soil organic carbon accumulation and carbon costs related to tillage, cropping systems and nitrogen...
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Soil & Tillage Research 94 (2007) 510–519
Soil organic carbon accumulation and carbon costs related
to tillage, cropping systems and nitrogen fertilization
in a subtropical Acrisol
J.A. Zanatta a, C. Bayer a,*, J. Dieckow b, F.C.B. Vieira a, J. Mielniczuk a
a Departamento de Solos, Universidade Federal do Rio Grande do Sul, P.O. Box 15100, 91501-970 Porto Alegre/RS, Brazilb Departamento de Solos e Engenharia Agrıcola, Universidade Federal do Parana, 80035-050 Curitiba/PR, Brazil
Received 16 October 2005; received in revised form 16 September 2006; accepted 7 October 2006
Abstract
Conservation management systems can improve soil organic matter stocks and contribute to atmospheric C mitigation. This
study was carried out in a 18-year long-term experiment conducted on a subtropical Acrisol in Southern Brazil to assess the
potential of tillage systems [conventional tillage (CT) and no-till (NT)], cropping systems [oat/maize (O/M), vetch/maize (V/M)
and oat + vetch/maize + cowpea (OV/MC)] and N fertilization [0 kg N ha�1 year�1 (0 N) and 180 kg N ha�1 year�1 (180 N)] for
mitigating atmospheric C. For that, the soil organic carbon (SOC) accumulation and the C equivalent (CE) costs of the investigated
management systems were taken into account in comparison to the CT O/M 0 N used as reference system. No-till is known to
produce a less oxidative environment than CT and resulted in SOC accumulation, mainly in the 0–5 cm soil layer, at rates related to
the addition of crop residues, which were increased by legume cover crops and N fertilization. Considering the reference treatment,
the SOC accumulation rates in the 0–20 cm layer varied from 0.09 to 0.34 Mg ha�1 year�1 in CT and from 0.19 to
0.65 Mg ha�1 year�1 in NT. However, the SOC accumulation rates peaked during the first years (5th to 9th) after the adoption
of the management practices and decreased exponentially over time, indicating that conservation soil management was a short-term
strategy for atmospheric C mitigation. On the other hand, when the CE costs of tillage operations were taken into account, the
benefits of NT to C mitigation compared to CT were enhanced. When CE costs related to N-based fertilizers were taken into
account, the increases in SOC accumulation due to N did not necessarily improve atmospheric C mitigation, although this does not
diminish the agricultural and economic importance of inorganic N fertilization.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Global warming; No-till; Cover crops; C accumulation; C costs; C mitigation
1. Introduction
Soil is an important terrestrial C reservoir which
plays a significant role in the global C cycle and,
depending on soil use and management, may function
* Corresponding author. Tel.: +55 51 33166040;
fax: +55 51 33166050.
E-mail address: [email protected] (C. Bayer).
0167-1987/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.still.2006.10.003
either as a C source or sink. Historically, soil organic
matter depletion has released 78 Pg � 12 Pg of C to the
atmosphere due to land use change and tillage (Lal,
2004b) and this, coupled with fossil fuel use, has
contributed to the rise in the concentration of atmo-
spheric C dioxide. However, under conservation
management soils may be able to accumulate 30–
60 Pg of C globally over a period of 25–50 years.
Besides increasing the organic matter stock of soil,
and thus improve soil quality and productivity,
J.A. Zanatta et al. / Soil & Tillage Research 94 (2007) 510–519 511
C accumulation could be a strategy for mitigating the
potential greenhouse effect (Lal, 2004b).
The adoption of no-till (NT) management in
subtropical Brazilian soils has lead to soil organic carbon
(SOC) accumulation rates of 0.19–0.81 Mg ha�1 year�1
in the 0–20 cm layer (Bayer et al., 2006), indicating that
NT soils can function as an atmospheric C sink. The less
oxidative environment and the physical protection
mechanism imparted by the stable aggregates of NT
soils reduce soil organic matter mineralization rates
(Gregorich et al., 1995; Feller and Beare, 1997; Six et al.,
1999) and allow SOC accumulation.
Besides the adoption of reduced tillage (RT)
practices, the cultivation of crops and cover crops
(especially legumes) with high potential for C-biomass
addition is another prerequisite for SOC accumulation
(Burle et al., 1997; Sisti et al., 2004). Bayer et al. (2006)
and Diekow et al. (2005) observed that soils subjected to
NT management for long period under low-addition
cropping systems did not accumulate SOC, although
NT legume-based cropping systems showed SOC
accumulation rates of around 0.8 Mg ha�1 year�1.
In many cropping systems, fertilization with N is a
key factor that controls biomass production and thus
may influence SOC storage patterns. Lovato et al.
(2004) showed that when N was applied to maize on an
oat/maize cropping system at an average rate of
139 kg ha�1 year�1 biomass production increased by
92% over the treatment without N. However, in a vetch/
maize system biomass production increased only 38%
with the same level of N fertilization, clearly indicating
that the legume winter cover crop may supply most of
the N required by the maize. Nitrogen fertilization can
enhance SOC accumulation but this accumulation
cannot be directly regarded as a net C mitigation (i.e.
the reduction of atmospheric C dioxide) because for
every kilogram of N-based fertilizer there is a hidden
cost of 1.3 kg of C equivalent (Lal, 2004a) which must
be taken into account when estimating the net C
mitigation potential resulting from N-based fertilizers.
Many other soil management practices (tillage, irriga-
tion, sowing, etc.) and inputs (herbicide, insecticide,
etc.) have energetic costs, which can be converted into C
equivalent (CE) costs (Lal, 2004b) and must also be
considered when estimating the C mitigation potential
of management practices.
The aim of this study was to assess the long-term
potential of tillage systems, cropping systems and N
fertilization for mitigating atmospheric C in a sub-
tropical Acrisol taking into account SOC accumulation
and the CE costs of the management systems, both
related to a reference soil management system.
2. Material and methods
2.1. Experimental site
The study was carried out in an 18-year long-term
experiment located at the Agronomic Experimental
Station of the Federal University of Rio Grande do Sul
(308 510 S and 518 380 W, altitude of 46 m), near the
town of Eldorado do Sul, in a typical gently sloping
landscape of the Central Depression of Rio Grande do
Sul State, Brazil. The soil is classified as a sandy clay
loam granite-derived Acrisol (FAO, 2002) containing
220, 240 and 540 g kg�1 of clay, silt and sand
respectively in the 0–20 cm layer. The clay fraction
is mainly composed of kaolinite (720 g kg�1 clay) and
iron oxides (109 g kg�1 clay) (Bayer et al., 2001). The
climate is classified as humid subtropical (Cfa, Koppen
climatic classification) with a mean annual temperature
of 19.4 8C, mean monthly temperature of 9–25 8C and
mean annual precipitation of 1440 mm uniformly
distributed throughout the year (Bergamaschi et al.,
2003).
The original vegetation was native grassland (mainly
Paspalum and Andropogon) but this was converted to
agricultural land in 1969, for cultivation of rape (Brassica
napus L.) in winter and sunflower (Helianthus annuus L.)
in summer for 16 years. Occasionally, the area was
cultivated with soybean (Glycine max L.) in summer and
left as fallow in winter. The soil was intensively ploughed
and disked and little concern was taken regarding
conservation practices. In 1985, when the soil was
showing visible signs of physical degradation (e.g. like
rill erosion and compaction) the current experiment was
established with the objective of investigating soil
management strategies to change this degradation
scenario to more sustainable agricultural production
systems.
The experiment (which is ongoing) involved three
tillage systems, three cropping systems and two levels of
N-based fertilization arranged in a split-plot randomized
blocks design with three replications. The main plots
(15 m � 20 m), subjected to no-till, reduced tillage and
conventional tillage, were divided in three subplots
(5 m � 20 m) each of which was cultivated on different
cropping system, involving oat (Avena strigosa Schreb)/
maize (Zea mays L.) (O/M), vetch (Vicia sativa L.)/maize
(V/M) and oat + vetch/maize + cowpea (Vigna unguicu-
lata [L.] Walp.) (OV/MC). Each block was divided into
two strips, one without N application (0 N) and other with
180 kg ha�1 (180 N) applied to the maize crop as urea.
From 1985 until 1994 this N rate was 120 kg ha�1, and
then 180 kg ha�1 until 2003, so that the average rate for
J.A. Zanatta et al. / Soil & Tillage Research 94 (2007) 510–519512
Table 1
Soil bulk density, obtained by core method, under native grassland and
in different tillage systems
Layers
(cm)
Native
grassland
No-tilld Conventional til-
laged
1998a 1990b 2001c 1990b 2001c
0–2.5 1.49 1.55 1.35 1.44 1.44
2.5–5 1.51 1.58 1.51 1.45 1.57
5–10 1.57 1.58 1.67 1.45 1.57
10–20 1.63 1.58 1.65 1.66 1.60
20–30 1.63 1.58 1.65 1.66 1.60
(a) Lovato et al. (2004); (b) Bayer et al. (2000a); (c) Silva et al. (2005);
(d) soil sampled on veth/maize cropping system.
this 18-year period corresponds to 150 kg ha�1 year�1.
Data from the reduced tillage system was not included in
the current analysis.
The winter cover crops (oat and vetch) were row-sown
in April or May (autumn in the southern hemisphere) of
each year at a seeding rate of 80 kg ha�1 when grown
individually and 40 kg ha�1 each when intercropped in
the OV/MC treatment. Maize was sown in September or
October (spring in the southern hemisphere) using a three
line seeding seed drill with a chisel-type furrow opener at
a density sufficient to provide a plant density of
60,000 ha�1. The summer cover crop (cowpea) was
planted between maize rows 20–30 days after sowing the
maize with a manual seed drill at a rate of 3–4 seeds per
hole and spaced 0.4 m between holes.
The CT plots were ploughed to a furrow-depth of
17 cm once a year in spring before maize sowing using a
three-disk plough and harrowed twice to a depth of
10 cm using a disk harrow mixing the crops residues in
this layer. At the same time, glyphosate-based herbicide
(Roundup, Monsanto) was applied in the NT plots at
1.4 kg ha�1 with respect to final glyphosate concentra-
tion, and two or three days later the winter cover crops
were managed with a knife-roller and the aboveground
residues left on the soil surface. More details about the
experiment are given in Amado et al. (1998) and Bayer
et al. (2000b, 2001).
2.2. Estimate of C addition
The mean annual C additions were estimated from the
cover crop data and the dry-matter maize yields during
the 18-year experimental period. The aboveground dry
matter yield of oat, vetch, oat + vetch and cowpea was
assessed in an area of one square meter per subplot for the
years 1986, 1987, 1988, 1990, 1991, 1995, 1997, 1998,
2002 and 2003. Samples were dried to constant weight at
60 8C, weighed and ground. The annual aboveground
maize dry matter yield was estimated using the grain
yield and aboveground maize residues ratio obtained by
Lovato et al. (2004), where aboveground maize dry
matter yield (Mg ha�1) = 0.96 � maize grain yield
(Mg ha�1) + 2.91 (r2 = 0.91, significant at P < 0.05).
To calculate the aboveground C addition, we multiplied
the dry matter yield (maize + cover crops) by a factor of
0.40, assuming that crops residues contains 40% C (Burle
et al., 1997; Bayer et al., 2000b). The root C addition was
assumed to be 30% of the aboveground C addition, which
is an average value calculated from the results of summer
and winter crops obtained by Fehrenbacher and
Alexander (1955), Buyanovsky and Wagner (1986),
Balesdent and Balabane (1992), Crozier and King
(1993), Bolinder et al. (1997) and Kissele et al.
(2001). Grain production of maize was removed from
the field each year.
2.3. Soil sampling
Triplicate soil samples were collected in October
2003 from various depths of a 10 cm � 30 cm sampling
area in each treatment and the native grassland adjacent
to the experimental field. Two sub-samples were
randomly taken from each plot and composited. Soil
samples from the first four layers (0–2.5, 2.5–5, 5–10
and 10–20 cm) were manually sampled with a spatula
while the 20–30 cm layer was sampled with an auger-
type sampling tool. The samples were air dried, ground
to pass through a 2-mm mesh and stored in plastic pots.
A portion of about 20 g was further ground to pass
through a 0.5-mm mesh and used for organic C
determination.
2.4. Carbon determination and estimate of soil C
stocks
The soil samples were analyzed for organic C using a
VCSH model total organic C (TOC) analyzer (Shi-
madzu). The total soil organic C in the 0–20 cm or 0–
30 cm layers of each treatment were calculated using
the equivalent mass of soil approach (Ellert and Bettany,
1995; Sisti et al., 2004), with the soil mass in each
treatment being adjusted to the mass of soil in the native
grassland to correct for soil compaction and differences
in soil bulk densities across treatments. To calculate the
mass of soil in each layer, we used the soil bulk density
evaluated in 1998 in grassland soil (Lovato et al., 2004)
and in 2001 in NT and CT systems under vetch/maize
cropping system (Silva et al., 2005) (Table 1). Soil bulk
density was determined using the core method (Blake
and Hartge, 1986).
J.A. Zanatta et al. / Soil & Tillage Research 94 (2007) 510–519 513
Fig. 1. Mean annual C addition across conventional tillage and no-till,
for oat/maize (O/M), vetch/maize (V/M) and oat + vetch/maize + cow-
pea (OV/MC) cropping systems subjected to two levels of N fertiliza-
tion, 0 kg ha�1 year�1 (0 N) and 180 kg ha�1 year�1 (180 N). Bars
indicate the standard error.
2.5. Soil organic C stocks in previous years
To track the temporal evolution of SOC stocks in the
0–20 cm layer we compared our results (2003) with
those obtained at the beginning of the experiment in
1985 (Medeiros, 1988), in 1990 (Bayer and Mielniczuk,
1997), in 1994 (Bayer et al., 2000a), and in 1998
(Lovato et al., 2004). In all these previous studies, soil
sampling was performed in September to October
(spring in the Southern Hemisphere), before tillage
operations, under the same procedures as used in 2003
(see above). All other experimental methodologies were
similar to those employed in 2003, except the analytical
C determination method. The SOC data of the previous
studies were obtained using the Walkley–Black
analytical method (Nelson and Sommers, 1996), so to
make them comparable with the data obtained by the
dry combustion TOC method employed in 2003 the
previous data were corrected by a factor of 0.9422. This
correction factor was determined in previous tests
where we compared results from Walkley–Black and
the dry combustion TOC method in a wide range of
SOC contents (data not shown). This temporal
evaluation was performed only for samples from the
treatments without applied N-based fertilizer because in
most of the previous determinations samples from the
plots fertilized with N had not been assessed for SOC.
The SOC data set for previous years was also
recalculated from the original values using the equivalent
soil mass method. For 1985, 1990 and 1994 we used the
values of soil bulk density evaluated in 1990 and reported
by Bayer et al. (2000a) (Table 1). For 1998, as well as for
2003, we used the values obtained in 2001 and reported
by Silva et al. (2005) (Table 1). In all these years (1990,
1998 and 2001), soil bulk density was determined using
the core method (Blake and Hartge, 1986).
2.6. Assessment of C mitigation
To estimate the annual atmospheric C mitigation rate
of each treatment, we considered the SOC accumulation
in the different soil management systems as well as the
hidden C equivalent costs due to tillage operations and N
fertilization. The CT O/M 0 N treatment was taken as
reference for calculating the SOC accumulation rates and
the C mitigation rates, because this treatment represents a
good approximation to the high soil disturbance and low
C addition soil management systems traditionally
adopted by farmers in Southern Brazil.
To calculate the CE costs due to tillage operations we
considered a C emission of 0.94 kg kg�1 of diesel
consumed (Lal, 2004b) and a diesel density of
0.85 Mg m�3 (Agencia Nacional do Petroleo, Brazil,
2003). For CE costs related to inorganic N fertilization
we considered a C emission of 1.3 kg kg�1 of applied N
(Lal, 2004a). The C costs of other operations (seeding,
spraying, etc.) and inputs (seeds, herbicides, lime,
phosphorus and potassium fertilizer, etc.) were not
taken into account because these operations and inputs
were the same across all investigated soil management
systems.
2.7. Statistical analysis
The significance of the treatment effects on SOC
concentrations and stocks were assessed using analysis
of variance (ANOVA) for each year. The difference
between means was analyzed using the Tukey-test
(P < 0.05). Linear regressions were adjusted between
SOC and the addition of C-biomass by cropping
systems.
3. Results and discussion
3.1. Carbon addition by cover crops and maize
Since the addition of C by cover crops and maize
plants did not differ significantly (P < 0.10) between
CT and NT systems (data not shown), the discussion
related to the C addition will be based only on the
average values between these two tillage systems. Total
C additions varied from 4.05 Mg ha�1 year�1 for the O/
M 0 N to 8.71 Mg ha�1 year�1 for the OV/MC 180 N
cropping system (Fig. 1). In the treatments without
J.A. Zanatta et al. / Soil & Tillage Research 94 (2007) 510–519514
Table 2
Statistical probability (P) regarding the effect of tillage systems, cropping systems and inorganic nitrogen fertilization on soil organic carbon (SOC)
concentration and stocks at different soil depths (cm)
Effect C concentration C stocks
0–2.5 2.5–5 5–10 10–20 20–30 0–20 0–30
Tillage systems (T) 0.001 0.005 0.332 0.023 0.610 0.025 0.157
Cropping systems (C) 0.013 0.017 0.002 0.012 0.502 0.003 0.002
N fertilization (N) 0.074 0.163 0.106 0.549 0.178 0.066 0.884
T � C 0.572 0.577 0.724 0.556 0.171 0.726 0.706
T � N 0.625 0.908 0.163 0.135 0.045 0.929 0.313
C � N 0.129 0.241 0.363 0.848 0.730 0.181 0.225
T � C � N 0.300 0.533 0.611 0.659 0.016 0.630 0.368
The P-value is less than the values in the Table.
added inorganic N the legume-based cropping systems
increased the C addition by 40% (V/M) and 87% (OV/
MC) compared to the grass-based system (O/M) and by
9% (V/M) and 37% (OV/MC) in the treatments that
received N fertilization (Fig. 1). Similarly, the increase
in C addition due to inorganic N was 56% in the O/M,
22% in V/M and 15% in OV/MC cropping systems.
The C addition due to the cover crops increased in
the order oat < vetch < oat + vetch/cowpea, in both
treatments with and without added inorganic N,
indicating the potential of multiple-cover crop systems
compared to single-cover crop systems in adding C to
the soil. The application of inorganic N to the maize had
no effect on the increase in C added by the cover crops
(Fig. 1).
The addition of C by maize in the subplots without
added inorganic N tended to be higher for the V/M and
OV/MC than the O/M cropping systems (Fig. 1), most
probably because the N symbiotically fixed by the
winter legume cover-crops was available to the maize
(Amado et al., 1998). However, this trend was not
observed in the treatments with added inorganic N,
possibly because in this case the fertilizer supplied the
N requirements of the maize (Fig. 1).
The maize crop was the major C contributor in most
management systems, being responsible for up to 73%
of the total addition of C in the O/M 180 N treatment
(Fig. 1). Other studies (Angers et al., 1995; Sisti et al.,
2004; Diekow et al., 2005) have also emphasized the
importance of the C addition by maize plants, with some
studies particularly emphasizing the contribution by
maize roots (Balesdent and Balabane, 1996; Bolinder
et al., 1999).
3.2. Soil organic C concentration and stock
The interactive effect of tillage systems, cropping
systems and inorganic N-based fertilizers on SOC
concentrations and stocks were not significant
(P < 0.10) (Table 2). Thus, the discussion of the effects
of tillage system, cropping system, and inorganic N-
based fertilizers will be based on average values for the
other two variables.
The NT soil contained higher concentrations of SOC
in the two top layers than the soil subjected to CT
system, but not in the layers deeper than 5 cm (Table 3).
In the NT soil the aboveground residues were left on the
surface and this created a decreasing SOC gradient with
depth while in the CT soils the disturbance imparted by
ploughing and disking led to the incorporation of crop
residues in the homogenized arable layer, promoting
more uniform SOC concentration from top to 30-cm
depth (Table 3).
For the three cropping systems and the two levels of
inorganic N fertilization (0 N and 180 N) the average
SOC stock of the 0–20 cm layer in the NT soil was
35.4 Mg ha�1, which was 4.1 Mg ha�1 higher than under
CT system, where the SOC content was 31.3 Mg ha�1
(Table 3), representing an average SOC accumulation
rate of 0.23 Mg ha�1 year�1. In the 0–30 cm layer the
average difference in SOC accumulation between these
two tillage systems increased to 5 Mg ha�1 while the
mean SOC accumulation rate increased to 0.28 Mg
ha�1 year�1. As commonly reported (Six et al., 1999;
Bayer et al., 2000b; Freixo et al., 2002; Sisti et al., 2004),
the higher SOC stocks in NT soil are principally due to
the reduced exposure of organic matter to more oxidizing
conditions and to the higher physical protection of
organic matter inside stable soil aggregates, which acts as
physical barriers against microbial enzyme attacks and
diminish the oxygen availability to aerobic microbial
activity (Sollins et al., 1996; Krull et al., 2003).
Cropping systems affected the SOC concentration up
to the 20 cm deep layer, and thus the total SOC in the 0–
20 cm and 0–30 cm layers (Table 3). As observed for the
addition of C (Fig. 1), the highest SOC concentrations
J.A. Zanatta et al. / Soil & Tillage Research 94 (2007) 510–519 515
Table 3
Soil organic carbon (SOC) concentration and stock at different soil depths (layers) as affected by tillage system, cropping system and the rate of
nitrogen fertilization
Tillage system a Cropping
systemb
N rate
(kg ha�1)
C concentration (g kg�1) C stock (Mg ha�1)
0–2.5 2.5–5.0 5–10 10–20 20–30 0–20 0–30
Native grasslandc 24.6 18.0 13.2 11.6 10.2 44.7 61.4
Beginning of the
experimentd
11.6 11.6 11.6 11.6 11.6 33.4 47.4
CT O/M 0 9.6 8.6 8.8 8.6 8.6 27.8 41.8
180 10.2 9.7 9.2 9.4 9.4 29.9 45.3
V/M 0 10.8 10.0 10.0 9.8 9.6 31.6 47.2
180 10.8 10.3 9.9 10.0 9.9 32.0 48.1
OV/MC 0 11.9 10.7 10.5 9.9 9.5 32.8 48.2
180 12.8 11.3 10.5 10.2 9.4 33.8 49.1
NT O/M 0 16.4 11.6 8.8 8.4 9.5 31.2 46.5
180 19.9 14.3 9.6 8.1 9.0 33.6 48.2
V/M 0 21.2 15.2 10.6 8.7 8.9 36.1 57.5
180 20.0 14.1 10.6 8.5 8.9 34.9 49.4
OV/MC 0 22.2 15.9 10.7 9.1 9.4 37.4 52.7
180 24.3 16.7 11.7 9.2 9.6 39.5 55.0
Average of tillage systeme
CT 11.0 b 10.1 b 9.8 a 9.7 a 9.4 a 31.3 b 46.6 a
NT 20.7 a 14.6 a 10.3 a 8.7 a 9.2 a 35.4 a 51.6 a
Average of cropping system e
O/M 14.0 b 11.0 b 9.1 b 8.6 b 9.1 a 30.6 b 45.5 b
V/M 15.7 ab 12.4 ab 10.3 ab 9.3 ab 9.3 a 33.7 a 50.6 a
OV/MC 17.8 a 13.7 a 10.8 a 9.6 a 9.5 a 35.9 a 51.3 a
Average of N fertilizatione
0 15.4 b 12.0 a 9.9 a 9.1 a 9.3 a 32.8 b 49.0 a
180 16.3 a 12.7 a 10.3 a 9.2 a 9.4 a 34.0 a 49.2 a
a CT: conventional tillage; NT: no-till.b O/M: oat/maize; V/M: vetch/maize; OV/MC: oat + vetch/maize + cowpea.c Data from Lovato et al. (2004).d Refers to 1985, data from Bayer et al. (2000a).e Means in the same column followed by the same letter do not differ significantly according to Tukey test (P < 0.10).
and stocks were observed for the OV/MC cropping
system, intermediate values for the V/M cropping system
and the lowest for the O/M cropping system (Table 3),
emphasizing the benefits of legume-based and multiple
cover crop-based systems to increase SOC stocks.
Compared to the O/M treatment, the legume-based
cropping systems had SOC stocks in the 0–20 cm layer
which were higher by 3.1 Mg ha�1 for the V/M and
5.3 Mg ha�1 for the OV/MC. In the 0–30 cm layer these
differences increased to 5.1 Mg ha�1 for V/M and
5.8 Mg ha�1 for OV/MC cropping systems. These results
reinforce the significant contribution of legume cover
crops to increase SOC stocks in subtropical soils under
NT management (Burle et al., 1997; Bayer et al., 2000b;
Sisti et al., 2004). Legumes may contribute to the addition
of organic C, either through their own phytobiomass
production or by increasing the soil N availability and
thus the phytobiomass yield of non-legume species such
as maize.
Nitrogen fertilization significantly increased the SOC
concentration in the top layer and tended to increase in
the deeper layers (Table 3). Thus, the average SOC stock
in the 0–20 cm layer across the two tillage systems and
the three cropping systems was 1.2 Mg ha�1 higher in the
treatments with added inorganic N than in the treatments
with no added inorganic N. In the grass-based no-till O/M
treatment the increase in SOC stock due to added
inorganic N was 2.4 Mg ha�1 (Table 3). These increases
in SOC indicated the positive effect of added inorganic N
on the addition of C by maize (Fig. 1). Several other
studies have reported that the addition of inorganic N
significantly increases SOC stocks (Campbell et al.,
1991; Lovato et al., 2004; Diekow et al., 2005; Campbell
et al., 2005).
J.A. Zanatta et al. / Soil & Tillage Research 94 (2007) 510–519516
Fig. 2. Relationship between soil organic C (SOC) stock in the
0–20 cm soil deep layer at the 18th year (2003) and C addition by
cropping systems subjected to two levels of fertilization with
inorganic N, 0 kg ha�1 year�1 (0 N) and 180 kg ha�1 year�1
(180 N), under conventional tillage (CT) and no-till (NT) systems.
(1) oat/maize (O/M) 0 N; (2) vetch/maize (V/M) 0 N; (3) oat + vetch/
maize + cowpea (OV/MC) 0 N; (4) O/M 180 N; (5) V/M 180 N; and
(6) OV/MC 180 N.
The combined effects of tillage system, cropping
system and N fertilization on SOC are shown in Fig. 2.
From the regressions between the annual addition of C,
as affected mainly by cropping systems and added
inorganic N, and the total SOC stock in the 0–20 cm
layer at the 18th year (Fig. 2), we estimated that the
annual addition of C required to maintain the initial
SOC stock (33.4 Mg ha�1) in the CT soil was
8.4 Mg ha�1, but it was only 5.4 Mg ha�1 (36% lower)
Fig. 3. Temporal evolution of (a) soil organic C (SOC) stocks and (b) SOC
maize (O/M), vetch/maize (V/M) and oat + vetch/maize + cowpea (OV/M
conventional tillage (CT) and no-till (NT) systems. The SOC accumulation
Source of data: 1985 (Medeiros, 1988), 1990 (Bayer and Mielniczuk, 1997)
square difference (LSD) between tillage systems under the same cropping sy
(LSDcrop). Tukey-test, P = 5%.
in the NT soil (Fig. 2). This means that to counter-
balance the higher SOC losses due to mineralization in
CT soil there must be an annual dry matter input of
about 21 Mg ha�1 (assuming dry matter C concentra-
tion of 40%), which is not always possible even under
the humid subtropical conditions of Southern Brazil
where more than two crops can be grown annually.
However, if proper rotation systems with legume
species such as those used in our study are adopted
the dry matter input of 13–14 Mg ha�1 y�1 needed to
fulfill the 5.4 Mg ha�1 of C requirement to maintain
SOC under NT can be easily achieved. Even so, it is
important to emphasize that if this requirement is not
met, the simple fact of no longer ploughing the soil will
not be enough to produce a NT management system
able to mitigate atmospheric C and, even worse, the soil
concerned could even contribute to a net emission of C
to the atmosphere.
In our study, the SOC stocks in the 0–20 cm layer of
the treatments without added inorganic N in the
previous 18-years were compared with the SOC stocks
observed in 2003. Since 1985 under CT system there
was a decrease in SOC of 1.8 Mg ha�1 in the V/M
cropping system and 5.7 Mg ha�1 in the O/M cropping
system, while the OV/MC cropping system was almost
able to maintain the initial SOC stock (�0.6 Mg ha�1)
(Fig. 3a). However, NT soil showed a negative SOC
balance (�2.3 Mg ha�1) only in the O/M cropping
system, while in the legume-based cropping systems the
SOC balance was positive with a SOC value of
+3.9 Mg ha�1 for the OV/MC cropping system and
+2.7 Mg ha�1 for the V/M cropping system. Theses
accumulation rates in the 0–20 cm deep soil layer as affected by oat/
C) cropping systems with no addition of inorganic N (0 N), under
was calculated in comparison to the reference CT O/M 0 N treatment.
, 1994 (Bayer et al., 2000b) and 2003 (this study). Bars show the least
stem (LSDtillage) or between cropping systems under the same tillage
J.A. Zanatta et al. / Soil & Tillage Research 94 (2007) 510–519 517
results underline the importance of high residue
addition cropping systems and emphasize that NT
per se is not enough to increase or maintain SOC stocks.
During the period between 1985 and 1990, little
changes were observed in SOC stocks in the NT
cropped with V/M and OV/MC cropping systems
(Fig. 3a). Similar behavior was observed for SOC
accumulation in the CT soil cropped with OV/MC
(Fig. 3a), where true SOC accumulation started to occur
only after this initial 5-year period. We hypothesize that
the observed behavior may be an ‘inertia effect’ due to
the CT practiced before the establishment of NT
management, so that this initial period was probably
Fig. 4. Annual soil organic C (SOC) accumulation rate (a), C costs (b) and C
maize (O/M), vetch/maize (V/M) and oat + vetch/maize + cowpea (OV/MC)
(180 N), under conventional tillage (CT) and no-till (NT) systems. Bars sh
required until soil mechanisms were developed allow-
ing SOC accumulation.
3.3. Atmospheric C mitigation
In CT system, the linear C accumulation rates,
calculated as the ratio between the difference of soil C
stocks at 2003 under the management systems in relation
to the reference treatment CT O/M 0 N and the time
duration of the experiment (18 years), varied from 0.09 to
0.34 Mg ha�1 year�1, which was half the SOC rate from
0.19 to 0.65 Mg ha�1 year�1 found in the NT soil
(Fig. 4a). The SOC accumulation rate for cropping
mitigation rate (c) in the 0–20 cm deep soil layer as affected by oat/
cropping systems without N (0 N) or 180 kg ha�1 of added inorganic N
ow the standard error.
J.A. Zanatta et al. / Soil & Tillage Research 94 (2007) 510–519518
systems increased in the order O/M < V/M < OV/MC,
while for the different inorganic N levels (0 N and 180 N)
higher SOC accumulation rates were generally found
when inorganic N was applied (Fig. 4a). The accumula-
tion rates found in conservation systems like NT
associated to V/M (mean of 0.44 Mg C ha�1 year�1)
or OV/MC (mean of 0.60 Mg C ha�1 year�1) cropping
systems had tendency to be higher than the range of 0.24–
0.40 Mg C ha�1 year�1 estimated for NT soils in
temperate regions of North America (Lal et al., 1999;
West and Marland, 2002), but they are comparable to the
average rate of 0.48 Mg C ha�1 year�1 estimated by
Bayer et al. (2006) for NT soils from the Brazilian
subtropical region.
We also calculated the average SOC accumulation
rates for the period between the beginning of the
experiment and 1990, 1994, 1998 and 2003 and found
that SOC accumulation was not linear or constant over
time (Fig. 3b). The SOC accumulation rates were higher
during the first years of a particular management system
but later decreased exponentially in both tillage systems.
The SOC accumulation rates showed two different
behaviors over time, SOC accumulation in the O/M and
V/M cropping systems being higher in first year while in
the OV/MC cropping system (which had the highest
organic C addition potential) peak SOC accumulation
occurred at the 9th year, which may have been related to
the ‘inertia effect’ mentioned above. The high initial SOC
accumulation rates, decreasing exponentially in subse-
quent years, emphasize that SOC accumulation in
agricultural soils is a short-term strategy for mitigating
increased atmospheric C (Lal, 2004b). However, it is
important to observe that SOC accumulation rates after
18 years were maintained at significant values varying
from 0.2 to 0.28 Mg ha�1 year�1 in the CT soil and from
0.2 to 0.5 Mg ha�1 year�1 in the NT soil.
We calculated the net effect of management practices
on atmospheric C mitigation by estimating the CE costs
due to tillage operations and the use of N-based
fertilizers. For these calculations we assumed that diesel
consumption in ploughing and disking operations for
the maize crop was about 30.2 L ha�1 y�1 according to
a previous survey in agricultural lands in Rio Grande do
Sul (Portella and Richardson, 1980) and that combus-
tion of each kilogram of diesel emits to atmosphere
0.94 kg C equivalent (Lal, 2004b), with a diesel density
of 0.85 Mg m�3. Our calculations show that the CE cost
due to tillage operations was about 24 kg ha�1 year�1
(Fig. 4b) while for inorganic N application it was
195 kg ha�1 year�1 (Fig. 4b), considering an average N
application of 150 kg ha�1 year�1 and a C emission of
1.3 kg kg�1 of applied N (Lal, 2004b).
Subtracting the CE costs from the SOC accumulation
rates and taking the CT O/M 0 N treatment as reference,
the net C mitigation rate varied from �0.13
Mg ha�1 year�1 in the CT O/M 180 N treatment to
0.56 Mg ha�1 year�1 in the NT OV/MC 0 N treatment
(Fig. 4c). The addition of C costs due to tillage operations
enhanced even more the C mitigation benefits of NT
system as compared to CT system. In contrast, when C
costs due to inorganic N application were taken into
account, the increases in SOC accumulation due to
inorganic N did not represent an atmospheric C
mitigation, depicting a clear example of how interpreta-
tions of the effects of a management practice on C
mitigation may change when C costs are considered. In
spite of the fact that inorganic N-based fertilizers did not
result in true atmospheric C mitigation the use of
inorganic N-based fertilizers is still recommended due to
its agricultural and economic benefits.
4. Conclusions
No-till soil management promotes atmospheric C
mitigation by reducing C costs due to tillage operations
and because it increases soil organic C accumulation in
comparison to soils which are conventionally tilled. Soil
organic C accumulation occurs mainly in the surface
soil layers and is proportional to the C input imparted by
cropping systems, which is improved by using legume
cover crops and inorganic N fertilization. The soil
organic C accumulation rates peaked during the first
years after these management practices were adopted
and then decreased exponentially during subsequent
years. The positive effect of fertilization with inorganic
N on soil C balance does not necessarily result in
atmospheric C mitigation because the benefits of
increasing soil organic C stocks may be counter-
balanced or surpassed by the C equivalent costs related
to the applied N-based fertilizers, although this does not
diminish the agricultural and economic benefits of
inorganic N fertilization.
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