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Published online 15 February 2008
Published in Soil Sci Soc Am J 72:541-547 (2008)
DOI: 10.2136/sssaj2007.0164
© 2008 Soil Science Society of America
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SOIL & WATER MANAGEMENT & CONSERVATION

Tillage Effects on Soil Organic Carbon Fractions in Mediterranean Dryland Agroecosystems

J. Álvaro-Fuentesa,*, M. V. Lópeza, C. Cantero-Martinezb and J. L. Arrúea

a Dep. de Suelo y Agua, Estación Experimental de Aula Dei, Consejo Superior de Investigaciones Científicas, Apartado 202, 50080 Zaragoza, Spain
b Dep. de Producció Vegetal i Ciencia Forestal, Universitat de Lleida–IRTA, Rovira Roure 177, 25198 Lleida, Spain

* Corresponding author (jalvaro.fuentes{at}gmail.com).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Under semiarid conditions, soil quality and productivity can be improved by enhancing soil organic matter content by means of alternative management practices. In this study, we evaluated the feasibility of no-till (NT) and cropping intensification as alternative soil practices to increase soil organic C (SOC). At the same time, we studied the influence of these management practices on two SOC fractions (particulate organic matter C, POM-C, and the mineral-associated C, Min-C), in semiarid agroecosystems of the Ebro River valley. Soil samples were collected from five soil layers (0–5-, 5–10-, 10–20-, 20–30-, 30–40-cm depth) during July 2005 at three long-term tillage experiments located at different sites in the Ebro River valley (northeast Spain). Soil bulk density, SOC concentration and content, SOC stratification ratio, POM-C, and Min-C were measured. Higher soil bulk density was observed under NT than under reduced tillage (RT), subsoil tillage (ST), or conventional tillage (CT). At the soil surface (0–5-cm depth), the highest total SOC concentration, POM-C, and Min-C were measured under NT, followed by RT, ST, and CT, respectively. In the whole soil profile (0–40 cm), similarly, slightly greater SOC content was measured under NT than under CT with the exception of the Selvanera site, where deep subsoil tillage combined with moldboard plowing accumulated more SOC than NT. In semiarid Mediterranean agroecosystems where CT consists in moldboard plowing, NT is a viable management practice to increase SOC.

Abbreviations: AG, Agramunt • PN-CF, cereal–fallow rotation at the Peñaflor site • CT, conventional tillage • Min-C, mineral-associated carbon • NT, no-till • PN-CC, continuous cropping system at the Peñaflor site • POM, particulate organic matter • POM-C, particulate organic matter carbon • RT, reduced tillage • SOC, soil organic carbon • SOM, soil organic matter • ST, subsoil tillage • SV, Selvanera


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil organic matter (SOM) is a key factor in semiarid agroecosystem productivity. Soils of semiarid regions are characterized by low SOC content, low water and nutrient retention, and thus low inherent soil fertility (Lal, 2004a). In these regions, low and erratic rainfall together with high evapotranspiration rates leads to a low crop biomass production and thus to a limited residue input into the soil. Bauer and Black (1994) quantified the contribution of SOM to productivity and observed that 1 Mg ha–1 of SOM increased wheat (Triticum aestivum L.) grain yield up to nearly 16 kg ha–1. They concluded that a loss of fertility explained the loss of productivity due to a depletion of SOM.

Reeves (1997), after compiling information from several long-term studies, concluded that cropping resulted in a general loss of SOC that can be reduced through rational soil management practices. The influence of different agricultural management practices on soil C storage or C sequestration has been reviewed by several researchers (Freibauer et al., 2004; Lal, 2004b). Enhancing SOC by soil management may be mainly achieved by means of reducing SOC decomposition, increasing residue inputs, or both (Paustian et al., 2000).

A reduction in the intensity of tillage has been widely recognized as a successful strategy to reduce SOC losses (Halvorson et al., 2002; West and Post, 2002; McConkey et al., 2003). West and Post (2002) analyzed the results from 67 long-term agricultural experiments and concluded that, on average, a shift from CT to NT can sequester nearly 60 g C m–2 yr–1. Moldboard plowing, in CT systems, accelerates SOM decomposition and C loss from soil to the atmosphere as CO2. Plowing creates residue and soil mixing, favoring physical contact between soil microorganisms and crop residues, and more optimal soil microclimatic conditions for crop residue decomposition (e.g., higher soil moisture content, temperature, and aeration) (Paustian et al., 1998; Bruce et al., 1999). In contrast, under NT systems, the absence of soil disturbance produces a modification of surface soil conditions, reducing microbial activity and therefore SOM decomposition (Mielke et al., 1986). Several studies have measured greater soil bulk density values after the adoption of NT (Kay and VandenBygaart, 2002). Increases in bulk density under NT are associated with reductions in soil porosity that may lead to a more limited O2 supply for heterotrophic decomposition. On the other hand, the intensification of cropping systems by means of a reduction in the long fallow period is associated with greater residue production and therefore with an increase in SOC content (Potter et al., 1997; Halvorson et al., 2002).

Soil organic matter is formed by various components with different structural complexities that differ in their chemical stability and, consequently, in their turnover rates (Christensen, 1996; Krull et al., 2003). Several SOC models have been developed in the last 30 yr (Jenkinson and Rayner, 1977; van Veen and Paul, 1981; Parton et al., 1987). One of the major limitations of these models is that they are comprised of conceptual C pools that do not correspond to experimentally verifiable fractions (Christensen, 1996). Accordingly, several attempts have been made to set up measurable C fractions that closely match the SOC pools described in those models (Cambardella and Elliot, 1992; Paul et al., 1999; Six et al., 2002). Cambardella and Elliot (1992) isolated the SOM pool of particulate organic matter (POM), which is more sensitive to soil management than total SOM. This fraction is mainly composed of fine root fragments and other organic debris (Cambardella and Elliot, 1992) and serves as a readily decomposable substrate for soil microorganisms (Mrabet et al., 2001). Wander et al. (1998) observed 25% greater SOC under NT than under CT. When POM-C was analyzed, however, this difference between tillage systems increased to 70%. Another measurable C fraction is the min-C, which is the SOM chemically stabilized on silt and clay surfaces (Hassink, 1997). This is a more stabilized SOM than the POM, however, and therefore less sensitive to soil management.

In semiarid Spain, several studies have been focused on the effect of soil management on SOM content (López-Fandos and Almendros, 1995; López-Bellido et al., 1997; Hernanz et al., 2002; Moreno et al., 2006). These studies mostly concluded that a reduction in tillage intensity increases SOM content, especially at the soil surface. In these studies, however, no attempt was made to estimate the effect of soil management on different SOM fractions.

In this study, we present SOM data from three different long-term tillage experiments located in semiarid Ebro River valley (northeast Spain). In this region, intensive soil tillage, with moldboard plowing as the main tillage implementation, and a cereal–fallow rotation have been the traditional agricultural practices for decades. We hypothesized that a shift from intensive tillage to more conservative tillage operations may lead to an increase in SOM, as it has been previously observed in other semiarid areas of Spain. At the same time, the removal of the fallow period in the rotation may help to raise the levels of SOM and thus to increase soil quality and productivity in the study area. In this respect, we consider it a major issue to quantify the different SOM fractions and to study the role that these fractions play on SOM dynamics. Therefore, our objectives were to investigate the influence of different soil tillage and cropping systems on SOC content and distribution of C between SOM fractions (POM-C and min-C).


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Cropping Systems and Locations
This experiment was conducted at three different long-term tillage experiments located across the semiarid Ebro River valley (northeast Spain). The Selvanera and Agramunt experimental sites, established in 1987 and 1990, respectively, are located in Lleida Province at dryland farms managed by the Agronomy Group of the University of Lleida. The third experimental site, Peñaflor, was established in 1989 at the dryland research farm of the Estación Experimental Aula Dei (Consejo Superior de Investigaciones Científicas) in Zaragoza Province. At the three sites, prior land use consisted in conventionally managed agriculture with intensive soil management. Selected site and Ap soil horizon characteristics are presented in Table 1 .


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  		Table 1. Site and soil (Ap horizon) characteristics.

 
In Selvanera (SV), the cropping system consisted of a wheat–barley (Hordeum vulgare L.)–wheat–rapeseed (Brassica napus L.) rotation with four tillage treatments: CT, ST, RT, and NT. The CT and ST treatments consisted of a subsoiler tilling at 50- and 25-cm depths, respectively, in August, followed in both cases by a pass with a field cultivator to a depth of 15 cm in October before sowing. The RT treatment was implemented every October with only one pass of a cultivator to a depth of 15 cm.

In Agramunt (AG), the cropping system consisted of a barley–wheat rotation with four tillage treatments: CT, ST, RT, and NT. The CT treatment consisted of a pass with a moldboard plow to a depth of 25 to 30 cm every October followed by a pass with a field cultivator to a depth of 15 cm. The ST treatment consisted of a subsoiler tilling at 25-cm depth every October followed by a field cultivator to 15-cm depth. The RT treatment was implemented with one or two passes of a cultivator to a 15-cm depth every October.

In Peñaflor (PN), two cropping systems were compared: a continuous barley cropping system (PN-CC) and a barley–fallow rotation (PN-CF). Three tillage systems were compared in both cropping systems: CT, RT, and NT. The CT treatment consisted of a pass with a moldboard plow to a depth of 30 to 35 cm plus a pass with a tractor-mounted scrubber as a traditional practice to break down large clods. The RT plots were chisel plowed to a depth of 25 to 30 cm. In the CT and RT plots of the PN-CC system, primary tillage was implemented every season in October followed by a pass of a sweep cultivator to a depth of 10 to 15 cm as secondary tillage. In the PN-CF rotation, however, primary tillage was implemented in March every other season, during the fallow phase of the rotation, while secondary tillage consisted of a cultivator pass to a depth of 15 to 20 cm in May. At the three experimental sites in the NT treatment, no tillage operations were done and, for sowing, a direct drill planter was used. In this treatment, the soil was kept free of weeds by spraying a total herbicide (glyphosate [N-(phosphonomethyl)glycine]).

At all sites, tillage treatments were arranged in a randomized complete block design, with three replicates at SV, PN-CC, and PN-CF and four replicates at AG. The size of each plot was 7 by 50 m at SV, 9 by 50 m at AG, and 10 by 33 m at PN-CC and PN-CF.

Soil Sampling and Analyses
Soil samples were collected at five depths (0–5, 5–10, 10–20, 20–30, and 30–40 cm) in July 2005 after crop harvest. For C analyses, a composite sample was prepared from two samples taken from each plot and depth. Once in the laboratory, the soil was air dried and ground to pass a 2-mm sieve. For soil dry bulk density determination, by the core method (Grossman and Reinsch, 2002), stainless steel cylinders (height 51 mm, diameter 50 mm, volume 100 cm3) were used for undisturbed soil sampling. Four soil cores were taken per plot and soil depth.

A 5-g subsample was used to determine the total SOC content by the wet oxidation method of Walkley and Black (Nelson and Sommers, 1982). The C content of the particulate organic matter (POM-C) and the mineral-associated organic matter (Min-C) were separated using a physical fractionation method adapted from Cambardella and Elliot (1992). Twenty-gram subsamples of soil from each depth, plot, and site were dispersed in 100 mL of 5 g L–1 sodium hexametaphosphate for 15 h on a reciprocal shaker. Then the samples were passed through a 53-µm sieve to separate the POM-C and Min-C. The material passing through the sieve (Min-C) was collected in aluminum pans and oven dried at 50°C overnight. The wet oxidation method of Walkley and Black was then used to measure the C concentration in the Min-C fraction. The total SOC and Min-C contents were expressed on a mass per unit area basis by multiplying the C concentration values obtained from the oxidation method by the corresponding soil bulk density values. The POM-C content was determined as

Formula 1[1]
Data were analyzed using the SAS statistical package (SAS Institute, 1990). To compare the effects of tillage treatments, ANOVA for a randomized block design was used. Differences between means were tested with Duncan's multiple range test.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Bulk Density
Soil bulk density ranged from 1.28 to 1.55, 1.25 to 1.67, 1.15 to 1.48, and 1.19 to 1.40 Mg m–3 at AG, SV, PN-CC, and PN-CF, respectively (Fig. 1 ). At all four fields, a general increase in soil bulk density was observed from the 0- to 5-cm layer to the 5- to 10-cm soil layer, especially under NT (Fig. 1).


Figure 1
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  		Fig. 1. Soil bulk density profile at Agramunt (AG), Selvanera (SV), and Peñaflor in a continuous barley cropping system (PN-CC) and in a barley–fallow rotation (PN-CF) as affected by tillage (CT, conventional tillage; ST, subsoil tillage; RT, reduced tillage; NT, no-till). Bars represent LSD (P < 0.05) for comparison among tillage treatments at the same depth, where significant differences were found. * Significant differences between PN-CC and PN-CF within the same tillage treatment and soil depth (P < 0.05).

 
At AG, PN-CC, and PN-CF the highest soil bulk density corresponded to the NT treatment, especially for the first 20 cm. At SV, however, differences among tillage treatments were only found in the 5- to 10-cm soil layer, where greater soil bulk density was measured under NT and RT than under CT and ST (Fig. 1). Several studies have observed greater soil bulk density under NT systems (Rhoton et al., 1993; Wander and Bollero, 1999; Lampurlanés and Cantero-Martínez, 2003).

Total Soil Organic Carbon
In the 0- to 40-cm soil depth, total SOC concentration values ranged from 5.3 to 22.5 g kg–1 at SV, from 3.7 to 18.8 g kg –1 at AG, from 8.0 to 13.7 g kg –1 at PN-CC, and from 7.3 to 11.6 g kg –1 at PN-CF (Fig. 2 ). At the soil surface (0–5-cm depth), a significantly greater SOC concentration was measured under NT at all the experimental sites. Below the 10-cm depth, on the contrary, the SOC concentration under this tillage treatment was similar (PN) or lower (SV and AG) than that measured in the other tillage treatments. Thus, at SV and AG, from the 0- to 5- to the 10- to 20-cm soil depth, the SOC concentration under NT decreased >60%. At PN-CC and PN-CF, this reduction was close to 40% (Fig. 2). In general, in the first 10-cm depth, the lowest SOC concentration corresponded to CT, but at deeper soil layers CT had the greatest SOC concentration at all the sites (Fig. 2). Several studies have reported greater SOC at the soil surface under NT than under other tillage systems (Potter et al., 1997; Deen and Kataki, 2003; Puget and Lal, 2005). In other similar experiments performed in semiarid Spain, SOC accumulation at the soil surface has also been observed when soil management shifted from conventional tillage to conservation tillage (Hernanz et al., 2002; Moreno et al., 2006). In NT systems, crop residues are left on the soil surface, implying much slower crop residue incorporation and decomposition compared with tilled systems in which crop residues are mechanically incorporated into the soil. This slower decomposition of crop residues under NT leads to the accumulation of SOC in the upper soil layers (Reicosky et al., 1995).


Figure 2
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  		Fig. 2. Vertical distribution of the soil organic C (SOC) concentration at Agramunt (AG), Selvanera (SV), and Peñaflor in a continuous barley cropping system (PN-CC) and in a barley–fallow rotation (PN-CF) as affected by tillage (CT, conventional tillage; ST, subsoil tillage; RT, reduced tillage; NT, no-till). Bars represent LSD (P < 0.05) for comparison among tillage treatments at the same depth, where significant differences were found. * Significant differences between PN-CC and PN-CF within the same tillage treatment and soil depth (P < 0.05).

 
The accumulation of SOC at the soil surface was considered to be a promising soil quality indicator by Franzluebbers (2002), who developed the so-called stratification ratio, defined as the proportion of SOC at the soil surface in relation to the SOC in deeper soil layers. This ratio permits an easy comparison between tillage treatments. Franzluebbers (2002) concluded that SOC stratification ratios >2 would be an indication that soil quality might be improving. In our experiment, NT showed the highest stratification ratio in all the experimental sites. The greatest stratification ratios were measured at SV, with values ≥2 for all the tillage treatments (Table 2 ). In contrast, at Peñaflor (PN-CC and PN-CF), we observed the smallest ratios, with values <2 for all the tillage treatments. At AG, the CT treatment showed a SOC stratification ratio <2, whereas NT showed a ratio >5 (Table 2). Greater SOC stratification ratios imply better soil conditions for crop growth due to the positive effects of SOM on soil surface processes such as erosion control, water infiltration, and nutrient conservation (Franzluebbers, 2002).


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  		Table 2. Soil organic C (SOC) stratification ratio (0–5 cm/30–40 cm) at Agramunt (AG), Selvanera (SV), and Peñaflor in a continuous barley cropping system (PN-CC) and in a barley–fallow rotation (PN-CF) for different tillage treatments (CT, conventional tillage; ST, subsoil tillage; RT, reduced tillage; NT, no-till).

 
When the whole soil profile (0–40 cm) was considered, at AG and PN-CF similar SOC contents were measured among tillage treatments (Table 3 ). At PN-CC, a significantly greater SOC content was measured under NT than under CT and RT throughout the whole soil profile (Table 3). On the contrary, the SOC value at SV was significantly greater under the tilled treatments (CT, RT, and ST) than under NT (Table 3). Therefore, at sites where the CT treatment consisted of moldboard plowing (AG and PN), similar or greater SOC content in the whole soil profile was measured in NT than CT. At SV, however, where CT consisted of subsoil plowing (without soil profile inversion), the SOC content was significantly higher in CT than NT. This fact would indicate that intensive tillage with moldboard plowing induces a greater disturbance than subsoil tillage, leading to greater SOM decomposition. Moldboard plowing compared with subsoil tillage caused a deeper distribution of SOM along the soil profile, greater soil microclimate condition modification (e.g., soil temperature, aeration, and water content), and aggregate breakage, releasing aggregate-protected SOM and leaving it susceptible to microbial attack (Paustian et al., 1997; Peterson et al., 1998).


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  		Table 3. Cumulative soil organic C (SOC) content at Agramunt (AG), Selvanera (SV), and Peñaflor in a continuous barley cropping system (PN-CC) and in a barley–fallow rotation (PN-CF) under different tillage treatments (CT, conventional tillage; ST, subsoil tillage; RT, reduced tillage; NT, no-till).

 
Since no differences in crop biomass existed among tillage treatments, differences in SOC were only the result of the effect of tillage on SOC decomposition. For the SV and AG sites, Cantero-Martínez et al. (2007) compiled crop biomass values since the beginning of the experiments. They observed similar averages among tillage treatments, with values ranging from 9034 to 10,681 kg ha–1 at AG and from 19,568 to 22,657 kg ha–1 at SV.

In our study, the intensification of the cropping systems did not significantly increase SOC content (Table 3). Moret et al. (2007), in the same experimental plots and during three cropping seasons (2000–2002), measured only <10% more aboveground crop biomass in the continuous cropping system than in the barley–fallow rotation. Therefore, low biomass production among cropping systems led to similar SOC contents

Soil Organic Matter Fractions
The SOM fractions (POM-C and Min-C) were determined only at SV, AG, and PN-CC. Following the same trend observed for total SOC concentration, the greatest POM-C was measured under NT at the soil surface (0–5 cm; Table 4 ). At this depth, POM-C ranged from 0.8 (in CT at PN-CC) to 6.4 Mg C ha–1 (in NT at AG; Table 4). These findings are in agreement with other studies measuring greater POM-C under NT than under CT at the soil surface (Wander et al., 1998; Hussain et al., 1999; Bayer et al., 2006; Sainju et al., 2006). Below the 10-cm depth in general, however, significantly greater POM-C was observed under CT (Table 4). Mrabet et al. (2001), in semiarid Morocco, measured slightly greater POM-C under CT than under NT at the 7- to 20-cm soil depth.


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  		Table 4. Distribution of particulate organic matter C content in the plow layer (0–40-cm depth) at Agramunt (AG), Selvanera (SV), and Peñaflor in a continuous barley cropping system (PN-CC) as affected by tillage (CT, conventional tillage; ST, subsoil tillage; RT, reduced tillage; NT, no-till).

 
The POM fraction has been defined as a labile SOM pool mainly consisting of plant residues partially decomposed and not associated with soil minerals (Cambardella and Elliot, 1992; Six et al., 2002). In our study, as we suggested for total SOC, the lack of soil disturbance under NT produced an accumulation of POM at the surface soil. When intensive tillage was applied (e.g., CT), however, two effects could have taken place: first, a redistribution of POM along the soil profile, which explains the increase in POM under CT compared with NT, and second, a faster mineralization of POM in the topsoil due to better soil microclimatic conditions for microbial activity.

Throughout the whole soil profile (0–40-cm depth), similar POM-C was measured among tillage treatments at all the three sites (Table 4). At SV, the greatest POM-C was measured under the CT treatment and the lowest under NT. At AG and PN-CC, however, where the CT treatment consisted of moldboard plowing, the opposite trend was observed. Therefore, as suggested above, at the sites where CT consisted in moldboard plowing, soil profile inversion accelerated POM decomposition. At the SV site, however, the pass of a subsoiler as CT implied lower tillage disturbance compared with moldboard plowing and also lower bulk density at soil depth compared with NT. We hypothesized that this fact resulted in better conditions for root development compared with NT, leading to greater root biomass in deep soil layers and thus greater POM-C accumulation in CT than NT at the SV site.

Regarding the Min-C content, this C fraction was significantly greater under NT than under the other tillage treatments at the soil surface (0–5-cm depth; Table 5 ). Mineral-associated C resulted from the decomposition of POM and its subsequent protection by silt and clay particles (Denef et al., 2004). Beare et al. (1994) found greater Min-C in soil aggregates of NT than CT in the surface soil (0–5 cm). They concluded that, besides POM, other soil C fractions were lost under CT compared with NT. Also, Cambardella and Elliot (1992) found greater Min-C under NT than a bare fallow treatment tilled with moldboard plowing from 0- to 20-cm depth. Therefore, in our experiment, soil surface (0–5-cm) NT compared with CT not only sequestering SOC as POM-C but also as Min-C. Due to the more humified and recalcitrant nature of the Min-C fraction, greater SOC accumulation as Min-C implies the stabilization of SOC in the long term in NT compared with CT.


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  		Table 5. Distribution of mineral-associated C content in the plow layer (0–40-cm depth) at Agramunt (AG), Selvanera (SV), and Peñaflor in a continuous barley cropping system (PN-CC) as affected by tillage (CT, conventional tillage; ST, subsoil tillage; RT, reduced tillage; NT, no-till).

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The NT system increased the SOC content only at the soil surface (0–10-cm depth) due to the accumulation of crop residues. When deeper soil layers were considered, however, the amount of SOC accumulated was greater under CT than under NT due to the placement of crop residues all along the soil profile. When the whole soil profile (0–40-cm depth) was considered, similar or slightly greater SOC content was measured under NT than under CT with the exception of the SV site, where CT consisted of subsoil tillage instead of moldboard plowing. Therefore, deep vertical subsoiling accumulated greater SOC in the whole soil profile compared with NT.

The POM pool, formed mainly by crop residues under different decomposition stages, increased on the soil surface under NT due to the accumulation of crop residues. At the same time, on the soil surface, the Min-C fraction that formed from the decomposition of the POM-C was also greater under NT than CT.

In semiarid agroecosystems of the Ebro River valley, enhancing soil organic C contents is a key factor to improve soil quality and productivity. The adoption of conservation tillage, especially NT, has a potential effect to sequester SOC in the dryland soils of this Mediterranean region. Nevertheless, after >15 yr of tillage testing, this beneficial effect of NT on SOC sequestration has been observed only in the first 10 cm of soil.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication May 4, 2007.


   REFERENCES
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 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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