2006 Carbon contents and aggregation Noellemeyer Soil Till Res.pdf

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STILL-2486; No of Pages 12

Carbon contents and aggregation related to soil physical and

biological properties under a land-use sequence in the

semiarid region of central Argentina

Elke Noellemeyer *, Federico Frank, Cristian Alvarez,German Morazzo, Alberto Quiroga

Facultad de Agronomıa, Universidad Nacional de La Pampa, C.C. 300, RA-6300 Santa Rosa, La Pampa, Argentina

Received 25 June 2007; received in revised form 11 December 2007; accepted 8 February 2008

Abstract

Land-use change affects vast areas of the semiarid region of central Argentina, where agriculture becomes predominant over

mixed farming systems, and large areas of permanent pastures (PAS) are being converted to agricultural land. This land-use change

causes loss of soil structure, but very little is known about the effect of changes in aggregate size distribution on soil physical,

chemical and biological properties. We decided to use dry sieved aggregates since this technique is commonly used in semiarid

regions. The study was carried out at Anguil, La Pampa, Argentina. The soil was a sandy loam Entic Haplustoll with a carbonate-

free A-horizon. The PAS site had been under weeping love grass for more than 40 years. Parts of this PAS were turned to cultivation

in 1989 (CULT14) and in 2001 (CULT2). Sampling was carried out at 0.6 m intervals to 0.18 m depth. Bulk density (BD), organic

carbon (OC), and water holding capacity and infiltration were determined on these samples. Dry aggregate size distribution and OC

content of the size fractions were determined on large undisturbed samples. Samples of pooled aggregate size fractions>4, 1–4, and

<1 mm, as well as corresponding samples of non fractionated soil were incubated and respiration was measured by CO2 evolved.

The soil of CULT2 had 29% lower contents of large (>4 mm) and 37% higher contents of very small (<1 mm) aggregates than PAS.

The intermediate size aggregates were not affected by the short-term effect of tillage. OC loss in CULT2 was 16% regarding PAS.

Longer term effects of cultivation were characterized by 30% loss of intermediate size aggregates, 22% increase of bulk density, 74

and 19% decrease in water infiltration and water retention, respectively of CULT14 compared to PAS. A 32% decrease of OC was

observed after 14 years of cultivation. Intermediate size aggregates had highest OC contents and no difference between treatments

was found, except for a lower value of large aggregates in CULT14. Respiration rates and total CO2 evolved was related to OC

contents of fractions; however, PAS respired more from its small aggregates than expected from their OC content. The results

showed that OC turnover and loss of aggregation was very fast in this soil, but soil hydraulic properties were affected in the longer

term. Dry aggregates were found to useful for studying soil degradation, and they showed similar trends as those indicated in the

literature for water stable aggregates.

# 2008 Elsevier B.V. All rights reserved.

Keywords: Land-use change; Semiarid Argentina; OC turnover; Dry aggregate size changes; Physical properties; Respiration rates

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Available online at www.sciencedirect.com

Soil & Tillage Research xxx (2008) xxx–xxx

* Corresponding author. Tel.: +54 2954 433093x104.

E-mail address: [email protected]

(E. Noellemeyer).

0167-1987/$ – see front matter # 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2008.02.003

Please cite this article in press as: Noellemeyer, E., et al., Carbon co

properties under a land-use sequence in the semiarid . . ., Soil Ti

1. Introduction

In the semiarid region of central Argentina agriculture

becomes increasingly dominant over the traditional

mixed farming and animal husbandry system, as large

ntents and aggregation related to soil physical and biological

llage Res. (2008), doi:10.1016/j.still.2008.02.003

mailto:[email protected]

http://dx.doi.org/10.1016/j.still.2008.02.003


E. Noellemeyer et al. / Soil & Tillage Research xxx (2008) xxx–xxx2

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areas of permanent pastures (PAS) are being converted to

cropland. Soil carbon losses due to cultivation and tillage

on virgin soils are reported to be in the order of 10–55%

(Brown and Lugo, 1990; Burke et al., 1989). This ample

range might be due to climate and texture differences,

with highest losses in sandy soils in semiarid arid regions

(Balesdent et al., 2000). Various authors reported soil C

losses in different semiarid regions ranging from 35 to

56% (Zach et al., 2006; Elberling et al., 2003) after 3–5

years of cultivation.

Cultivation also changes the soils’ aggregate size

distribution and stability, which has been related to soil

organic matter and microbial activity in numerous studies

(Balesdent et al., 2000; Paustian et al., 2000; Six et al.,

2000a, 2000b; Beare et al., 1994; Dinel et al., 1992). In

general, water stable aggregates smaller than 250 mm

obtained by wet sieving have been used in these studies,

and conceptual frameworks about aggregate hierarchy,

related pore sizes and the relationship between aggregate

turnover and C dynamics have been developed (Six et al.,

2004). However, the soil’s surface in its natural state is

formed by dry aggregates ranging in size from less than

1 mm to more than 8 mm. This is specifically true for

sandy loam and loam soils in semiarid regions that

developed under grassland vegetation. Pena Zubiate et al.

(1980) described the soil structure in the semiarid Pampa

as predominantly medium to large sub-angular blocks.

Dry sieved aggregates could therefore be considered to

represent more truly the actual state of aggregation and

soil structure, and differences in size of these aggregates

have been associated with the effect of different tillage

practices (Hevia et al., 2007). They are obtained by gentle

hand manipulation followed by sieving (Douglas and

Goss, 1982; Chepil, 1953) as in contrast to water stable

aggregates fractionated by suspension and wet sieving.

Dry sieved aggregates have mainly been used in wind

erosion studies (Diaz Zorita et al., 2002; Zobeck, 1991)

and few references exist about their use to evaluate the

effect of management on soil aggregate stability (Eynard

et al., 2004; Martens, 2000) and on C sequestration in

these aggregates (Holeplass et al., 2004).

In the context of the semiarid Pampa of central

Argentina the most common land-use change is the

cultivation of very old PASs (mainly Eragrostis

curvula) for cash cropping. The climatic variations

that occurred during the past three decades, character-

ized by higher rainfall during the summer (Sierra et al.,

2001), induced farmers to increase the area of summer

crops at the expense of these long established PASs.

Based on the concepts developed for water stable

aggregates we supposed that in the sandy to sandy loam

soils of this region, the conversion of PAS into



cultivated lands would cause rapid change of dry

aggregate size distribution, carbon loss and associated

deterioration of soil physical properties such as the

soil’s pore system, water dynamics and susceptibility to

erosion. We also hypothesized that C losses would

affect the biological activity of the soil. The objective of

the present study therefore was to evaluate short and

longer term effects of cultivation on soil structure and

related physical properties, carbon stocks and their

distribution in different size dry aggregates.

2. Materials and methods

2.1. Study area

The study was carried out in 2003 on three adjacent

fields at INTA (National Institute for Agricultural

Technology) Experimental Station in Anguil

(3683501900S and 6385704600W), in the center of the

semiarid Pampa of Argentina (Fig. 1). The soil was a

sandy loam Entic Haplustoll with a carbonate-free A-

horizon of approximately 25 cm depth.

The PAS site had been under weeping love grass for

more than 40 years. A 25-ha part of this PAS was

ploughed in 1989 (CULT14) and then cultivated with a

rotation of wheat (Triticum aestivum); oats (Avena

sativa); sunflower (Elianthus annuus) and alfalfa

(Medicago sativa) PAS with conventional tillage (disk

plough). This field had been under alfalfa PAS for the

last 3 years when the sites were sampled. Another 12 ha

part of the original PAS was ploughed in 2001 and

cultivated with one crop of forage oats and one crop of

sunflower, both with conventional tillage before

sampling (CULT2). The three treatments thus reflected

a land-use time sequence under the same soil and

climatic conditions. Soil properties of the three sites are

shown in Table 1.

2.2. Soil sampling and analysis

All soil samples were collected at six points at 20 m

distance each, along an N–S linear transect at each site.

This sampling method was chosen in order to create

pseudo-replicates of each treatment, since true repli-

cates were impossible to obtain given that only one field

represented each treatment. Sampling was carried out

with cylinders (471.24 cm3 volume) at 0–0.06, 0.06–

0.12 and 0.12–0.18 m depth, all within the limits of the

A-horizon and within the tillage depth of the disk plow.

Samples were air dried, passed through a 2 mm sieve

and weighed, and bulk density (BD) of the soil was

calculated. Organic carbon (OC) was determined by wet




E. Noellemeyer et al. / Soil & Tillage Research xxx (2008) xxx–xxx 3

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Fig. 1. Map of La Pampa, Argentina.

oxidation with sodium dichromate and sulphuric acid at

120 8C and titration of the CO2 trapped in NaOH

(Snyder and Trofimov, 1984).

Water infiltration was also measured at the six

sampling points in each treatment using the double ring

method as described by Fernandez et al. (1971).

Infiltrated water was determined after 1, 10, 20, 30,

40, 50 and 60 min, and the results were expressed as

accumulated infiltration (mm h�1). Initial soil water

contents of the treatment sites were 10.5, 10.2 and

12.3% in the first 0.20 m for PAS, CULT2 and CULT14,

respectively. Water holding capacity was determined in

1 m2 plots in each treatment, at the same points where



Table 1

Basic soil properties of the A-horizon in the three adjacent fields

corresponding to the three treatments studied

CULT14 CULT2 PAS

C (g kg�1) 12.3 b 12.8 b 16.5 a

N (g kg�1) 0.86 b 0.93 b 1.1 a

P (mg kg�1) 10.4 a 4.8 b 6.7 b

Ca (mmol p+ kg�1 soil) 99 b 122 a 131 a

Mg (mmol p+ kg�1 soil) 30 a 23 a 30 a

K (mmol p+ kg�1 soil) 3.7 a 2.6 b 3.7 a

Na (mmol p+ kg�1 soil) 29 a 23 b 23 b

CEC (mmol p+ kg�1 soil) 191 b 200 a 203 a

Base saturation (%) 84.5 a 87.7 a 90.3 a

infiltration was measured. After the infiltration assay,

when the soils were saturated to at least 0.40 m depth,

the soil surface was covered with polyethylene to

prevent evaporation. Samples were collected after 3, 6,

8 and 12 days and their volumetric water content was

determined by weighing moist and oven-dried (60 8C)

samples.

Large undisturbed samples to a depth of 0.20 m

(1413.72 cm3 cylinders) were collected at six points

along the sampling transect for aggregate size

distribution determination. Air-dried samples were

manually disaggregated with very gentle pressure

through rupture across the natural planes of weakness

(Arshad et al., 1996). The same technician skilled in this

procedure processed all samples. Then the samples

were shaken through a battery of 8, 4, 3, 2 and 1 mm

diameter sieves during 30 min. Soil mass retained by

each sieve was weighed and C content of these

aggregate size fractions was determined by the same

method as for OC.

100 g samples of pooled aggregate size fractions>4,

1–4, and <1 mm, as well as corresponding samples of

non fractionated (entire) soil were incubated in closed

recipients in a growth chamber at 24 8C and at 80% of

their water holding capacity. The respired CO2 was

trapped in 1N NaOH and the excess was titrated with

0.1N HCl. Determination of CO2 was realized daily at





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Table 2

Dry aggregate size distribution

Size classes (mm) Dry aggregate size distribution (g kg�1 soil)

>8 4–8 3–4 2–3 1–2 <1

CULT14 281.4 a 156.4 a 85.8 b 68.4 b 76.9 b 331.1 b

CULT2 160.4 b 114.6 b 116.7 ab 84.1 ab 113.9 a 409.6 a

PAS 226.9 a 147.6 b 147.4 a 95.6 a 122.6 a 259.8 b

References: soil mass (g kg�1) in the aggregate size classes. Different letters indicate significant differences ( p <0.05) within a column (n = 6).

the beginning of the experiment and less frequently

when respiration rates slowed down. When rates had

stabilized no further measurements were taken.

2.3. Statistical analysis

Mean values of the six replicates of all variables

were compared using one-way analysis of variance

and separated by the Fisher LSD test at the 95%

confidence level. CO2 incubation data were log-

transformed when necessary, and regression lines

were compared according to the method proposed by

Sokal and Rohlf (1968).

3. Results

3.1. Soil physical properties

Dry aggregate size distribution (Table 2) differed

according to the history of land-use of the soil. The soil

with a short period of cultivation (CULT2) had lower

contents of large (>4 mm) and higher contents of very

small (<1 mm) aggregates than both the PAS and long-

term cultivated (CULT14) soil. The intermediate size

aggregates were very little affected by the short-term



Fig. 2. Accumulated water infiltration in the field (mm) after 1 h.

References: error bars represent standard error of means (n = 6).

effect of tillage, since values for CULT2 and PAS were

similar. However, CULT14 showed significantly lower

values, which might have been due to the longer term

effect of tillage on aggregate size distribution. The

decrease in medium size aggregates in this site was

considerably higher than that of CULT2, with losses of

42, 29 and 37% for the 3–4, 2–3 and 1–2 mm size

classes, respectively. The relatively high values of large

aggregates in CULT14 may be attributed the effect of

the alfalfa PAS, which covered this soil for the last 3

years.

The highest water infiltration rate was found in

CULT2, whereas CULT14 had a considerably lower

rate than both other treatments (Fig. 2), even during the

first 10 min. Accumulated infiltration of CULT14 was

74% lower than that of PAS, and CULT2 showed a 38%

increase (Table 3) with respect to the latter.

Soils reached steady state of water movement

between 8 and 12 days, when no further water drainage

occurred due to gravity forces. Soil water content at this

moment (Table 3) can be considered as the water

holding capacity as measured in field conditions

(Fuentes Yague, 1996). Water holding capacity was

22.5% (w/w) in CULT14, whereas both CULT2 and

PAS had significantly higher values of 25.5 and 27.6%,

respectively. However, when these values were con-

verted to volumetric water contents, due to the

differences in BD between CULT14 and both other

treatments, no significant difference was found (27.4%,

25.5% and 28.7% for CULT14, CULT2 and PAS

respectively).



Table 3

Soil physical properties

BD (Mg m�3) WC 12 (%, w/w) I (mm h�1)

CULT14 1.22 a 22.5 b 100.7 c

CULT2 1.00 b 25.5 a 540.5 a

PAS 1.04 b 27.6 a 391.3 b

References: bulk density (BD), water content 12 days after saturation

(WC 12) and infiltration (I). Different letters indicate significant

differences ( p < 0.05) within a column (n = 6).



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Table 4

Organic carbon (C)

Depth (cm) C content (g kg�1) C mass (Mg ha�1)

0–6 6–12 12–18 0–6 6–12 12–18

CULT14 11.5 c 10.4 a 8.5 a 8.4 b 7.4 a 6.0 a

CULT2 14.3 b 12.0 a 9.4 a 8.6 b 7.6 a 6.2 a

PAS 17.0 a 12.8 a 9.7 a 10.6 a 9.1 a 6.4 a

References: total C content (g kg�1) and volumetric C mass (Mg ha�1) of the soil. Different letters indicate significant differences ( p < 0.05) within

a column (n = 6).

Table 5

Organic carbon (C) of different aggregate size fractions

Size classes (mm) C content (g kg�1) C mass (Mg ha�1)

>4 1–4 <1 >4 1–4 <1

CULT14 8.6 b 23.6 a 10.9 a 9.2 a 13.1 a 8.8 b

CULT2 11.7 a 16.9 a 12.3 a 6.4 b 10.6 b 10.1 a

PAS 12.1 a 20.7 a 12.4 a 9.2 a 15.4 a 6.6 a

References: total C content (g kg�1) and volumetric C mass to a depth of 20 cm (Mg ha�1) of the different aggregate size fractions. Different letters

indicate significant differences ( p < 0.05) within a column (n = 6).

Table 6

Accumulated CO2 production after 17 days

Aggregate size class Treatment CO2-17 (mg C kg�1 soil)

>4 mm CULT14 177.0 b

CULT2 178.2 b

PAS 302.3 a

1–4 mm CULT14 565.4 a

CULT2 377.8 b

PAS 479.5 ab

<1 mm CULT14 182.0 b

CULT2 211.0 b

PAS 466.1 a

Entire soil CULT14 351.4 a

CULT2 414.7 a

PAS 386.6 a

References: accumulated CO2 production after 17 days of incubation

(mg C kg�1 soil). Different letters indicate significant differences

( p < 0.05) within a column and within an aggregate size fraction

(n = 6).

3.2. Organic matter

Organic C in the uppermost depth layer (0.00–

0.06 m) decreased from 17.0 g kg�1 in PAS to

14.3 g kg�1 in CULT2 and 11.5 g kg�1 in CULT14

(Table 4). Carbon losses in this layer due to cultivation

were 16% for CULT2 and 32% for CULT14. However,

there were no significant differences among treatments

in the two deeper layers (0.06–0.12 and 0.12–0.18 m).

When these values were transformed to mass per

hectare, OC losses were 4.3 and 3.7 Mg ha�1 for

CULT14 and CULT2, respectively.

The OC content of aggregate size fractions ranged

from 8.6 to 12.1 g kg�1 in>4 mm aggregates; from 16.9

to 23.6 g kg�1 in 1–4 mm aggregates, and from 10.9 to

12.4 g kg�1 in<1 mm aggregates (Table 5). There were

no significant differences in OC content of aggregate size

fractions among treatments, with the exception of the

lower value of >4 mm aggregates in CULT14. The

highest OC contents were found in the 1–4 mm size class

in all treatments, with an average value of 20.4 g kg�1.

The two other size classes had very similar averagevalues

of 11.9 and 10.8 g kg�1 for<1 and>4 mm, respectively.

We then calculated OC mass per hectare contained in

these aggregate fractions by multiplying the OC

concentration in each fraction by its weight proportion,

considering the sampling depth of 0.20 m and the average

bulk density: OC (kg Mg�1) � weight of aggregate frac-

tion (kg Mg�1) � 2000 m3 ha�1 � average bulk density

(Mg m�3). Theresultsarealso shown inTable 5. This way



of expressing the data made long- and short-term effects

more evident: CULT2 had the highest amount of OC in

the<1 mm fraction and least in the largest aggregates. On

the other hand, CULT14, showed a similar distribution of

aggregate size classes as PAS.

3.3. Biological properties

In order to characterize the biological activity and

availability of C pools contained in the different size





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Fig. 3. Respiration of different aggregate size classes and entire soil. References: values represent the average of six replicates.

fractions incubation assays for pooled fractions of

large (>4 mm), intermediate (1–4 mm) and small

(<1 mm) aggregates were carried out. We expected

that soil fragments would show different respiration

rates according to their difference in C contents. The

total amounts of CO2-C evolved after 17 days of

incubation (Table 6) confirmed this hypothesis. The

intermediate (1–4 mm) aggregate size fraction showed

the highest values with an average of 474.2 mg C kg�1

soil for all three treatments, while >4 and <1 mm size

classes had averages of nearly half that value (219.2

and 286.4 mg C kg�1 soil, respectively). CULT2 had

the lowest CO2-C production (377.8 g C kg�1 soil) in

the intermediate aggregate size fraction while both

CULT14 and PAS had similar and higher values.

This trend coincided with the respective aggregate

size OC contents of the treatments. No such trend

was observed in the largest size fraction where

PAS had significantly higher CO2-C production

(302.3 mg kg�1) than CULT14 (177.0 mg kg�1) and



CULT2 (178.2 mg kg�1), whereas the OC contents of

this fraction were similar between PAS and CULT2. In

the <1 mm fraction PAS also produced significantly

more CO2-C (466.1 mg kg�1) than both other treat-

ments, which also could not be predicted from its OC

contents. Accumulated CO2 respiration from the

entire soil did not differ among treatments, and the

values were considerably lower compared to the sum

of aggregate size fraction CO2-C respiration for each

treatment. These sums were 924, 766 and

1247 mg C kg�1 for CULT14, CULT2, and PAS,

respectively, while the values for entire soil of these

treatments were 351, 414, and 386 mg C kg�1,

respectively.

Within each site, the aggregate size fractions also

differed in their respiration rates (Fig. 3). A steep curve

and very rapid depletion of evolved CO2 was observed

in the>4 mm class. On the contrary,<1 mm aggregates

showed the lowest slope and also the longest period of

respiration.





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Please cite this article in press as: Noellemeyer, E., et al., Carbon contents and aggregation related to soil physical and biological

properties under a land-use sequence in the semiarid . . ., Soil Tillage Res. (2008), doi:10.1016/j.still.2008.02.003

Fig. 4. Regression lines for CO2 production of entire soil and different aggregate size fractions.



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Within each fraction, we also found different slopes

for PAS, CULT2 and CULT14 (Fig. 4). With the

exception of >4 mm aggregates of CULT2 and

CULT14, all data sets had to be log-transformed in

order to obtain regression lines that adjusted to a linear

model with R2 ranging from 0.85 to 0.96. Statistical

comparison of these regression lines revealed that only

>4 mm aggregates of CULT2 and CULT14 had

comparable regression lines ( p = 0.60), with similar

slopes ( p = 0.87) that could be considered superposed

( p = 0.85). All other regression lines were different

between treatments.

The contribution of total CO2-C produced in 17 days

by each aggregate fraction was calculated considering

their weight proportion and CO2-C evolved during

incubation (Fig. 5). The 1–4 mm aggregate fraction

contributed most to respiration in all treatments, ranging

from 42% in PAS to 49% in CULT14. Larger aggregates

contributed similar amounts in PAS and CULT14 (28

and 29%, respectively) while in CULT2 this fraction

made up only 19%. These trends were expected

considering the OC content and weight proportion of

these fractions. On the other hand, smaller than 1 mm

aggregates decreased in their contribution according to

land use from 30% in PAS to 22% in CULT14. This was

not related to their OC content and aggregate size

distribution.

The total CO2-C production was calculated by the

sum of aggregates’ contributions (Fig. 5), considering

the partial contributions of each class, and it showed

great differences among treatments. PAS had signifi-

cantly higher values than both cultivated soils, and there

were no significant differences between the latter. When

compared to the entire soil, only PAS had similar values

of aggregates’ sum and undisturbed (entire) sample. On

the other hand, the CO2-C released by CULT14 and



Fig. 5. Contribution of different aggregate size classes to total CO2

(mg C kg�1 soil) produced in 17 days, compared to the entire soil

sample.

CULT2’s entire soil was higher than their sum of

aggregates’ respiration.

4. Discussion

Land-use change from a permanent PAS to cultiva-

tion caused important changes in the size distribution of

aggregates. After 2 years of tillage, large aggregates had

decreased by 29% compared to the PAS. At the same

time, small aggregates (<1 mm) increased by 37%. The

losses of aggregates in the intermediate size classes

ranged from 21, 12 and 7% for 3–4, 2–3 and 1–2 mm

aggregates, respectively. This suggested that larger soil

fragments contributed more to the loss of soil structure,

and that cultivation produced an immediate increase of

small soil particles, especially those that are susceptible

to wind erosion (<0.84 mm). The longer term effect of

cultivation on this soil was more related to intermediate

size aggregates in the 1 to 4 mm size classes, which

were significantly lower in CULT14 than in the

permanent PAS, whereas the proportion of largest

and smallest aggregates was similar between these

treatments. The relatively high values of large

aggregates in CULT14 may be attributed the effect of

the alfalfa PAS, which covered this soil for the last 3

years. A recuperation of soil structure under PAS and

under no-till agriculture has often been observed. For

instance, Paustian et al. (2000) and Six et al. (2000b)

found increases in water stable aggregates and soil OC

in soils that were not tilled. Our data indicated that the

same might hold true for dry sieved aggregates. The

distribution of aggregate size classes among our land-

use sequence treatments was very different since the

size fraction which represented the smallest amount of

soil mass was different in each treatment: PAS had 26%

of small aggregates, CULT2 had 28% of large

aggregates and 23% of the soil under CULT14 was

found in intermediate ones.

It has been reported by various authors that tillage

destroys water stable aggregates and specifically affects

the larger soil aggregates, which are considered to be

less stable (Beare et al., 1994; Kay, 1990; Elliott, 1986;

Tisdall and Oades, 1982; Van Veen and Paul, 1981).

This could be applied to our results as a short-term

consequence of cultivation. However, the longer term

effect on intermediate size aggregates we observed

could not be compared with results obtained with water

stable aggregates. There are few studies on the

dynamics of dry sieved aggregates to be found in the

literature. For instance, Eynard et al. (2004) found

smaller mean weight diameters in dry aggregates of

perennial PASs on Ustolls and Uderts of South Dakota





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than in corresponding no-till and tilled plots. Drury

et al. (2004) also found higher proportion of larger

aggregates in continuous corn than under rotation corn

in a clay loam, typic Argiaquoll. The apparent

contradiction with our results might be explained by

the climatic regime and clay contents that favor high

carbon stocks and corresponding fine granular struc-

tures in Ustolls and Argiaquolls under land-use systems

that have relatively high inputs of plant residues. In

Udolls of the semiarid Pampa large sub-angular blocks

are formed as predominant structural units under PASs

and native vegetation, while carbon losses due to

cultivation lead to predominance of smaller less stable

aggregates and soil compaction (Quiroga et al., 1998).

Thus, the effect of texture and environmentally

determined carbon stocks of the soil have to be

considered when comparing the results of dry sieved

aggregate size distributions and these have to be related

to the structure type conditioned by soil genesis. Drury

et al. (2004) already suggested that their results showed

that indigenous soil properties exerted a greater

influence on aggregate size distribution than cropping

history.

After 14 years of cultivation, soil water holding

capacity decreased by nearly 19%. This change

considerably affects the soils potential for crop

productivity, especially in a semiarid region where

long drought periods are common. The loss of water

retention might be related to the significantly lower

proportion of aggregates in the 1–4 mm size class in

CULT14. Following Elliott and Coleman’s (1988)

theory of hierarchical pore categories that correspond

to aggregate size classes as their mirror images, this

would indicate that the intermediate size aggregates

have associated pore sizes which play an important role

in defining water retention. Dexter (2004) showed that

soil microstructure is responsible for most physical soil

properties and that structural porosity is directly related

to water holding capacity. In the case of the sandy loams

of the semiarid Pampa, water holding structural porosity

is apparently more related to medium size aggregates

(1–4 mm). The decrease of structural porosity under

cultivation (CULT14) was also confirmed by a 22%

increase of bulk density in CULT14 relative to PAS and

CULT2.

For the case of water infiltration, the short-term

effect of cultivation was positive, facilitating water

capture and translocation in the soil. The longer term

effect of agricultural use however, greatly diminished

the soil’s capacity to infiltrate water and could lead to

runoff losses and decreased water use efficiency. Thus,

water retention, aggregate size class, bulk density and



water infiltration data revealed significant reduction of

structural porosity due to 14 years of cultivation. On the

other hand, the short-term effect of cultivation on

structural porosity and soil hydraulic properties was

very small, despite the considerable loss of large and

increase of very small aggregates that are most

susceptible to wind erosion.

Organic C losses were 4.3 and 3.7 Mg ha�1 for

CULT14 and CULT2, respectively. About half of this

decrease was due to the OC loss in the uppermost 0.06 m

soil layer (2.2 and 2.0 Mg ha�1 for CULT14 and CULT2,

respectively). The apparent OC loss rate during the first 2

years of cultivation was 1.85 Mg C ha�1 year�1, while

the average rate after 14 years of cultivation under a

rotation with alfalfa PAS was 0.31 Mg C ha�1 year�1.

Considering 26.1 Mg ha�1 as a stable OC content in this

soil its average half life would be approximately 42 years,

but during the first few years of cultivation a significantly

lower half life of 7 years was found. These data confirmed

that OC turnover rates in soils of the semiarid Pampa are

high, with half lives of approximately 10 years and no

evidence of long-term stabilized C (Zach et al., 2006).

Apparently, most of the C was lost during a short period

following cultivation of the PAS, and even under

rotations that include alfalfa (as in CULT14) a further

decrease was observed. Angers et al. (1992) reported an

increase in OC content upon conversion of continuous

corn to alfalfa PAS, which might indicate that under

continuous cropping, C degradation would have been

even more severe than that observed in CULT14.

Carbon contents in aggregates size fractions were

very similar among treatments, and only CULT14 had

significantly lower OC contents in the largest aggregate

fraction. While our results did not show a clear effect of

land use on OC contents of aggregate fractions, Drury

et al. (2004) reported higher OC contents in all

>0.25 mm aggregate fractions under rotation, com-

pared with corn monoculture, and Holeplass et al.

(2004) also found higher OC concentration in these

aggregate sizes under grain – PAS rotation than under

all – grain production systems.

The highest OC content among aggregate fractions

was found in the pooled 1–4 mm size class in all

treatments. For water stable aggregates, Holeplass et al.

(2004) found a trend of increasing OC concentration

with decreasing aggregate size, while Saroa and Lal

(2003) reported that OC increased with increasing

aggregate size. This trend reflects the concept of

aggregate hierarchy proposed by Tisdall and Oades

(1982). Our results, however, did not agree with this

theory, since intermediate size aggregates had higher

OC contents than both other fractions. Zotarelli et al.





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(2005) explained the lack of aggregate hierarchy as

shown by a similar OC content across aggregate size

fractions as the effect of principal binding agents

other than organic matter. In their study on water

stable aggregates in low activity clay oxisols under

different cropping systems, these authors conclude

that the OC loss from natural vegetation to conven-

tional tillage can only partly be explained by the loss

of C-rich macroaggregates and an increase in C-poor

microaggregates, since both fractions did not differ in

their OC concentrations. Thus, several authors

reported divergences from the aggregate hierarchy

model under different soil conditions even for

aggregates separated by wet sieving. Our data

suggested that for the dry sieved aggregate size

classes we studied, the most important fraction for

OC stabilization would be the intermediate class

(1–4 mm), due to its higher OC concentration and the

observed decrease of this aggregate size fraction after

prolonged agricultural use.

The lower OC contents of >4 and <1 mm fractions

(not significant) in CULT14 would suggest that in these

dry sieved aggregate classes the conceptual model of

Six et al. (1998), which states that in tillage agro-

ecosystems less C is sequestered due to higher macro

aggregate turnover that lowers the rate of micro

aggregate formation within macro aggregates also

could be valid.

Incubation assays showed that on the average the OC

content of the aggregate size fraction was related to the

amount of CO2-C evolved by the fraction. The

intermediate size class had highest OC contents and

also produced most CO2-C. Both for OC contents and

CO2-C production this fraction showed values almost

twice as high as both >4 and <1 mm fraction. These

results do not agree with those of Drury et al. (2004),

who also used dry sieved aggregates to evaluate the

impact of aggregation on biological processes under

rotation and continuous cropping. They found an

increase of CO2 production with decreasing aggregate

size, but they also reported a strong relation between

CO2 respiration and POC contents, especially under

rotation. Our results however did not show a clear effect

of treatment on the OC content of the aggregate size

fractions, nor on respiration. Whereas the low CO2-C

production of CULT2 in the 1–4 mm fraction could be

associated to its low OC content, this was not true for

CO2-C in the >4 and <1 mm fractions. In both cases

OC contents were similar between CULT2 and PAS,

while CO2-C production was similar for CULT14 and

CULT2. This might indicate a similar nature of OC in

these fractions between CULT14 and CULT2. Short and



longer term effects of cultivation on the biological

behavior of the aggregate size fractions therefore could

not be distinguished.

The comparison between the CO2-C production of

the entire soil and the sum of the aggregate size

fractions revealed that size fractions produced far more

CO2-C. The smallest difference was found in CULT2

(414.7 and 767.0 mg kg�1, for entire soil and aggregate

sum, respectively, which represents 1.85 times the

amount), while in the case of CULT14, CO2-C

production of aggregate sum was 2.63 times that of

the entire soil; and for PAS this values was 3.23 times.

The lower CO2-C production of entire soil could be

attributed to protection of OC due to the undisturbed

soil matrix, while aggregate fractionation by dry sieving

might have released otherwise protected organic

material. Similarly, Drury et al. (2004) found that

grinding the aggregates reduced the differences of CO2

production between aggregate size classes and gen-

erally enhanced CO2 production.

In an attempt to evaluate the contribution of each

aggregate size fraction to soil respiration we

calculated these amounts pondered by the aggregates’

size fraction’s weight proportion and found that only

PAS had similar values between the sum of pondered

aggregate contribution and entire soil CO2 produc-

tion. Both cultivated treatments had higher entire soil

CO2 production than the pondered sum of aggregates

production. This result was astonishing, since the sum

of CO2 production of separate aggregate fractions was

considerably higher than entire soil respiration.

However, taking into account their contribution to

total soil mass this trend is reversed and no

explication for this apparent contradiction could be

adventured.

A steep curve and very rapid depletion of evolved

CO2 was observed in the >4 mm class, which might be

related to a more labile nature of C. On the contrary,

<1 mm aggregates showed the lowest slope and also the

longest period of respiration. Zhang et al. (2007) studied

the respiration rates of different OC fractions and found

that labile fractions showed steeper slopes than heavy C

fraction and whole soil OC. The difference of

respiration rates of aggregate size fractions among

treatments that became evident in the comparison of

regression lines therefore could indicate differential

substrate availability and/or differences in microbial

community. The major differences were between PAS

and both cultivated treatments, suggesting that despite

the positive effect of the alfalfa PAS on OC contents and

aggregate size distribution in CULT14, the biological

activity had not been recovered.





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5. Conclusions

Land-use change from permanent PAS to arable

agriculture greatly affected OC stocks and all tested

physical and biological soil properties. Cultivation

caused a decrease of structural porosity, which was

related to higher bulk density and decreased infiltra-

tion rate. This process might be divided into a short-

term disruption of large aggregates, while the long-

term effect appeared to be the decrease of inter-

mediate size ones. The latter was reflected in an

important reduction of soil’s water retention and

infiltration, which could have a negative impact on

soil quality and productivity. OC losses were similar

between short- and long-term cultivation, which

indicated a very fast rate of C turnover in the studied

soil. The intermediate size aggregates had the highest

OC contents, and also contributed most to total

respiration. Differences in land-use history only

became evident in aggregate size fractions, while

entire soil samples did not show different respiration

rates or total CO2 production. Our results showed that

dry sieved aggregates provide meaningful fractions

for studies of the impact of land use on soil physical

and biological properties.

Acknowledgement

This work was financed by the Interamerican

Institute for Global Change Research (IAI) within the

collaborative research networks CRN 001 and 2031.

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