cm)

ÿ32aba

3.37ab

9.73a

ÿ3.67b

ÿ18.09a
ÿ12.16d
ÿ17.33ab

ÿ16.99ab
ÿ14.76bc
ÿ15.25b

ÿ14.01c
ÿ14.79bc
ÿ15.41b

ÿ15.48ab
ÿ14.51bc
ÿ17.66bc

ÿ17.13bc
ÿ15.47d
ÿ17.93abc

ÿ16.87c
ÿ15.73cd
ÿ17.65abc

ÿ16.28cd
ÿ16.20d
ÿ17.66b

ÿ16.99bc
ÿ15.98d
ÿ18.31a

C-labeled
d13 C
1996

1997

d13C (-)
Corn residues
Bulk soil
SOC fractions
Loose-POM
Occluded-POM
HF

ÿ11.70ba
ÿ16.85ab

+159.36a ±
±
ÿ16.28a ÿ16.36a ÿ17.05b

ÿ15.26
ÿ19.45
ÿ17.31

+23.45ab ÿ6.56b ÿ13.13b
ÿ16.15a ÿ14.06b ÿ16.41a
ÿ16.82a ÿ15.85b ÿ17.89a

a
Values within rows followed by di€erent letters were signi®cantly
di€erent at P < 0.05.
b
Means are based on samples from both depths from all labeled
plots; where n, including subsamples, is equal to 16 in 1995, 48 in
1996, and 144 in 1997.

Loose POM
Mean of all dates
Occluded POM
1995
1996
1997
Humi®ed OM (HF)
1995
1996
1997

a
Means comparisons were made across tillage by depth (LPOM)
and tillage by depth by date (OPOM and HF) combinations; means
within fraction categories not followed by the same letter are signi®cantly di€erent at P < 0.05.

M.M. Wander, X. Yang / Soil Biology & Biochemistry 32 (2000) 1151±1160

1157

, std=47.5-). Tillage and depth interacted with time
to a€ect dOPOM and fOPOM (Table 1, Fig. 3). Initially,
the dOPOM and fOPOM were ranked NT 0±5 cmrNT 5±
20 cm r MP 5±20 cm r MP 0±5 cm and both dOPOM
and fOPOM were greater in the NT 0±5 cm than in
either MP depth and were greater in the NT 5±20 cm
than in the MP 0±5 cm depth. By spring 1996, dOPOM
and fOPOM had increased signi®cantly in the MP 0±5
cm and slightly in the MP 5±20 cm depths and had
declined (0±5 cm) or remained unchanged (5±20 cm) in
the NT plots. The following spring, dOPOM and fOPOM
of the MP 5±20 and NT 0±5 cm depths remained
unchanged but had declined in the MP 0±5 and NT 5±
20 cm depths. The decline in dOPOM in the NT 5±20 cm
depth was not signi®cant. Temporal trends in the dHF
and fHF were generally similar to trends in OPOM
(Table 3, Fig. 3). Values for both dHF and fHF were initially greatest in the 0±5 cm depth of NT and least in
the 0±5 cm depth of MP treatments. By May 1996,
dHF and fHF had not changed in the NT 0±5 cm depth

and had increased in the other samples; the increase in
fHF in the MP 5±20 cm depth was not signi®cant. As
was true for OPOM, the relative increase in dHF and
fHF that occurred during the ®rst winter was greatest in
the 0±5 cm depth of the MP plots. After soybean production, the dHF of all samples and the fHF of the NT
5±20 cm depth had declined signi®cantly.
Our ability to recover label within the LPOM fraction did not vary signi®cantly among tillage treatments, soil depths or sampling dates. The in¯uence of
those factors on C dynamics is revealed by label recovery within the OPOM and HF fractions (Table 1,
Fig. 4). Shortly after harvest, percent recovery in the
OPOM and HF from the 0±5 cm depth of the MP
plots was less than that recovered in fractions from
other treatment by depth combinations. The amount
of label-derived C recovered as OPOM and HF
increased between Dec. 1995 and May 1996 in both
depths of the MP and in the 5±20 cm depth of the NT
plots. Within a year, the amount of label recovered in

Fig. 3. Temporal change in the fractional abundance of label-derived
C recovered in the (a) loose- ( fLPOM, see inset) and occluded-particulate organic matter ( fOPOM), and (b) humi®ed fractions ( fHF) of the
0±5 and 5±20 cm depths of moldboard plowed (MP) and no-tilled
(NT) plots. For individual fractions, di€erences between letters identify means that di€er at P < 0.05.

Fig. 4. Temporal change in tillage and depth e€ects on the percent
recovery of label-derived C in the loose- and occluded-particulate organic matter (LPOM and OPOM), and humi®ed (HF) fractions.
Di€erences between lower case letters identify OPOM means and
upper case letters identify HF means that di€er at P < 0.05.

1158

M.M. Wander, X. Yang / Soil Biology & Biochemistry 32 (2000) 1151±1160

the 0±5 cm depth of the MP and 5±20 cm depth of the
NT plots had declined to 1995 values, while recovery
in the OPOM and HF from the MP 5±20 and NT 0±5
cm depths remained unchanged. In general, about
60% of the label initially recovered was lost between
Dec. 1995 and spring 1997; a much greater proportion
of the label was initially recovered in the NT plots
than was lost from the MP plots.
Temporal trends in fOPOM and fHF were in¯uenced by
the interaction between tillage practices and label origin (Fig. 5). Initially, fOPOM and fHF were greatest in
the root-labeled NT plots. By May 1996, fOPOM had
increased in all MP and in the shoot-labeled NT plots,
while it declined in the root-labeled NT plots. Similarly, the fHF of the shoot-labeled MP and NT plots
increased during that time period. The fHF of the rootlabeled plots had increased, but not signi®cantly, by
May 1996. Only fOPOM and fHF of shoot labeled MP
plots declined signi®cantly during the 1996±1997 season.

4. Discussion
4.1. Labeling and fractionation of total, loose and
occluded SOC
During the 1995 growing season whole soil d
increased by 1.4±16.3- in unlabeled plots due to corn
production. The increase in 13C isotopic abundance is
consistent with the magnitude of change (1.6-)
observed after one corn crop by Qian and Doran
(1996). Pulse labeling only increased the whole soil d
values by an additional 0.58- but increases in
dcorn residue and dLPOM were sucient to allow shortterm C dynamics (1.5 years) to be assessed.
Our fractionation scheme revealed the in¯uence of
aggregate protection on the short-term decay dynamics
of C derived from newly-incorporated residues.
Reduced label recovery in the LPOM between Dec.
1995 and May 1996 coincided with increased total
SOC recovered in fractions and increased label recovery in the OPOM and HF fractions. The temporal
trends in all fractions were in¯uenced by the large
sieve size (8 mm) used to initially process the soil.
Macroorganic matter typically removed from soils
before quanti®cation of SOC was a focal point of this
study. The May 1996 peak in OPOM, HF and total C
in the MP and NT 5±20 cm depths likely re¯ects the
movement of C, that was in residues too large to
measure in Dec. 1995, into measured particulate and
humi®ed forms. Additionally, there was a net transfer
of label-derived C during the winter from LPOM and
crop residues, not yet incorporated into SOC fractions
in Dec. 1995, into the OPOM and HF fractions. Temporal changes in d, f, and percent label recovery indicate that LPOM decayed rapidly while OPOM and
HF were less dynamic. These ®ndings are consistent
with those of Beare et al. (1994), Gregorich et al.
(1995), Besnard et al. (1996) and Jastrow et al. (1996)
who showed that aggregate occluded-particulate organic matter (POM) has a slower turnover rate than
does POM that is loose and not protected by mineral
aliation.
4.2. Tillage and C dynamics

Fig. 5. Temporal change in the fractional abundance of C derived
from labeled shoots or roots in (a) occluded-particulate organic matter ( fOPOM) and (b) humi®ed fractions ( fHF) of moldboard plowed
(MP) and no-tilled (NT) plots. For individual fractions, di€erences
between letters identify means that di€er at P < 0.05.

At the onset of this experiment, we had anticipated
that the inputs and decay rates of crop-derived C
would be lesser in the NT plots. This assumption was
not supported by our ®ndings. In Dec. 1995, approximately twice as much label-derived C was recovered
from NT than MP plots, suggesting that less residuederived C had been added to the MP plots. The
amount of residue-derived C contained in the MP
plots could have been underestimated if residues were
buried below the depth of sampling in Dec. 1995. Chisel plowing of those plots in Dec. 1997 is likely to

M.M. Wander, X. Yang / Soil Biology & Biochemistry 32 (2000) 1151±1160

have further diluted our microplots with unlabeled
soil. Rapid mineralization of residues following their
incorporation by plowing could also explain the low
recovery of label-derived C from the MP plots (Prior
et al., 1997). By mixing residues into the soil in a fairly
concentrated mass, plowing can shift microbial decomposer communities from autochthonous, high eciency
species to more zymogenous populations with lower
metabolic eciencies (Bradley and Fyles, 1995).
Regardless of whether lower C-additions to the MP
plots were caused by mineralization or dilution of C
by unlabeled soil, less residue-derived C was initially
accounted for in the 0±20 cm depth of the MP plots.
During the next 1.5 years, more of the label initially
recovered was ultimately lost from the NT plots, indicating that during that period residue decay rates in
the top 20 cm depth were actually faster than rates
occurring in the MP plots.
4.3. The in¯uences of tillage, physical protection and
roots on SOC
We had expected that aggregate protection of
macroorganic matter and root contributions to SOC
would be greater in the NT than in MP plots. Our
®ndings indicate that the placement pattern, composition or origin of residue interacted with physical protection to in¯uence SOC sequestration. Initially, more
root- than shoot-derived C was incorporated into
OPOM and HF fractions at the surface of the NT
plots. This ®nding is consistent with the results of
Balesdent and Balabane (1996), who observed that
SOC derived from corn root-derived C was 1.5 times
that of stalks + leaves. They attributed this to
enhanced belowground production and to relatively
slow biodegradation of root-derived materials. In our
work, the relatively low fractional abundance of rootderived C in the OPOM and HF fractions recovered in
Dec. 1995 from tilled plots might re¯ect dilution by
mixing of label rich surface soil by depleted subsoil or
relatively low root productivity. The low fOPOM and fHF
of root-derived C in the 5±20 cm depth of the NT
plots at that time likely re¯ects limited inputs by roots.
The relatively high fOPOM and fHF in the 0±5 cm depth
of the NT plots then, probably re¯ect root concentration and shoot contributions to SOC. Between May
1996 and April 1997, the fractional abundance of rootderived C in OPOM and HF fractions did not decline.
The fOPOM and fHF of shoot-derived C increased signi®cantly by May 1996 in MP and to a lesser degree in
NT plots and declined again by April 1997 to equal
1995 values. While Richter et al. (1990) found that
root-derived C was a dominant factor in the C balance
of tilled soils and argued that tillage induced SOC depletion was associated with the preferential loss of
root-derived C, our data suggest that the tilled soil

1159

failed to conserve shoot-derived C. This could stem
from the incorporation of shoot residues as a mass, as
the proportion of newly-added C conserved in SOC
has been shown to decrease as the concentration of C
added increases (Bremer and Kuikman, 1994; JansHammermeister et al., 1997). Collectively these results
suggest shoot- and root-derived residues move between
SOC fractions at di€erent rates, that root-derived materials are more rapidly occluded by aggregates and
are likely to contribute to humic materials where roots
are concentrated. Besnard et al. (1996) showed that
aboveground and root residues were probably not incorporated homogeneously in organic matter fractions
and that more root-like macroorganic matter was preserved within aggregates than was retained in the free
POM. Overall, our ®ndings suggest that root-derived
SOC in OPOM and HF fractions may be more persistent in the long-term. Even though the total amount of
residue-derived C retained after 1.5 years did not di€er
in the MP and NT soils, there were fundamental
di€erences in the depth, decay dynamics and distribution of C newly-assimilated into SOC fractions.

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