Change in carbon and nitrogen stocks in soil under 13

Soil & Tillage Research 76 (2004) 39–58
Change in carbon and nitrogen stocks in soil under 13 years of
conventional or zero tillage in southern Brazil
Claudia P.J. Sisti a , Henrique P. dos Santos b , Rainoldo Kohhann b ,
Bruno J.R. Alves c , Segundo Urquiaga c , Robert M. Boddey c,∗
a
Departamento de Fitotecnia, Universidade Federal Rural do Rio de Janeiro, 23890-000 Seropédica, Rio de Janeiro, Brazil
b Embrapa Trigo, Caixa Postal 569, Passo Fundo, 99001-970 Rio Grande do Sul, Brazil
c Embrapa Agrobiologia, Caixa Postal 74.505, Seropédica, 23890-000 Rio de Janeiro, Brazil
Received 30 July 2002; received in revised form 6 August 2003; accepted 11 August 2003
Abstract
The objective of this study was to determine in a long-term experiment (13 years) the effect of three different crop rotations
(R1: wheat (Triticum aestivum)–soybean (Glycine max), R2: wheat–soybean–vetch (Vicia villosa)–maize (Zea mays), and
R3: wheat–soybean–oat (Avena sativa)–soybean–vetch–maize) under zero tillage (ZT) and conventional tillage (CT) on the
stocks of soil organic matter (SOM) in a clayey Oxisol soil of Passo Fundo, Rio Grande do Sul. At the end of 13 years, soil
samples were taken to a depth of 100 cm, and analysed for bulk density, chemical composition and 13 C natural abundance.
Under a continuous sequence of wheat (winter) and soybean (summer) the stock of soil organic C to 100 cm depth under ZT
(168 Mg ha−1 ) was not significantly different (LSD at P = 0.05 of 11 Mg ha−1 ) to that under CT (168 Mg ha−1 ). However,
in the rotations with vetch planted as a winter green-manure crop (R2 and R3), soil C stocks were approximately 17 Mg ha−1
higher under ZT than under CT. Between 46 and 68% of this difference occurred at 30–85 cm depth. The 13 C abundance
data indicated that under ZT the decomposition of the original native SOM was not affected by the different composition of
crops in the different rotations, but under CT the rotations R2 and R3, which included vetch and maize, stimulated the decay
of the original native SOM compared to the continuous wheat/soybean sequence (R1). It appears that the contribution of N2
fixation by the leguminous green manure (vetch) in the cropping system was the principal factor responsible for the observed
C accumulation in the soil under ZT, and that most accumulated C was derived from crop roots.
© 2003 Elsevier B.V. All rights reserved.
Keywords: 13 C; Crop rotation; Green-manure crop; Soil organic matter; Tillage methods
1. Introduction
In the 1970s it had become common practice in
southern Brazil to plant both a summer and winter
crop, and hence fields were being ploughed twice a
year. This frequent ploughing stimulated loss of soil
∗ Corresponding author. Tel.: +55-21-2682-1500;
fax: +55-21-2682-1230.
E-mail address: [email protected] (R.M. Boddey).
organic matter (SOM) and in many regions soil erosion
was becoming a serious problem. For this reason some
innovative farmers, with the help of pesticide companies and state and federal research organisations,
started the introduction of zero tillage (ZT). Until the
early 1990s adoption of this system was relatively
slow, reaching approximately 1 M ha in 1992. However, gradually farmers began to appreciate that ZT
required less field operations, thus fuel consumption
was considerably lower, and crops could be planted
0167-1987/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.still.2003.08.007
40
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
earlier than under conventional tillage (CT). For these
reasons, once farmers became accustomed to the new
system, there was a very rapid increase in its adoption,
so that today more than 70% of the soybean-based
crop rotations in the southern region are estimated
to be under ZT, and country-wide >14 M ha of grain
and fibre production use this system (Embrapa,
2002).
In North America many studies have shown that the
conversion from CT to ZT promotes the accumulation
of SOM (Kern and Johnson, 1993; Lal, 1997), and
hence recently much attention has been given towards
the use of conservation tillage as a method to mitigate agricultural emissions of CO2 . In Brazil most,
but not all, studies also indicate that the introduction of ZT increases SOM. Early studies by Muzilli
(1983) on maize or soybean as summer crops in rotation with wheat as a winter crop at two sites in
the state of Paraná for 4 and 5 years, suggested that
only continuous maize/wheat at one site had C concentrations in the soil significantly higher under ZT
than CT to a depth of 30 cm. However, Sidiras and
Pavan (1985) recorded higher C concentrations under
ZT than under CT to a depth of 20 and 40 cm at
two sites in Paraná where the legume perennial soybean (Glycine wightii) was included in the rotations.
More recent studies in Rio Grande do Sul covering
periods from 5 to 12 years (Bayer and Mielniczuck,
1997; Bayer and Bertol, 1999; Amado et al., 1999,
2001; Bayer et al., 2000a,b) have all shown that soil
C stocks under a variety of crop rotations managed
under ZT were higher than under the same rotations
under CT. In these studies the soils were sampled
to only 17.5 or 20.0 cm depth, with one exception—
30 cm (Bayer et al., 2000b). In contrast, under continuous soybean/wheat, and a 2-year rotation of soybean/wheat followed by vetch/maize (V/M), Machado
and Silva (2001) and Freixo et al. (2002) reported
that there were no differences in soil C stocks to
30 or 40 cm depth between CT and ZT in two further studies conducted in Paraná and Rio Grande
do Sul.
At a site in Paraná (Ponta Grossa) at the end of 22
years of cropping under ZT, soil C stocks to 40 cm
depth were 19 Mg ha−1 higher than under CT (de Sá
et al., 2001). However, this study was based on a
chronosequence and not on a long-term experiment
like the other studies cited above, and the sequence
of crops under the two tillage systems was not
identical.
Two studies performed in the tropical central savanna region of Brazil (Centurion et al., 1985; Corazza
et al., 1999) showed that while soil C stocks under
ZT were higher than under CT in the surface 0–20 or
0–30 cm depth intervals, when sampling was extended
to 100 cm depth these differences disappeared due to
lower C content in the 30–100 cm depth interval under ZT. These results suggest that it may be advisable
to sample to a depth of 100 cm or more.
One powerful tool to study changes in soil organic
carbon under different crops/tillage treatments comes
from the determination of the 13 C natural abundance
of the soil C. Old-growth forests in both temperate
and tropical regions are predominantly composed of
plants possessing the C3 photosynthetic pathway. Such
plants have a 13 C abundance of approximately −26
to −28‰ (Smith and Epstein, 1971) and this is reflected in the surface layers of the soil (e.g. Flexor
and Volkoff, 1977; Balesdent et al., 1988). Crops such
as maize, sorghum (Sorghum vulgaris) and sugarcane
(Saccharum spp.) and many pastures grasses of tropical origin, possess the C4 photosynthetic pathway and
plant tissues usually show a 13 C abundance of between
−10 and −12‰. Where the original SOM was derived
predominantly from just one type of vegetation (e.g.
C3 ), and the crops planted are predominantly of the
other type, it is often possible to quantify the C input
to the soil derived from the crop residues (Cerri et al.,
1985; Balesdent et al., 1988; Tarré et al., 2001).
Our objective was to investigate changes in soil C
and N stocks to a depth of 100 cm in response to different rotation and tillage treatments. Soil was analysed for 13 C abundance in an attempt to separate C
derived from crop residues from that derived from the
native vegetation.
2. Materials and methods
2.1. Experimental design
This study was conducted at the Experimental Station of the Embrapa Wheat Research Centre in the
Municipal district of Passo Fundo (28◦ 15 S, 52◦ 24 W,
altitude 684 m above sea level). The soil at the site was
an Oxisol of clayey texture (1% coarse sand, 23% fine
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
sand, 13% silt, 63% clay—in the Brazilian classification: Latossolo Vermelho distrófico). The experiment
was initiated in November of 1985 when the soil was
ploughed and limed with 7 Mg ha−1 of dolomitic lime.
Until the sampling of these plots for this present study
no further lime additions were made.
The experiment had four tillage treatments:
(1) ZT, (2) minimum cultivation, (3) CT with
a disc plough and (4) CT with a mouldboard
plough to, and three crop rotations of, respectively, 1, 2 and 3 years duration. These crop
rotations were: R1, wheat/soybean (W/S); R2,
wheat/soybean–hairy vetch/maize (W/S–V/M); and
R3, wheat/soybean–white oat/soybean–vetch/maize
(W/S–O/S–V/M). The wheat, vetch and oat were
planted in winter and the soybean and maize in summer. In the rotations containing maize, for just 1
year (1996 for rotation R2 and 1995 for rotation R3),
maize was substituted by sorghum (owing to a problem with theft of the maize cobs). The experiment was
arranged in a randomised complete block (split plot)
design with three replicates; tillage treatments were
in main plots and rotations in subplots of 10 m × 4 m.
From 1987 onwards all summer crops were direct
drilled into the straw remaining from the preceding
winter crop, and ploughing with the disc plough to a
depth ∼20 cm before planting was only practised for
the winter crops in the CT treatments. Seeding, weed
and disease control were carried out as recommended
for each crop in this region.
2.2. Grain and crop residue production
All soybean, wheat and maize crops were harvested
for grain with a special small-plot harvester. From
1994 to 1996 total dry matter yields were recorded and
harvest indices of the maize, sorghum, wheat and soybean were calculated. These indices were then used
to calculate total crop aerial production for all years.
Tomm (1996) estimated total dry matter accumulation
of the oat and vetch for these years and the mean values were used for the whole experiment.
In 2000, samples of residues (aerial tissue) of all
crops in the rotations (soybean, wheat, vetch, oat and
maize) were taken from the soil surface in the ZT
treatments immediately after harvest of the grains, or
in the case of vetch and oat, the day after knocking
down the oat crop with a knife roller. These samples
41
were dried and ground to a fine powder (<0.15 mm)
using a roller mill similar to that described by Smith
and Myung (1990). Aliquots containing between 200
and 400 ␮g total C were analysed for 13 C abundance
using a continuous-flow isotope-ratio mass spectrometer (Finnigan DeltaPlus mass spectrometer coupled
to the output of a Carlo Erba EA 1108 total C and N
analyser—Finnigan MAT, Bremen, Germany).
2.3. Soil samples
Soil samples were collected from all three replicates of all three rotations under ZT and CT with
a disc plough. Composite samples consisting of six
sub-samples were taken at depths of 0–5, 5–10,
10–15, 15–20, 20–30, 30–40, 40–55, 55–70, 70–85
and 85–100 cm in each sub-plot and from three areas
(six sub-samples per area) separated by approximately
30 m in a neighbouring area of native vegetation
(Araucaria woodland) in May 1999. Soil bulk density
was estimated by opening one sampling trench per
sub-plot, and four samples were taken at each depth
interval sampled using a bevelled ring of known volume (length 7.4, diameter 5 cm = 145 cm3 ). The soil
from the rings was removed and weighed after drying
at 110 ◦ C for 72 h.
Soil samples were air dried, passed through a 2 mm
sieve, thoroughly homogenised, and sub-samples
taken for analyses of soil fertility indices (exchangeable K, Ca, Mg, Na, Al; pH and available P [Mehlich
I]) according to standard procedures (Embrapa, 1999).
For the analyses of total N and C, and the C isotopic
abundance, sub-samples were further ground to a fine
powder (<0.15 mm) using the same roller mill used
to grind the crop residues (Smith and Myung, 1990).
Total N concentration was determined on aliquots
of 1.0 g of soil using semi-micro-Kjeldahl digestion
in heated aluminium digester blocks (36 samples,
two standards and two blanks) followed by steam
distillation using a Tecator Kjeltec model 3100 (Tecator, Höganäs, Sweden) automatic titration/distillation
unit as described by Urquiaga et al. (1992). Total C
analysis was performed on ∼150 mg aliquots of the
samples using a total C and N analyser (LECO model
CHN 600, Leco Corp., St. Joseph, MI). The 13 C isotopic abundance of the soil samples was determined
on aliquots containing between 200 and 400 ␮g total
C as described above.
42
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
2.4. Calculations
2.4.1. Correction for soil compaction
To correct for the compaction of the soil induced by
cultivation and traction of agricultural machinery, the
total C and N stocks in the soil were estimated as the
total C or N content of the same weight of soil as that
present to 100 cm depth in the adjacent forest (mean
of three profiles) using the procedure originally recommended by Vallis (1972). It was assumed that soil
compaction due to mechanical operations was most
significant in the surface layers of the profiles so that
the C and N stocks were calculated by subtracting the
total C and N content of the extra weight of soil in
the deepest (85–100 cm) layer sampled of each profile
(Neill et al., 1997). This correction can be expressed
mathematically as
Cs =
n−1
i=1
CTi + MTn −
n
i=1
MTi −
n
MS i
C Tn
i=1
(1)
where Cs is the total C stock (Mg C ha−1 ) in soil to a
depth equivalent to
the same mass of soil as that in the
reference profile, n−1
i=1 CTi the sum of the total carbon
content (Mg ha−1 ) in the layers 1 (surface)to layer
n − 1 (penultimate) in the treatment profile, ni=1 MSi
the sum of the mass of soil (Mg ha−1 ) in layers 1 (surface)
n to n (greatest depth) in the reference soil profile,
−1
i=1 MTi the sum of the mass of soil (Mg ha ) in
layers 1 (surface) to n (greatest depth) in the treatment
profile, MTn the mass of soil in the deepest layer in the
treatment profile and CTn the concentration of carbon
(Mg C Mg per soil) in the deepest layer in the treatment profile. The reference profile is often a profile
of neighbouring native vegetation, but if no such area
is available, or no information is available as to the
history of the area prior to the installation of the experiment, any one treatment (preferably that with the
lowest soil mass in the profile) can be used to correct
the others (Balesdent et al., 1990).
2.4.2. Estimation of the proportion of soil C derived
from original native vegetation
Balesdent et al. (1990) developed a technique that
was applied in this study to determine the proportions
of C derived from SOM under the original vegetation
and that derived from crop residues under the different experimental rotation and tillage treatments. The
technique is based on a simple mixing model for two
sources of carbon A and B, with values of δ13 C of δA
and δB , respectively. It follows that the 13 C abundance
of the mixture will be described by the equation
δ = fδA + (1 − f)δB
(2)
If the total organic carbon content (g C kg per soil)
is known, then the amounts of carbon from the two
sources are CA = fC and CB = (1−f)C, respectively.
Eq. (2) can then be written as
1
δ=
(3)
(δA − δB )CA + δB
C
For this study A represents the organic carbon remaining in the soil derived from the native vegetation, and B the organic carbon derived from the crop
residues/roots.
A regression of δ (the measured 13 C abundance) for
each depth interval versus 1/C will give the intercept
on the Y-axis of the δ13 C of the organic carbon derived
from the crop residues/roots (δB ), and the slope will
be (δA − δB )CA . Furthermore, Balesdent et al. (1990)
showed that if these regressions were plotted for different cropping systems which were installed in a uniform area under the same vegetation, the point where
the linear regression lines cross can be considered as
defining the 13 C natural abundance of the organic carbon common to the different systems, i.e. the organic
carbon remaining in the soil derived from the original vegetation at the time of the installation of the experiment. This procedure involves the assumption that
the 13 C abundance of the soil derived from the native
vegetation is constant with depth.
For rotation 1, which consisted of only two C3 crops
(and the δ13 C abundance of the residues of all crops
was estimated), once the 13 C abundance of the soil derived from the original vegetation has been estimated
from the regression procedure, it is possible to calculate the proportion of C derived from the crop residues
and from the original native SOM using the standard
mixing model utilised by Vitorello et al. (1989) and
others
fCdnv =
δ − δC3
δNV − δC3
(4)
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
where fCdnv is the proportion of soil carbon derived
from the original native SOM and the subscripts A
and B have been substituted by ‘NV’ for the original
native vegetation and ‘C3 ’ for the C3 crop residues,
respectively.
For the crop rotation treatments R2 and R3 the proportions of soil C derived from the original native vegetation and from the crop residues (which included
both C3 crops and the C4 maize), two different techniques were applied:
1. In the first the 13 C abundance of soil C derived
from the original native vegetation was taken as
the value estimated from the regression technique
of Balesdent et al. (1990) applied to the ZT data
only. That is, the δ13 C value (δNV ) registered where
the regression of 1/C with δ13 C for R1 intercepted
the same regressions for R2 and R3 (see above).
The mean values of δ13 C of the soil C derived
from the crop residues (δres ) were taken from the
intercepts of the regressions with the Y-axis for R2
and R3 in the ZT treatment. Utilising these values,
the proportion of C derived from the original native
vegetation (fCdnv ) was calculated for each depth
interval to 30 cm again using the standard mixing
model equation
fCdnv =
δ − δres
δNV − δres
43
C derived from all crop residues (Cres ) and the
original native vegetation (CNV ) can be calculated
by subtracting the total C in the depth interval
from the total C derived from the native vegetation
under the W/S rotation (CNVR1 ). This for rotation 2
becomes
CresR2 = CR2 − CNVR1
(6)
where CresR2 is the total C derived from the crop
residues (including those from both the C3 crops
and maize) in R2 and CR2 the total soil C in the
respective depth interval in this rotation treatment.
For R3 the same equation was used except that
the subscripts R2 were substituted by R3.
To make a comparison between the estimates of
Cres and CNV derived from the use of the two different techniques, the values of the 13 C abundance
of the residues, which in the first technique was estimated from the regression technique of Balesdent
et al. (1990), can also be calculated from an isotopic
mass balance which for R2 becomes
(CresR2 δresR2 ) + (CNV δNV ) = CR2 δR2
(7)
The assumptions involved in the application of these
techniques are discussed in Section 3.5.
(5)
where δ is the value of the 13 C abundance of the soil
in each depth interval. Using the C concentration
of the soil in each layer the quantities of C derived
from the crop residues and the native vegetation
under each rotation were computed.
2. The second technique was based on the hypothesis
of Cadisch and Giller (1996) that the rate of decomposition of the C derived from the original native
vegetation would not be affected by the composition of crops in the rotation. This model was originally developed to estimate the proportion of C
derived from a legume in a mixed Brachiaria/forage legume pasture established at the same time as
a grass-alone Brachiaria pasture, on a site derived
from tropical forest, and they assumed that the
presence of the legume in the pasture made no significant effect on the rate of decomposition of the
original forest derived C. Adapting their model to
this present study, for each depth interval the total
2.4.3. Statistical analyses
All data for total C and N and 13 C isotopic
abundance of soil were subjected to standard analysis of variance procedures (split plot design with
tillage treatments in main plots and crop rotations
in sub-plots) using the software package MSTAT-C
(Michigan State University, USA). To evaluate
whether there were statistically significant effects
of the tillage regime (ZT or CT) on the accumulation of C and N under the individual crop rotations, it was necessary to compare sub-plot treatment means for different main plots. This was
achieved using the procedure described by Little and
Jackson-Hills (1978): the calculation of the least significant difference between means (LSD, Student)
becomes
LSD0.05 = tab {2[(b − 1)Ea + Eb ]/rb }
(8)
where b is the number of subplot treatments, r the
number of replicates, Ea and Eb are the mean squares
44
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
of the sub-plot and main plot errors, respectively, and
tab the weighted t value for main plots and subplots
calculated as described by Little and Jackson-Hills
(1978). This procedure was performed on the software SISVAR, produced by the Federal University of
Lavras (UFL), Lavras, Minas Gerais.
A statistical comparison of slopes of the regressions
of δ13 C versus 1/C (the technique of Balesdent et al.,
1990) was made with the Student’s t-test using the
program SigmaStat® version 1.0 (SPSS Inc., Chicago,
IL).
3. Results and discussion
3.1. Soil fertility
The chemical soil fertility data are presented for
just the 0–20 cm layer in the profiles under the three
crop rotations and the neighbouring native vegetation
(Table 1). The initial addition of 7 Mg lime ha−1 raised
pH of the soil at the end of 13 years of cropping by
approximately 0.5 pH units above that registered in
the unlimed area under native vegetation. The effect
Table 1
Soil fertility parameters under the neighbouring native vegetation or after 13 years of cultivation of three different crop rotations under
either no-tillage or CTa
Tillage system
Al (cmol dm−3 )
Ca (cmol dm−3 )
Mg (cmol dm−3 )
K (mg dm−3 )
P (mg dm−3 )
(0.10)1
(0.17)
(0.10)
(0.08)
0.4
0.4
0.4
0.4
(0.15)
(0.08)
(0.10)
(0.07)
4.9
4.5
5.0
4.9
(0.19)
(0.33)
(0.14)
(0.20)
1.9
2.0
2.1
2.4
(0.28)
(0.19)
(0.17)
(0.12)
238
168
148
118
(7.7)
(7.6)
(3.6)
(8.3)
37
23
11
6
(0.49)
(2.49)
(1.74)
(0.47)
5.0
5.2
5.3
5.2
(0.08)
(0.14)
(0.12)
(0.09)
0.0
0.1
0.1
0.3
(0.03)
(0.03)
(0.02)
(0.14)
4.1
4.3
4.4
3.6
(0.34)
(0.29)
(0.45)
(0.54)
2.0
2.0
2.3
2.1
(0.17)
(0.14)
(0.30)
(0.21)
223
137
130
122
(15)
(7.8)
(5.1)
(3.6)
19
17
14
13
(1.42)
(1.11)
(0.14)
(0.62)
5.1
5.0
5.2
4.7
(0.14)
(0.14)
(0.19)
(0.07)
0.7
0.7
0.8
1.3
(0.07)
(0.07)
(0.12)
(0.12)
3.8
3.4
3.3
3.4
(0.26)
(0.60)
(0.68)
(0.23)
1.6
1.6
1.8
1.7
(0.08)
(0.05)
(0.26)
(0.14)
218
155
143
122
(4.1)
(5.8)
(4.7)
(4.9)
28
18
13
8
(1.42)
(1.74)
(1.91)
(0.82)
5.2
5.0
5.1
4.8
(0.12)
(0.09)
(0.14)
(0.07)
0.5
0.6
0.5
0.6
(0.12)
(0.12)
(0.08)
(0.12)
4.2
4.5
4.2
3.2
(0.22)
(0.35)
(0.36)
(0.28)
1.8
1.9
2.0
2.3
(0.26)
(0.14)
(0.15)
(0.22)
244
137
125
113
(7.8)
(2.7)
(7.8)
(5.7)
17
18
14
10
(1.09)
(1.63)
(1.78)
(1.30)
5.0
5.2
5.1
4.9
(0.07)
(0.17)
(0.15)
(0.07)
0.3
0.4
0.6
0.5
(0.07)
(0.07)
(0.05)
(0.01)
4.2
3.2
3.3
3.8
(0.20)
(0.57)
(0.40)
(0.36)
1.8
1.5
1.6
2.0
(0.14)
(0.03)
(0.09)
(0.15)
248
175
135
88
(7.2)
(4.3)
(3.5)
(6.5)
29
24
11
8
(2.45)
(0.62)
(1.19)
(0.98)
0–5
5–10
10–15
15–20
4.9
5.1
5.0
4.5
(0.07)
(0.14)
(0.09)
(0.07)
0.5
0.6
0.6
0.6
(0.03)
(0.03)
(0.05)
(0.10)
3.9
4.2
4.5
4.5
(0.27)
(0.22)
(0.38)
(0.28)
2.2
2.0
1.8
2.2
(0.08)
(0.14)
(0.12)
(0.22)
242
136
133
120
(3.6)
(3.3)
(5.4)
(8.0)
22
18
12
7
(1.78)
(0.98)
(1.44)
(0.62)
0–5
5–10
10–15
15–20
4.8
4.5
4.3
4.3
Depth
(cm)
pH (H2 O)
0–5
5–10
10–15
15–20
4.9
5.2
5.0
4.7
0–5
5–10
10–15
15–20
Rotation 2: W/S–V/M
No-till
0–5
5–10
10–15
15–20
Rotation 1: W/S
No-till
Conventional
Conventional
0–5
5–10
10–15
15–20
Rotation 3: W/S–O/S–V/M
No-till
0–5
5–10
10–15
15–20
Conventional
Neighbouring
native
vegetation
a
0.6
2.5
2.7
3.3
5.3
2.5
1.5
1.1
1.9
1.6
1.1
1.0
Values are means of three replicates. Values in parentheses indicate standard errors of the means.
102
51
34
31
4
3
3
3
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
Table 2
Mean annual applications (kg ha−1 per year) of N, P and K as
fertiliser, to the different crop rotations during the 13-year perioda
Crop rotationsb
N (kg ha−1
per year)
P (kg ha−1
per year)
K (kg ha−1
per year)
W/S
W/S–V/M
W/S–O/S–V/M
53
60
48
45
37
40
98
76
83
a
Fertiliser applications were the same for both tillage regimes.
Codes for crops are as follows: W, wheat; S, soybean; V,
vetch; M, maize; O, white oats.
b
of liming on the availability of Al3+ , was more significant, with values under native vegetation approximately 2.8 cmol dm−3 in the 5–15 cm layers compared with values generally <0.8 cmol dm−3 under
cropping. Likewise Ca2+ levels were raised to approximately 4 cmol dm−3 down to 20 cm depth, whereas
under native vegetation values were <1.5 cmol dm−3
below 10 cm depth.
Table 2 shows the mean annual quantities of N,
P and K added as fertiliser during the 13 years of
cropping. Fertiliser additions were identical for each
rotation whether under ZT or CT.
There was a distinct concentration of P in the top
5 cm under ZT compared to CT (Table 1) which illustrates the immobility of this nutrient in these acid
soils and earlier observations by many authors (e.g.
Muzilli, 1983; Sidiras and Pavan, 1985; Selles et al.,
1997). This higher concentration of plant-available
P in the surface horizon under ZT compared to CT,
eventually results in a greater efficiency of P utilisation under ZT, as under CT the mixing of P derived
from plant residues with a greater volume of soil leads
to enhanced P fixation. Gassen and Gassen (1996)
have reported that after some years, demand for P
fertiliser is lower (up to 50%) for the same P uptake
of crops under ZT for this reason. In contrast, regardless of the fact that the ZT plots were never ploughed,
the concentration of potassium down the profiles of
the rotations managed under either tillage system was
very similar due to the much higher mobility of K in
the soil.
3.2. Crop yields and residue C and N inputs
Grain yields were recorded throughout the experiment and in general yields of both soybean and wheat
45
were higher in more diversified rotations (R2 and
R3) than in the simple annual rotation of wheat and
soybean (Table 3) reflecting the lower incidence of
diseases, particularly of the roots in the former (dos
Santos et al., 1990; dos Santos and Lhamby, 1998,
2000). The mean yields of soybean obtained in the
different rotations compare well with the mean national yield of this crop which was 2400 kg ha−1 , for
the 1999/2000 season (IBGE, 2002). Soybean and
wheat yields were considerably higher than the mean
yields for the state of Rio Grande do Sul for the same
season (1590 and 1600 kg ha−1 , respectively).
In the period 1995–1996, total dry matter production of all crops in the three rotations and the N content of the residues were recorded (Table 4). Utilising
these data along with the grain yields for each year
and assuming that all residues contained 45% C, it
was possible to make an approximate estimate of the
total input of C and N in the crop residues during the
13-year period (Table 5).
The total C and N inputs in the crop residues
(aerial tissue) were higher where maize and vetch
were included in the rotations. This was partially
because mean maize yields were between 6 and
7.5 Mg ha−1 , considerably higher than either soybean
or wheat (approximately 2.4 and 3.0 Mg ha−1 , respectively, Table 3), and in these rotations all of the vetch
biomass was left as residue for the next crop (mean
yields between 3 and 4 Mg ha−1 ; G.O. Tomm, unpublished data). While for R1 (W/S) soybean yielded
10% less under ZT than under CT, wheat yields were
approximately 11% higher under ZT and had almost
twice the C content of the soybean. In R2 (W/S–V/M)
under ZT soybean yields were 5% lower, wheat yields
11% higher and maize yields 20% higher, than under
CT. In R3 (W/S–O/S–V/M) under ZT soybean yields
were 6% higher, wheat yields 3% higher and maize
yields 10% higher than under CT. In summary total
C inputs for rotations 1, 2 and 3 were, respectively,
6, 8 and 4% higher under ZT than CT. Total residue
N inputs were slightly less affected by the tillage
system, being 5, 5 and 4 % higher under ZT for the
3 rotation systems, respectively. It should be pointed
out that these inputs do not include below-ground
residues (dead roots or root exudates) and that the
total N inputs are partially derived from soil (recycled
N) and partially from fertiliser and biological nitrogen
fixation (inputs from outside the soil/plant system).
46
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
Table 3
Influence of the crop rotation and form of tillage (ZT, CT) on grain yields per crop (kg ha−1 ) of soybean, wheat and maize during the
13-year period
Year
Soybean
Maizea
Wheat
R1b
R2c
R3d
R1
ZT
CT
ZT
CT
ZT
CT
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
1795
1101
2897
1900
945
1955
2533
2972
3508
3151
2907
2202
2002
1788
1189
2916
2512
1028
3144
2423
2894
3515
3053
2617
2082
2080
1733
1562
1866
1181
1928
1244
3491
3467
Mean
S.E.M.g
2298
217
2403
208
ZT
CT
ZT
CT
3177
2972
1947
3040
1936
2646
2919
1061
2136
1904
1530
1791
693
–
3448
3255
3405
3204
2189
1927
2968
3114
2135
217
3016
2843
2136
2308
2249
2129
2607
351
2739
407
2492
267
2351
273
2417
236
1238
3445
3901
3273
3853
3520
3102
3565
2830
3109
2648
R2
CT
3224
3052
1158
2576
1126
R3
ZT
3291
2982
2064
3020
3060
2469
3664
1207
3124
2242
1504
1703
1085
–
2565
1667
R2
3299
2317
4142
4295
3455
3032
4648
4878
2764
2601
R3
ZT
CT
6110
5327
7724
7372
5277
4278
NAe
NA
4474f
3084
ZT
CT
8088
7697
4879
4628
7273f
7216f
1887
1777
1269
1092
1732
1280
8796
6674
9243
7049
2813
392
2528
406
3103
474
3015
579
6476
885
5347
871
7371
924
6648
687
a
Includes sorghum grain substituted for maize in 1995 and 1996.
R1: 1 year rotation—wheat planted in winter followed by soybean in summer.
c R2: 2-year rotation—W/S followed by V/M.
d R3: 3-year rotation—W/S followed by O/S followed by V/M.
e Data not available owing to theft of maize grain.
f Sorghum planted in 1995 and 1996 to avoid theft of grain.
g Standard error of the annual means for each crop.
b
3.3. Soil C and N stocks
In this study the soil under the native vegetation
neighbouring the experiment had a high C and N
concentration (37 g C and 3.1 g N kg per soil) in the
Table 4
Harvest index and concentration of nitrogen in the residues of the
six crops utilised in the three rotations. Data from G.O. Tomm,
unpublished research report, Embrapa Wheat Centre, Passo Fundo,
Rio Grande do Sul
Crop
Harvest
indexa
N concentration in
crop residues (g kg−1 )
Soybean
Wheat
Maize
Oats
Sorghum
Vetch
0.43
0.30
0.41
0.38
0.43
–
17
10
10
10
10
35
a
Total grain dry matter yield/total aerial tissue dry matter yield.
surface 0–5 cm, probably derived from plant litter of
slow decomposition rate (Fig. 1). C and N concentration declined to approximately half of these values at
10–15 cm depth. As was to be expected, the C concentration of the surface 5 cm of soil was considerably
higher in all three rotations managed with ZT than
with CT, although this was still not as high as under
the native vegetation (Fig. 2).
In the continuous annual rotation of W/S (R1),
only in the 0–5 cm layer there was a significantly
(P < 0.01) higher C concentration under ZT than CT.
However, in this same rotation at a depth of 15–30 cm
there was a greater C concentration under CT than
ZT, reflecting the fact that disc ploughing of the soil
once a year buried surface residues to this depth.
At lower depth intervals there were no significant
differences in C concentration. The same pattern of
significantly (P < 0.001) higher C concentration in
the 0–5 cm layer under ZT than CT was also evident
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
47
Table 5
Estimates of the total carbon and nitrogen deposited in the crop residues in the three crop rotations under ZT and CT during the 13-year
period of the experiment
W/S
ZT
Carbon content of residues
Soybean
Wheat
Maize/sorghum
Oats
Vetch
Total
C3 plant
C4 plant
Ratio C3 C:C4 C
W/S–O/S–V/M
CT
ZT
CT
ZT
CT
15500
28700
–
–
–
44200
16200
25350
–
–
–
41550
9500
18000
18700
–
12150
58350
10000
16200
15600
–
12150
53950
11600
14200
16000
8200
8100
58100
11000
13800
14500
8200
8100
55600
44200
–
–
41550
–
–
39650
18700
2.12
38350
15600
2.46
42100
16000
2.63
41050
14500
2.83
(kg C ha−1 )
Nitrogen content of residues (kg N ha−1 )
Soybean
585
Wheat
640
Maize/sorghum
–
Oats
–
Vetch
–
Total
1225
Mean C:N ratio
W/S–V/M
36.1
610
560
–
–
–
1170
35.5
under both the other rotations (R2 and R3, Fig. 2).
However, under R2 (W/S–V/M), C concentration
was significantly higher (P < 0.05) under ZT than
CT lower in the profile (30–40, 40–55, 55–70 and
70–85 cm). Under R3 (W/S–O/S–V/M), C concentration was significantly higher in the ZT treatment in
the 0–5, 40–55 and 55–70 cm depth intervals. Patterns
of total N concentration were similar to those of C
concentration (Fig. 3).
Greater accumulation of SOM at depth under ZT
means that if sampling were restricted only to the 20
or 30 cm depth (the maximum depth affected by the
disc ploughing under CT), in this study the gain in C
and N stocks by the adoption of ZT compared to CT
would be grossly underestimated. Soil C and N stocks
were estimated using the soil bulk density data at each
depth interval (Table 6) to calculate equivalent weights
of soil to the same depth (using Eq. (1), and the mean
values under the native vegetation as the standard).
When C and N stocks were calculated to a depth of
30 cm, it was apparent that there was no significant
difference in the quantity of SOM under ZT and CT
in R1 (Table 7), but there were significantly greater
C and N stocks in the soil under ZT compared to CT
358
400
420
–
950
2128
376
360
350
–
950
2036
27.4
24.5
440
315
350
180
630
1915
415
305
320
175
630
1845
30.1
30.1
under the other two rotations (R2 and R3), amounting
to differences of 5.3 and 9.1 Mg C ha−1 and 0.31 and
1.38 Mg N ha−1 , for R2 and R3, respectively.
The mild climate and year-round rainfall in the more
southerly states of Brazil (as well as more northerly
Table 6
Soil bulk density in the profiles (0–100 cm) of the soil below
the plots managed under ZT or CT and the neighbouring native
vegetationa,b
Depth (cm)
Native vegetation
(Mg m−3 )
ZT
(Mg m−3 )
CT
(Mg m−3 )
0–5
5–10
10–15
15–20
20–30
30–40
40–55
55–70
70–85
85–100
0.97
0.99
1.09
1.15
1.17
1.18
1.16
1.15
1.15
1.15
1.10
1.35
1.40
1.38
1.35
1.28
1.21
1.20
1.20
1.21
1.10
1.30
1.41
1.40
1.38
1.32
1.20
1.20
1.20
1.21
a
b
b
b
b
b
b
ns
ns
ns
ns
a
a
a
a
a
a
ns
ns
ns
ns
a
a
a
a
a
a
ns
ns
ns
ns
Values are means of three replicates.
Means followed by the same letter in the same row are not
significantly different at P = 0.05 (Student’s t-test, LSD).
b
48
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
g C kg soil-1
-1
g C kg soil
0
5
10
15
20
25
30
35
0
40
Total carbon
15
20
25
30
20
Depth (cm)
20
Depth (cm)
10
0
0
40
60
40
Rotation Wheat/Soybean
60
Zero tillage
Conventional tillage
80
Native vegetation
0
100
0
20
Depth (cm)
80
Total nitrogen
20
40
Rotation: Wheat/Soybean
- Vetch/Maize
60
80
40
Zero tillage
Conventional tillage
0
60
Native vegetation
20
80
100
0.0
0.5
1.0
1.5
2.0
2.5
g N kg soil
3.0
3.5
4.0
-1
Depth (cm)
Depth (cm)
5
40
Rotation: Wheat/Soybean
- Oat/Soybean
- Vetch/Maize
60
80
Zero tillage
Conventional tillage
100
Fig. 1. Total carbon (A) and nitrogen (B) concentration (g kg per
soil) of soil to a depth of 100 cm under the native vegetation neighbouring the tillage experiment. Means of three replicate profiles.
Error bars indicate standard errors of the means.
regions of Argentina and most of Paraguay) allows
cropping throughout the year. This means that the results of studies on the effects of ZT on soil fertility
and N and C stocks are not directly comparable with
those reported in numerous similar studies performed
in Canada and most of the USA. In these latter regions difference in soil C stocks between ZT and CT
recorded in long- and medium-term experiments are
rarely greater than 0.5 Mg ha−1 per year (Lal, 1997;
Paustian et al., 2002).
The reason that C stocks did not increase under ZT
compared to CT under the W/S (R1) rotation could
be attributed to the fact that for there to be an accumulation of SOM there must be not only a C input from crop residues but a net external input of N.
Recent 15 N and ureide abundance studies by Alves
0
5
10
15
20
25
30
g C kg soil-1
Fig. 2. Total carbon concentration (g C kg per soil) of soil to a
depth of 100 cm under three crop rotations managed for 13 years
under either zero or CT. Means of three replicate profiles. Error
bars indicate least significant differences between means (LSD,
Student, P = 0.05).
et al. (2002) have shown that while up to 80% of soybean N can be derived from biological N2 fixation
(BNF) in similar rotations and conditions in southern Brazil (Londrina, Paraná State), the proportion of
soybean N exported from the field as grain is usually very close to this value. As the fertiliser input for
wheat (mean 53 kg N ha−1 per year) was only approximately 20 kg ha−1 per year greater than that exported
in the grain, the overall external annual input of N was
marginal and possibly negative under CT where soybean BNF is partially inhibited by the stimulation of
the mineralisation of SOM provoked by tillage (Alves
et al., 2002, 2003).
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
g N kg soil-1
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0
Depth (cm)
20
40
Rotation Wheat/Soybean
60
80
Zero tillage
Conventional tillage
0
Depth (cm)
20
40
Rotation: Wheat/Soybean
- Vetch/Maize
60
80
Zero tillage
Conventional tillage
0
Depth (cm)
20
40
Rotation: Wheat/Soybean
- Oat/Soybean
- Vetch/Maize
60
80
Zero tillage
Conventional tillage
100
0.0
0.5
1.0
1.5
g N kg soil
2.0
2.5
3.0
-1
Fig. 3. Total nitrogen concentration (g N kg per soil) of soil to a
depth of 100 cm under three crop rotations managed for 13 years
under either zero or CT. Means of three replicate profiles. Error
bars indicate least significant differences between means (LSD,
Student, P = 0.05).
The higher C and N stocks under ZT than under
CT in R2 and R3 to a depth of 30 cm are of similar magnitude to those reported in other studies conducted in Brazil and North America (Lal, 1997; Dick
et al., 1998; Paustian et al., 2002). In these two rotations the N2 -fixing green-manure crop, vetch, was included and the entire crop was left as residues for the
subsequent maize crop. It therefore seems reasonable
to conclude that this N input was the key to the observed SOM accumulation or conservation under ZT.
49
Furthermore, under CT this input was not apparent either because the BNF input was reduced by soil mineral N released by the disc ploughing that immediately
preceded this crop (Alves et al., 2002), and/or much
of the N was lost by leaching (NO3 − ) or in gaseous
forms (via NH3 volatilisation or denitrification) again
due to SOM mineralisation stimulated by tillage.
Other reports from Brazil where no legume was
included in the rotation (Muzilli, 1983) or the only
legume in the system was soybean (Machado and
Silva, 2001; Freixo et al., 2002), also indicate that C
stocks were not greater under ZT than CT. However,
when a green-manure legume was included in the rotation, C stocks under ZT were significantly higher
than under CT (Sidiras and Pavan, 1985; Bayer and
Mielniczuck, 1997; Bayer and Bertol, 1999; Amado
et al., 1999, 2001; Bayer et al., 2000a,b). De Maria
et al. (1999) compared the effects on SOM content
(0–30 cm) of 9 years of maize or soybean in summer,
with oat as the winter crop, under either ZT or CT. Despite the fact that maize yields (5.6–5.7 Mg ha−1 ) were
more than double those of soybean (2.1–2.4 Mg ha),
the much larger residue input under maize did not result in any significantly higher accumulation of SOM
under either tillage system, and over the study period SOM levels declined in all treatments. As the fertiliser N application to maize was only 91 kg N ha−1
and at least 70 kg N ha−1 (5.6 Mg grain ha−1 × 1.3%
N) was exported in the grain, the lack of C accumulation under the ZT or CT treatments appears to be
related to the lack of an external input of N to the
system.
Little attention has been paid to this question of
overall N inputs either in studies performed in Brazil
or in North America (Kern and Johnson, 1993; Lal,
1997). It may be that in systems fuelled by positive
N balances resulting from generous N fertiliser usage,
due to low cost, greater conservation of C and N stocks
can be achieved with reduced tillage.
Because of the significantly higher soil C and N
concentrations under ZT at depths greater than 40 cm,
the differences in C and N stocks between ZT and CT
calculated to 100 cm depth were even greater, amounting to differences of 16.9 and 16.7 Mg C ha−1 and
1.2 and 2.0 Mg N ha−1 , for R2 and R3, respectively
(Table 7). There were still no significant difference in
C stocks to this depth below the annual W/S rotation
(R1). The difference in C stocks for the depth interval
50
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
Table 7
C and N stocks in different depth intervals beneath three crop rotations managed under ZT or CTa
Rotation
Soil C stockb,c (Mg ha−1 )
Tillage treatment
ZT
Depth interval: 0–30 cm
R1
R2
R3
Mean
Coefficient of variation (%)
Depth Interval 0–100 cm
R1
R2
R3
Mean
Coefficient of variation (%)
Depth interval: 30–70 cm
R1
R2
R3
Mean
Coefficient of variation (%)
60.9
64.7
69.6
65.0
Mean
CT
a
a
a
a
4.3
168.0
178.2
179.4
175.2
Soil N stockb,c (Mg ha−1 )
62.2
59.3
60.5
60.7
ZT
a
b
b
b
61.6 A
62.0 A
65.0 A
4.3
a
a
a
a
167.5
161.3
162.7
163.8
5.0
5.3
6.2
5.5
Mean
CT
a
a
a
a
4.8
4.9
4.8
4.9
a
a
b
b
4.9 C
5.1 B
5.5 A
a
a
a
a
13.5
13.6
13.2
13.4
a
b
b
b
13.6 B
14.2 A
14.2 A
a
a
a
a
5.4
5.4
5.2
5.3
a
b
b
b
5.4 B
5.7 A
5.5 B
2.4
a
b
b
b
167.7 A
169.7 A
171.0 A
4.1
67.9
71.9
70.0
69.9
Tillage treatment
13.8
14.8
15.2
14.6
1.1
a
a
a
a
67.8
64.7
64.8
65.8
a
b
b
b
4.6
67.9 A
68.3 A
67.4 A
5.5
5.9
5.8
5.7
1.7
Values of individual treatments (rotation × tillage) are means of three replicates.
b Means in the same row followed by the same lower case letter are not significantly different at P < 0.05.
c Means in the same column followed by the same upper case letter are not significantly different at P < 0.05.
a
30–100 cm constituted 68 and 46% of the total difference (0–100 cm) between ZT and CT for R2 and R3,
respectively. For the N stocks these values were 75
and 31% for R2 and R3, respectively.
The large difference in SOM content between ZT
and CT at depths below the plough layer has been observed by other authors: Doran et al. (1998) found that
with 22 years of winter wheat in Nebraska, USA, the C
stock to a depth of 122 cm under ZT was 10.6 Mg ha−1
higher than under CT, but if only the 0–30.5 cm interval were considered, the difference between tillage
treatments was only 4.7 Mg C ha−1 . No other studies
in the southern region of Brazil have examined C and
N stocks below 40 cm depth. However, in the Cerrado (central edaphic savanna) region, Centurion et al.
(1985) and Corazza et al. (1999) showed that while
in the surface layers of the soil (0–20 and 0–30 cm,
for the two studies, respectively) under soybean, significantly greater accumulation of SOM occurred under ZT than CT, but this trend was reversed at greater
depths, such that there was no significant difference
in SOM content when the whole profile to a depth
of 100 cm was considered. These experiments were at
sites which had only been cleared of native vegetation
for a few years. The soils of this region suffer from
(inter alia) Ca deficiency and Al toxicity which must
be corrected down the profile if roots of soybean or
other crops are to penetrate to depth and resist dry periods (Ritchey and Sousa, 1997). Several authors have
reported that crop rooting depth may be restricted under ZT due to poor percolation of Ca from surface applied lime (Ritchey et al., 1980; Pavan et al., 1984). As
this problem is less acute under CT, this may explain
why at depths greater than 20–30 cm more C accumulated under CT than ZT. In contrast, in our study the
increased C accumulation below 30 cm could best be
explained by greater root density at depth under ZT
than under CT, but until further investigations are conducted we cannot suggest any reason why this should
occur.
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
3.4.
13 C
natural abundance data
δ13C (‰)
-16
13 C
-18
-20
-22
-24
-26
0
Depth (cm)
20
40
Rotation: Wheat/Soybean
60
80
0
Depth (cm)
20
40
Rotation: Wheat/Soybean
- Vetch/Maize
60
Native vegetation
Zero tillage
Conventional tillage
80
0
20
Depth (cm)
natural abundance of the crop residues (aerial
tissue) were as follows: soybean, −27.70 ± 0.22‰;
wheat, −28.68 ± 0.09‰; vetch, −28.34 ± 0.14‰; oat,
−28.55 ± 0.05‰; maize, −11.06 ± 0.03‰ (means of
four analyses, ±standard error).
As SOM becomes increasingly humified, 12 CO2 is
lost in preference to 13 CO2 , and as SOM deeper in
the profile is more humified the 13 C signal becomes
less negative with depth, usually amounting to 1–2
δ13 C units (‰) between the surface and 100 cm depth
(Trumbore et al., 1995; Tarré et al., 2001). A further factor that may be responsible for this increase
in 13 C abundance with depth is the fact that during
the last 150 years increased burning of fossil fuels of
low 13 C abundance has lowered average atmospheric
δ13 C by at least 1.3‰ (Friedli et al., 1984; Stuvier and
Braziunas, 1987), and age of SOM (time since deposition) increases with soil depth.
In this study the difference in the 13 C abundance between the surface layer (0–5 cm) under the native vegetation and that in the 85–100 cm depth interval was
approximately 5‰ less negative (Fig. 4), far greater
than could be explained by processes described above
(Cerri et al., 1985). These data suggest that in recent times in the area of native vegetation adjacent
to the experiment the vegetation was predominately
C3 , but there was a much larger proportion of tropical grasses of the C4 photosynthetic pathway present
in earlier times. Similar observations and conclusions
were made by Vitorello et al. (1989) in a study in forest
area near Piracicaba, São Paulo State, where δ13 C values were approximately 8‰ units less negative in the
50 to 70 cm depth interval than in the surface 30 cm.
The 13 C abundance under all three rotations under both ZT and CT of the soil in the deepest layer
(85–100 cm) was between 1.6 and 2.0‰ more negative
than that of the soil under the native vegetation at the
same depth (Fig. 4). If the soil under the native vegetation had been of exactly the same type as that under
the experiment at the time of its installation, it would
be expected that at this depth, where there should have
been little influence of crop residues/roots, that the
13 C abundance would have been very similar. These
data suggest that the 13 C abundance under the native
vegetation did not represent that under the experiment
prior to its installation.
51
40
Rotation:
Rotation:Wheat/Soybean
Wheat/Soybean
- -Oats/Soybean
Oat/Soybean
- -Vetch/Maize
Vetch/Maize
60
80
100
-16
-18
-20
-22
-24
-26
δ C (‰)
13
Fig. 4. 13 C natural abundance of soil to a depth of 100 cm under
three crop rotations managed for 13 years under either zero or
CT. Means of three replicate profiles. Error bars indicate least
significant differences between means (LSD, Student, P = 0.05).
The data for the 13 C abundance of the soil under the
continuous W/S (R1) indicated even more clearly that
the distribution of 13 C abundance with depth under the
neighbouring native vegetation was completely different from that under the experimental area (Fig. 4A).
The 13 C abundance of the soil under the neighbouring
native vegetation was −25.3‰ in the 0–5 cm depth
interval. If the 13 C abundance of soil under the experiment had been originally identical to that of the native vegetation, then the incorporation of C3 residues
(mean 13 C abundance ∼−28.2‰) into the soil would
have resulted in a 13 C abundance of between −25.3
52
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
and −28.2‰. However, under both tillage treatments
the 13 C abundance of the soil was considerably less
negative than that under the native vegetation down
the whole 100 cm of the profile. Furthermore, when
the C concentration of the soil in any depth interval
was significantly lower (Fig. 2), which could only be
reasonably explained by the presence of less (C3 ) crop
residues, the 13 C abundance was less negative than
the soil under the native vegetation. From this it was
concluded that the soil profile under the neighbouring native vegetation did not represent that under the
experiment prior to its installation and that this was
also likely to be true for total C stock. The fact that
the bulk density of the soil below 40 cm under the native vegetation was consistently (although not significantly) lower than under the experimental treatments,
also contributes to this conclusion.
13 C abundance was significantly different among
rotations and between tillage treatments (Fig. 4). Differences in δ13 C between R1 and the other two rotations were to be expected as in both the latter, maize,
a C4 crop (δ13 C = −11.1‰), was included in the rotations, whereas R1 consisted of only wheat and soybean (both C3 crops). As was mentioned above, the
fact that δ13 C was almost 2‰ more negative under
ZT than CT in the surface 0–5 cm of R1 indicates
that the greater C concentration in this layer under
ZT was due to the incorporation of more C3 residues.
Similarly, the soil C in the 10–15 cm depth interval was more negative reflecting the accumulation at
this depth of wheat and soybean crop residues buried
by the disc plough. There were no significant differences in 13 C abundance of the soil between CT and
ZT under continuous W/S (R1) at any depth interval
below 15 cm.
In R2 and R3 there was a strong tendency for the
13 C abundance of the soil below 40 cm under ZT to be
less negative. The most obvious explanation for this
was that there was more maize derived carbon (δ13 C =
−11.06‰) in the profile at depths below 30 cm. However, as there was a tendency (albeit weaker) for this
to be true also for R1, where no C4 crop was present,
this suggests that under ZT some of the SOM of the
surface layers of the soil, which was predominately C4
(δ13 C higher than −17‰), moved down the profile to
a greater extent under ZT than under CT due perhaps
to increased faunal activity, water infiltration or clay
illuviation etc.
3.5. Estimation of the proportion of soil C derived
from original native vegetation
As the 13 C data showed that the soil under the neighbouring vegetation was not representative of the soil
under the experiment at the time of its installation,
soil δ13 C data from the neighbouring native vegetation could not be used to calculate the proportions of
C derived from crop residues and the original native
vegetation. For this reason we resorted to the regression procedure of Balesdent et al. (1990) to predict
the 13 C abundance (and hence subsequently the total
C content) of the C derived from the native vegetation
prior to the installation of the experiment. To apply
this procedure, it is necessary to assume that the 13 C
abundance of the C derived from the original native
vegetation was approximately constant with depth for
the surface layers (0–30 cm) and the same in all rotation and tillage treatments to this depth.
The regressions for the three rotations under ZT and
under CT are displayed in Fig. 5, and it is apparent
that the results for the two tillage systems are very
different. The regression coefficients for the three rotations under ZT were uniformly high (from R2 =
0.94–0.98). Using the t-test to compare the slopes of
the three regressions, it was found that for R1 this was
significantly different to that of R2 at P = 0.049, and
significantly different to R3 at P = 0.025. The regression for the continuous W/S (R1) in the ZT treatment yielded an intercept value of −28.6‰ for the 13 C
abundance of the residues of these two crops, which
was bracketed by the measured values for the aerial
tissues of these crops (soybean, −27.70‰ and wheat,
−28.7‰). This adds support to the validity of the use
of this regression procedure.
In contrast, the regression coefficients for the different rotations under CT were considerably lower (for
R1 and R2, 0.66 and 0.60, respectively). The reason
for this appears to be that under ZT there is a steep
gradient of C concentration in the soil down the profile to 30 cm depth, hence the data are spread widely
along the regression plot. Under CT, where the soil
is thoroughly mixed once a year in the plough layer
there is only a narrow range of C concentrations (and
hence of 1/C) down to 30 cm depth so that extrapolation to the Y-axis cannot be regarded as reliable. For
this reason the estimate of the δ13 C of the original native vegetation derived from the regressions (−15.5‰)
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
Fig. 5. Regressions of the reciprocal of the total C concentration
(g C kg per soil) versus the 13 C abundance for soil samples taken
to a depth of 30 cm (0–5, 5–10, 10–15, 15–20, 20–30 cm) under
three crop rotations managed under either: (A) ZT or (B) CT.
Procedure of Balesdent et al. (1990), see Section 2. ∗∗ indicates
regressions significant at P < 0.01, ns indicates regressions not
significant at P < 0.05.
under ZT was utilised for subsequent calculations. It
is assumed that this value for the 13 C abundance of the
original native SOM was the same under CT and ZT.
For R1 the two source mixing model (Eq. (4)) was
applied using the value of −15.5‰ for δ13 C of the
original native SOM and −28.2‰ for the wheat and
soybean residues (arithmetic mean of the two measured values (−28.7 and −27.7‰) for these residues,
respectively). The resulting estimates of the proportion of C derived from the two sources (Table 8) indicate that under ZT almost half (44%) of the C in
the top 0–5 cm of the soil were derived from the crop
residues (CC3 ), but this gradually decreased with depth
to just under 15% in the 20–30 cm depth interval. In
53
this same rotation treatment (R1), under CT the mixing caused by the annual tillage led to a gradual decline in CC3 with depth from approximately 29% in
the surface 0–5 cm to 17% in the 20–30 depth interval.
In the other two rotation treatments, R2 and R3, crop
residues were composed of a mixture of residues of
both C3 crops and maize (C4 ) in unknown proportions
so that the overall 13 C abundance (δres ) could not be
determined directly from the individual values of the
C3 and C4 crops. In the first technique described in
Section 2.4.2 the mean values of δres were taken from
the intercepts of the regressions for R2 and R3 with
the Y-axis for the ZT treatment. Both intercepts had
the value of −24.5‰ (Fig. 5). To calculate the value
of δres for these rotations under CT it was assumed
that the 13 C abundance of the residues under CT was
equal to that under ZT. This implies that the ratio of the
quantities of C3 residues to C4 residues (both above
and below-ground) and their rates of decomposition
were the same under the two tillage systems. The data
in Table 5 show that the ratio of above-ground C3 to
C4 residues under ZT was 2.12 for R2 and 2.46 for R3,
and under CT, 2.63 for R2 and 2.83 for R3. No data
are available for the below-ground residues to support
the assumption that the relative rates of decomposition
of the C3 and C4 residues were unchanged.
As for R1, to calculate the proportions of C derived
from the crop residues and the native vegetation for
the rotations R2 and R3 under both tillage systems,
the two source mixing model (Eq. (5)) was applied
to for each depth interval from 0 to 30 cm using: (a)
the measured values of the 13 C abundance of the soil
(δ) in each depth interval, (b) the value of −15.5‰
for δ13 C of the original native SOM, and (c) −24.5‰
for the δ13 C of the residues. These calculations indicate that under ZT the quantity of C derived from the
original native vegetation was almost the same (46.0,
46.2 and 47.0 Mg C ha−1 for R1, R2 and R3, respectively) regardless of the composition of the crops in
the rotations (Table 8). Under CT, however, it appears
that the mixing of the residues with the native SOM
promoted changes in the quantity of original native
SOM, such that in R2 and R3, where vetch and maize
were included in the rotations (and also oat in R3),
the quantities of native SOM were lower than under the continuous W/S (R1) (Table 8). This suggests
the mixing, provoked by disc ploughing, of the native
SOM with the residues of lower C:N ratio in R2 and
54
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
Table 8
13 C natural abundance and total C content of soil (0–30 cm) under three different crop rotations at the end of 13 years of cropping under
either ZT or CTa
Tillage system/
rotation
Depth
(cm)
δ13 C of
soil C (‰)
ZT/R1
0–5
5–10
10–15
15–20
20–30
−21.21
−19.43
−18.37
−17.81
−17.45
Percentage of C derived from:
Native SOM
Crop residues
56.4
70.0
78.1
82.4
85.1
43.6
30.0
21.9
17.6
14.9
Total
ZT/R2
0–5
5–10
10–15
15–20
20–30
−19.87
−18.56
−18.02
−17.48
−17.05
51.4
66.0
72.0
78.0
82.8
48.6
34.0
28.0
22.0
17.2
Total
ZT/R3
0–5
5–10
10–15
15–20
20–30
−20.70
−18.90
−18.07
−17.63
−17.07
42.2
62.2
71.4
76.3
82.6
57.8
37.8
28.6
23.7
17.4
Total
CT/R1
0–5
5–10
10–15
15–20
20–30
−19.36
−19.48
−19.60
−18.41
−17.73
70.5
69.6
68.7
77.8
83.0
29.5
30.4
31.3
22.2
17.0
Total
CT/R2
0–5
5–10
10–15
15–20
20–30
−18.96
−18.86
−18.63
−18.30
−18.13
61.6
62.7
65.2
68.9
70.8
38.4
37.3
34.8
31.1
29.2
0–5
5–10
10–15
15–20
20–30
−19.26
−19.22
−18.72
−18.23
−17.41
58.2
58.7
64.2
69.7
78.8
41.8
41.3
35.8
30.3
21.2
Total
CT/R3
Total
a
Total C
(Mg C ha−1 )
Mg C ha−1 derived from:
Native SOM
Crop residues
11.96
9.77
9.83
9.91
19.49
6.75
6.84
7.68
8.16
16.59
5.21
2.93
2.15
1.75
2.90
60.96
46.01
14.94
12.65
10.15
10.72
11.09
20.06
6.51
6.70
7.72
8.65
16.60
6.14
3.45
3.00
2.44
3.45
64.67
46.18
18.49
14.98
12.18
11.51
10.72
20.20
6.33
7.58
8.22
8.18
16.67
8.66
4.60
3.29
2.54
3.52
69.58
46.98
22.60
9.38
9.80
10.64
11.13
21.28
6.61
6.82
7.31
8.66
17.66
2.76
2.98
3.33
2.47
3.62
62.23
47.06
15.15
8.80
9.27
10.23
10.61
20.44
5.42
5.81
6.67
7.31
14.46
3.38
3.46
3.56
3.30
5.97
59.34
39.67
19.67
9.43
9.73
10.60
10.83
19.94
5.49
5.71
6.81
7.54
15.71
3.94
4.02
3.79
3.28
4.23
60.53
41.26
19.27
The proportions of C derived from the original native SOM and from the W/S residues were calculated from Fig. 5.
R3 stimulated a faster decomposition of native SOM
than under R1, where the mean C:N ratio was higher
(Table 5). However, without knowledge of the quantities and quality of the root residues in each rotation
system, this explanation remains speculative.
In applying the second technique based on the assumption of Cadisch and Giller (1996), that the crop
composition of the rotation had no effect on the rate of
decomposition of the original native SOM, under ZT
the estimates of total C derived from the crop residues
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
55
Table 9
Total C, 13 C natural abundance and estimates of C derived from the original native SOM and all crop residues in rotations R2 and R3
under both ZT and CT, based on the assumption of Cadisch and Giller (1996) that the quantity of original native SOM remains the same
within each tillage treatment regardless of the composition of the crops in the rotation sequence, see Section 2.4.2
Tillage system/
rotation
Depth
interval (cm)
Total C
(Mg ha−1 )
δ13 C of soil
(‰)
Mg C ha−1 derived from:
ZT/R2
0–5
5–10
10–15
15–20
20–30
12.65
10.15
10.72
11.09
20.06
−19.87
−18.56
−18.02
−17.48
−17.05
6.74
6.84
7.68
8.16
16.59
5.91
3.31
3.04
2.93
3.47
46.01
18.66
0–5
5–10
10–15
15–20
20–30
14.98
12.18
11.51
10.72
20.20
6.74
6.84
7.68
8.16
16.59
8.24
5.34
3.83
2.56
3.61
46.01
23.57
6.61
6.82
7.31
8.66
17.66
2.19
2.45
2.92
1.95
2.78
47.07
12.28
6.62
6.82
7.31
8.66
17.66
2.81
2.91
3.29
2.17
2.28
47.06
13.47
Total (mean)
ZT/R3
64.67
Total (mean)
CT/R2
69.59
0–5
5–10
10–15
15–20
20–30
Total (mean)
CT/R3
Total (mean)
−20.70
−18.90
−18.07
−17.63
−17.07
8.80
9.27
10.23
10.61
20.44
−18.96
−18.86
−18.63
−18.30
−18.13
59.35
0–5
5–10
10–15
15–20
20–30
9.43
9.73
10.60
10.83
19.94
−19.26
−19.22
−18.72
−18.23
−17.41
60.53
and the native SOM for R2 and R3 were very similar
to those obtained using the first technique (Table 9).
However, applying this second technique for the R2
and R3 rotations under CT the estimates of the quantities of C derived from the crop residues were much
lower. When the further step was taken to estimate the
13 C abundance of the crop residues using the assumption of Cadisch and Giller (1996) using Eq. (7), rather
than solely the regression technique of Balesdent et al.
(1990), the values of δres for the rotations R2 and R3
under ZT were close to the estimate of −24.5‰ from
the regression technique (means for all depth intervals to 30 cm, −24.3 and −24.1‰ for R2 and R3,
respectively). However, for these rotation treatments
under CT the values of δres were more negative than
Native SOM
Crop residues
δ13 C of C derived from
crop residues (‰)
−24.86
−24.88
−24.38
−22.99
−24.46
(−24.31)
−24.96
−23.25
−23.22
−24.42
−24.29
(−24.03)
−29.43
−28.22
−26.47
−30.73
−34.85
(−29.94)
−28.10
−27.95
−25.87
−29.12
−32.20
(−28.65)
−24.5‰, and some of them (impossibly) more negative than the measured values of the residues of any
of the crops. This means that to use the supposition
of Cadisch and Giller (1996) that native SOM decomposed at the same rate under CT and ZT is clearly
incorrect in the case of this study.
Under ZT there was a good agreement of the two
different techniques and this suggests that, when the
soil is not disturbed by ploughing, the composition of
the crop residues has little effect on the rate of decomposition of the original native SOM. However, under
CT this assumption did not appear to hold, suggesting
that different crop residues mixed into the soil through
ploughing will have different effects on the rate of decomposition of native SOM.
56
C.P.J. Sisti et al. / Soil & Tillage Research 76 (2004) 39–58
4. Conclusions
At the end of 13 years under a continuous sequence
of W/S (R1) there was no significant difference in soil
C stocks between ZT and CT. However, in the rotations where the N2 -fixing legume vetch was planted
as a winter green-manure crop (R2 and R3), soil C
stocks under ZT were increased by approximately
10 Mg ha−1 . Furthermore, the soil C stocks (to 100 cm
depth) in R2 and R3 were 17 Mg ha−1 higher under
ZT than under CT.
These results confirm many earlier studies that ZT
can promote the conservation of SOM in comparison
to CT, but also indicate that where the net N balance
of the whole crop rotation system is close to zero, no
long-term accumulation of soil C is to be expected
even under ZT. This applies to rotations that include
soybean as the only legume, as available evidence suggests that owing to the large export of N from this
crop in the harvested grain there is little or no overall
N gain in the soil/plant system.
Where soil C stocks under ZT were higher than
under CT, much of the N gain was at depths below
the plough layer. This suggests that most of the accumulated soil C is derived from root residues and
emphasises the importance of studies of root biomass
and turnover to further an understanding of the driving forces behind C accumulation/sequestration under
ZT systems.
Finally, the 13 C abundance data indicated that under
ZT the decomposition of the original native SOM was
not affected by the different composition of crops in
the different rotations, but under CT the rotations R2
and R3, which included vetch and maize, stimulated
the decay of the original native SOM, compared to the
continuous W/S sequence (R1).
Acknowledgements
This study was principally funded by Embrapa,
FINEP (contract No. 64.00.0348.00) of the Brazilian
Ministry of Science and Technology and IAEA (contract No. 10953/R1). The first author CPJS gratefully
acknowledges the Brazilian Ministry of Education
for an M.Sc. research fellowship and, HPdoS, BJRA,
SU and RMB research fellowships from the Brazilian National Research Council (CNPq). The authors
thank Roberto G. de Souza, Altiberto M. Baêta and
José Vincente Alves at Embrapa Agrobiologia for
their diligent technical assistance of the 13 C abundance and total N analyses, Dra Janaina R. Costa for
valuable assistance with the statistical analyses, and
Dr. George Cadisch of Imperial College at Wye (UK)
and one of the anonymous referees for their valuable
suggestions.
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