soil organic carbon accumulation and carbon costs related to tillage, cropping systems and nitrogen...

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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, Brazil b Departamento de Solos e Engenharia Agrı ´cola, 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 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, www.elsevier.com/locate/still Soil & Tillage Research 94 (2007) 510–519 * 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

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Page 1: Soil organic carbon accumulation and carbon costs related to tillage, cropping systems and nitrogen fertilization in a subtropical Acrisol

www.elsevier.com/locate/still

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,

Page 2: Soil organic carbon accumulation and carbon costs related to tillage, cropping systems and nitrogen fertilization in a subtropical Acrisol

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

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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).

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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

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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

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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).

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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

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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.

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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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