Carbon accumulation at depth in Ferralsols under zero-till subtropical agriculture

Carbon accumulation at depth in Ferralsols under zero-tillsubtropical agriculture

R O B E R T M . B O D D E Y *, C L A U D I A P. J A N T A L I A *, PA U L O C . C O N C E I C A O w , J O S I L E I A A .

Z A N A T T A w , C I M E L I O B AY E R w , J O A O M I E L N I C Z U K w , J E F E R S O N D I E C K O W z,H E N R I Q U E P. D O S S A N T O S § , J O S E E . D E N A R D I N § , C E L S O A I T A } , S A N D R O J .

G I A C O M I N I } , B R U N O J . R . A LV E S * and S E G U N D O U R Q U I A G A *

*Embrapa Agrobiologia, km 47, Antiga Rodovia Rio – Sao Paulo, Seropedica, 23890-000, RJ, Brazil, wDepartment of Soil Science,

Federal University of Rio Grande do Sul, PO Box 15100, 91.501-970 Porto Alegre, RS, Brazil, zDepartment of Soil Science and

Agricultural Engineering, Federal University of Parana, 80.035-050 Curitiba, PR, Brazil, §Embrapa Wheat Research Centre, Caixa

Postal 569, Passo Fundo, 99001-970, RS, Brazil, }Department of Soil Science, Federal University of Santa Maria, Faixa de Camobi,

km 9, 97105-900, Santa Maria, RS, Brazil

Abstract

Conservation agriculture can provide a low-cost competitive option to mitigate global

warming with reduction or elimination of soil tillage and increase soil organic carbon

(SOC). Most studies have evaluated the impact of zero till (ZT) only on surface soil layers

(down to 30 cm), and few studies have been performed on the potential for C accumula-

tion in deeper layers (0–100 cm) of tropical and subtropical soils. In order to determine

whether the change from conventional tillage (CT) to ZT has induced a net gain in SOC,

three long-term experiments (15–26 years) on free-draining Ferralsols in the subtropical

region of South Brazil were sampled and the SOC stocks to 30 and 100 cm calculated on

an equivalent soil mass basis. In rotations containing intercropped or cover-crop

legumes, there were significant accumulations of SOC in ZT soils varying from 5 to

8 Mg ha�1 in comparison with CT management, equivalent to annual soil C accumulation

rates of between 0.04 and 0.88 Mg ha�1. However, the potential for soil C accumulation

was considerably increased (varying from 0.48 to 1.53 Mg ha�1 yr�1) when considering the

soil profile down to 100 cm depth. On average the estimate of soil C accumulation to

100 cm depth was 59% greater than that for soil C accumulated to 30 cm. These findings

suggest that increasing sampling depth from 30 cm (as presently recommended by the

IPCC) to 100 cm, may increase substantially the estimates of potential CO2 mitigation

induced by the change from CT to ZT on the free-draining Ferralsols of the tropics and

subtropics. It was evident that that legumes which contributed a net input of biologically

fixed N played an important role in promoting soil C accumulation in these soils under

ZT, perhaps due to a slow-release of N from decaying surface residues/roots which

favored maize root growth.

Keywords: carbon sequestration, conventional tillage, crop rotations, Ferralsol, legume cover crops,

maize, soil depth, soybean, zero tillage

Received 29 January 2009; revised version received 25 May 2009 and accepted 3 June 2009

Introduction

For the last decade or so a general consensus has

reigned, especially in the United States, that a change

from conventional tillage (CT) to zero tillage (ZT)

would lead to a net accumulation of soil carbon (West

& Post, 2002; Lal, 2003; Lal et al., 2003). In a review of

275 comparisons at different sites around the world

(mostly in North America) of reduced, or zero, tillage

with CT, West & Post (2002) found that, on average,

stocks of carbon (C) in the soil were increased by

57 g m�2 yr�1 (570 kg C ha�1 yr�1) under reduced til-

lage/ZT compared with CT. However, recently this

conclusion has been challenged by Baker et al. (2007)

Correspondence: Robert M. Boddey, tel. 1 55 21 2682 1427 or 1 55

21 8703 1411, fax 1 55 21 3441 1230, e-mail:

[email protected]

Global Change Biology (2010) 16, 784–795, doi: 10.1111/j.1365-2486.2009.02020.x

784 r 2009 Blackwell Publishing Ltd

mailto:[email protected]

who pointed out that in none of the trials was the soil

sampled to a depth 430 cm, and in 68% of the com-

parisons, to 20 cm or less. These authors presented

evidence that in studies, particularly in Canada (Van-

denBygaart et al., 2003) where soils had been sampled to

greater depths, more C was found at depth under CT,

suggesting that the apparent soil C accumulation under

ZT was an artifact of the sampling depth. The same

conclusion was made by Blanco-Canqui & Lal (2008)

from studies at 11 sites in three States of the USA,

although these were on-farm studies and the crop

sequences and management practices in the compari-

sons were not identical.

Adoption of ZT in Brazil has been widespread such

that today over 27 Mha of mechanized crop production

uses this system and the soils in most of this area are

free-draining Ferralsols. A considerable number of

studies on medium- to long-term field experiments

have been published. Most of the earlier studies per-

formed in southern Brazil also sampled the soil to

o30 cm depth and in most cases soil C concentration

or stocks were considerably higher under ZT that

under CT after more than 5 years of the different crop

rotations (Sidiras & Pavan, 1985; Bayer & Mielniczuk,

1997; Bayer & Bertol, 1999; Bayer et al., 2000; Amado

et al., 2001).

In a study in Parana (southern region), Sa et al. (2001)

found that the difference in soil C stocks between ZT

and CT were 23% higher when the soil was sampled to

40 cm rather than 20 cm depth, but these results were

obtained from a chronosequence as opposed to more-

reliable long-term plot experiments. On the other hand,

two studies performed in the tropical central-savanna

(Cerrado) region of Brazil (Centurion et al., 1985; Cor-

azza 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 low-

er C content below 30 cm depth under ZT. These results

suggested that it is advisable to sample to a depth

approaching 100 cm.

In a 13-year-old experiment at the Embrapa Wheat

Research Centre it was found that when a winter

legume (hairy vetch – Vicia villosa) was included in crop

rotations under ZT or CT management with soybean

(Glycine max), wheat (Triticum aestivum) and maize (Zea

mays), the C accumulation in ZT soil was much higher

to a depth of 100 cm (approximately 17 Mg C ha�1) than

to a depth of 30 cm (from 5.7 to 9.1 Mg C ha�1), suggest-

ing a large C retention in the 30–100 cm soil layers of the

highly weathered Ferralsol (Sisti et al., 2004). Thus, this

result indicated a distinct behavior of C accumulation at

depth in tropical and subtropical soils in contrast to

most results for North American soils.

The results of a study by the team at the Federal

University of Rio Grande do Sul (UFRGS) led to simi-

lar conclusions (Diekow et al., 2005). The effects of

three different crop sequences managed under ZT on

the recovery of soil organic carbon (SOC) stocks were

examined: a continuous oats-maize sequence, an inter-

crop of lablab (Lablab purpureum) with maize or an inter-

crop of pigeon pea (Cajanus cajan) with maize. All were

managed under ZT and where the summer legumes

were present in the inter-crops, soil C stocks to 17.5 cm

depth increased by approximately 8 Mg C ha�1 over the

17 years of the experiment. When the change in soil

C stocks was evaluated to a depth of 107.5 cm, the

increase in soil C stocks were 13.1 and 21.2 Mg C ha�1

for the maize intercropped with lablab and pigeon pea,

respectively.

The objective of this study was to investigate further

long-term experiments to assess the impact of ZT

management on C accumulation in deeper layers of

free-draining subtropical Ferralsols in comparison with

conventionally tilled soils, as well as to evaluate the

influence of crop rotations on soil C accumulation.

Materials and methods

The experimental sites

General. The geographical co-ordinates, altitude, soil

type and clay content and the annual rainfall for each

site are given in Table 1. In all experiments winter crops

were planted in April/May and summer crops in

October/November. Experimental design in all cases

was randomized complete blocks. Crop sequences and

management (fertilizer addition etc.) were identical for

the ZT and CT treatments in all three experiments.

Soil samples to 1 m depth were taken 15, 17 and

26 and years after installation of the experiments for

the sites 1 (Passo Fundo), 2. (Cruz Alta) and 3 (Santo

Angelo), respectively. At all sites soil bulk density

was estimated by opening one sampling trench (1.0�1.0� 1.2 m deep) per subplot, and four samples were

taken, one from each trench wall, in the center of each

depth interval (see information for individual sites,

below) sampled using a beveled ring of known

volume [4.5 cm internal diameter (i.d.) for sites 1 and

2 and 8 cm i.d. for site 3]. The soil from the rings was

carefully removed and weighed after drying at 110 1C

for 472 h. For analyses of total C, N and fertility

parameters, further soil samples were taken uniformly

from the whole of each depth interval from the four

walls of the trenches. These were air dried before

further processing. Samples of surface residues were

not taken.

Z E R O T I L L I N D U C E S C I N C R E A S E AT D E P T H I N F E R R A L S O L S 785

r 2009 Blackwell Publishing Ltd, Global Change Biology, 16, 784–795

Site 1. Embrapa wheat research centre, Passo Fundo. This

experiment was installed in 1984. Before this the area

had been under continuous wheat (winter)/soybean

(summer) under CT for at least 20 years. Before this

the area was open araucaria (Parana pine – Araucaria

angustifolia) woodland with a ground cover of mainly

C4 grasses (e.g. Aristida longiseta).

Cropping was in a 3-year rotation with (Year 1) black

oats (Avena strigosa) in winter and soybean in summer,

(Year 2) barley (Hordeum vulgare)/soybean followed by

(Year 3) hairy vetch/maize. In the ZT treatment all

crops were direct drilled, and for the CT treatment

tillage was performed before planting both the winter

and summer crops with one pass of a disk plough

followed by two passes of a harrow. The addition of

fertilizer N to each maize crop was 70 kg N ha�1, and to

barley 50 kg N ha�1. No N fertilizer was added to vetch,

oats or soybean. Mean annual additions of P and K

were, respectively, 23 kg P and 44 kg K ha�1. The

experiment had four replicate blocks with tillage

treatments in plots of 5� 10 m.

After 15 years, the soil from the plots was sampled

for depth intervals of 0–5, 5–10, 10–15, 15–20, 20–30, 30–

40, 40–55, 55–70, 70–85 and 85–100 cm. Approximately

200 m from the site was an area of native vegetation and

four trenches were opened in this area for sampling for

bulk density and soil C analysis in the same manner

and at the same sampling depths.

Site 2. FUNDACEP, Cruz Alta. This experiment was

installed in 1985. This area had also been originally

open araucaria woodland until the 1960s, then plowed

and used for continuous wheat/soybean under CT until

1985. There were two crop rotations:

Rotation 1 (R1) – a continuous sequence of wheat in

winter followed by soybean in summer.

Rotation 2 (R2) – (Year 1) common vetch (Vicia sativa)

mixed with black oats in winter and maize in summer,

(Year 2) wheat/soybean followed by (Year 3) black

oats/soybean.

In the ZT treatment all crops were direct drilled, and

for the CT treatment tillage for the oats and maize was

using a heavy disc plough followed two passes of

a harrow, and for the other crops was rotavated to

� 16 cm depth followed by a heavy disc plough and

one pass of a harrow. In both rotations the addition of

fertilizer N to each wheat crop was 60 kg N ha�1. In R2,

the maize crop received 90 kg N ha�1. R1 was fertilized

with an annual mean of 52 kg P and 75 kg K ha�1, and

R2 received annually 62 kg P and 105 kg K ha�1. The

experiment had three replicate blocks with tillage

treatments in main plots and rotations in subplots of

20� 15 m.

After 17 years the soil was sampled for depth

intervals of 0–5, 5–10, 10–15, 15–20, 20–30, 30–40, 40–

60, 60–80 and 80–100 cm.

Site 3 Centro de Atividades Agrıcolas e Florestais da

COTRISA, Santo Angelo. This experiment was installed

in 1979. The site was originally native grassland, with

Paspalum notatum and Aristida pallens as the pre-

dominant species. In 1964, it was plowed and until

1979 was under continuous wheat/soybean with CT

(disk plowing and harrowing). The crop rotations

were:

R1 – from 1979 to 1998 a 1-year rotation with wheat

(T. aestivum) in winter and soybean (G. max) in summer

and from 1999 to 2004 (winter/summer) wheat/

soybean – wheat/maize (Z. mays) – black oats (A.

strigosa)/soybean – oil radish (Raphanus sativus var.

olerifera)/maize – wheat/soybean – oil radish/maize.

R2 – from 1979 to 1985 a 1-year rotation with wheat

in winter and soybean in summer, from 1986 to 1998 a

1-year rotation with black oats in winter and maize in

summer and from 1999 to 2004 the same sequence as in

R1 for this period (wheat/soybean – wheat/maize –

Table 1 Locations and descriptions of sites of the long-term experiments

Site 1 2 3

Location Passo Fundo Cruz Alta Santo Angelo

Geographical co-ordinates 281150S,

521240W

281290S,

531360W

281300S,

541270W

Altitude 684 m 470 m 300 m

Soil type (US Soil Taxonomy Typic Haplorthox Typic Haplorthox Typic Haplorthox

Soil type (FAO) Rhodic Ferralsol Rhodic Ferralsol Rhodic Ferralsol

Slope 1% 1% o1%

Soil clay content* 630 g kg�1 640 g kg�1 630 g kg�1

Annual rainfall 1800 mm 1750 mm 1850 mm

*0–30 cm depth interval.

786 R . M . B O D D E Y et al.


black oats/soybean – oil radish/maize – wheat/

soybean – oil radish/maize).

In the ZT treatment all crops were direct drilled and

for the CT treatment tillage was performed (one pass of

a disk plough followed by two passes with a harrow)

for both winter and summer crops. In both rotations,

the N addition was 60 kg N ha�1 for wheat and

80 kg ha�1 for maize, while no N fertilizer was

applied to soybean or the cover crops (black oat and

oil radish). An average of 60 kg ha�1 P and 60 kg ha�1 K

were applied only to the cash crops. The experiment

had two replicate blocks with tillage treatments in main

plots (20� 40 m) and rotations in subplots (10� 40 m)

After 26 years, the soil was sampled for depth

intervals of 0–2.5, 2.5–5, 5–10, 10–20, 20–30, 30–40, 40–

60, 60–80 and 80–100 cm.

Analyses

All air-dried soil samples were passed through a 2 mm

sieve and then ground to a fine powder using a roller

mill similar to that of Arnold & Schepers (2004). Sam-

ples were analyzed for total C content using dry com-

bustion. For samples from sites 1 and 2 a LECO CHN

600 total CN analyzer was used and for samples from

sites 3 and 4 a model VCSH total organic C analyzer

(Shimadzu, Tokyo, Japan).

Samples from all sites were analyzed for 13C natural

abundance. Aliquots of the soil samples containing

between 200 and 400 mg total C were analyzed for 13C

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).

Calculation of total crop residue carbon

Crop yield data was collected for all grain crops. To

calculate the total dry matter of crop residues deposited

on the soil surface, the harvest indices measured by Dr.

Gilberto Tomm of Embrapa Trigo cited in Sisti et al.

(2004 – Table 4) were utilized. The indices used for

soybean, wheat, maize and barley were 0.41, 0.30, 0.41

and 0.30, respectively. It was further assumed that the

residues all contained 450 g C kg DM�1. For the noncash

crops (oats, vetch and oil radish), all biomass was taken

from areas of all plots ranging from 1.6 to 2.4 m2

immediately after application of herbicide or knife roll-

er. Subsamples were weighed and dried and once again

it was assumed that C content was 450 g C kg DM�1.

The annual mean C deposited as crop residues on the

soil surface was calculated from the total C deposited

over the whole of each experiment divided by its

duration.

Soil C data treatment

As contrasting tillage treatments, ZT and CT, may result

in different profiles of soil compaction (Ellert & Bettany,

1995), the bulk density measurements were used to

calculate the mass of soil present in each interval and

then the quantity of C in each depth interval. It was

assumed that soil compaction due to mechanical opera-

tions was most significant in the surface layers of the

profiles. In all cases it was found that to 30 or 100 cm

depth there was a greater mass of soil (i.e. there was

more compaction) in the CT treatment. Firstly the total

mass of soil in the profile of the soil down to 30 cm

under both the ZT and CT treatments was calculated.

Then C stocks to 30 cm were calculated by subtracting

the total C content of the extra mass of soil in the 20–

30 cm layer under the CT treatment from that under the

ZT treatment. For the stocks to 100 cm depth the same

procedure was followed but the total C in the extra

mass of soil in the deepest layer (85–100 or 80–100 cm)

was used. This procedure was expressed mathemati-

cally by Sisti et al. (2004).

Fig. 1 Soil carbon concentrations (g kg�1) to 100 cm depth after

15 years of a 3-year crop rotation of white oats/soybean –

barley/soybean – hairy vetch/maize managed under either ZT

or CT, and under the neighboring native vegetation, at the

Embrapa Wheat Research Center (Site 1, Passo Fundo, RS). Error

bars for the native vegetation indicate standard errors of the

means, and for the experimental area the least significant differ-

ence (LSD – Student, Po0.05) between ZT and CT means. Values

are means of four replicates.



Statistical analyses

All data for the soil C stocks were subjected to standard

analysis of variance procedures using the software

package MSTAT-C (Michigan State University, USA).

At sites 2 and 3, tillage treatments were in main plots

and crop rotations in subplots (split plot design). For

the data from these sites in order to evaluate whe-

ther there were statistically significant effects of the

tillage regime (ZT or CT) on the accumulation of C

under the individual crop rotations, it was necessary to

compare subplot treatment means for different main

plots. This was achieved using the procedure descri-

bed by Little & Jackson-Hills (1978): The calculation of

the least significant difference between means (LSD –

Student) becomes:

LSD0:05 ¼ tab

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 b� 1ð ÞEa þ Eb½ �=rbf g

p; ð1Þ

where b is the number of subplot treatments, r the

number of replicates, Ea and Eb are the mean squares

of the subplot and main plot errors, respectively, and tab

is the weighted t value for main plots and subplots

calculated as described by Little & Jackson-Hills (1978).

This procedure was performed on the software SISVAR,

produced by the Federal University of Lavras (UFL),

Lavras, Minas Gerais.

Results

Crop yields

Mean grain yields of soybean for the duration of all

three experiments ranged from 2 to 2.6 Mg ha�1 (Table

2), close to the means for Rio Grande do Sul for 2007

(2.5 Mg) and 2008 (2.0 Mg ha�1) and considerably above

mean yields from a few years ago (Alves et al., 2003;

IBGE, 2009). Likewise, maize yields were well above the

state and national averages. Wheat and barley yields

were similar to State means for the same years. The

yields of these two crops are low by international

standards as farmers are reluctant to invest much

fertilizer and other agro-chemicals in crops with such

low prices on the international markets.

Soil C stocks

Site 1 – Passo Fundo. Only at this site was there a

neighboring area of native vegetation close (� 200 m)

to the experiment which could be used as reference to

evaluate changes in bulk density and carbon (C) stocks.

The bulk density data show clearly the very significant

compaction of the soil caused by cultivation and

traction down to a depth of 40 cm (Table 3).

The soil C stocks under the ZT and CT treatments,

corrected for the same mass of soil to 100 cm under the

Table 2 Mean annual yields of grain crops (kg ha�1) and mean annual deposition of C in aboveground residues (Mg C ha�1) by

grain crops and cover crops at the three long-term experiment sites. Details of crop rotations/sequences given in Materials and

methods

Crop

Site 1 Passo

Fundo Site 2 Cruz Alta Site 3 Santo Angelo

R1 R1 R2 R1 R2

ZT CT ZT CT ZT CT ZT CT ZT CT

Grain yield

Soybean 2478 2598 2467 2238 2588 2382 2025 1889 2058 1892

Wheat 1722 1759 2003 1979 1331 1457 1421 1453

Maize 6094 6088 5786 4874 6461 6461 5590 5407

Barley 2742 2893

C-biomass input

Barley 2.88 3.04

Black Oat 1.58 1.58 1.73 1.94 2.25 2.25 2.49 2.75

Black Oat 1 Vetch 2.11 1.84

Vetch 1.97 1.97

Wheat 1.81 1.85 2.10 2.08 1.40 1.53 1.49 1.53

Oil radish 1.06 1.06 0.90 0.90 0.90 0.90

Soybean 1.48 1.55 1.47 1.34 1.54 1.42 1.21 1.13 1.23 1.13

Maize 3.95 3.94 3.75 3.16 4.18 4.18 3.62 3.50

Mean* 4.12 4.22 3.28 3.19 4.31 4.03 3.11 3.13 3.38 3.34

*Annual mean for the whole duration of each experiment considering the number of occurrences of each crop in each rotation.



neighboring native vegetation, were calculated to be

167.1 and 150.4 Mg ha�1 compared with 171.1 Mg C ha�1

under the former. This suggests that even cropping

under ZT caused some loss of soil carbon (Fig. 1), and

CT management caused much greater losses. However,

this correction was performed based on the assumption

that the C stock under the neighboring native

vegetation was representative of that present at the

time of planting the experiment in 1984. The 13C

natural abundance data suggest that this may not be

true (Fig. 2). The d13C signal for the area of native

vegetation show that in recent times the vegetation

has been dominated by C3 plants. It is to be expected

that there would be a somewhat higher (less negative)

d13C signal under the cropped area as maize (d13C ffi�12%) was included in the rotation. However, if

15 years previously the composition of the vegetation

at the site of the experiment had been the same, it

would be expected that the d13C signals of the soil

under the experimental plots and the area of native

vegetation would converge at depth. As it appears that

the vegetation before establishment of the experiment

was perhaps not the same as the native vegetation

sampled 200 m away, it cannot be assumed with

complete confidence that the soil C stocks were

exactly the same. Whether the cropping increased or

decreased the total C stocks over the 15-year period of

the study cannot be conclusively deduced from the

results.

While all soil samples were analyzed for 13C natural

abundance, the differences in d13C between CT and ZT

yielded little information of interest as, not only were

nearly all the rotations composed of both C3 and C4

crops, but the sites all had prior histories of C4 grasses

under C3 trees. In addition, except for this site at Passo

Fundo, there were no neighboring native vegetation

Table 3 Variation in bulk density (kg dm�3) of soil with

depth under after 15 years of the 3-year crop rotation bar-

ley/soybean, vetch/maize, black oats/soybean, and under an

neighboring area of native vegetation, Site 1, Passo Fundo

Depth

interval (cm)

Native

vegetation

Crop management

ZT CT

0–5 0.97 � 0.01* 1.14 � 0.03w 1.14 � 0.03

5–10 0.99 � 0.01 1.35 � 0.04 1.33 � 0.02

10–15 1.09 � 0.02 1.41 � 0.06 1.42 � 0.02

15–20 1.15 � 0.01 1.38 � 0.01 1.40 � 0.02

20–30 1.17 � 0.01 1.32 � 0.03 1.37 � 0.02

30–40 1.18 � 0.01 1.26 � 0.05 1.33 � 0.02

40–55 1.16 � 0.01 1.20 � 0.03 1.22 � 0.03

55–70 1.15 � 0.01 1.20 � 0.01 1.20 � 0.01

70–85 1.15 � 0.01 1.20 � 0.01 1.20 � 0.01

85–100 1.15 � 0.01 1.21 � 0.01 1.21 � 0.01

*Values following means are � SEM.

wIn no depth interval were the means of bulk density signifi-

cantly different between ZT and CT at Po0.05.

Fig. 2 13C natural abundance (%) of soil to 100 cm depth after

15 years of a 3-year crop rotation of white oats/soybean –

barley/soybean – hairy vetch/maize managed under either ZT

or CT, and under the neighboring native vegetation, at the

Embrapa Wheat Research Center (Site 1, Passo Fundo, RS). Error

bars for the native vegetation indicate standard errors of the

means, and for the experimental area the least significant differ-



Table 4 Comparison of the estimates of total C deposited on

the soil surface as crop residues under zero tillage manage-

ment, with estimates of the mean annual gain* in soil organic

C to 0–30 and 0–100 depth in a total of five experiments at

three sites in southern Brazil

Site

Crop

rotation

Mean annual

residue C

deposition

(Mg C ha�1)

Soil C

accumulation

(Mg C ha�1)

0–30 cm 0–100 cm

Passo

Fundo

R1 4.12 0.25 1.11

Cruz

Alta

R1 3.28 �0.24 �0.22

R2 4.31 0.21 0.52

Santo

Angelo

R1 3.11 0.35 0.85

R2 3.38 0.04 0.48

*This gain in organic soil C is calculated as the difference

between soil C stocks under ZT and under CT divided by the

number of years from establishment to soil sampling of each

experiment.



reference sites to see the effects of the deposition of

belowground crop residues on C3 and C4 carbon. For

these reasons these data for the other sites/rotations are

not presented.

However, what is clear from the results from this

site, is that the soil C stocks (corrected for equal mass

of soil) under ZT were significantly higher than under

CT management (Fig. 3). The difference was only

3.8 Mg C ha�1 when the stocks were evaluated to

30 cm, but was much higher, 16.7 Mg C ha�1 when

stocks were evaluated to 100 cm depth.

Site 2 – Cruz Alta. At this site there were two rotations

within the same experimental design, both managed

under either ZT or CT. The data for C concentration

down the profile show that for R1 (continuous wheat/

soybean), there was little difference in C concentration

with depth, with a tendency for there to be higher C

levels under CT in the depth intervals from 5 to 20 cm

(Fig. 4a). The C concentration under ZT only tended to

exceed that under CT in the 0–5 cm depth interval.

Under the 3-year rotation (R2 – wheat/soybean – oat/

soybean – vetch/maize), there was a strong tendency

for soil C concentration to be higher under ZT than

under CT in the 0–5 cm and 20–60 cm depth intervals

(Fig. 4b).

The total soil C stocks under R1 to depths of 30 cm

were 4.1 Mg C ha�1 greater under CT than under ZT

(Fig. 5a), but the difference did not increase and was not

significant when the stocks were calculated for 0–

100 cm (Fig. 5b). In contrast, for the 3-year rotation

(R2) the soil C stocks to 30 cm under ZT were

3.6 Mg C ha�1 higher than under CT, and this

difference increased to almost 9 Mg C ha�1 when C

stocks were calculated for 0–100 cm (Fig. 5c and d).

Site 3 – Santo Angelo. This study also compared two

rotations under both ZT and CT. The sampling was

made 26 years after installation of the experiment. In

R1, the first 20 years was continuous wheat/soybean,

followed by the sequence wheat/maize – black oats/

soybean – oil radish/maize – wheat/soybean – oil

radish/maize. In R2 the first 7 years was again

continuous wheat/soybean, then for the next 13 years

(a) (b)

Fig. 3 Soil carbon stocks (Mg C ha�1) to (a) 30 cm and (b)

100 cm after 15 years of a 3-year crop rotation of white oats/

soybean – barley/soybean – hairy vetch/maize managed under

either ZT or CT at the Embrapa Wheat Research Center (Site 1,

Passo Fundo, RS). Error bars indicte the least significant differ-



(a) (b)

Fig. 4 Soil carbon concentrations (g kg�1) to 100 cm depth after 17 years of (a) continuous wheat/soybean, and (b) a 3-year crop rotation

of wheat/soybean – oat/soybean – oat 1 hairy vetch/maize, managed under either ZT or CT, at FUNDACEP (Site 2, Cruz Alta, RS).

Error bars indicate the least significant difference (LSD – Student, Po0.05) between ZT and CT means. Values are means of three

replicates.



a 1-year rotation of black oats/maize, then for the last

6 years the same sequence as in R1 for this period

(wheat/soybean – wheat/maize – black oats/soybean

– oil radish/maize – wheat/soybean – oil radish/

maize). The main difference between the sequence of

the crops was 13 years of oat/maize in R2 from

substituting 13 years of wheat/soybean (1986–1998).

The soil C stocks to 30 cm depth under ZT in R1 were

9 Mg C ha�1 higher than under CT but in R2 there was

virtually no difference in C stocks between ZT and CT

(Fig. 6a and c). However, in both rotations there were

greatly increased differences in soil C stocks between ZT

and CT when they were estimated to a depth of 100 cm.

In the case of R1 this difference was over 22 Mg C ha�1,

and for R2 just over 12 Mg C ha�1 (Fig. 6b and d).

Discussion

It is estimated that over 70% of mechanized crop

production in Brazil is situated on Ferralsols (‘Latosso-

los’ in the Brazilian classification – Manzatto et al.,

2002). It is therefore not surprising that the 3 long-term

experiments reported in this paper, and two others

conducted by the same team (Sisti et al., 2004; Diekow

et al., 2005) were all situated on this soil type. Crop

yields at all sites were close to, or above, national and

state averages. This suggests that the experiments were

representative of well-managed farms in the southern

region of Brazil.

For lack of suitable reference areas, none of the

experiments at the three sites reported here show

whether long-term ZT practice increased since the

establishment of the experiments or if soil C stocks will

increase over and above those present under the origi-

nal native vegetation. In Rio Grande do Sul most farm-

ers have tilled their land for many years before they

made the change from CT to ZT, and all the long-term

experiments in this study were sited on areas which had

been under CT for at least 15 years. The results reported

(a) (b)

(c) (d)

Fig. 5 Soil carbon stocks (Mg C ha�1) to (a) 30 cm and

(b) 100 cm after 17 years of continuous wheat/soybean, or to

(c) 30 cm and (d) 100 cm under 17 years of a 3-year crop rotation

of wheat/soybean – oat/soybean – oat 1 hairy vetch/maize,

managed under either ZT or CT, at FUNDACEP (Site 2, Cruz

Alta, RS). Error bars indicate the least significant difference (LSD

– Student, Po0.05) between ZT and CT means. Values are means

of three replicates.

(a) (b)

(c) (d)

Fig. 6 Soil carbon stocks (Mg C ha�1) to (a) 30 cm and

(b) 100 cm after 26 years of crop sequence R1 or to (c) 30 cm

and (d) 100 cm under 26 years of crop sequence R2 managed

under either ZT or CT, at COTRISA (Site 3, Santo Angelo, RS).

Both crop sequences were composed of wheat, soybean, maize,

black oats, and oil radish – for full details see Materials and

methods. Error bars indicate the least significant difference (LSD

– Student, Po0.05) between ZT and CT means. Values are means

of two replicates.



here show that in most cases considerably higher soil C

stocks are attained under ZT management than would

be present if CT had been continued. The data on crop

yields, and the crop residue C deposited on the soil

surface, show that these inputs were very similar for CT

and ZT plots. Only at the site 2 (Cruz Alta) was there

any tendency for residue C to be greater under ZT than

CT and even then this difference was only 7%. Thus, it

does not appear that the higher soil C accumulation

observed under ZT was due to higher residue inputs,

assuming that the quantities of surface residues reflect

the more-relevant belowground inputs from roots.

The soil texture (clay content) was very similar at all

sites. This accounts for the fact that the soil C stocks (to

100 cm) under either ZT (from 154 to 172 Mg C ha�1) or

CT (132–163 Mg C ha�1) were not very different be-

tween sites (Feller et al., 1991; Feller & Beare, 1997). In

this case it is interesting to examine the mean annual

difference between soil C stocks under ZT and CT with

the quantity of crop residue carbon deposited on the

soil surface (Table 4). The highest rates of soil C accu-

mulation were under the experiment at Passo Fundo

and under rotation R1 at Santo Angelo. Passo Fundo

had the highest C residue input and rotation R1 at Santo

Angelo the lowest, which suggests that there is no

apparent relationship between soil C accumulation

and the input of C in residues.

There was a linear relationship between soil C accu-

mulation to 30 cm with that to 100 cm (C100 5 0.307 1

(1.97�C30), r2 5 0.83, Po0.05) suggesting that on aver-

age the soil C stocks to 100 cm were 97% greater than

those estimated to a depth of 30 cm. When the data from

this study was added to those from our two previous

studies (Sisti et al., 2004; Diekow et al., 2005), the

regression was a closer approximation to linear (r2 5

0.87, Po0.001), and showed that on average for these

14 comparisons the evaluation of C accumulation to a

depth of 100 cm were 59% greater than those estimated

to a depth of 30 cm (Fig. 7).

These results contrast with most recent findings in

North America where several authors have suggested

that the apparently higher C stocks under ZT than CT

diminish, or disappear, when soils are sampled to

depths below the plough layer (Baker et al., 2007; Gal

et al., 2007; Blanco-Canqui & Lal, 2008; Poirier et al.,

2009). However, Omonode et al. (2006) reported much

higher soil C stocks to 100 cm depth under ZT

(151 Mg C ha�1) than under annual chisel-plough tillage

(108 Mg C ha�1) in a long-term experiment on a silty

clay loam (Typic Endoaquoll) in Indiana (USA) regard-

less of whether the rotation was continuous maize,

soybean/maize or soybean/maize/wheat. Insufficient

studies to 430 cm depth are available at present to

determine which crop rotation and edaphoclimatic

factors determine whether the change from CT to ZT

will result in net C accumulation when measured to

depths well below the plough layer.

There were no significant difference between soil C

stocks under ZT and CT management for the contin-

uous 1 year cycle of soybean/wheat at Cruz Alta (R1 –

Fig. 5, C1 – Fig. 7) or in this same cropping sequence in

the study of Sisti et al. (2004) (S1 – Fig. 7). The study of

Diekow et al. (2005) estimated soil C accumulation

under ZT only compared with samples taken 17 years

earlier, and here the comparisons D1 and D2 were for

continuous oats (winter) and maize (summer) without

(D1) or with (D2) a mean addition of � 150 kg N

fertilizer ha�1 yr�1 added to the maize. All other com-

parisons which showed C accumulation rates ranging

from 0.5 to 1.5 Mg C ha�1 yr�1 were for rotations which

included N2-fixing legumes (vetch, lablab or pigeon

pea) either as winter crops or intercropped with maize

(D3–D6). In these rotations simple N budgets have been

shown to be positive, but not when the only legume

present is soybean. Because of the high N content

(6–7%) of soybean grain, the export of N in grain is

often in excess of that accumulated from BNF (Alves

et al. 2003), which may explain the lack of C accumula-

tion under ZT under the continuous wheat/soybean

sequence. Drinkwater et al. (1998) have shown that N

for maize derived from cover crop legumes in rotation,

is far more efficient for building SOC than fertilizer N

applied to maize monocultures.

At the sites sampled in Rio Grande do Sul it is evident

that intercropped or cover-crop legumes promoted soil

C accumulation under ZT which was not observed

Fig. 7 Regression of soil C accumulation under ZT to 0–30 cm

and 0–100 cm depth for this present study (P1 Passo Fundo, C1

and C2 Cruz Alta – R1 and R2, respectively and A1 and A2 Santo

Angelo – R1 and R2) and from the study of Sisti et al. (2004 – S1 to

S3 for rotations 1–3, respectively) and from the study of Diekow

et al. (2005 – D1–D6 for rotations 1–6, respectively).



under CT. Several studies have shown that the intro-

duction of forage legumes into a grass-alone pasture

can promote significant increases in soil C stocks to

100 cm depth (Fisher et al., 1994; Tarre et al., 2001;

Mortenson et al., 2004). Legume trees have also been

shown to be highly effective in accumulating soil C

(Resh et al., 2002; Binkley, 2005; Macedo et al., 2008). The

reasons for this are at present not well understood.

It is probable legumes promote a larger microbial

populations in the rhizosphere than do cereal crops

(Chen et al., 2008). If root/arbuscular mycorrhizal asso-

ciations are favored by legumes, it has been suggested

that they facilitate the binding of aggregates and other-

wise promote soil C accumulation in undisturbed soils

(Rillig & Mummey, 2006).

Perhaps a more likely explanation why legume crops

favored C accumulation in these studies may be that the

slow-release of N from legume residues/roots under ZT

favors the growth of maize roots. In CT the crop

residues and a large proportion of the residual roots

are ploughed into the soil thus stimulating the rapid

release of mineral N (Drinkwater et al., 2000). High

levels of mineral N have been shown to decrease the

root shoot : ratio of cereal crops (e.g. Sen & Chalk, 1996),

so that under CT it may be expected that the root

biomass of maize is lower. As tillage or N fertilizer

addition increases soil mineral N levels in the upper

fraction of the soil profile, maize roots may have a more

superficial distribution under CT, or when N fertilizer is

added, than is the case under ZT with no N fertilizer

addition. This is consistent with the report of Diekow

et al. (2005) who found that maize fertilized annually

with between 120 and 180 kg N ha�1, failed to accumu-

late soil carbon under ZT, whereas when intercropped

with lablab or pigeon pea up to 22 Mg C ha�1 were

accumulated in the soil over a 17-year period.

All Ferrasols are renowned for their high content of

ferric oxides and, at least at the sites in Rio Grande do

Sul reported here, they were free-draining. Wright et al.

(2007) suggested that translocation of soluble carbon

compounds could be an important process for C accu-

mulation in deeper soil layers. If such soluble C com-

pounds originate from surface residues left undisturbed

under ZT and form organo-mineral complexes with iron

oxides as suggested by Eusterhues et al. (2005) for

podzolic soils in Bavaria, this may help to explain the

accumulation of C at depth under ZT. An alternative or

additional explanation may be that in ZT managed soils

old root channels and those made by soil fauna, which

are destroyed by tillage, may facilitate deeper rooting.

These results have very significant implications for

soil C sequestration potential in the humid and sub-

humid tropics. In these regions there exists approxi-

mately 750 million ha of Ferralsols (US classification –

Oxisols). At present very significant proportions of

these soils, especially in sub-Saharan Africa, are under

low-input CT for cereal grain production, which has

severely depleted their SOM reserves (Stoorvogel et al.,

1993; Batiano et al., 2007). Their management under

zero-till crop rotations would lead to the virtual elim-

ination of soil erosion (Derpsch & Benites, 2003). If

climatic (especially rainfall) and economic conditions

permitted the incorporation of legume cover crops

under ZT in these regions this would not only lead to

increased cereal production but perhaps also to C-

sequestration at higher rates than possible under mod-

ern intensive agriculture in temperate regions.

Increasing sampling depth from 30 cm as presently

recommended by the IPCC (2006) to 100 cm as reported

here, may decrease the estimates of the potential benefit

of the change from CT to ZT in cropping systems in

temperate climatic regions, and increase them signifi-

cantly in ZT managed rotations on the free-draining

Ferralsols of the tropics and subtropics.

Acknowledgements

The authors gratefully acknowledge funding from the Interna-tional Atomic Energy Agency (Vienna), Embrapa, FINEP, theNational Research Council (CNPq), the Rio de Janeiro StateResearch Foundation (FAPERJ), and the Rio Grande do Sul StateResearch Foundation (FAPERGS) for funding of their research oncarbon sequestration and greenhouse gas emissions. R. M. B., H.do. S., J. M., C. B., B. J. R. and S. U. gratefully acknowledgeresearch fellowships from CNPq. We thank Dr. Amando DallaRosa and Dr. Joao Becker (COTRISA) and Dr. Jackson Fiorin andDr. Jose Rueddel (FUNDACEP) for allowing the access to theexperimental sites under their responsibility, and R. G. Souzaand A. M. Baeta for technical assistance with the stable isotopeand total N and C analyses.

References

Alves BJR, Boddey RM, Urquiaga S (2003) The success of BNF in

soybean in Brazil. Plant and Soil, 252, 1–9.

Amado TJC, Bayer C, Eltz FLF, Brum AC (2001) Potencial de

culturas de cobertura em acumular carbono e nitrogenio no

solo no plantio direto e a melhoria da qualidade ambiental.

Revista Brasileira da Ciencia do Solo, 25, 189–197.

Arnold SL, Schepers JS (2004) A simple roller-mill grinding

procedure for plant and soil samples. Communications in Soil

Science and Plant Analysis, 35, 537–545.

Baker JM, Ochsner TE, Venterea RT, Griffis TJ (2007) Tillage and

carbon sequestration – What do we really know? Agriculture,

Ecosystems and Environment, 118, 1–4.

Batiano A, Kihara J, Vanlauwe B, Waswa B, Kimetu J (2007) Soil

organic carbon dynamics, functions and management in West

African agro-ecosystems. Agricultural Systems, 94, 13–25.

Bayer C, Bertol I (1999) Caracterısticas quımicas de um Cambissolo

humico afetadas por sistemas de preparo, com enfase a materia

organica. Revista Brasileira da Ciencia do Solo, 23, 687–694.



Bayer C, Mielniczuk J (1997) Nitrogenio total de um solo sub-

metido a diferentes metodos de preparo e sistemas de cul-

turas. Revista Brasileira da Ciencia do Solo, 21, 235–239.

Bayer C, Mielniczuk J, Amado TJC, Martin-Neto L, Fernandes SV

(2000) Organic matter storage in a sandy clay loam Acrisol

affected by tillage and cropping systems in southern Brazil.

Soil and Tillage Research, 54, 101–109.

Binkley D (2005) How nitrogen-fixing trees change soil carbon.

In: Tree Species Effects on Soils: Implications for Global Change (eds

Binkley D, Menyailo O), pp. 155–164. Kluwer Academic Pub-

lishers, Dordrecht, the Netherlands.

Blanco-Canqui H, Lal R (2008) No-tillage and soil-profile carbon

sequestration: an on-farm assessment. Soil Science Society of

America Journal, 72, 693–701.

Centurion JF, Dematte JLI, Fernandes FM (1985) Efeitos de

sistemas de preparo nas propriedades quımicas de um solo

sob cerrado cultivado com soja. Revista Brasileira de Ciencia do

Solo, 9, 267–270.

Chen M, Chen B, Marschner P (2008) Plant growth and soil

microbial community structure of legumes and grasses grown

in monoculture or mixture. Journal of Environmental Sciences,

20, 1231–1237.

Corazza EJ, da Silva JE, Resck DVS, Gomes AC (1999) Compor-

tamento de diferentes sistemas de manejo como fonte ou

deposito de carbono em relacao a vegetacao de Cerrado.

Revista Brasileira da Ciencia do Solo, 23, 425–432.

Derpsch R, Benites JR (2003) Situation of conservation agriculture in

the World. Second World Congress on Conservation Agricul-

ture 1, Foz de Iguacu, 11-8-2003, pp. 125–135.

Diekow J, Mielniczuk J, Knicker H, Bayer C, Dick DP, Kogel-

Knabner I (2005) Soil C and N stocks as affected by cropping

systems and nitrogen fertilisation in a southern Brazil Acrisol

managed under no-tillage for 17 years. Soil & Tillage Research,

8, 87–95.

Drinkwater LE, Janke RR, Rossoni-Longnecker L (2000) Effects of

tillage intensity on nitrogen dynamics and productivity in

legume-based grain systems. Plant and Soil, 227, 99–113.

Drinkwater LE, Wagoner P, Sarrantonio M (1998) Legume-based

cropping systems have reduced carbon and nitrogen losses.

Nature, 396, 262–265.

Ellert BH, Bettany JR (1995) Calculation of organic matter and

nutrients stored in soils under contrasting managment re-

gimes. Canadian Journal of Soil Science, 75, 529–538.

Eusterhues K, Rumpel C, Kogel-Knabner I (2005) Organo-miner-

al associations in sandy acid forest soils: importance of specific

surface area, iron oxides and micropores. European Journal of

Soil Science, 56, 753–763.

Feller C, Beare MH (1997) Physical control of soil organic matter

dynamics in the tropics. Geoderma, 79, 69–116.

Feller C, Fritsch E, Poss R, Valentin C (1991) Effets de la texture

sur le stockage et la dynamique des matieres organiques dans

quelques sols ferrugineux et ferrallitiques (Afrique de l’Ouest,

en particulier). Cahiers de ORSTOM, Serie Pedologie, 26,

25–36.

Fisher MJ, Rao IM, Ayarza MA, Lascano CE, Sanz JI, Thomas RJ,

Vera RR (1994) Carbon storage by introduced deep-rooted

grasses in the South American savannas. Nature, 371,

236–238.

Gal A, Vyn TJ, Micheli E, Kladivko EJ, McFee WW (2007)

Soil carbon and nitrogen accumulation with long-term

no-till versus moldboard plowing overestimated with

tilled-zone sampling depths. Soil & Tillage Research, 96,

42–51.

Instituto Brasileiro de Geografia e Estatıstica (IBGE) 2009.

Available at http://www.sidra.ibge.gov.br/bda/default.asp?

t=5&z=t&o=1&u1=1&u2=1&u3=1&u4=1&u5=1&u6=1&u7=1&

u8=1&u9=3&u10=1&u11=26674&u12=1&u13=1&u14=1 (ac-

cessed 4 March 2009)

Intergovernmental Panel for Climate Change (IPCC) (2006).

Guidelines for National Greenhouse Gas Inventories. Avail-

able at http://www.ipcc-nggip.iges.or.jp/public/2006gl/

index.htm (accessed November, 2007).

Lal R (2003) Global potential of soil carbon sequestration to

mitigate the greenhouse effect. Critical Reviews Plant Science,

22, 151–184.

Lal R, Follet RF, Kimble JM (2003) Achieving soil carbon seques-

tration in the United States: a challenge to policy makers. Soil

Science, 168, 827–845.

Little TM, Jackson-Hills F (1978) Agricultural Experimentation.

John Wiley, New York, NY, USA.

Macedo MO, Resende AS, Garcia PC et al. (2008) Changes in soil

C and N stocks and nutrient dynamics 13 years after recovery

of degraded land using leguminous nitrogen-fixing trees.

Forest Ecology and Management, 255, 1516–1524.

Manzatto CV, Freitas Junior E, Peres JRR (2002). Uso agrı-

cola dos solos brasileiros. Embrapa Solos, Rio de Janeiro,

p. 179.

Mortenson MC, Schuman GE, Ingram LJ (2004) Carbon seques-

tration in rangelands interseeded with yellow-flowering alfal-

fa (Medicago sativa ssp. falcata). Environmental Management, 33,

S475–S481.

Omonode RA, Gal A, Stott DE, Abney TS, Vyn TJ (2006) Short-

term versus continuous chisel and no-till effects on soil

carbon and nitrogen. Soil Science Society of America Journal,

70, 419–425.

Poirier V, Angers DA, Rochette P, Chantigny MH, Ziadi N,

Tremblay G, Fortin J (2009) Interactive effects of tillage and

mineral fertilization on soil carbon profiles. Soil Science Society

of America Journal, 73, 255–261.

Resh SC, Binkley D, Parrotta JA (2002) Greater soil carbon

sequestration under nitrogen-fixing trees compared with Eu-

calyptus species. Ecosystems, 5, 217–231.

Rillig MC, Mummey DL (2006) Mycorrizas and soil structure.

New Phytologist, 171, 4–53.

Sa JC, de M, Cerri CC, Dick WA, Lal R, Filho SPV, Piccolo MC,

Feigl BE (2001) Organic matter dynamics and carbon seques-

tration rates for a tillage chronosequence in a Brazilian Oxisol.

Soil Science Society of America Journal, 65, 1486–1499.

Sen S, Chalk PM (1996) Stimulation of root growth and soil

nitrogen uptake by foliar application of urea to wheat and

sunflower. Journal of Agricultural Science, 126, 127–135.

Sidiras N, Pavan MA (1985) Influencia do sistema de manejo do

solo no seu nıvel de fertilidade. Revista Brasileira da Ciencia do

Solo, 9, 249–254.

Sisti CPJ, de Santos HP, Kochhann RA, Alves BJR, Urquiaga S,

Boddey RM (2004) Change in carbon and nitrogen stocks in



http://www.ipcc-nggip.iges.or.jp/public/2006gl/index.htm

http://www.ipcc-nggip.iges.or.jp/public/2006gl/index.htm

soil under 13 years of conventional or zero tillage in southern

Brazil. Soil and Tillage Research, 76, 39–58.

Stoorvogel JJ, Smaling EMA, Janssen BH (1993) Calculating soil

nutrient balances in Africa at different scales. I. Supra-national

scale. Fertilizer Research, 35, 227–235.

Tarre RM, Macedo R, Cantarutti RB et al. (2001) The effect of

the presence of a forage legume on nitrogen and carbon

levels in soils under Brachiaria pastures in the Atlantic

forest region of the South of Bahia, Brazil. Plant and Soil, 234,

15–26.

VandenBygaart AJ, Gregorich EG, Angers DA (2003) Influence of

agricultural management on soil organic carbon: a compen-

dium and assessment of Canadian studies. Canadian Journal of

Soil Science, 83, 363–380.

West TO, Post WM (2002) Soil organic carbon sequestration rates

by tillage and crop rotation: a global data analysis. Soil Science

Society of America Journal, 66, 1930–1946.

Wright AL, Dou F, Hons FM (2007) Crop species and tillage

effects on carbon sequestration in subsurface soil. Soil Science,

172, 124–131.



Carbon accumulation at depth in Ferralsols under zero-till subtropical agriculture

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