Aggregation and Soil Organic Matter Accumulation in Cultivated and Native Grassland Soils

Abstract

Tillage intensity affects soil structure and the loss of soil organic C and N. We hypothesized that no-tillage (NT) and conventional tillage (CT) differentially affect three physically defined particulate organic matter (POM) fractions. A grassland-derived Haplustoll was separated into aggregates by wet sieving. Free light fraction (LF) and intra-aggregate POM (iPOM) were isolated. Natural abundance 13C was measured for whole soil C, free LF C, and iPOM C. The mean residence time of soil C under CT (44 yr) was 1.7 times less than in NT (73 yr). The amount of free LF C was 174, 196, and 474 g C m-2 for CT, NT, and NS, respectively. Total iPOM C amounts in CT, NT, and NS were 193, 337, and 503 g C m-2, respectively. The level of fine iPOM C (53-250 micrometer) level in macroaggregates (250-2000 micrometers) obtained after slaking was five times greater in NT vs. CT and accounted for 47.3% of the difference in total POM C between NT and CT. The amount of coarse iPOM C (250-2000 micrometers) was only 2.4 times greater and accounted for only 21% of the difference in total POM C. Sequestration of iPOM was observed in NT vs. CT, but free LF was not influenced by differential tillage. We conclude that differences in aggregate turnover largely control the difference in fine iPOM in CT vs. NT and consequently SOM loss is affected by both the amount of aggregation and aggregate turnover.

Full text

Aggregation
and
Soil Organic Matter Accumulation
in
Cultivated
and
Native Grassland Soils
J.
Six,*
E.T.
Elliott,
K.
Paustian,
and
J.
W.
Doran
ABSTRACT
Tillage
intensity
affects
soil structure and the loss of
soil
organic
C
and
N.
We
hypothesized that no-tillage (NT)
and
conventional
tillage
(CT)
differentially
affect
three physically defined
particulate
organic
matter (POM) fractions.
A
grassland-derived
Haplustoll
was
separated into aggregates
by wet
sieving. Free light fraction
(LF)
and
intra-aggregate
POM
(iPOM)
were isolated. Natural abundance
"C
was
measured
for
whole soil
C,
free
LF C, and
iPOM
C. The
mean
residence
time
of
soil
C
under
CT (44
yr)
was 1.7
times less than
in
NT (73
yr).
The
amount
of
free
LF C was
174,196,
and 474
g
C
m~2
for
CT, NT, and
NS,
respectively. Total iPOM
C
amounts
in CT, NT,
and
NS
were 193, 337,
and 503 g C
nr2,
respectively.
The
level
of
fine
iPOM C
(53-250
jim)
level in
macroaggregates
(250-2000
urn)
obtained
after
slaking
was
five
times greater
in NT vs. CT and ac-
counted
for
47.3%
of the
difference
in
total
POM C
between
NT and
CT.
The
amount
of
coarse iPOM
C
(250-2000
(Jim)
was
only
2.4
times
greater
and
accounted
for
only
21% of the
difference
in
total
POM
C.
Sequestration
of
iPOM
was
observed
in NT vs. CT, but
free
LF
was
not
influenced
by
differential tillage.
We
conclude
that differences
in
aggregate turnover largely control
the
difference
in
fine
iPOM
in
CT vs. NT and
consequently
SOM
loss
is
affected
by
both
the
amount
of
aggregation
and
aggregate turnover.
I
NCREASING
TILLAGE INTENSITY
enhances
the
turnover
of
soil organic matter (SOM)
and
decreases soil
ag-
gregation. The relationship between tillage intensity,
soil structure,
and SOM
dynamics
has
recently gained
much
attention
(Beare
et
al.,
1994;
Golchin
et
al.,
1994a,b;
Jastrow,
1996).
It has
been proposed that soil
aggregates physically protect certain
SOM
fractions,
re-
sulting
in
pools with longer turnover times
(Adu
and
Oades,
1978).
Since soil aggregates
are
sensitive
to
man-
agement practices, increased aggregate disruption due
to
tillage
may
lead
to
increased decomposition
of
SOM.
Reduced aggregation
and
increased turnover
of ag-
gregates
in CT vs. NT is a
direct
function
of the
immedi-
ate physical disturbance due to plowing and several
indirect
effects
on
aggregation (Paustian
et
al.,
1997).
First, tillage continually exposes
new
soil
to
wet-dry
and
freeze-thaw
cycles
at the
soil
surface
(Beare
et
al.,
1994; Paustian et
al.,
1997), thereby increasing the
susceptibility
of
aggregates
to
disruption. Second, plow-
ing
changes
the
soil conditions (e.g., temperature, mois-
ture, aeration)
and
increases
the
decomposition rates
of
the litter
(Rovira
and
Greacen,
1957;
Cambardella
and
Elliott, 1993). Third,
the
microbial
community
is
affected
by
plowing
and
litter
placement (Holland
and
Coleman,
1987).
No-tillage
may
promote
fungal
growth
and the
proliferation
of
fungal
hyphae
that contribute
to
macroaggregate
formation (Beare et
al.,
1993).
J.
Six, E.T. Elliott,
K.
Paustian, Natural Resource Ecology
Lab.,
Colo-
rado State Univ.,
Fort
Collins
CO
80523.
J.W.
Doran,
USDA-ARS,
Univ.
of
Nebraska, Lincoln,
NE
68583.
Received
21
Nov. 1997. *Corre-
sponding
author (johan@nrel.colostate.edu).
Published
in
Soil
Sci.
Soc.
Am. J.
62:1367-1377
(1998).
Defining
SOM
pools that relate
to
soil structure
and
delineating
SOM
fractions that
are
functionally mean-
ingful
are
important challenges
for
research
and are
necessary
for a
better
understanding
of SOM
dynamics.
For
example,
Elliott
et al.
(1996)
and
Christensen
(1996)
described
the
importance
of
differentiating
the
free
and
intraaggregate
SOM in
conceptual models
of
physically
based
SOM
pools.
Intraaggregate organic matter
is in-
corporated
and
physically stabilized within macroaggre-
gates (Cambardella
and
Elliott, 1992), while
free
organic
matter
is
found between aggregates. This
difference
in
position within
the
soil matrix
and the
resultant acces-
sibility
of SOM to
soil organisms leads
to
pools
that
differ
in
stability
and
dynamics (Golchin
et
al.,
1994a,b).
In
addition, mineralization studies
of
crushed
vs. in-
tact
aggregates indicate that aggregate-protected pools
of
C are
more labile than unprotected pools (Cambar-
della
and
Elliott, 1993) because protected pools
are
less
exposed
to
microbial decay (Beare
et
al.,
1994).
This
suggests
that
the
free
and
occluded
POM
(i.e.,
POM
within
aggregates)
are
functionally
different
pools
caused
by
positional
differences
within
the
soil matrix
(Golchin
et
al.,
1994a,b). Golchin
et al.
(1994a,b)
isolated
two
structurally defined fractions
of
POM:
a
fraction
occluded within aggregates
and a
free
fraction.
The
occluded
POM had
higher
C and N
concentrations
than the
free
POM. The higher
alkyl-C
and lower
O-
alkyl-C
content
in
occluded
POM
suggested
a
highly
decomposed state
in
comparison
with
free
POM.
In a
subsequent study, Golchin
et al.
(1995) suggested that
macroaggregates
are
stabilized mainly
by
carbohydrate-
rich
root
or
plant debris occluded
within
aggregates.
Angers
and
Giroux
(1996) provided
further
evidence
that
slake-resistant
macroaggregates
are
stabilized
by
recently
deposited residue. Jastrow (1996) suggested
that
the
intramacroaggregate
POM is an
important
agent that
facilitates
the
binding
of
microaggregates
into
macroaggregates.
The
objectives
of
this study were
(i)
to
isolate
SOM
fractions
that relate
to
soil structure,
(ii)
to
determine
the
main driving variables
for the
decomposition rate
of
these
SOM
fractions,
and
(iii)
to
study
the
influence
of
tillage intensity
on
processes related
to the
stabiliza-
tion
of
these fractions.
MATERIALS
AND
METHODS
Sampling
Mixed grassland-derived soil
was
collected from
an
agricul-
tural
tillage
experiment
located
at
Sidney,
NE
(41°14'
N,
103°
00'
W). The
soil type
is a
Duroc
loam
(fine-silty,
mixed,
mesic
Abbreviations:
CT,
conventional tillage;
LF,
light
fraction;
iPOM,
intraagregate
particulate organic matter;
MRT,
mean residence time;
mSOC,
mineral-associated organic
C; NS,
native
sod;
NSI,
normalized
stability
index;
NT, no
tillage; POM, particulate organic matter; SOM,
soil organic matter;
1367

1368
SOIL
SCI.
SOC.
AM.
J.,
VOL.
62,
SEPTEMBER-OCTOBER
1998
Table
1.
Some
general
differences
in
chemical
and
physical
properties
between
native
sod
(NS),
no-tillage
(NT),
and
conventional
tillage
(CT)
treatments
in the
Duroc
loam
at
Sidney,
NE.
Depth
Treatment
Texture
Organic
C
Organic
N
Sand
Clay
Silt
Bulk
density
MRTf
cm
0-5
5-20
0-20
NS
NT
CT
NS
NT
CT
NS
NT
CT
0 P 1
g
*-
'
1437
at
1129
b
699 c
2653
a
2299
a
2208
a
4090
a
3428
ab
2907
b
m
2
131 a
108 b
82 c
265 a
258 a
230 a
396 a
366 a
312 a
16
14
35
31
33
32
-
_
-
36
34
24
25
24
24
_
_
-
48
52
41
44
43
44
_
_
-
gem
3
0.82
all
1.05
b
1.18
b
1.12
a*
1.22
b*
1.28
b
_
_
-
yr
-
33
44
-
93
45
-
73
44
t
MRT
is the
mean residence time
of
total
soil
C
(yr).
I
Values followed
by a
different
lowercase letter
within
a
depth
are
significantly
different
(P
>
0.05) according
to
Tukey's
HSD
mean separation test.
II
Values followed
by * at the 5- to
20-cm
depth
are
significantly
different
compared
with
corresponding values
at 0 to 5 cm.
Pachic
Haplustoll)
developed
on
mixed loess
and
alluvium.
The site was a native grassland prior to 1969, when the experi-
ment
was
established. Within native
sod
(NS), three manage-
ment
treatments were established:
NT,
stubble mulch
(SM),
and
CT
(moldboard
plow, cultivator, rotary
rod-weeder)
in a
winter
wheat
(Triticum
aestivum
L.)-fallow rotation Fertilizer
was
not
applied.
The
experiment design
was a
randomized
complete block design with three
field
replicates.
For
further
details of site and experiment treatment characteristics see
Doran
et
al.
(1998).
Samples
from
NS, NT, and CT (SM
treatments were
not
sampled)
were taken
in
November 1995 with
a
5.5-cm
diam.
steel
core
to a
depth
of 20 cm.
Eight cores
per
plot were taken
along
a
transect
in the
middle
of the
plot
to
avoid edge
effects.
The litter layer was removed and the soil
cores
were divided
into
two
depth increments:
0 to 5 and 5 to 20 cm.
Once
in the
laboratory,
the
soil
was
passed through
an
8-mm
sieve
by
gently breaking apart
the
soil. Sieved soil
from
the
eight cores
per
plot were composited, weighed,
and a
10-g
subsample
was
taken
for
gravimetric measurement
of the
moisture content.
The
sieved soil
was air
dried
and
stored
at
room temperature.
General characteristics
of the
soil
are
given
in
Table
1.
Aggregate
Separation
The
method
for
isolation
of the
free
LF
(POM occurring
between aggregates),
iPOM
(POM occurring within aggre-
gates), and mineral-associated
SOM
(C associated
with
the
mineral fraction)
is
shown
in
Fig.
1.
Aggregate separation
was
done
by wet
sieving.
Two
pretreatments
were applied before
wet
sieving:
air
drying followed
by
rapid immersion
in
water
(slaked)
and air
drying plus capillary
rewetting
to
field
capacity
plus
5%
(kg/kg)
(rewetted).
Aggregate stability
is
maximum
at a
moisture content
of
field
capacity plus
5%
(kg/kg)
(Cam-
bardella
and
Elliott,
1993).
Slaking,
on the
other hand, disrupts
aggregates because
of
internal
air
pressure,
and
aggregates
that
resist slaking are more stable than rewetted aggregates
(Elliott,
1986).
The
soils were
wet
sieved through
a
series
of
three sieves (2000, 250,
and 53
(Am).
The
method used
for
aggregate size separation
was
adapted
from
Cambardella
and
Elliott
(1993).
A
100-g
subsample (air dried
or
capillary wet-
ted)
was
submerged
for 5
min
in
room temperature
deionized
water,
on top of the
2000-u.m
sieve. Aggregate separation
was
achieved
by
manually moving
the
sieve
up and
down
3 cm
with
50
repetitions during
a
period
of 2
min. After
the
2-min
cycle,
the
stable
>2000-u,m
aggregates were gently back-
washed
off the
sieve into
an
aluminum pan. Floating organic
material (>2000
jjim)
was
decanted
and
discarded because
this
large organic material
is, by
definition,
not
considered
SOM. Water plus soil that went through
the
sieve
was
poured
onto
the
next sieve
and the
sieving
was
repeated,
but
floating
material was retained. The aggregates were oven dried (50°C),
weighed,
and
stored
in
glass jars
at
room temperature.
Based
on the
aggregate distribution
of the
slaked
and
rewet-
ted
aggregates,
and the
sand distribution
of
rewetted aggre-
gates,
a
normalized stability index
(NSI),
modified
from
van
Steenbergen
et al.
(1991),
was calculated. The stability index
is
on a
scale
of 0 to 1 and is
normalized
by
taking
the
ratio
of
the
observed soil stability
and the
maximum stability
of
the
soil.
Size
Density
Fractionation
The
method
for
separation
of
free
LF and
iPOM
was
modi-
fied
from
Elliott
et al.
(1991),
Cambardella
and
Elliott
(1993),
Golchin
et al.
(1994a),
and
Jastrow
(1996).
Aggregate size
fractions
were oven dried
(110°C)
overnight prior
to the
analy-
sis.
After cooling
in a
desiccator
to
room temperature,
a
5-g
subsample
was
weighed
and
suspended
in 35 mL of
1.85
g
cm~3
sodium
polytungstate
in a
50-mL
graduated conical centrifuge
tube.
The
suspended subsample
was
mixed without breaking
the
aggregates
by
slowly
reciprocal
shaking
by
hand
(10
strokes).
If 10
strokes were
not
enough
to
bring
the
whole
sample into suspension,
a few
more strokes were done rather
than increasing
the
speed
or
force
of
shaking, thereby avoiding
aggregate disruption. The material remaining on the cap and
sides
of the
centrifuge tube were washed into suspension with
10 mL of
sodium polytungstate.
The
sample
was
then
put
under
vacuum (138
kPa)
for 10 min to
evacuate
air
entrapped
within
the
aggregates. After
20 min
equilibration,
the
sample
was
centrifuged
(1250
g) at
20°C
for 60
min.
The
floating
material (free
LF) was
aspirated onto
a
20-u,m
nylon filter,
rinsed
thoroughly with deionized water
to
remove sodium
polytungstate
(C.A.
Cambardella, 1996, personal communica-
tion), transferred into
a
small aluminum pan,
and
dried
at
50°C.
The
heavy fraction
was
rinsed twice
with
50 mL of
deionized
water
and
dispersed
in
0.5%
hexametaphosphate
by
shaking
for 18
h
on a
reciprocal shaker.
The
dispersed
heavy
fraction
was
passed through
a
2000-, 250-, and/or
53-
jjim
sieve depending
on the
aggregate size being analyzed.
The
material remaining
on the
sieve,
iPOM
+
sand
(53-250,
250-2000,
and
>2000
(im
size), was dried
(50°C)
and weighed.
The
iPOM
+
sand
in the
size classes
250 to
2000
and
>2000
(jim
that were derived
from
the
>2000
|j,m
aggregates were
pooled
for
determination
of C and N
concentrations
of
iPOM.
Carbon
and
Nitrogen
Analyses
Carbon
and
nitrogen concentrations were measured with
a
LECO
CHN-1000
analyzer
(Leco
Corp., St. Joseph, MI) for
the
aggregate size fractions and the
iPOM.
Due to smaller
sample
sizes
for the
free
LF, C
concentrations
for
free
LF
were
measured on a Carlo
Erba
NA
1500
CN
analyzer (Carlo

SIX
ET
AL.:
AGGREGATION
AND
SOIL ORGANIC MATTER ACCUMULATION
IN
GRASSLAND SOILS1369
100g
air
dried
soil
(110°C)
wet
sieving
LF
=
light
fraction
HF
=
heavy
fraction
IPOM
=
intraaggregate
paniculate
organic matter
mSOC
=
mineral
associated soil organic
C
HMP
=
hexametaphosphate
HMP
dispersion
+
sieving
mSOC
<
53
urn
iPOM
+
sand
iPOM
+
sand iPOM
+
sand
53-250
Jim
53-250
urn
250-2000
urn
(53) fine iPOM
coarse
iPOM
(250a)
(250b)
iPOM
+
sand
iPOM
+
sand
53-250
urn
>
250
urn
(2000a)
(2000b)
Fig.
1.
Fractionation
sequence.
Erba,
Milan, Italy), which requires less
C for
analysis.
We
preferred
to use the
LECO
for
samples
for
which
there
was
adequate material
to
minimize potential error associated with
subsampling.
The
mineral-associated soil organic
C
concentra-
tion
is
calculated
by
difference. Because purchased sodium
polytungstate
is
contaminated with
C, it was
cleaned before
use
(Six
et
al.,
1999).
Textural
differences
between size fractions
and the
fact
that there
is
little
or no
binding
of
organic matter with sand
particles, makes
it
necessary
to
correct
for the
sand content
(Elliott
et
al.,
1991) when comparing
the
aggregation,
C and
N
concentrations
of
iPOM,
free
LF, and
aggregate size
frac-
tions.
It is
important
to
note
the
difference
in
texture between
the two
depths
in NT and
NS
(Table
1) and the
homogenous
texture
for
both
layers
in the
CT
as a
result
of
plowing, which
exemplifies
the
necessity
for
sand correction
in
order
to
make
appropriate interpretations in treatment comparisons. Sand-
free
C and N
concentrations were calculated
with
the
following
formula:
The
proportion
of
wheat-derived
C,
/,
was
calculated using
the
equation
f
=
(Qt
-
80)
(Sw
-
80)
[3]
where
8,
=
813C
at
time
t,
8W
=
813C
of
wheat straw,
and
80
=
initial
813C
of the
grassland-derived
SOM
(at
time
0,
1969)
and
/
=
fraction
of
wheat-derived
C in the
soil.
The
fraction
of
grassland-derived soil
C is (1
—
/).
The
initial
80
was
deter-
mined
on
archived soil samples taken
at the
initiation
of the
field
experiment
(R.F.
Follett,
1997, personal communication).
The
signature
of the
wheat straw
is an
average
of
collected
residue of the three NT
field
replicates
(8W
=
-27.57).
The
turnover rates
for
whole soil
C
were calculated using
a
first-order decay model:
[4]
sandfree
(C or
N)fraction
=
(C or
N){raction
1
-
(sand
proportion)fractio
[1]
Isotope Analyses
Carbon-isotope ratios
for the SOM
fractions
and
whole soil
were
determined
using
a
Carlo
Erba
NA
1500
CN
analyzer
coupled
to a
Micromass
VG
isochrom-EA
mass spectrometer
(Micromass
UK
Ltd., Manchester,
UK)
(continuous
flow
mea-
surement).
Isotope
ratios were expressed
as
813C
values:
Rl3r,
_
|"(13C/12C
sample
-
13C/12C
reference)]
innn
O
\~s
— I
————————~————————————————————
I
_LUUU
L
13C/12C
reference
J
[2]
where
Ai
is the
grass-derived
C at
time
t
[A,
=
,(1
~~
f)C-
content
at
time
t],
A0
is the
grass-derived
C at
time
0
[A0
=
the
value
of the
native grassland treatment],
t is the
time since
conversion
of
grass
to
wheat
(26
yr),
and k is the
specific rate
of
decomposition
(year"1).
The
mean residence time
(MRT)
for
total soil C is calculated as
Ilk.
The
MRTs
for individual
SOM
fractions were
not
calculated
here
because
the
model
does
not
account
for the
transfer
of old C
between
the
fractions
during
the
period
since
cultivation
(Angers
and
Giroux,
1996;
Golchin
et
al.,
1995).
Statistical
Analyses
The
data were analyzed,
as a
complete randomized block
design, using
the
SAS
statistical package
for
analysis
of
vari-
ance
(ANOVA-GLM,
SAS
Institute,
1990).
Within depth,
tillage
treatment
was the
main factor
in the
model, with size

1370
SOIL
SCI.
SOC.
AM.
J.,
VOL.
62,
SEPTEMBER-OCTOBER
1998
fraction
and
replicate
as
secondary factors. Separation
of
means
was
tested
using
Tukey's
honestly
significant
difference
with
a
significance
level
of P
<
0.05.
RESULTS
AND
DISCUSSION
Whole
Soil
Characteristics
At both depths, the order of total organic
C
and
N
was:
NS
>
NT
>
CT
(Table
1).
However,
the
differences
were only
significant
(P
<
0.05)
in the 0- to
5-cm
depth.
The C and N
concentrations
were
similar
at
both
depths
of
the CT
treatment
due to
mixing
by
plowing. Similarly,
the
MRT
of
whole soil
C was the
same
at
both depths
for
CT (44
yr).
In
contrast,
the MRT in the
lower depth
of
NT was
three
times longer than in the surface layer
of
NT (Table 1). In order to compare the MRT of C in
CT and NT, it is necessary to compare across the total
plow depth
and on a
volumetric basis.
For the
total
depth,
we
observed approximately
a
doubling
of the
MRT in NT
compared with
CT
(Table
1).
Aggregate
Distribution, Carbon,
and
Nitrogen
The
NSI
was
significantly
different
across
the
treat-
ments and between depths (Fig. 2). Native sod had the
highest
NSI
value
(0.71)
and the NSI
increased with
depth (0.85). Conventional tillage had the lowest NSI
(0.07)
and
there
was no
difference between
the two
depths. Increasing aggregation
with
depth
in NS and
NT
suggests that there
are
processes
near
the
soil sur-
face,
such
as
wet-dry
and
freeze-thaw
cycles
(Rovira
and
Greacen,
1957;
Adu
and
Oades,
1978;
Hadas,
1990;
Degens
and
Sparling, 1995), which disrupt aggregates.
Sand-free
organic
C and N
concentrations
of the
wet-
sieved aggregates were generally higher
in NS
than
in
CT and NT in
both
rewetted
and
slaked treatments
at
both depths (Fig. 3 and 4), except for the 5- to
20-cm
slaked aggregates, which
did not
show
any
differences
(Fig.
3).
Beare
et
al.
(1994) also reported
no
differences
in
aggregate
C and N
concentrations
in the
lower depth
(5-15 cm) of NT vs. plowed treatments. Rewetted aggre-
gates
from
NT had
significantly
higher
C and N
concen-
trations than
CT
aggregates, especially
in the 0- to
5-cm
depth. However, when slaked, there were
no
differences
between
CT and NT in
either
of the
depths, except
for
the
microaggregates
(53-250
u,m)
at the 0- to
5-cm
depth. This
is
similar
to
observations
by
Elliott
(1986).
The C and N
concentration tended
to
increase with
aggregate size for all management treatments in both
rewetted
and
slaked aggregates, except between
the two
largest aggregate sizes. However, there
was
less
differ-
ence
in C and N
concentration between aggregate sizes
at
depth than
at the
surface.
Intraparticulate
Organic Carbon
and
Nitrogen
At the
surface,
the
total
iPOM
C
concentration
for
rewetted aggregates
was
strongly influenced
by
tillage
intensity,
but
there were
no
significant
differences
in
the 5- to
20-cm soil layer between
NT and CT
(Fig.
5).
Coarse iPOM
(250-2000
urn)
C and N
concentrations
in
slaked
macroaggregates
were similar for CT and NT
Normalized
Stability Index
0-5 cm
NT
CT
treatment
s
Fig.
2.
Effect
of
management
on the
normalized stability index (NSI)
at
the 0- to 5- and 5- to
20-cm depths.
NS
=
native sod;
NT
=
no-
tillage;
CT
=
conventional
tillage.
Values
followed
by a
different
uppercase
letter
within
a
depth
are
significantly different. Values
followed
by * for the 5- to
20-cm depth
are
significantly
different
from
corresponding values
in the 0- to
5-cm depth. Statistical sig-
nificance
determined
at P
>
0.05 according
to
Tukey's
HSD
mean
separation test.
(Fig.
6).
However,
in
rewetted macroaggregates,
the
order
of
coarse iPOM
C
concentrations was:
NS
>
NT
>
CT
(Fig.
5).
This
difference
in
trend
of
coarse iPOM
concentrations between slaked
and
rewetted aggregates
across tillage treatments suggests that
the
coarse
iPOM
is
part
of the
intermicroaggregate
organic
C
(Elliott,
1986;
Elliott
and
Coleman,
1988) which stabilizes macro-
aggregates.
In
addition,
the
order
of
total iPOM levels
in
stable slaked aggregates
was NS
>
NT
>
CT
(Table
2),
indicating that
the
amount
of
soil
in the
aggregate
fractions
is the
main determinant
for the
levels
of
total
iPOM per unit soil. In other words, the amount of total
iPOM
per
unit soil
is
primarily
a
function
of
aggregation.
In
contrast,
the
similar concentration
of C of the
coarse
iPOM
per
unit aggregate
in NT and CT
suggests that
the concentration of coarse
iPOM
in the aggregate is
related
to the
stability
of the
aggregate.
While
the
concentrations
of
coarse iPOM
(250-2000
|xm)
were
not
affected
by
tillage treatment, there were
substantial
differences
for
fine
iPOM
(53-250
p,m)
be-
tween
NT and CT. The
fine
iPOM
in CT was
almost
three
times lower than that
for NS,
whereas
the
fine

SIX
ET
AL.:
AGGREGATION
AND
SOIL
ORGANIC
MATTER
ACCUMULATION
IN
GRASSLAND
SOILS
1371
Slaked
Carbon
60
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30
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0-5 cm
a A
B
<53
53-250
250-2000
>2000
60
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5-20
cm
6-1
5-20
cm
<53
53-250
250-2000
>2000
<53
53-250
250-2000
>2000
aggregate size class
(urn)
aggregate
size
class (urn)
Fig.
3.
Total
C and N for
slaked aggregate size fractions
(g
kg"'
aggregate, sand-free basis).
NS
=
native sod;
NT
=
no-tillage;
CT
=
conventional
tillage. Values followed
by a
different uppercase letter within
a
management treatment
or
depth
and
among aggregate size fractions
are
significantly
different. Values followed
by a
different lowercase letter within
an
aggregate size
or
depth
and
among management treatments
are
significantly different. Values followed
by * for the 5- to
20-cm
depth
are
significantly different from corresponding values
in the 0- to
5-cin
depth. Statistical significance determined
at
P
>
0.05 according
to
Tukey's
HSD
mean separation test.
iPOM
level
in NT was
only one-third lower than
in NS
(Fig.
6).
These
observations,
and the
assumption
that
POM
tends
to
decrease
in
size
as it
decomposes
(Bal-
dock
et
al.,
1992;
Guggenberger
et
al.,
1994), suggest
that
macroaggregates
are
formed around coarse iPOM
at
similar rates in both treatments, but due to more
intensive
tillage
in CT,
macroaggregates turn over faster
in
CT
than
in NT.
Hence, there
is
less formation
and
stabilization
of
finer,
more decomposed particles
(fine
iPOM) within
CT
macroaggregates.
In NT,
aggregates
are not
disrupted
by
tillage,
and
fine
iPOM
is
formed
and
stabilized through physical protection.
The
same
pattern
is
observed
in the
lower depth,
but the
magni-
tude
is
less pronounced.
The
similarity between
the two
depths
in CT is
probably more
a
result
of
mixing
due
to
plowing than
an
occurrence
of the
same processes
at
both depths.
The
release
of
coarse iPOM particles upon
the
disintegration
of
aggregates
in CT may
result
in a
faster
decomposition rate
and
subsequent loss
of
these
particles.
Therefore,
the
small
difference
in
free
LF
C
content
between
CT and NT may
result
from
a
faster
decomposition rate
for
free
LF in CT
than
in NT
(Ta-
ble 2).
At the
surface,
the
concentration
of
fine
iPOM
was
6.4
g C kg
l
aggregate
in NS,
compared with
4.4 g C
kg"1
aggregate
in NT,
whereas
microaggregate
iPOM
was
9.7 g C
kg"1
aggregate
in NS,
compared with
3.4 g
C
kg"1
aggregate in NT (Fig. 6). This result suggests
that
fine
iPOM
is
less vulnerable
to
cultivation than
microaggregate
iPOM, since
fine
iPOM
is
probably pro-
tected
by
microaggregates
within macroaggregates.
Upon cultivation, macroaggregates break into micro-
aggregates
and
<53-(jun
particles
(Tisdall
and
Oades,
1982).
However,
we
found
that
the
weight
of
<53-fjun
particles
was
similar
in NS and NT,
while microaggre-
gate weight
was
19.7% higher
in NT;
macroaggregate
weight
was
10.3%
lower
in NT
compared with
NS
(data
not
shown).
The
aggregate distribution data
and the
much
lower level
of
microaggregate iPOM
in NT
(Fig.
6)
suggest that macro- and microaggregates break into
smaller
microaggregates
and
iPOM
is
lost, resulting
in
less
iPOM associated with these microaggregates. There-
fore,
it
appears that microaggregates
are
more vulnera-
ble to
disruption
from
cultivation
(Jastrow,
1996) than
originally
hypothesized
by
Tisdall
and
Oades
(1982).
One
suggestion
has
been
to
differentiate
two
sizes
of
microaggregates:
20 to 90 and 90 to 250
u,m
(Oades
and
Waters, 1991; Waters
and
Oades,
1991).
The
larger

1372
SOIL
SCI.
SOC.
AM.
J.,
VOL.
62,
SEPTEMBER-OCTOBER
1998
Rewetted
Carbon
Nitrogen
<53
53-250
250-2000
>2000
<53
53-250 250-2000
>2000
aggregate size class
(urn)
aggregate size class
(urn)
Fig.
4.
Total
C
and
N
for
rewetted
aggregate size fractions
(g
kg"1
aggregate, sand-free basis).
NS
=
native sod;
NT
=
no-tillage;
CT
=
conventional tillage. Values
followed
by a
different uppercase letter within
a
management treatment
or
depth
and
among aggregate size
fractions
are
significantly different. Values followed
by a
different lowercase letter within
an
aggregate size
or
depth
and
among management
treatments
are
significantly different.
Values
followed
by * for the 5- to
20-cm
depth
are
significantly different from
corresponding
values
in
the
0- to
5-cm
depth. Statistical significance determined
at
P
>
0.05 according
to
Tukey's
HSD
mean separation test.
microaggregates
(90-250
u.m)
have
a
nucleus
of
plant
residues with
a
distinct cellular anatomy, whereas small
microaggregates
(20-90
urn)
show only
a few
distinct
organic
entities (Waters
and
Oades,
1991).
Our
data
suggest
that
the
differentiation
of two
sizes
of
microag-
gregates could
be
valuable
in
future
studies,
because
of
the
break
up of
larger microaggregates into smaller
microaggregates
on
cultivation. Therefore,
the
dynam-
ics of the two
kinds
of
microaggregates could
be
differ-
ent
and
valuable
to
study.
In
both
NT and CT, the
coarse
iPOM
had the
youn-
gest
age
(most negative
13C
ratio) (Table
3),
indicating
the
relatively recent incorporation
of
this material into
aggregates.
The
hypothesis that
the
formation
of
macro-
aggregates occurs at a similar rate in NT and CT is
supported
by the
similar signatures
of the
coarse iPOM
in
NT and CT
even though
the
amount
of
macroaggre-
gates
is
different.
The
8-value
of
fine
iPOM indicates
that
it was
older (less negative value) than coarse iPOM
in
the
macroaggregates,
but was
younger (more negative
value)
than that
for the
microaggregate
iPOM (Table
3). Macroaggregates indeed turn over faster than mi-
croaggregates
(Jastrow
et
al.,
1996), and microaggreg-
ates
are
thought
to
function
as
building blocks
for the
formation
of
macroaggregates
(Tisdall
and
Oades,
1982). Therefore,
we
hypothesize that
the
fine
iPOM
pool
is a
mixture
of
newly
formed,
fine
iPOM within
the
macroaggregate
and
fine
iPOM
within microaggregates
that are incorporated into macroaggregates.
The
extremely high
N
concentration
of
coarse iPOM
in
slaked
aggregates
in
both
depths
of CT
(Fig.
6) is an
interesting
result,
but we do not
have
a
satisfactory
explanation
for it.
Free
Light Fraction Carbon
There
was a
small
and
nonsignificant
difference
in
amounts
of
free
LF
C
between
NT and CT
(Table
2),
but a large difference of free LF C when compared with
NS.
The
difference
in
free
LF C
between
NT and CT
accounted
for
13.7%
of the
total
POM
difference
be-
tween
these
two
treatments,
whereas
the
fine
iPOM
difference
accounted
for
47.3%
of the
total
POM
differ-
ence.
In
contrast,
the
difference
in
free
LF C
difference
between
NS and NT was
47.5%
and the
fine
iPOM
difference
was
12.9%
of the
total
POM
difference.
The
lower
level
of
free
LF in
cultivated soils than
in
grassland
soils
is in
agreement with
Besnard
et al.
(1996),
who
observed
the
greatest decline
in
free
POM
(=free
LF)
compared
with
intraaggregate
POM
after
the
conver-

SIX
ET
AL.:
AGGREGATION
AND
SOIL
ORGANIC
MATTER
ACCUMULATION
IN
GRASSLAND
SOILS
1373
Rewetted
Carbon
Nitrogen
0-5
cm
53
250a
250b
2000a2000b
250a
250b
2000a
2000b
53
size
class
250a
250b
2000a2000b
size
class
Fig.
5.
Effect
of
management
and
depth
on
intraaggregate
participate
organic matter
(iPOM)
C
and N
(g
kg"1
aggregate, sand-free
basis)
in
rewetted
aggregate size fractions.
NS
=
native sod;
NT
=
no-tillage;
CT
=
conventional tillage; 250a
and
2501)
=
fine
and
coarse intraaggregate
particulate
organic matter, respectively; 2000a
and
2000b
=
iPOM
in
>2000-|un
macroaggregates
with
a
size
of 53 to 250 and
>250
urn,
respectively. Values followed
by a
different lowercase letter within
an
iPOM size fraction
or
depth
and
among management treatments
are
significantly
different. Values followed
by * for the 5- to
20-cm
depth
are
significantly different
from
corresponding values
in the 0- to
5-cm
depth. Statistical significance determined
at
P
>
0.05 according
to
Tukey's
HSD
mean separation test.
sion
of
native forest
to
corn
(Zea
maize
L.)
cultivation.
This suggests that
the
nonaggregate-protected
free
LF
is
mostly
affected
by the
residue input
and
microclimatic
soil
and
surface conditions
and is not
influenced
by
aggregation.
The
microclimatic soil
and
surface condi-
tions
are
different
between
NT and CT, but not as
great
as
those between
NS and
either
NT or CT. In NS,
residue
inputs
are
higher
and
drier soil conditions prevail
(Pau-
stian
et
al.,
1997). Residue quality
and
root
system archi-
tecture are
other
factors that
differ
between NS and
wheat-fallow management systems
(NT and
CT). These
factors
probably
affect
the
level
of
free
LF and may
explain
the
great differences
in
free
LF
between grass-
land
and
cultivated soils
vs. the
small differences
in
free
LF with differential tillage.
Interestingly,
the
free
LF had an
older
13C
signature
than
the
iPOM
of the
same size (Table
3),
especially
the 250- to
2000-n,m
size. Therefore, the
free
LF is not
only
recently deposited residue,
but a
mixture
of
fresh
residue
and
older inert plant material.
Cadisch
et al.
(1996)
also reported that
the LF is not a
uniform
pool
and
noted that
it
consists
of
undecomposed
and
partly
decomposed
root
and
plant fragments
and
charcoal par-
ticles.
The
presence
of
charcoal
in LF and POM has
been
reported
by
other authors
(Skjemstadt
et
al.,
1990;
Molloy
and
Spear, 1977;
Elliott
et
al.,
1991;
Cambardella
and
Elliott, 1992). Charcoal could
be
partly responsible
for
the
older signature
of the
free
LF.
Yet,
no
satisfac-
tory method
has
been devised
for the
isolation
of
char-
coal,
and the
magnitude
of the
confounding
effect
of
charcoal
is
unknown
(Golchin
et
al.,
1997). Inert mate-
rial other than charcoal
may
also result
in an
older
signature.
The
relatively greater presence
of
older
and
relatively inert plant material in the
free
LF when com-
pared with
the
iPOM
may be due to the
fact
that inert
material does
not
form
a
nucleation
site
for
aggregate
formation,
because
there
is
little
microbial
activity asso-
ciated with this material. Hence,
the
inert material
has
a
relatively lower probability
of
being incorporated into
an
aggregate
and is
more likely
to
remain part
of the
free
LF. In
contrast,
Gregorich
et al.
(1997) observed
a
younger
13C-signature
for
free
LF
organic matter than
for
protected LF organic matter.
Mineral
Associated Soil Organic Carbon
The
mineral-associated soil organic
C
(mSOC)
did
not
vary
by
treatment
for
slaked aggregates, except that
the
mSOC concentration
of
microaggregates
in the
0-
to
5-cm layer
was
lower
in
both
NT and CT
than
in

1374
SOIL
SCI.
SOC.
AM.
J.,
VOL.
62,
SEPTEMBER-OCTOBER
1998
Slaked
Carbon
Nitrogen
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0.0
I
T
I
53
250a 250b
2000a2000b
size
class
size class
Fig.
6.
Effect
of
management
and
depth
on
intraaggregate
participate
organic
matter
(iPOM)
C and N
(g
kg'1
aggregate,
sand-free
basis)
in
slaked
aggregate size
fractions.
N/A
=
not
analysed because
not
enough
material
was
available;
NS
=
native sod;
NT
=
no-tillage;
CT
=
conventional
tillage; 250a
and
250b
=
fine
and
coarse
intraaggregate
particulate
organic
matter,
respectively; 2000a
and
2000b
=
iPOM
in
>2000-(tm
macroaggregates
with
a
size
of 53 to 250 and
>250
fJim,
respectively. Values
followed
by a
different
lowercase
letter
within
an
iPOM
size
fraction
or
depth
and
among management
treatments
are
significantly
different.
Values followed
by
*
for the 5- to
20-cm
depth
are
significantly
different
from
corresponding
values
in the 0- to
5-cm depth.
Statistical
significance
determined
at
P
>
0.05
according
to
Tukey's
HSD
mean separation test.
NS
(Fig.
7). The
mSOC
concentration
was
significantly
lower
in the 5- to
20-cm
depth
compared
with
the 0- to
5-cm
depth
for NS, but
there were
no
differences
in NT
and
CT
between these respective depths.
Conceptual Explanation
for
Aggregate
Formation
and
Soil
Organic
Matter
Accumulation
Based
on our
results,
we
suggest that
the
rate
of
mac-
roaggregate
formation
is
probably similar
in NT and CT.
When residue is applied to the soil,
microbial
activity
increases and available C is assimilated. Fresh residue
contains
a
high
percentage
of
easily
available
C and is
favored
for use by the
soil biota. Extracellular
polysac-
charides
are
deposited during assimilation
by
microbes,
which
leads
to
aggregate formation
(Chaney
and
Swift,
1986a,b;
Haynes
and
Francis,
1993).
Extracellular poly-
saccharides
do not
diffuse
far
from
the
site
of
production
(Oades,
1984)
but may
diffuse
into nearby
micropores.
Therefore, newly applied residues function
as
nucle-
ation
sites
for the
growth
of
fungi
and
other soil
mi-
crobes
(Puget
et
al.,
1995; Angers
and
Giroux,
1996;
Jastrow,
1996), resulting
in the
binding
of
residue
and
soil
particles
into
macroaggregates.
During this
process,
coarse iPOM both forms
and is
incorporated into macro-
aggregates, probably concomitant with
a
deposition
of
microbial products
and
other forms
of
SOM
on
mineral
surfaces.
Since
the
residue input
in NT and CT
(Doran
et
al.,
1998)
are
similar
and
soil biota
are
active
in
both
systems,
it is
likely that aggregates
are
formed
at the
same rate.
During
the
incorporation
of
fresh
residue, microbes
utilize
the
easily decomposable carbohydrates, leaving
Table
2.
Comparison
of
particulate organic matter fractions (free
LF
=
free light fraction; iPOM
=
intraaggregate particulate
organic matter) between native
sod
(NS), no-tillage (NT),
and
conventional
tillage (CT) over
the
whole plow depth
(0-20
cm).
Treatment
Fine
iPOM
Coarse
iPOM
Free
LF
Total
POM
NS
NT
CT
174 at
99
b
21 c
Ilia
60 b
25 c
474 a
196 b
174 b
1116
a
533 b
367 c
t
Values
followed
by a
different
lowercase
letter
are
significantly
different
(P
>
0.05)
according
to
Tukey's
HSD
mean
separation
test.

SIX
ET
AL.:
AGGREGATION
AND
SOIL
ORGANIC
MATTER
ACCUMULATION
IN
GRASSLAND
SOILS
1375
Table
3.
8"C
ratios
(per
mille)
of
light fraction
(LF)
and
intraaggregate
particulate
organic matter (iPOM) associated with slaked
aggregates
in the
surface
layer
(0-5
cm) of
no-tillage
(NT)
and
conventional tillage
(CT).
More negative values indicate younger
material
and a
greater proportion
of
crop derived
C.
__
Treatment
iPOM
fraction
813C
iPOM
LF
fraction
5UC
LF
NT
CT
Microaggregate
iPOM
Fine iPOM
Coarse iPOM
Microaggregate
iPOM
Fine iPOM
Coarse iPOM
-23.43
±
0.09fl
-23.69
±
0.05
-25.27
±
0.17
-22.02
±
0.18
-23.54
±
0.35
-24.54
±
0.55
53-250
250-2000
53-250
250-2000
-23.2
±
0.05
-24.3
±
0.05
-22.3
±
0.05
-23.2
±
0.12
1[
Average
ratio
±
standard
deviation.
behind
the
more recalcitrant iPOM that
has a
higher
proportion
of
alkyl
C
(Golchin
et
al.,
1994a, 1995). Nev-
ertheless, the coarse iPOM is
further
decomposed and
fragmented
into
fine
iPOM. Decomposition occurs at a
slower
rate when held within
macroaggregates
(Elliott,
1986),
partly
due to the
physical protection within
the
aggregate
and
probably
due
partly
to the
more chemi-
cally
recalcitrant nature
of the
partially decomposed
iPOM. We hypothesize that more
fine
iPOM accumu-
lates
in NT
macroaggregates than
in CT
macroaggre-
gates because
of
slower
macroaggregate
turnover
in the
absence
of
soil disturbance
from
tillage. Conventional
tillage
disrupts macroaggregates
and
reduces
the
accu-
mulation
of
fine
iPOM within macroaggregates.
In
addi-
tion,
the
amount
of
macroaggregates
is
lower
in CT
than
in
NT,
which results
in an
even larger
difference
in
fine
iPOM between
CT and NT on a
whole soil basis.
Fragmented iPOM probably becomes encrusted with
clay
particles
and
microbial
byproducts, forming
mi-
croaggregates
within macroaggregates and leading to an
increased physical
protection
of the
fine
iPOM
(Oades,
1984;
Elliott
and
Coleman,
1988;
Oades
and
Waters,
1991;
Beare
et
al.,
1994;
Golchin
et
al.,
1995;
Jastrow,
1996).
Hence,
the
formation
of
fine
iPOM
and
inclusion
into stable
microaggregates
contributes
to the
seques-
tration
of
SOM
in NT.
Skjemstadt
et al.
(1990) also
demonstrated that some
of the
stability
of SOM was a
result
of its
incorporation
into
microaggregates.
Over time with
further
decomposition,
the
labile con-
stituents
of the
iPOM
are
consumed, microbial produc-
tion
of
binding agents diminishes,
and the
degree
of
association between
the
coarse iPOM particles
and the
soil matrix decreases (Golchin
et al.
1994a, 1995).
Mac-
roaggregates break down
and
release
microbially
pro-
cessed particles
and
microaggregates.
On
release,
the
partially
processed coarse iPOM and
fine
iPOM become
part
of the
free
LF and are
decomposed more quickly
than when they were protected within the aggregate. In
addition,
the
free
LF is
probably lost faster
in CT
than
in
NT
because
of
microclimatic
differences between
the
two
tillage practices.
The
same degradation processes
that occur
in
macroaggregates also occur
in
microag-
gregates (Golchin
et
al.,
1994a,
1995;
Jastrow, 1996).
Therefore, more chemically inert materials
are
mixed
with
recently deposited
but
unincorporated residues
to
become part
of the
free
LF.
Once
the
microaggregates
are no
longer protected within
the
macroaggregates,
they
are
more susceptible
to
disrupting factors
and
break into smaller microaggregates with
a
lower iPOM
content
(Fig.
6).
Microaggregates released
from
macro-
aggregates are probably, in the next macroaggregate
formation
cycle, incorporated into
new
macroaggre-
gates
(Tisdall
and
Oades,
1982).
CONCLUSIONS
We
found
that tillage strongly
affected
soil aggrega-
tion
and the
amount
and
type
of
particulate organic
matter
associated with soil aggregates. Our results
sug-
Carbon
(Slaked)
0-5 cm
53-250
250-2000
30
-i
5-20
cm
53-250250-2000
size
class
(urn)
Fig.
7.
Effect
of
management
and
depth
on the
mineral associated
soil
organic
C
(mSOC)
(g
kg'1
aggregate, sand-free basis)
in
slaked
size
fractions.
NS
=
native sod;
NT
=
no-tillage;
CT
=
conventional
tillage.
Values followed
by a
different lowercase letter within
a
mSOC
size fraction
or
depth
and
among management treatments
are
significantly different. Values followed
by * for the 5- to 20-
cm
depth
are
significantly different
from
corresponding values
in
the 0- to
5-cm
depth.
Statistical
significance determined at
P
>
0.05 according
to
Tukey's
HSD
mean separation test.

1376
SOIL
SCI.
SOC.
AM.
J.,
VOL.
62,
SEPTEMBER-OCTOBER
1998
gest
that coarse
iPOM
is
part
of the
intermicroaggregate
organic
C
that stabilizes
macroaggregates,
and
thus,
the
amount
of coarse iPOM is closely tied to the amount
of
aggregation.
In
contrast,
the
amount
of
fine
iPOM
appears to be mainly a function of aggregate turnover.
When aggregates
are
frequently disrupted,
as in
CT,
there
is
less formation
of
fine
iPOM,
and the
subsequent
encrustation
of
fine
iPOM with clay particles leading
to
the
formation
of
stable
microaggregates
within macro-
aggregates
is
inhibited.
The
iPOM, which
is not
incorpo-
rated
and
protected
within
microaggregates,
is
rapidly
decomposed
on
release
from
the
macroaggregates.
We
conclude that fine iPOM is a fraction that is lost under
CT but is
sequestered within aggregates under
NT be-
cause
of the
slower aggregate turnover
in NT
than
in CT.
The
nonaggregate-protected
free
LF
is a
labile
frac-
tion that was most sensitive to cultivation but was not
affected
by
different
tillage practices within annual crop-
ping systems.
Our
results
suggest
that
nonprotected
free
LF is
mainly influenced
by
residue input rates
and
soil
temperature and moisture conditions.
ACKNOWLEDGMENTS
Thanks
to
Clay
Combrink,
Scott
Pavey,
Matthew
Nemeth,
and
Justin
Bolten
for the
laboratory
assistance
and
especially
for
all the
hours
of
sieving.
All the
suggestions
and
hints
from
Dan
Reuss
during
the
development
of the
methodologies
and
laboratory
work
are
greatly
appreciated.
The
many
discussions
with
Serita
Frey,
Roel
Merckx,
and
Georg
Guggenberger
were
very
useful.
This
research
was
supported
by a
grant
(DEB-
9419854)
from
the
National
Science
Foundation.

BEN-HUR
ET
AL.:
COMPACTION, AGING,
AND
RAINDROP-IMPACT
EFFECTS
IN
VERTISOLS
1377