European Journal of Soil Science, December 1996, 47,485-493
The significance of organic separates to carbon dynamics and
its modelling in some cultivated soils
J . BALESDENT
Unite‘ de Science du Sol, INRA,78026 Versailles Cedex, France; and Laboratoire de Bioge‘ochimie Isotopique, INRA -CNRS, Unniversite‘
P. et M. Curie, ccl20, 4 Place Jussieu, 75252 Paris, Cedex 05, France
Summary
Soil organic matter (SOM) dynamics are usually described by compartmental models. We have
sought SOM separates that might be related to SOM dynamic compartments. The turnover of C in
various separates from long-term field experiments with maize was measured using the natural 13C
labelling technique. The Rothamsted carbon model gave a good prediction of the observed C
turnover. Primary particle-size fractions coarser than SO pm had short lives, and could be associated with the plant structural compartment of models. Water-extractable components, are enriched
in young C but cannot be associated with labile compartments. None of the chemical separates
obtained by acid hydrolysis, wet oxidation, thermic oxidation, pyrolysis or alkaline extraction,
were enriched either in young or old C. The results showed neither a sequential relation between
fulvic acids and humic acids nor a resistance of nonhydrolysable material. The range of lifetimes of
soil C seems to be determined more by physical position and protection than by the chemical nature
of SOM.
Fractionnements des matiZres organiques: apport I’Ctude de la dynamique du carbone
de sols cultivks et a sa modelisation
RCsumC
Les modeles de la dynamique du carbon des sols rkpartissent le carbone du sol en compartiments
fonctionnels, de durkes de vie tres diffkrentes. On a cherchk B skparer matkriellement les composks
de durkes de vie diffkrentes, afin de proposer des estimateurs de ces compartiments. Les vitesses de
renouvellement sont mesurkes par la mkthode du marquage nature1 en 13C, dans des sols en culture
ckrkaliere (mabs) provenant d’expkrimentations de longue durke. Le modtle de Rothamsted donne
une bonne reprksentation de la dynamique observke. Les fractions granulomktriques grossitres
permettent de quantifier le compartiment structural d’origine vkgktale des modtles. Les composks
extractibles B l’eau sont enrichis en composks jeunes, mais ne sont pas identifiables aux compartiments labiles. Concernant les mati2res organiques < SO pm, aucune des mkthodes classiques
kvalukes (hydrolyses acides, oxydations humides, oxydations thermiques, pyrolyses, extraction
alcalines) ne permettent de skparer des composks de vitesses de renouvellement diffkrentes.
L’analyse permet de rejeter l’hypothbse de formation des acides humiques par condensation des
acides fulviques. La localization et la protection physique des matieres organiques apparaot comme
plus dkterminante pour leur vitesse de biodbgradation que leur appartenance B une famille chimique.
Introduction
The quantitative aspect of soil organic carbon dynamics is
described by compartmental models such as those of Jenkinson
Correspondence: E-mail: [email protected]
Received 18 December 1995; revised version accepted 11 March 1996
0 1996 Blackwell Science Ltd.
& Rayner (1977) and Parton et al. (1987). An important aim of
studies of soil organic matter has been to separate the
mathematical or conceptual compartments of these models.
On one hand such separation could provide a means of
evaluating or parameterizing compartment sizes at a given
place. On the other hand it could help elucidate the pathways
485
Carbon dynamics in cultivated soil
in water with glass beads. The resulting particle-size fractions
200-2000 pm and 50-200 pm were separated by wet sieving.
From these fractions, light-density organic debris was
separated from mineral sand by repeated flotation-panning in
water (after Feller, 1979). This senaration gave organic debris
and mineral sand fractions. The mineral fractions consisted
mainly of quartz. Their carbon content was negligible in most
cases ( <0.1 % of soil C at Boigneville), but contained some
mine-coal at La Minihe. These fractions were not taken into
account in the study. The suspension containing the particles
finer than 50 pm was then dispersed by ultrasonic treatment
and size-separated. The comparison with the particle-size
distribution obtained from the reference method demonstrated
that dispersion was complete: the weight of inorganic material
in each fraction did not differ from the weight obtained by the
reference method by more than 10 mg g-' soil. The advantage
of the method is that ultrasonic treatment is not applied on
particulate organic matter >50pm, because it has been
demonstrated that the energy required for complete clay
dispersion resulted in the splitting of more than half the
organic fraction > 50 pm by weight in C, into finer fractions
(Balesdent et al., 1991). The carbon balance of the separation,
including solubilized organic carbon, was 99.5 If: 1.0%.
Chemical separations were done on the 0-50 pm-sized
fraction of five samples. The samples were: La Minihe
(4 years), Boigneville (20 years and 20N) and the two C3
references. The 0-50-pm suspension was flocculated with
1 g 1-' CaC12. The flocs were freeze-dried then passed
through a 0.1 mm sieve before subsampling for further extraction. The supernatant was defined as 'water-soluble' and
freeze-dried for isotopic determination.
Water extraction at 120°C (Autoclaving) was done as
follows. Four-gram samples were added to 50 ml water in
centrifuge stainless tubes and maintained at 120°C and
0.2 MPa in a autoclaving apparatus. Each sample was treated
six times to obtain cumulative extraction periods of 1,4, 8, 12,
16 and 20 h. At each time step, the supernatant was centrifuged, re-extracted twice with water, and freeze-dried for
analysis.
Alkaline extraction was performed as follows. Twenty-gram
samples were treated by four repeated extractions with 200 ml
0.1 M N%P207+ 0.1 M NaOH. The residue was washed and
defined as 'humin'. Fine clay particles in the extract were
flocculated by addition of 1 M KCl and added to humin.
Extracts were acidified to pH 1.5. The precipitate was defined
as 'humic acid'. The supernatant, defined as 'fulvic acid', was
briefly dialysed against water in MWCO 1000 membranes to
remove salts. All fractions were freeze-dried.
Hydrolysis, oxidation and pyrolysis were each done
successively, up to five or six times, so as to attack approximately 10, 25, 40, 60, 75% of the initial organic carbon,
respectively. This was obtained by treating a batch of subsamples with reagents of varying concentration and reactions
for varying duration and temperature.
0 1996 Blackwell
Science Ltd, European Journal of Soil Science, 47,485-493
487
Acid hydrolysis was done using the method of hydrolysis
described by Cheshire (1979) for total sugar extraction. Some
reaction conditions were changed to obtain incomplete
hydrolysis. Six different extractions were carried out under
the following conditions of time, H2S04 concentration and
temperature: 0.5 h 0.5 M 20°C; 0.5 h 12 M 20°C; 2 h 12 M
20°C; 16 h 12 M 20°C; 16 h 12 M 20°C+2 h 1 M 100°C;
16 h 12 M 20°C + 8 h 1 M 100°C. Residues were washed and
freeze-dried for mass, carbon and isotope determination. The
carbon content in the supernatant was measured for carbon
balance then discarded.
The carbon was oxidized with hydrogen peroxide oxidation
as follows. Five-gram samples were added to 300 ml water
and a variable volume of a H 2 0 2 solution at 0.27 g g-', left for
16 h at 20°C then heated at 96°C for 40 min and finally at
105°C for 4 h. Five or six separate extractions were done with
an increasing quantity of H202 added to the sample, i.e. 3.5,
8.8, 13.2, 26.4 or 44.1 mmol. The residues were freeze-dried,
together with solubilized carbon.
Thermal oxidation under air was done thermogravimetrically on 0.5-g samples heated at the rate of 2°C min-' up to
200, 230, 260, 290, 325 or 370°C. Pyrolysis under nitrogen
was done similarly up to 270, 300, 380,425 or 600°C.
Analysis
Total carbon of solids was determined either by dry combustion with the evolved COz measured colometrically, or by
dry combustion in an autoanalyser (Erba NA1500). Total
organic carbon in solution was determined by high temperature
catalytic combustion, followed by CO2 NDIR measurement
(Dohrmann DC 190). Stable isotope ratios were analysed
either in a SIRA 9 (VG-Instruments) on COz obtained by
combustion in sealed quartz tubes with CuO at 850"C, or by
dry combustion in an autoanalyser (Erba NA1500) coupled
with a SIRA 10 (VG-Instruments) Isotope Ratio Mass Spectrometer. 6I3C was calculated as:
where R = 13C+ 12C.The reference is the PDB standard.
Calculation of the proportion of new organic carbon
in the fractions
The proportion of C derived from maize in a soil sample or
fraction was calculated according to Balesdent & Mariotti
( 1996):
6 =f . A + 6,f
or f = (6 - 6,f)
-F
A,
(2)
where f stands for the proportion of maize-derived C in the
sample, 6 for the measured 6I3C of the sample, Grefforthe 6I3C
of the corresponding sample or fraction separated from the C3
reference soil, and A for the difference between maize plant
material and C3 plant material of the reference plot, during the
488 J. Balesdent
experiment. Equation 2 is unbiased even if I3C enrichment
occurs in the soil during C decomposition, provided that the
two soils to be compared have similar C dynamics. Such a
condition is reached at La Minitre and Boigneville, since
maize introduction on these cereal soils induced no quantitative change in C content, and probably no qualitative change
regarding decay or the biochemistry of the decay products
(Broder & Wagner, 1988). At Auzeville, Equation 2 may be
slightly biased due to a possible isotopic difference between
reference grassland soil and initial experiment conditions.
For the sites of La Minitre and Boigneville, 6,f is the 613C
of the equivalent fraction separated from the reference plot.
Since some extractions cause isotopic enrichment (i.e. wet
oxidation and acid hydrolysis), 6,f was calculated as the 6I3C
of the reference sample after the same extend of reaction as
in the treatment of the sample with maize. The quantity A
was estimated in Boigneville as the difference between maize
and wheat material grown in these fields in 1987, i.e. - 12.5(-26.3) = 13.8%0.At La Minih-e A was estimated from
mean soil organic material > 2 mm (dead leaves and roots),
i.e. -12.8 - (-27.6) = +14.8%0. At Auzeville the mean
maize value was -12.3%0 in 1985, and C3 plant material
value was estimated from soil material coarser than 2 mm,
giving A = -12.3 - (-26.6) = +14.3%0.
+
had been estimated previously as 446 g a-I, split into 290 g
ax' as shoots returned in October and 156 g a-' incorporated
by the roots (Balesdent & Balabane, 1992), regularly from
June through September. This set of values leads to an overestimation of total carbon. To fit total carbon (4.0 kg m-*), I
estimated the yearly carbon input to the soil as 368 g mP2 a-I.
The model was run with a monthly time-step over 25 years
with total OM and with new OM from maize only to compute
f. The upper layer with no tillage was simulated assuming an
additional carbon input to fit total carbon concentration in the
layer at the date of analysis. The time scale throughout this
paper started on October preceding the first maize planting.
Results and discussions
Turnover of particle-sizefractions
Figure 1 shows the progressive enrichment of particle-size
fractions in new C. Since the total C did not vary, this
describes the turnover of the fraction. The proportion of
residual old, C3-derived, OM would be represented by the
inverse curves. The turnover of material coarser than 2 mm
was complete in less than 1 year. The turnover rates decreased
with decreasing particle-size. The data was first fitted to single
exponential kinetics according to the least squares criterion
Errors on f
Only the residual carbon was analysed after step-by-step
exFaction or attack, by hydrolysis, wet oxidation, dry oxidation or pyrolysis. In these cases, the isotopic composition of
the extracted or lost material was calculated by mass balance.
The systematic error was based on an error of 0.05X on each
613C used in the calculation off. When f is obtained by direct
measurement its error is 2 x 0.05IA. When it is calculated by
difference the error is larger. The error in A was not taken into
account in Table 3 because A is common to all extracts.
Rothamsted carbon model
To relate the different fractions to current model compartments
the Rothamsted model of Jenkinson & Rayner (1977) was
used, as described in Jenkinson et af. (1992) (ROTHC-26.3).
The soil was estimated to be in steady state on a yearly basis.
Monthly parameters to run the model were calculated for
the mean site and climate conditions for La Minikre and
Boigneville. The DPM: RPM and the BIO : HUM formation
ratios were equal to 1.44 and 0.85, respectively. The ratio of
CO2-C to BIO HUM was set equal 3.71, according to
average soil clay content, 18.5%. The 'plant retainment rate
modifying factor' was set to 0.6 in June to August inclusive,
and 1.O otherwise. The yearly average decay rates were finally
k ~ =p9.1~a-', k ~ =p0.27
~ a-I, kero = 0.60a-',
=
0.018a-', k ~ =o 0 ~a-I. IOM was arbitrary set at 380 g C
m-2 (Jenkinson et al., 1992). The carbon returned to the soil
+
0
5
10
15
Time /years
20
25
dig. 1 Evolution with time of the proportion of new carbon, as
estimated from the I3C natural abundance, in particle-size fractions
from French maize-cultivated soils. Fitting procedures are described
in text.
0 1996 Blackwell Science Ltd, European Journal of Soil Science, 47, 485-493
Carbon dynamics in cultivated soil
Table 2 The turnover of C in the sizefractions. The quantity f is the proportion of
new C estimated from I3C abundance, t is
time, and k is the reciprocal of the mean age
of C in the fraction, as obtained from the
regression, followed by 95% confidence
interval and number of observations
Size
IPm
> 2000”
200-2000a
50-200”
0-50
20-50
2-20
0.2-2
0-0.2
Average C
Cb
Ikg rn-’
Irng g-‘
0.08
0. I8
0.4 1
3.40
0.27
1.09
I .27
0.78
292
366
280
8.8
1.2
7.0
39.6
28.5
489
C:Nb
ratio
Regression
year Ik-l
43
20.2
14.9
7.9
11.6
11.6
9.0
7.5
f = I -exp(-kt)
f = 1 -exp(-kt)
f = 1 -exp(-kt)
f = 1 -exp(-kr)
f=kt
f=kt
2.3k0.5
0.32 k0.03
0.054 f0.005
0.016 f0.001
0.018 f0.005
0.016f0.004
0.012f0.004
0.015 f0.004
f=kt
f=kt
”Light fraction.
bValues are means from La Miniere.
(Table 2). This assumes that each compartment is homogenous. The mean turnover times would be 0.5, 3, 18 and 63
years for fractions > 2000, 200-2000, 50-200 and 0-50 pm,
respectively. In fact, fractions 200-2000 pm and 50-200 pm
exhibited little heterogeneity. These fractions could contain
some carbon with a slow turnover. The quantities of slow
carbon were estimated by fitting the data with a linear
combination of two exponentials, one rapid, the other slow,
with the same rate constant as fraction 0-50 pm. The slow
material accounted for 15% of fraction 200-2000 pm and 35%
of fraction 50-200 pm. The results of these adjustments are
those shown in Fig. 1. The incorporation of new C in fraction
50-200 pm was slower in the two first years than thereafter.
This shows that the fraction is largely fed by the 200-2000 pm
fraction. The incorporation of new C into 0-50 pm fraction
was progressive, almost linear over the 23 years. Silt- and
clay-sized fractions were separated from a few samples,
mainly from the site of Auzeville. The incorporation of new C
(Table 2) is very similar in the four fractions, with a slightly
more rapid incorporation in coarse silts and fine clay. Watersoluble C obtained from particle-size fractionation accounted
for approximately 1.5% of soil C (Table 3). This fraction is
younger than organo-mineral associations < 50 pm, but contains considerable old C. After 20 years, more than half of the
soluble C was still derived from old OM.
Turnover of chemical separates from fraction 0-50 pm
(Table 3)
Water at 120°C solubilized up to 30% of the C. Extracts were
enriched in young C in the first hour, and then the age of the
extracted C increased progressively, towards the age of the
residue after 24-h extraction.
All fractions obtained by alkaline extraction had similar
kinetics of enrichment in new C. The youngest components are
humic acids; fulvic acids are older. Since 30% was lost
(smallest molecules) during dialysis, the bulk fulvic acids
might have been younger. This result, nevertheless, does not
accord with the theory that postulates fulvic acids are progressively condensed into humic acids. The result agrees better with
0 1996 Blackwell
Science Ltd, European Journal of Soil Science, 47, 485-493
a theory of humic molecule genesis by simultaneous condensation of small molecules and degradation of large molecules
(Kogel-Knabner, 1992). Residual humin had the same characteristics as the bulk sample. Humin is classically described as
heterogeneous, containing plant and microorganisms debris, as
well as strongly humified material (Anderson & Paul, 1984). It
is concluded that a scheme that would link humin, HA and FA
by any sequential relation would be false if applied to these
separates, each taken as a whole, in this soil. Their separation
can be of no help in the investigation of C turnover.
Acid hydrolysis has often been proposed for concentrating
old resistant OM. It is based on the notion that resistance of
OM to biological attack is due to chemical resistance. The
most biodegradable substrates (i.e. carbohydrates and proteins)
are easily hydrolysable (Golchin et al., 1995). In the present
study, the differences between fractions are small. First
hydrolysates appear enriched in young C, and the residue
enriched in old C only in the 20-year-old samples. There is
here no concordance between biodegradability and hydrolysability. There are several possible explanations for these
findings. Non-hydrolysable material could contain both highly
condensed, old humic material and unaltered, relatively young
labile lignin components, as well as aliphatic components of
various ages (Schulten & Hempfling, 1992; Kogel-Knabner,
1992). In my method, the recondensation of hydrolysabfe
material during the extraction was prevented by a two-step
extraction. With regard to hydrolysable material, a substantial
part of the carbohydrates (cellulose and hemi-cellulose) was
first removed by particle-size fractionation before the treatment. Carbohydrate or proteinaceous chains may be protected
from biodegradation by association with either humic material
or inorganic colloids (Oades, 1995).
Wet oxidation has been suggested as a way to attack
accessible OM, leaving OM that would be inaccessible to exoenzymes. Interlayer OM would thus resist wet oxidation (Righi
et al., 1995). The interpretation of the results is difficult
because 5- and 20-year-old samples behave in a contrary way.
The method might separate young OM in the early stage of
attack. The final residue is enriched in old OM, but is nevertheless far from corresponding to very old or inert OM. The
490 J. Balesdent
Age of treatment /years
4.6
20.1
20.1 (N)
C
/ g kg-I soil C -
4.6
20.1
20.1 (N)
Proportion of young C
1%
-
~
Rothamsted model compartments
DPM
0
RPM
136
BIO
20
HUM
748
IOM
96
148
21
728
93
19
274
36
623
47
100
66.3
62.0
4.4
0
Particle-size fractions
> 50 pm
0-50 pm
Water soluble
178
804
18
438
538
24
166
82 I
13
10
100
99.7
90.7
25.9
100
0
99.9
97.3
56.4
0
45.0
3.6
13.6
72.9
25.5
45.3
74.9
43.8
60.8
Table 3 Proportion of young C in particlesize fractions and in chemical subfractions of
fraction 0-50 pm, as estimated from the
enrichment in I3C under maize cultivation,
and in the Rothamsted model compartments.
Sample dated 4.6 years is from La Minibre.
Samples dated 20.1 years are from Boigneville. Sample 20.1(N) is the upper layer of a
treatment with no tillage, thus relatively
enriched in young C. Errors in parenthesis
are systematic errors based on a systematic
error of 0.05% on each 6I3C used in the
calculation. Error is indicated when the
isotope composition was calculated by mass
balance. It is otherwise equal to 0.7%
Chemical separates of fraction 0-50 p n
Autoclaving ( H 2 0 , 120°C)
0-1 h
1-4 h
4-16 h
16-24 h
Residue
63
50
132
28
728
64
47
128
34
728
86
63
132
28
69 1
8.6
6.1
4.4
3.4
3.4
33.4
29.2
26.0
23.5
22.3
51.6
49.3
46.3
42.5
39.7
Alkaline extracts
Fulvic acids
Humic acids
Humin
157
133
655
180
132
626
159
176
5 27
3.O
6.8
3.2
20.7
28.3
24.6
40.6
46.4
43. I
Sequential acid hydrolysis (H2SO4 )
Extracted, 2 h 20"Ca
257
247
286
Extracted. 8 h 100°C'
306
388
355
3.4
(4.6)
1.7
Residue, 8 h 100°C
437
366
359
5.0
(0.7)
27.5
(5.11
26.7
(1.6)
22.9
(0.7)
48.9
(4.3)
46.6
( 1.9)
37.0
(0.7)
Sequential wet oxidation ( H z 0 2 )
Lost, 2 mmoI g-Ia
381
233
206
Lost, 10 mmol g-la
373
497
548
Residue, 10 mmol g-'
247
270
247
0.4
(2.9)
8.7
(2.1)
0.7
(0.7)
45.8
(5.5)
17.7
(1.4)
22.5
(0.7)
69.3
(6.3)
34.2
(1.3)
44.1
(0.7)
Thermal oxidation
Lost, 25-260°C"
321
360
316
Lost, 260-370°C'
469
444
488
Residue 370°C
210
196
196
2.2
(3.6)
5.4
(1.6)
2.4
(0.7)
24.6
(3.3)
29.4
(1.9)
18.6
(0.7)
42.7
13.9)
46.8
(1.7)
38.5
(0.7)
Pyrolysis ( N 2 )
Lost, 25-300°C"
205
206
199
Lost, 300-5 10"ca
473
505
392
Residue 510°C
322
289
409
-0.3
(6.0)
6.7
(1.3)
2.9
(0.7)
23.0
(6.3)
27.7
(1.3)
23.5
(0.7)
42.7
(6.6)
45.6
(1.5)
42.7
(0.7)
(1.8)
(02)
"Isotope composition calculated by difference from mass balance.
0 1996 Blackwell Science Ltd, European Journal of Soil Science, 47, 485-493
Carbon dynamics in cultivated soil
results are compatible with a relation between resistance to
H202and resistance to biodegradation, but further investigations with other tracers are required. It was difficult to analyse
the extracted material by mass balance because of an isotope
fractionation by oxidation, the extent of which is greater than
the effect being studied.
The two thermal attacks, oxidation and pyrolysis, gave
similar results: the younger products were always found in
products released at intermediate temperatures, but the age
differentiation was weak. Thermal decomposition of soil
carbon has been related to structure, with for instance some
decarboxylation occurring at low temperature, and aromatic
material generally oxidized at higher temperature than
aliphatic carbon, but the temperature of decomposition is also
affected by inorganic bonding (Schnitzer & Kahn, 1972). The
results from the sequential thermal degradation of whole soil
carbon are poorly related to soil carbon age.
Relating separates to compartments of Rothamsted model
and 4). According to their physical nature, fractions >50 pm
might contain the RPM compartment and fraction 0-50 pm
might contain BIO+HUM+IOM. The location of the
microbial biomass in the 0-50 pm fraction is supported by
experiments of inorganic N immobilization at La Minitre.
Microbially immobilized N was almost absent from the
fractions >50 pm (Balabane & Balesdent, 1995). Figure 3
shows the correspondence between new C in fractions
>50 pm and new C in RPM. The rather good agreement of
the shape of the kinetics indicated a good correspondence of
the life-time of fraction >50 pm and of RPM. The similar
plateau value indicated that most of RPM was in the fraction
>50 pm. Figure 4 shows the proportion of new C in the
separates compared to that in the model compartments. The
fraction > 50 pm also contains some BIO HUM IOM,
as discussed above. This is also in agreement with the
observation that the fraction > 50 pm contains more carbon
(0.65 kg C m-2) on the average than RPM (0.56 kg C m-2).
In Fig. 3 we see that fraction 0-50 pm agrees well with
+
The Rothamsted carbon model was run for the conditions of
the sites. Total soil carbon content in the model was first fitted
to actual content (4 kg C m-2) by adjustment of the carbon
input to soil. Under this condition the model gave an excellent
prediction of the proportion of new C in the soil (Fig. 2). Data
for the first 4 years were a little less than predicted by the
model. This could be due to dead plant material such as coarse
stems and root crowns not being taken into account in our
measurement of total C.
The enrichment of size fractions in new C was further
compared with the enrichment of model compartments (Figs 3
0.4
0
491
0.2
+
-c :
z2
0.3
3
z"
0
5 0.2
c
E
R
2
0.1
0
0
5
10
15
20
25
Timebears
Timebears
Fig. 2 The proportion of new C in total soil carbon, as measured in
maize-cultivated soils (m) and as predicted by the Rothamsted carbon
model (--).
C input in the model was adjusted to fit soil total soil C
content.
0 1996 Blackwell Science Ltd, European Journal of Soil Science, 47,485-493
Fig. 3 The incorporation of new C in particle-size fractions
compared to that in compartments of the Rothamsted carbon model.
and RPM (--).
(b) Fraction <50 pm
(a) Fraction > S O bm
(m) and HUM IOM B10 (--).
+
(m)
+
492 J. Balesdent
1.0 1
1
C
.-0
9 0.8
)
L.
al
d
.C
0
z
0.6
0.4
C
.-0
I2
8
0.2
n
0
5
0
10
20
15
25
Timebears
Fig. 4 The turnover (proportion of new C) in particle-size fractions
compared to that in compartments of the Rothamsted carbon model.
Fraction >50 pm (m) and RPM (--).
(b) Fraction <50 pm (0)
and HUM + IOM + BIO (- - -).
+
+
BIO HUM IOM. Fraction 0-50 pm may nevertheless
contain some RPM (on the average 0.1 kg C m-*). To
summarize, the fraction >50 pm contains 0.45 kg RPM and
0.20 kg BIO HUM IOM, and fraction 0-50 pm 0.1 kg
RPM and 3.3 kg BIO HUM IOM.
In the Rothamsted model HUM + BIO is almost equally
supplied by DPM and RPM. The observed incorporation of
new C in the first years in fraction 0-50 pm (Fig. 3) is in good
agreement with such a hypothesis: HUM BIO supplied
either by RPM alone or by DPM alone would not tally with my
findings.
The chemical separates, also, were compared to Rothamsted
model compartments (Table 3). Water-soluble C may contain
one part of DPM, but from its kinetics of enrichment in new C
we can conclude that at least 70% of water soluble C is
extracted from the HUM IOM compartments. Most of the
young compartments (DPM, BIO, part of RPM) may be found
in the C extracted by autoclaving (Table 3), but this extract is
essentially derived from HUM IOM.
+
+
+
+
+
+
+
Conclusions
Physical fractionation of SOM in primary particle-size or
density separates fractions has been of interest for SOM
dynamics studies for decades (Christensen, 1992). The strong
correlation observed between particle-size and age demonstrates the usefulness of separating the coarsest fractions such
as ‘Free OM’ (HCnin et al., 1959) or ‘Particulate Organic
Matter’ (Cambardella & Elliott, 1992). Separated particulate
organic matter coarser than 50 pm by mild agitation without
ultrasonic treatment agrees well with the compartment of plant
structural material, named ‘RPM’ by Jenkinson & Rayner
(1977), and it would probably do for the ‘STRUC’ compartment of Century (Parton et al., 1987). Organic matter extracted
by cold water or autoclaving contains some of the plant metabolic components and the microbial biomass, but it also
contains such a large amount of old organic carbon that its
separation appears useless for quantifying these compartments.
None of the other chemical separation methods I used allowed
the concentration of either the young metabolic material or the
old inert OM. Resistance to wet oxidation may reflect resistance to biodegradation. Alkaline extraction and acid hydrolysis cannot be recommended for investigating soil C turnover.
On the one hand, the kinetics of enrichment of new C confirms
that most of soil OM can be represented as being very
homogeneous so far as C dynamics are concerned. Thus, the
HUM compartment of Jenkinson & Rayner (1977) appears to
be supplied both by the progressive alteration of plant debris
and by rapid incorporation of soluble or microbial C. On the
other hand, the results of these rough chemical separations
agree with those obtained by more sensitive methods of
chemical characterization of OM (Capriel et al., 1992;
Schulten & Hempfling, 1992). They support the view that
variations in rate of C turnover in soils may be less due to
chemical composition of OM than to the localization and the
physical protection of OM (Oades, 1995; Golchin et al., 1994).
Acknowledgements
Long-term field experiments are essential for such studies.
I thank the Domaine ExpCrimental de La Miniere (INRA),
the Station d’Agronomie de Toulouse (INRA) and the Institut
Technique des CCrCales et de Fourrages for their contributions,
and J. P. PCtraud, C. Picot, M. Grably for their friendly help
in physical fractionation, chemical extraction and isotope
analysis, respectively.
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