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Published online 3 May 2006
Published in Agron J 98:637-643 (2006)
DOI: 10.2134/agronj2005.0254
© 2006 American Society of Agronomy
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Production Papers

Crop and Soil Response to Wheel-Track Compaction of a Claypan Soil

Daniel W. Sweeneya,*, M. B. Kirkhamb and J. B. Sissonc

a Kansas State Univ., Southeast Agric. Res. Center, P.O. Box 316, Parsons, KS 67357
b Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506
c Real Life Enterprises, 9643 W. Compton Ct., Star, ID 83669

* Corresponding author (dsweeney{at}oznet.ksu.edu)

Received for publication September 2, 2005.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Annual row crop production on the naturally occurring claypan soils of the eastern Great Plains may require field operations during somewhat wet conditions and this potentially results in soil compaction by the commonly-used, heavy-weight tractors and equipment. The objectives of this experiment were (i) to determine if compaction reduced yield and growth of soybean [Glycine max (L.) Merr.] and grain sorghum [Sorghum bicolor (L.) Moench] grown on a claypan soil (fine, mixed, thermic Mollic Albaqualf) and (ii) to determine the effect of wheel tracks on selected soil properties and whether chisel plow tillage could reduce wheel-track compaction. Compaction treatments were (i) ALL—all of the plot compacted, (ii) WT—wheel-track compaction, (iii) WTC—wheel-track compaction followed by a chisel tillage operation, and (iv) NO—no intentional compaction. In general, it took until the third year of annually repeated compaction in the ALL treatment to reduce crop growth and yields compared with the NO compaction treatment. Even though nearly half of the area was compacted each year in the WT treatment, few measured crop parameters decreased. In wheel tracks, soil penetrometer resistance and bulk density increased and air permeability decreased compared with out of tracks. However, chisel tillage appeared to eliminate the compaction by reducing penetration resistance and bulk density and increasing air permeability to values similar to out of tracks. Thus, compaction of claypan soils may not often be a problem for producers in this area, especially if occasional chisel tillage is included to remove possible compacted zones.

Abbreviations: ALL, all of plot compacted • LAI, leaf area index • NO, no intentional compaction • WT, wheel-track compaction • WTC, wheel-track compaction followed by chisel plowing


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
DURING recent decades as machinery size has increased to allow farming of larger areas with fewer passes, the weight of tractors and other machinery has also increased. The use of heavy-weight machinery potentially may contribute to compaction of soils and may affect crop growth and production. In the northern Corn Belt, Voorhees et al. (1978) found that wheel traffic increased soil bulk density and penetrometer resistance. High soil strength and bulk density can confine crop root growth (Laboski et al., 1998), alter root distribution (Kaspar et al., 1991), and result in a shallower root system (Oussible et al., 1992). When soil compaction suppresses total root length, shoot growth may also be reduced (Montagu et al., 2001). On clay soils, a single compaction by heavy field traffic can reduce yields and N uptake of crops for several years (Alakukku and Elonen, 1995). Soil compaction can decrease nodulation of soybean plants and subsequent yield and protein content of seed (Katoch et al., 1983). In contrast, compaction treatments applied to some soils may not affect soil bulk density (Lal, 1999) or average corn and soybean yields (Bicki and Siemens, 1991). Effects of soil compaction are complex and can be affected by load and tire inflation pressure (Abu-Hamdeh et al., 2000), weather events (Unger and Kaspar, 1994), crop root reinforcement of soil (Ess et al., 1998), and periodic chiseling and controlled traffic (Ishaq et al., 2001).

Crop production in much of the eastern Great Plains is greatly influenced by poor rainfall distribution and less-favorable soil types. Two of the major summer annual row crops in this area are soybean and grain sorghum. Unfortunately, cultural practices for these crops occasionally require field operations during somewhat wet conditions. Johnson et al. (1990) found that climate influenced the response of soybean and that compaction decreased yield during a wet year in Minnesota. Many of the soils of the eastern Great Plains are claypan soils consisting of approximately 30 cm of silt loam overlying a high clay content subsoil that often exceeds 1 m. Low crop root numbers have been recorded below 30 cm (Grecu et al., 1988) and, thus, the claypan's contribution to annual crops may be small. As a result, mechanical operations that affect the top 30 cm of these claypan soils may significantly affect soil properties, plant growth, and crop production. Producers continue to be concerned that compaction by some of the heavy-weight farm equipment may reduce yields of crops grown on the claypan soils of the area. However, data are limited regarding compaction of claypan soils and its effect on annual row crops. Thus the objectives of this experiment were (i) to determine the effect of compaction of a claypan soil on soybean and grain sorghum growth and yield and (ii) to determine how compaction tracks affect selected characteristics of a claypan soil and whether chisel tillage can reduce compaction effects.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The experiment was conducted from 1987 through 1989 on the same site on the Columbus field of the Southeast Agricultural Research Center of Kansas State University. The topsoil was a Parsons silt loam (fine, mixed, thermic Mollic Albaqualf) of approximately 30 cm overlying a "claypan" B horizon. The topsoil has an available water-holding capacity of approximately 5 cm, and the subsoil (>1 m) has a low percolation rate of <0.15 cm h–1 (Fleming et al., 1985). Selected background soil chemical analyses in the 0- to 15-cm depth were 6.7 pH (1:1 soil/water), 12 mg kg–1 P (Bray-1), 55 mg kg–1 K (1 M NH4C2H3O2 extract), and 17 g kg–1 soil organic matter analyzed by North Central Region recommended procedures (Dahnke, 1980).

The experimental design was a randomized complete block with a split-plot arrangement of treatments in three replications. In each replication, whole plots were compaction treatments and subplots were two soybean varieties and one grain sorghum hybrid. The compaction treatments were: (i) ALL, all of the plot compacted, (ii) WT, wheel-track compaction, (iii) WTC, wheel-trackcompaction followed by chisel plowing, and (iv) NO, no intentional compaction. Except when compacting soil in the WT, WTC, and ALL treatments, light-weight tractors of <1.5 Mg axle–1 were used for field operations. For compaction treatments, a four-wheel drive tractor was used that weighed approximately 4.5 Mg axle–1 with tire size of 18.4 x 38 inflated to 105 kPa. In the ALL treatment, continuous, side-by-side, double passes were made with the four-wheel drive tractor perpendicular to the subsequent planting direction until the entire surface of the plot was compacted. In the WT and WTC treatments, side-by-side, double passes were made with the same tractor perpendicular to the subsequent planting direction (Fig. 1 ). Thus, four sets of tracks, each approximately 1-m wide, were made across each WT and WTC plot compacting approximately 44% of the area. The areas were marked and compaction each year was applied to the same tracks. Soil compaction in the WT, WTC, and ALL treatments occurred on 20 Mar. 1987, 4 Apr. 1988, and 3 Apr. 1989 when gravimetric soil moisture levels each year were about 0.22 kg kg–1 in the 0- to 15-cm depth (about –0.1 MPa based on data by Grecu, 1988). In the WTC treatment, plots were tilled on 22 Apr. 1987, 22 Apr. 1988, and 21 Apr. 1989 to a depth of 15 to 20 cm in the same direction as subsequent planting using a chisel plow with spring-action, curved shanks, and straight spear chisel points. Subplots were: (i) Williams 82, a maturity group III soybean, (ii) Bay, a maturity group V soybean, and (iii) Pioneer 8585, a mid-maturity grain sorghum hybrid. Crops were planted on 16 June 1987, 23 June 1988, and 21 June 1989. Individual subplot size was 3 by 9.1 m.


Figure 1
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  		Fig. 1. Plot diagram of wheel-track treatments.

 
General tillage operations in the four compaction treatments before planting consisted of discing and field cultivation. Each year at planting, soybean varieties received 10 kg N ha–1, 17 kg P ha–1, and 32 kg K ha–1 and grain sorghum received 97 kg N ha–1, 14 kg P ha–1, and 26 kg K ha–1 applied 5 cm to the side of the row and 5 cm below the soil surface with the planter's dry fertilizer attachment. Soybean was planted at 350 000 seeds ha–1 and grain sorghum at 160 000 seeds ha–1 in 0.76-m rows. Weed control followed recommended herbicide rates.

The center two rows of each plot were harvested with a small-plot combine. Soybean yield was adjusted to 130 g kg–1 moisture content and grain sorghum yield was adjusted to 125 g kg–1 moisture content. Plant population for soybean was counted at R4 (full pod) and head count for grain sorghum was taken at the soft dough stage. Average seed weight was determined from duplicate random samples of 100 seed for soybean and 1000 seed for grain sorghum taken from the harvest and adjusted to the same moisture content as yield. Seeds per plant were calculated from yield, weight per seed, and population data from each plot. In 1987, aboveground parts of four whole plants were collected at random from each plot at the R4 growth stage for soybean and at boot stage for grain sorghum. In 1988 and 1989, six soybean plants were sampled. Leaf area index (LAI) was measured using a leaf area meter (Model LI-3000 with LI-3050A transport belt conveyor accessory, LI-COR, Lincoln, NE). After LAI measurements, saved leaves and stems from the sample were dried at 60°C, weighed for dry matter determination, and ground to pass a 1-mm screen. Plant samples were colorimetrically analyzed for N (Crooke and Simpson, 1971) after a H2SO4–H2O2 digestion (Linder and Harley, 1942). Nitrogen uptake was calculated as the product of dry matter production and N concentration. In the WT and WTC plots, measurements were collected from both in-track and out-of-track areas and weighted averages (percent of total area: 44% in-track, 56% out-of-track) for population, plant height, LAI, dry weight, N concentration, and N uptake were calculated.

To determine the effect of compaction that occurs in the tracks of heavy-weight equipment compared with out of the tracks, several soil parameters were measured in and out of the tracks in the WT and WTC treatments in the Williams 82 soybean subplots. Soil bulk density and air permeability were measured in all 3 yr and soil penetration resistance was measured in 1988. Bulk density was determined by the core method (Blake, 1965) from the 5 to 10 cm and 10- to 15-cm depths using a hammer-type sampler with removable cylinders (Art's Manufacturing & Supply, Inc., American Falls, ID). Bulk density samples were taken on 16 July 1987, 27 Sept. 1988, and 26 Sept. 1989. Air permeability was measured in situ in the 0- to 7.5-cm zone by the steady-state method of Grover (1955) on 16 July 1987, 10 Oct. 1988, and 26 Sept. 1989. Soil penetration resistance was measured with an electronically-controlled, portable, cone penetrometer (Christensen et al., 1998) on 27 Sept. 1988. Force measurements were made at 1-cm increments to 30-cm depth. Data from 3-, 9-, 15-, 21-, and 27-cm depths were selected for statistical analysis.

All data were analyzed using the PROC MIXED procedure of the Statistical Analysis System (Littell et al., 1996). Because of year-by-treatment interactions, measured plant parameters were reanalyzed within years. Because of crop differences, soybean data were analyzed separately from grain sorghum data. Soybean data were analyzed as a split-plot with compaction as whole plots and varieties as subplots, whereas, compaction was the only factor analyzed for data from the single hybrid grain sorghum. Grain sorghum data from 1987 were not used because of extensive bird damage. Soil data were analyzed as a split-plot with compaction treatments (WT and WTC) as whole plots and track (IN and OUT) as subplots. Air permeability data were transformed to log10 (x) values before analysis to improve normalilty of the distribution (Roseberg and McCoy, 1992) and to reduce heterogeneity of variances. Treatment effect means for measured parameters were compared using Fisher's LSD.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Climatic Conditions
Climatic conditions in the eastern Great Plains are often variable during the growing season of annual row crops. Climatic conditions during the growing seasons in 1987, 1988, and 1989 were also varied (Fig. 2 ). During late July and early August of 1987, maximum air temperatures were equal to or greater than 35°C nearly half of the time and rainfall was <5 cm for that period. In 1988, there were two periods of high temperatures, in late June and August. However, rainfall totaled 11 cm in late June, but was <5 cm for the month of August. Rainfall amount was more than 17 cm in late September of 1988, but likely was too late in the growing season to overcome effects of earlier high temperatures and low rainfall. In 1989, rainfall amounts were low in late June and early August, but the crops may not have been as stressed as in previous years because no days were recorded with maximum air temperatures equal to or greater than 35°C. Typical climatic conditions for each half-month period from June through September are that average rainfall ranges from about 4 to 6 cm and maximum air temperature ranges from 23 to 33°C (30-yr data not shown).


Figure 2
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  		Fig. 2. Rainfall (bars) and number of days that maximum air temperature was ≥35°C (squares) during the growing season from 1987 through 1989 at Columbus, KS.

 
Crop Response
An analysis of variance for the effects of compaction on soybean and grain sorghum yield, yield components, and other selected plant characteristics is shown in Table 1. The effect of compaction on any soybean parameter was not affected by an interaction with soybean variety. So, even though plant characteristics of the two varieties were often different, any effects on soybean by compaction treatments were similar for both.


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  		Table 1. Analysis-of-variance significance levels for the effect of compaction and variety for soybean and compaction for grain sorghum on yield and selected plant characteristics.

 
Compaction treatments did not affect any measured soybean plant characteristics in 1987 and few parameters in 1988 (Table 1). Soybean yield was unaffected by compaction during the first 2 yr of the study (Fig. 3 ). However, because yields were low in 1988 from low rainfall and high temperatures in August (Fig. 2), differences due to compaction treatments may have been difficult to discern. In 1989, the third year of imposing compaction treatments, the ALL compaction treatment yielded <80% of the yield with NO (Fig. 3). The WT compaction tended (P = 0.14) to decrease soybean yield by nearly 10% compared with NO compaction. On the claypan soil used in our study, compaction effects on soybean yield were not immediate and took 3 yr of annually repeated compaction to reduce yields. Lindemann et al. (1982) did not find significant soybean yield reductions in either year of the test from up to three passes with a tractor of near 2 Mg axle–1 weight. However, Gray and Pope (1986) found that soybean yield of one of two varieties was reduced in the first year and yield of both varieties was reduced in the second year by compaction done by a tractor of nearly 3 Mg axle–1.


Figure 3
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  		Fig. 3. Effect of compaction treatments [ALL = all area compacted; WT = wheel-track compaction; WTC = wheel-trackcompaction followed by chisel plowing to remove tracks; and NO = no intentional compaction] on average soybean yield during 3 yr. Within years, bars with the same letter are not statistically different at P < 0.05 according to the LSD test.

 
The decrease in soybean yield from ALL compaction in our study in 1989 (Fig. 3) was likely because of nearly 30% fewer plants per hectare (Table 2). Saini and Singh (1980) found reduced seedling emergence and emergence rate from soil compaction. If compaction can reduce emergence rate, then dry surface conditions from low rainfall in late June, 1989 in our study may have further reduced plant stand. The decrease in dry matter measured at the R4 growth stage from ALL compaction was greater than the decrease in population. This suggested that not only were there fewer plants with ALL compaction, but the soybean plants were smaller too. This was supported by similar reductions in LAI and, to a lesser degree, plant height. Tu and Buttery (1988) found an inverse relationship between soil compaction and plant biomass, total leaf area, and plant height for both soybean and white bean (Phaseolus vulgaris L.), even though the effect appeared more pronounced in white bean. Depression in N uptake mirrored dry matter production for soybean at the R4 growth stage (Table 2). Compaction has been shown to inhibit soybean nodulation (Voorhees et al., 1976) and nodulation efficiency (Tu and Buttery, 1988). In our study, ALL compaction did reduce N uptake compared with the other treatments. This may have been related to poor N2 fixation, but also may have been because of poor growth since the response to compaction was similar to dry matter depressions.


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  		Table 2. Effect of compaction treatments on soybean plant population, dry weight, N uptake, and leaf area index (LAI) taken at the R4 growth stage and on plant height taken at harvest in 1989.

 
For 1988 and 1989, grain sorghum yields were affected by compaction treatments at P < 0.10 (Table 1). In 1988, ALL compaction reduced grain sorghum yields by nearly 15% compared with other compaction treatments (Fig. 4 ). In 1989, the reduction exceeded 30% compared with the NO compaction treatment. Yield in the WT treatment appeared to be reduced by nearly 20% compared with yield from the WTC (P = 0.11) or the NO compaction treatment (P = 0.16). Corn grain yield depressions may be infrequent with axle loads of 7.5 Mg (Lal and Ahmadi, 2000), but the frequency may increase with heavier axle loads (Al-Adawi and Reeder, 1996). In our study, the axle loads used were more representative of typical four-wheel drive tractors and may not be as severe as axle loads from other heavy-weight equipment such as fertilizer or grain carts.


Figure 4
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  		Fig. 4. Effect of compaction treatments [ALL = all area compacted; WT = wheel-track compaction; WTC = wheel-trackcompaction followed by chisel plowing to remove tracks; and NO = no intentional compaction] on grain sorghum yield during 3 yr. Within years, bars with the same letter are not statistically different at P < 0.10 according to the LSD test.

 
Similar to soybean, most of the measured parameters for grain sorghum were not affected until 1989, the third year of our study (Table 1). The decrease in head count from ALL compaction was <20% compared with the other compaction treatments (Table 3). However, ALL compaction decreased dry weight production at the boot stage by nearly 40% compared with the NO compaction treatment. The ALL compaction also reduced LAI and plant height compared with the NO treatment. Similarly, grain sorghum N uptake was less in the ALL compaction treatment compared with NO, but N uptake by sorghum in the WT treatment was also less than in NO compaction. This suggests that, even though the sorghum plant can compensate somewhat in dry matter and leaf production for track compaction, N uptake mechanisms may be affected more.


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  		Table 3. Effect of compaction treatments on grain sorghum dry weight, N uptake, and leaf area index (LAI) taken at boot growth stage and on head count and plant height taken at harvest in 1989.

 
Soil Response
To understand the effect of wheel tracks, several selected soil properties were examined in and out of tracks in both the WT and WTC compaction treatments. Across years, bulk density was about 10% greater in both the 5- to 10-cm and 10- to 15-cm depths in the tracks of the WT treatment than out of the tracks (Fig. 5 ). Chisel operations to reduce soil compaction in the tracks of the WTC treatment effectively lowered bulk density to values similar to out of tracks in either the WT or WTC treatments. Abu-Hamdeh (2003) found that subsoiling a clay loam soil improved bulk density of compacted plots. However, on some soils, soil bulk density may not be as influenced by compaction or tillage as by other factors (Lal, 1999). Air permeability in the top 7.5 cm was significantly lower in the tracks of the WT treatment than out of the tracks or in or out of the tracks in the WTC treatment in which a chisel operation was included to remove track compaction (Fig. 6 ). Roseberg and McCoy (1992) also found that wheel traffic can decrease air permeability. This may be because of reduced total porosity with compaction (Al-Adawi and Reeder, 1996; Abu-Hamdeh et al., 2000). Chisel operations in the WTC treatment restored air permeability to values similar to out of tracks in either the WT or WTC treatments.


Figure 5
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  		Fig. 5. Average soil bulk density in and out of wheel tracks in no chisel and chisel plow treatments during 1987 through 1989. Treatments are: WT-IN (in wheel tracks—no chisel plow), WT-OUT (out of wheel tracks—no chisel plow), WTC-IN (in wheel tracks—chisel plowed), and WTC-OUT (out of wheel tracks—chisel plowed). Within depths, bars with the same letter are not statistically different at P < 0.10 according to the LSD test.

 

Figure 6
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  		Fig. 6. Average air permeability in and out of wheel tracks in no chisel and chisel plow treatments during 1987 through 1989. Treatments are: WT-IN (in wheel tracks—no chisel plow), WT-OUT (out of wheel tracks—no chisel plow), WTC-IN (in wheel tracks—chisel plowed), and WTC-OUT (out of wheel tracks—chisel plowed). Bars with the same letter are not statistically different at P < 0.10 according to the LSD test.

 
Soil penetration resistance was measured as cone index values in 1988 (Fig. 7 ). At shallow depths there were no differences in penetration resistance among the compaction treatments. However, deeper measurements showed a significant increase in soil penetration resistance in the WT-IN treatment that appeared to peak near the 10-cm depth at more than 2 MPa and this was about twice the values in the other treatments at that depth. In a review, Atwell (1993) reported that there was limited agreement between experiments but mechanical impedance >2 MPa likely limits root growth by at least 50%. Voorhees et al. (1978) found wheel traffic to increase penetrometer resistance by as much as 400%. However, the differences in penetrometer resistance between non-wheel-track and wheel-track areas may be influenced by the soil moisture content at the time of compaction (Reicosky et al., 1981). In our study, the soil depth zone from about 8 to 15 cm was where wheel-track compaction was the greatest (Fig. 7). In this zone, a chisel plow operation was able to mitigate the effect since the WTC-IN treatment was not significantly different than out of track measurements in the WT-OUT and WTC-OUT areas. Radcliffe et al. (1989) found that in-row chiseling disrupted a hardpan to a depth of 26 cm. In our study, below 15 cm, the effect of the wheel track diminished and at 20 cm there was no difference between WT-IN and WT-OUT and WTC-IN treatments. On a silty clay loam soil in Nebraska, Liebig et al. (1993) found that "the influence of tractor wheel traffic on soil properties was largely dissipated by the 15- to 30-cm depth." Additionally in our study, from 20 cm and extending to 30 cm, the WTC-OUT was significantly lower than WT-IN, WT-OUT, or WTC-IN (Fig. 7). This suggests that the chisel operation, which was done to a 15- to 20-cm depth, in the absence of wheel-track compaction may reduce soil strength deeper than the actual operation.


Figure 7
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  		Fig. 7. Cone penetration resistance in and out of tracks in chisel and chisel plow treatments. Treatments are: WT-IN (in wheel tracks—no chisel plow), WT-OUT (out of wheel tracks—no chisel plow), WTC-IN (in wheel tracks—chisel plowed), and WTC-OUT (out of wheel tracks—chisel plowed). At selected depths, bars with the same letter are not statistically different at P < 0.05 according to the LSD test.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Producers often perceive that compaction by some of the heavy-weight farm equipment may reduce yields of following row crops grown on naturally-occurring claypan soils. In general, it took until the third year of annually repeated compaction of the claypan soil with approximately 4.5 Mg axle–1 loads used in this study to reduce crop growth and yields. Both soybean and grain sorghum appear to be able to compensate somewhat for track compaction at these axle weights. Even though nearly half of the area was compacted each year in the WT treatment, few measured crop responses decreased. The ALL treatment usually reduced crop responses by the third year, but this treatment would not be expected to represent actual field conditions unless compaction continued indefinitely with no attempt to ameliorate the compacted zone. The four-wheel drive tractor used in this study to compact the soil should be typical of tractors used in the area, but may underestimate the effects from axle weights of large grain and fertilizer carts. Effects of wheel-track compaction can be long lasting (Sharratt et al., 1998) and more severe without deep tillage (Larney and Kladivko, 1989). However, a chisel tillage operation reduced penetration resistance and bulk denity, and increased air permeability in a compacted wheel-track zone in our study. Thus, compaction of claypan soils in the eastern Great Plains may not often be a problem for producers, especially if occasional chisel operations are done to eliminate compacted soil zones.


   ACKNOWLEDGMENTS
 
This research was funded in part by the Kansas Soybean Commission. The authors would like to acknowledge and thank Ted Wary, Robert Black, and David Kerley for their assistance with this project.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Kansas Agric. Exp. Stn. Contribution no. 06-72-J. Company names are included for the benefit of the reader and do not imply any endorsement or preferential treatment by Kansas State University.


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





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