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Organic Carbon Origin and Stock in Cultivatedand Grassland Soil of the Argentine PampaAlejandro Costantini a , Helvécio De‐Polli b , Roberto Pereyra Rossiello c &

Maria Cristina Plencovich aa Faculty of Agronomy , University of Buenos Aires , Buenos Aires, Argentinab Emprapa, Agrobiologia , Seropédica, Brazilc Soil Department , Rural Federal University of Rio de Janeiro , Seropédica,BrazilPublished online: 13 Nov 2007.

To cite this article: Alejandro Costantini , Helvécio De‐Polli , Roberto Pereyra Rossiello & Maria CristinaPlencovich (2007) Organic Carbon Origin and Stock in Cultivated and Grassland Soil of the Argentine Pampa,Communications in Soil Science and Plant Analysis, 38:19-20, 2767-2778, DOI: 10.1080/00103620701663016

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Organic Carbon Origin and Stock inCultivated and Grassland Soil of the

Argentine Pampa

Alejandro Costantini

Faculty of Agronomy, University of Buenos Aires,

Buenos Aires, Argentina

Helvecio De-Polli

Emprapa, Seropedica, Agrobiology, Brazil

Roberto Pereyra RossielloSoil Department, Rural Federal University of Rio de Janeiro,

Seropedica, Brazil

Maria Cristina Plencovich

Faculty of Agronomy, University of Buenos Aires,

Buenos Aires, Argentina

Abstract: A study was carried out in the Argentine Pampa. Plots under continuous maize

and maize–wheat/soybean–soybean rotation were used. Three control plots on grassland

with different undisturbed periods were also used. The objective was to show that C3 and

C4 plants have a different effect on the quantity of carbon retained in the soil when different

crop sequences are used. Total organic carbon was determined, and mass spectrometry

techniques were used to assess the natural variation of the abundance of 13C and 12C to

trace carbon fate in the soil. No differences were observed in the carbon stock at 90 cm

deep across cultivated plots. Maize monoculture represented an important contribution

to the soil organic matter when compared to the grassland areas, but the comparison

Received 6 March 2006, Accepted 2 December 2006

Address correspondence to Alejandro Costantini, Facultad de Agronomıa, Univer-

sidad de Buenos Aires, Av., San Martın 4453, 1417 Buenos Aires, Argentina. E-mail:

costanti@agro.uba.ar

Communications in Soil Science and Plant Analysis, 38: 2767–2778, 2007

Copyright # Taylor & Francis Group, LLC

ISSN 0010-3624 print/1532-2416 online

DOI: 10.1080/00103620701663016

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through the initiald13C from reference plots did not allow an assessment of the original soil

carbon in the plot under rotation.

Keywords: d13C, C3 and C4-plants, crop sequences, grassland, soil organic carbon stock

INTRODUCTION

The Argentine Pampa comprises more than 10 million hectares (Hall et al.

1992). Soils are primarily Mollisols developed on loess as original material,

under grass vegetation, and in a temperate humid climate. The region is

mainly under agricultural activity, and technical advisers often recommend

grassland rotations for soil conservation, but farmers do not always follow

these recommendations because agricultural products have better prices

than those related to the livestock production (Costantini 2003).

The Pampa northern area has few land-use restrictions. Traditionally, the

area has been under crop–livestock rotations for variable periods, depending

on soil and economic conditions. In the seventies, soybean was introduced in

this main maize-growing area and had a profitability that attracted farmers.

Thus, maize-planting area decreased, and soils showed some degradation,

mainly in organic matter and aggregate stability.

Balesdent, Mariotti, and Guillet (1987) mentioned that many studies

about the organic carbon (C) dynamics are based on plot comparisons with

different levels of organic manure or on the incorporation of carbon-labeled

plant material. The natural abundance of the 13C stable isotope can be in

some cases a good natural tracer for the organic incorporation of material to

the soil.

The isotopic ratio is d13C (d13C % ¼ [(R sample/R standard) 2 1], where

R is the 13C/12C isotopic ratio of the higher plants and their metabolic

pathways, which has been emphasized by Bender (1968). The isotope effect

induced by the enzymatic reaction in the primary carboxylation is much

higher for C3-plants than for C4-plants. Then, from atmospheric CO2

(whose isotopic composition is about 27%, C3-plants have d13C values

ranging from 223 to 240%, with a most frequent value of about 227%,

whereas C4-plants have d13C values ranging from 29 to 219%, with a

more frequent value of about 212%. When plants having these two photosyn-

thetic patterns and grown in the same environment are compared, there is an

average difference of 12 to 14% in d13C (Smith and Epstein 1971).

Almost all temperate species and all trees belong to the C3-plant group.

Most of the tropical species from Chenopodiaceae and Gramineae botanic

family belong to the C4-group. When C4-plants grow on a soil that has pre-

viously been under C3-plants, this can be considered as an in situ labeling

of the organic matter incorporated into the soil. Because of the relative

weakness of the signal of this tracer, it can be best used in the case of

several successive crops (Balesdent, Mariotti, and Guillet 1987).

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Using the 13C natural abundance labeling technique to examine the soil

organic matter turnover, Balesdent, Wagner, and Mariotti (1988) demon-

strated the existence of a large pool of very stable organic matter in an

Alfisol developed under grassland vegetation. After approximately 100

years of cropping, the proportion of the organic matter of grassland origin

was about 50%. Balesdent, Wagner, and Mariotti (1988) stated that this

technique is applicable when there are changes in the type of vegetation

(from C3- to C4-plants or vice versa).

Agricultural practices should optimize the use of carbon dioxide (CO2) in

photosynthesis to increase both crop productivity and yields, and especially

the accumulation of C in soil organic matter. Alternative options include

high residue production, cover crops, bare fallow reduction, tillage limiting

depth and intensity of the mechanical soil disturbance, and fertilization

practices (Batjes 1998).

Carbon sequestration potential in ecosystems is not unbounded. If lands

were removed from agriculture, C sequestration could continue only until

reaching a new balance in the soil in 50 to 100 years (Mosier 1998).

In temperate areas, there is previously cultivated land that has been

removed from agricultural use by government programs. In these areas,

there is a significant C sequestration that will continue until a new equilibrium

is reached (Mosier 1998). Soils naturally tend to equilibrium if environmental

factors and soil-management practices remain constant. For example, after a

shift in soil management, the balance is disturbed, and the system moves

slowly toward a new balance characterized by the new management con-

ditions (Anderson and Domsch 1989).

Rice (2001) mentioned that the C amount that may be sequestered in

the soil depends on several factors, such as climate variations and the soil

clay content. Many grassland areas in the central region of the United

States and Canada are now devoted to agriculture. Grassland has the

capacity to store most C in the soil, which may eventually become soil

C. Many cropping practices have diminished the C soil content, but the

advances in crop production techniques and soil management have

enhanced the agriculture potential to increase C in the soil. Additional C

gains could be achieved by reducing the seedbed preparation and the

stubble handling in cropping areas.

Some studies have shown the usefulness of 13C natural abundance to

estimate the C turnover and dynamics in soils when the photosynthetic

pathways of the original vegetation change (Balesdent, Mariotti, and Guillet

1987; Martin et al. 1990). Andriulo, Guerif, and Mary (1999) stated that

this technique is very suitable to identify the origin of C in the soil. C3-

plants discriminate against 13C in the photosynthesis cycle, originating a

tendency so that the 13C/12C ratio is exhausted when compared to the C4-

plants. The isotopic composition of the organic C reflects the type of

material from which it originates. Therefore, the incorporation of vegetation

with a different photosynthetic pathway provides an in situ labeling,

Carbon Origin and Stock in an Argentine Soil 2769

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allowing quantification of both the net loss of the original organic matter and

the net input rate of the new C pool (Jastrow, Boutton, and Miller 1996).

The present study aimed to show that C3- and C4-plant metabolisms have a

different contribution for the C amount retained in the soil system in crop

rotations. It is further aimed to establish the contribution of different plant

species in crop rotations through their differential capacity to fix C and contri-

bute to the soil C stock, when comparing plots having maize as a monoculture.

MATERIALS AND METHODS

This study was carried out in plots located at Marcos Juarez Experimental

Station of INTA (Instituto Nacional de Tecnologıa Agropecuaria), in

Cordoba, Argentina (328 420 S, 628 070 W). The location has a temperate

and subhumid climate, with a mean annual temperature of 178C and annual

rainfall of 924 mm, without a dry season, but with a tendency to a greater

rain distribution from October to March.

Soil is a typic Argiudoll, Marcos Juarez series, with high silt content. This

particular characteristic of granulometric composition originates a relatively

high structural fragility. Soils are dark and deep, with good drainage and

very good agronomic aptitude (INTA 1978).

Data obtained of two trials established at the Experimental Station were used.

The first one, established 9 years ago, has plots with maize (Zea mays L.)–wheat/soybean (Triticum aestivum L./Glycine max L.)–soybean rotation under no

tillage. This crop sequence and tillage system are widely used in the region at

present. The second one has plots with maize monoculture under no tillage for

11 years. In both cases, six replications were obtained when maize was

growing. In both plots, soils showed great homogeneity in their taxonomic type.

These data were compared with three control plots with 6 replications:

1. An undisturbed 10-yr plot sown at the same time as trial 1 and located close

to it, which had been left undisturbed after establishment. It was first a

mixed grassland of gramineous and leguminous crops (mainly alfalfa,

Medicago sativa L.). At sampling, there was very little alfalfa left

because of great grass competition. Bromus unioloides HBK (C3),

Festuca arundinacea Schreb. (C3), and Phalaris augusta L. (C3) were

abundant, and there was some Cynodon plectostachyum Pilger (C4) present.

2. An undisturbed 30-yr plot. In this plot, Cynodon dactilon L. and

Paspalum dilatatum Poir. (both C4-plants) were abundant. The site had

some ornamental trees, mainly Melia and Ulmus species.

3. An undisturbed 100-yr plot, which had a more diversified floristic com-

position and more C3-plants, except for abundant Sorghum halepensis

L. The most important species were Aristida sp., Brisa sp., Hordeum

sp., Artemisia sp. (compound), Torilis nodosa (umbelliferous), Rhynch-

osia sp. (leguminous), Solidago chilensis Meyen (compound), and

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Melica brasiliana Ard. Some Eragrostis polytricha Nees, a native plant

with C4 leaf anatomy, was also found. The C metabolism was checked

by d13C with a mass spectrometer. Soil type in grassland plots was

very similar to soil of cultivated plots.

Soil samples for all plots were taken, using a bore at 0–5, 5–10, 10–20,

20–30, 30–50, 50–70, and 70–90 cm deep. Soil density was determined with

a cylinder. Samples were analyzed for carbon isotopic ratios, using 13C natural

abundance by means of a mass spectrometer.

Total organic C content was determined by the Nelson and Sommers

(1982) method. The oxidizable C value was then corrected by a factor

obtained by Richter, Massen, and Mizuno (1973) for total organic C.

Soil C stock can be estimated in two ways: the most traditional one

requires sample taking at a certain depth and then estimating the C amount.

However, with time, agricultural practices and machinery traffic in cropping

areas have increased the density of the soil top layer in comparison with

natural areas under native vegetation (Sisti 2001) and with different

intensity practices. In such cases, if we considered the same depth in two

sites (i.e., one under cropping and the other one under grassland), or two agri-

cultural areas with different soil activity intensity, soil mass would be greater

under the system that increased the density of the soil the most. Then, at esti-

mating C amount under these two conditions, more soil contents may be found

in those denser areas simply because they have greater soil amounts. This may

lead to an erroneous result interpretation (Veldkamp 1994; Neill et al. 1997).

To compare C stocks, soil layer weights were estimated, taking into

account the soil density of the sample fractions. To obtain the reference

values and estimate the error margin, C content was also determined only

by depth for a critical analysis.

Mass spectrometry techniques, allowing for a minor natural variation of

the abundance of two stable C isotopes (13C and 12C), were used to trace C

fate in the soil.

For the C quantification of organic matter of the crop (C4), that is, the loss

rate of original organic matter and the net input rate of the new plant, we used

the d13C of the 100-yr plot, the d13C of the soil under agricultural use in mono-

culture (A), and the d13C of the agricultural crop residue (R) (maize), with

d13C of approximately 212%.

Based on these data, the following calculation was made:

d13C ð%Þ ¼ð13C=12CÞsample

ð13C=12CÞreference

� 1� � 1000

where

X is the C fraction of the soil organic matter derived from the previous

condition.

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(1 2 X) is the C fraction of the organic matter derived from the soil of the new

vegetation introduced in the area.

A ¼ FXþ R (1 2 X), or A ¼ FXþ R 2 RX, or A ¼ RþX (F 2 R), with F

being the d13C of the soil used as reference, and R the d13C of crop residues.

X ¼ (A 2 R)/(F 2 R) By substituting letters, we have, for example, the C

fraction calculation derived from the original organic matter (OM) 2 C3

where

d13C mixture ¼ C-isotopic composition (C3- and C4-plant residue mixture

determining a 13C PDB (%) of the SOM (soil organic matter) in the

present condition);

d13C3 ¼ C-isotopic composition of the SOM derived from native vegetation

(C3-plants, mostly) or from plant composition previous to vegetation

change—d 13CPDB (%);13C4 ¼ C-isotopic composition of residue from the plants (in this case C4)

introduced in d13C PDB (%);

X ¼ % residue from C3-plants in C3- and C4-mixtures;

100 2 X ¼ % Residue from C4-plants in C3- and C4-mixtures.

Analysis of variance (ANOVA) was performed, and significant differ-

ences were localized by Tukey’s test.

RESULTS AND DISCUSSION

The transformation of plant residues into SOM is associated with a very small13C enrichment, usually in the range of 0.5 to 1.5% d units (Balesdent,

Mariotti, and Boigontier 1988). After estimating all treatments and depths,

total organic C showed significant differences (ANOVA) for ‘plot’ and

‘depth’ (P , 0.01). Soils under cultivation presented a smaller soil organic

C content in the layers at 0–5 and 5–10 cm, even though the no tillage

system was used in the two different rotation sequences (Table 1).

Therefore, under any of the rotation sequences analyzed, there was a

decrease of soil organic C compared to soils under grassland, even with the

10-yr plot without cultivation.

In general, we may state that the distribution of the total organic C by

treatment was similar to that observed by Sisti (2001), working on Rio

Grande do Sul (Brazil) soils.

As for the C stock estimation, it is nowadays practically impossible to find

a soil under completely original conditions in the Argentine Pampas, because

even though there are some uncultivated areas, their floristic composition has

changed in relation to the original one. Thus, the 100-yr plot was used as if it

were under natural conditions, because it was the least disturbed.

Based on the organic C and density data, the soil kg ha21 was estimated.

Nevertheless, as cultivation changes soil density, adopting a fixed depth for all

the treatments would lead to an overestimation of the C stocks in those

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treatments with more dense soils (in this study, soils under cropping practices)

having more soil mass per volume unit.

The 10-yr plot and the 30-yr plot had almost the same soil density.

Conversely, the 100-yr plot had variable density values, but as a whole, it

had less soil mass in the first 90 cm, so it was chosen as a reference plot.

Table 2 shows the C stock for each depth in Mg ha21, independent of

density. Based on these data, mass unit values were obtained.

By summing the C (stock) values in the 0 to 90-cm layer, independent of

the soil mass value, it is demonstrated that cultivated soils at that depth have

Table 1. Total organic C (g kg21) in five plots studied at seven depths

Depth

(cm) Rotation Monoculture 10-yr plot 30-yr plot 100-yr plot

0–5 14.5 a + 4.9 14.3 a + 3.5 24.7 b + 6.2 42.1 c + 8.3 30.0 b + 2.1

5–10 13.8 a +2.6 11.8 a + 2.3 16.0 ab + 2.3 24.0 c + 5.0 20.9 bc + 1.1

10–20 11.6 ns + 2.1 11.8 ns + 1.6 16.3 ns + 5.8 16.1 ns + 2.7 18.8 ns + 4.7

20–30 6.5 ab + 2.4 5.8 a + 2.4 12.6 bc + 3.7 12.4 bc + 4.1 14.1 c + 3.9

30–50 5.5 ns + 2.0 6.1 ns + 1.8 5.5 ns + 2.1 5.8 ns + 1.6 7.7 ns + 1.4

50–70 3.7 ns + 2.8 2.8 ns + 1.3 3.9 ns + 0.7 3.7 ns + 0.8 4.4 ns + 1.3

70–90 2.3 ns + 2.0 4.0 ns + 1.7 2.9 ns + 1.1 3.3 ns + 0.8 3.7 ns + 1.2

Note: Values followed by the same letter are not significantly different within the

same row (Tukey test, P , 0.05). “ns” indicates that there are no significant

differences.

Table 2. Organic carbon stock (Mg ha21) in five plots studied at seven depths, by soil

volume

Depth

(cm) Rotation Monoculture 10-yr plot 30-yr plot 100-yr plot

C stock per depth

0–5 8.219 a 8.101 a 11.977 ab 21.172 c 15.707 c

5–10 8.826 a 7.529 a 9.902 ab 15.238 c 13.084 bc

10–20 14.637 ns 14.837 ns 19.008 ns 18.630 ns 22.594 ns

20–30 8.560 ns 7.684 ns 14.273 ns 14.887 ns 14.364 ns

30–50 14.063 ns 12.949 ns 13.458 ns 14.517 ns 19.891 ns

50–70 8.976 ns 6.888 ns 9.336 ns 8.892 ns 10.611 ns

70–90 5.460 ns 6.557 ns 6.503 ns 7.459 ns 8.293 ns

Total 68.741 a 64.545 a 84.457 ab 100.795 b 104.544 b

C stock for the same soil mass

62.557 a 64.798 a 81.675 ab 96.094 b 104.544 b

Note: Values followed by the same letter are not significantly different within the

same row (Tukey test, P , 0.05). “ns” indicates that there are no significant

differences.

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the lowest C contents and that the oldest grassland has the highest. Organic C

amounts for the same soil mass are shown in the lower row (Table 2).

The treatment under cultivation that included the most diversified crops

did not have any significant differences, either in C buildup at 90 cm or in

the comparison at equal mass (Table 2). Undoubtedly, even when there is

organic matter buildup with regard to other establishing systems (Derpsch

2000), no-tillage systems do not contribute to the carbon buildup in the soil

as grassland does and are not capable of maintaining it, at least under these

conditions, contrary to what has been found by Pereira (2001).

The effect of different management on sol C buildup could be positive or

negative, depending on the initial condition (Dick et al. 1991). Thus, we

should be cautious before generalizing these results.

As for the C origin, the distribution of d13C natural abundance for the

treatments under cultivation and the grassland plots can be observed in

Figures 1 and 2, respectively. Plots under cultivation had higher values for

monoculture than for rotation sequences. The monoculture used was maize,

a C4-plant. As for rotation, the plots under grassland had more similar

values than the plots under monoculture. In general, d13C tend to be less

Figure 1. d13C values for cultivated treatments (maize as a monoculture and under

rotation) at 0–90 cm deep.

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negative as depth increases. In the plot under rotation, values were slightly

more negative for the top layer, probably because the plot had been under a

double-crop system—wheat/soybean—and previously under soybean, which

are C3-plants.

Grassland plots had similar tendencies; nevertheless, the 30-yr plot had

the most negative values in d13C (Figure 2). The analysis results from the

plant material (clippings) left over the soil were not in accord with the

observed data, presenting d13C of 210.68, which confirmed the predominance

of C4-plants observed in the plot, such as Cynodon dactilon and Paspalum

dilatatum. This may be due to the organic matter contribution from the

trees planted in the area, as mentioned previously.

In the case of the 10-yr plot, the soil sample results for d13C agreed with

the results from the plant material collected in the place (mixed grassland of

gramineous and leguminous, which degraded until the latter almost disap-

peared). A predominance of C3-gramineous was observed with abundant

Cynodon plectostachyum (C4).

The 100-yr plot showed d13C of 220.6, which demonstrated the presence

of a mixture of C3-species and abundant C4-plants, such as Eragrostis sp.

Figure 2. d13C values by plot (10, 30, and 100-yr) at 0–90 cm deep.

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(native) and a large amount of Sorghum halepensis, the most significant weed

in the Pampean region.

Depth values for all cases were similar, and in some cases even higher

than those obtained by Sisti (2001) in Rio Grande do Sul, Brazil. This

subsuperficial C seems to originate in C4-plants.

It is difficult to identify a plot that can be taken as a reference considering

the d13C natural abundance, because although the natural grassland of the

Pampean region is mainly composed of C3-gramineous plants, there was a

major adaptation of some C4-plants, which in some cases even became

major weeds. When using d13C as a natural marker, it is remarkable to find

the highest values coming from the major and continuous contribution of

material from maize (C4).

According to Hauman (1928), before the major floristic changes occurred

in the Pampa area due to agriculture, the climax composition of the Pampean

vegetation was approximately 50% stipa (C3), 40% bothriocloa (C4), and 10%

paspalum (C4). This represents a d13C isotopic-composition of 220.6%.

The value mentioned by Andriulo, Guerif, and Mary (1999) agrees with the

values for the superficial layer found in this work. These authors observed a

d13C of 219.6% for the A horizon of Pergamino soils, Buenos Aires

Province, Argentina (typic Argiudolls, with more clay and less silt than

those from the Marcos Juarez series).

A long-term experiment in Missouri, United States, allowed Balesdent,

Mariotti, and Guillet (1987) to study the SOM turnover. Historical records

indicated that the area was originally a natural prairie with C4-grass preva-

lence. Thus, one could study organic-matter turnover accompanying the cul-

tivation of C3-crops, such as wheat (Triticum aestivum) or red clover

(Trifolium pratense). The authors collected samples representing a

temporary sequence. However, there are studies showing opposite results,

with C4-plants in original grassland with C3-plant prevalence, where one

can see the change in the isotopic composition of the soil organic C

(Balesdent, Wagner, and Mariotti 1988).

In the Pampas, the original grassland is made up of a mixed population of

C3- and C4-plants. Thus, the soil presents an intermediate isotopic compo-

sition, with d13C values ranging from 218 to 221%, according to the

latitude (Andriulo, Guerif, and Mary 1999). For these authors, the initial

state has the advantage, in theory at least, of being able to detect a decline

of d13C following the introduction of C3-plants and its enrichment after the

introduction of C4-plants.

In this work, the C percentage of C3-plant residue under monoculture is

41.51%. In the plot under rotation, the C-isotopic composition does not show

any significant differences in relation to the reference plot. The C amount con-

tributed by the monoculture (maize) was 36.93 Mg ha21. Balesdent, Mariotti,

and Boigontier (1990) observed that after many years of continuous monocul-

ture, long after the removal of the original vegetation, crops carry out a stable

degradation of the original SOM. Balesdent, Mariotti, and Boigontier (1990)

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found that under conditions of a temperate climate, on a Hapludalf, maize as a

monoculture preserved larger amounts of original organic matter of soils

derived from C3-plants. In this work, the original material differs in its

natural marking from that of organic material with that metabolism.

Although Andriulo, Guerif, and Mary (1999) mentioned that d13C-

grassland composition with C3- and C4-mixtures may be ideal to detect C

increases or decreases, they used rotations (or monocultures) in their trials,

including only plants with either one of the two metabolisms. Thus, they

were able to distinguish all the new C fractions from the old ones. In this

work, both cropping treatments include maize (C4), but the rotation

includes a C3- and C4-plant mixture that does not show any remarkable differ-

ence from the reference plot. Even though the use of d13C natural abundance

has some disadvantages, it is an appropriate technique to study C sinks in soils

under different cropping systems.

CONCLUSIONS

No significant differences were observed in the carbon stock at 90 cm deep

across cultivated plots (monoculture vs. rotation maize–wheat/soybean–

soybean, under no tillage).

Grassland areas presented more C stock at equal depth, compared with

cultivated areas.

Corn monoculture represented an important contribution to the soil

organic C when compared to the grassland areas used as a reference.

The comparison through the initial d13C from reference plots did not

allow assessment of what happened to the original soil C in the plot under

rotation, because this rotation is made up of a C3- and C4-plant mixture, as

well as the those from the reference plots.

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