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Published online 2 February 2006
Published in Soil Sci Soc Am J 70:448-453 (2006)
DOI: 10.2136/sssaj2005.0096
© 2006 Soil Science Society of America
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Forest, Range & Wildland Soils

Wildfire-Produced Charcoal Directly Influences Nitrogen Cycling in Ponderosa Pine Forests

T. H. DeLucaa,*, M. D. MacKenziea, M. J. Gundalea and W. E. Holbenb

a Dep. of Ecosystem and Conservation Sciences, The Univ. of Montana, Missoula, MT 59812
b Division of Biological Sciences, The Univ. of Montana, Missoula, MT 59812

* Corresponding author (tom.deluca{at}cfc.umt.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Fire is the primary form of disturbance in temperate and boreal forest ecosystems. However, our knowledge of the biochemical mechanisms by which fire stimulates forest N cycling is incomplete. Charcoal is a major byproduct of forest fires and is ubiquitous in soils of most forest ecosystems, yet the biological function of charcoal in soils of forest ecosystems has been greatly overlooked. We conducted a suite of laboratory experiments on soils from ponderosa pine (Pinus ponderosa Laws) forests to determine the influence of charcoal on soil N dynamics and in particular, nitrification. The addition of NH4+ to forest soils had absolutely no effect on nitrification demonstrating that this process is not substrate limited. The amendment of these soils with NH4+ and field collected charcoal (1% w/w) significantly increased the nitrification potential, net nitrification, gross nitrification, and decreased the solution concentrations of plant secondary compounds (phenolics). Charcoal had no effect on nitrification in soils (from a grassland site) that had naturally high rates of nitrifier activity. The increase in gross nitrification in forest soils and lack of effect on grassland soils suggests that charcoal may alleviate factors that otherwise inhibit the activity of the nitrifying microbial community in forest soils. These results reveal the biological importance of charcoal and advance our mechanistic understanding of how fire drives nutrient cycling in temperate and boreal ecosystems.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
CHARCOAL is one of the most significant byproducts of forest fire and has the potential to influence physical, chemical, and biotic processes in soil (Tyron, 1948; Ishii and Kadoya, 1994; Zackrisson et al., 1996; Pietikäinen et al., 2000; Glaser et al., 2001); however, there is virtually no understanding of the effect of natural fire deposited charcoal on forest productivity or soil biotic processes. Recent studies allude to the potential importance of charcoal in soil ecosystems by demonstrating that soils amended with charcoal and manure centuries ago sustain some of the highest biodiversity and productivity of any soils within the Amazon basin (Glaser et al., 2001, 2002; Mann, 2002). The mechanism by which charcoal enhances this long-term soil productivity is not clear, but may be linked to the base saturation status (Bélanger et al., 2004), exchange potential (Glaser et al., 2002), or surface adsorption potential of organic compounds (Zackrisson et al., 1996). The lack of information regarding charcoal in soils is a noted oversight given the sheer volume of charcoal that accumulates in soils (Schmidt et al., 1999; Skjemstad et al., 2002) and the potential biological importance of charcoal (Zackrisson et al., 1996).

Charcoal is generated by the partial combustion of organic materials and is created during most natural fire events. The polycyclic, aromatic structure of charcoal or "black carbon (C)" makes it chemically and biologically stable allowing it to persist in the environment for centuries. Both activated C (industry produced charcoal) and charcoal C effectively function as adsorptive surfaces for organic compounds. This functional aspect of charcoal has resulted in its use in a myriad of applications from water purification (Ivancev-Tumbas et al., 1998) to odor reduction.

The majority of ecological literature available regarding charcoal has focused on its use as a paleoecological tool (Clark, 1990) or as a form of recalcitrant C that is sequestered in marine sediments (Massiello and Druffel, 1998; Dickens et al., 2004) and soil organic matter (Schmidt et al., 1999). There are only a limited number of studies that address the biological or biochemical importance of charcoal in ecosystem function. Charcoal may enhance seedling germination (Keeley et al., 1985; Keeley and Pizzorno, 1986), increase N uptake by certain plant species (Wardle et al., 1998), increase nutrient availability (Tyron 1948; Glaser et al., 2001), and enhance humus formation (Glaser et al., 2001). Charcoal or activated C has the capacity to deactivate phytotoxic compounds via adsorption to the char surface (Callaway and Aschehoug, 2000; Nilsson et al., 2000; Yang and Sheng, 2003). Although charcoal additions to soil do not appear to influence total microbial biomass or activity (Tyron, 1948; Wardle et al., 1998; Steiner et al., 2004), there is some indication that charcoal may alter the presence or activity of specific soil microorganisms (Wardle et al., 1998; Pietikäinen et al., 2000). We currently have only very limited understanding of the influence of charcoal on soil processes and specifically those involving N transformations.

Natural forest ecosystems are often considered to be N limited (Vitousek and Howarth, 1991) and respond rapidly to the addition of inorganic N (e.g., Tamm, 1991; Mandzak and Moore, 1994). Although little NO3– accumulates in natural forest ecosystems via the autotrophic process in which NH4+ is converted to NO3–, it is thought to be formed and rapidly immobilized by microorganisms (Stark and Hart, 1997), taken up by plants, or lost to leaching and denitrification (Neary et al., 1999). Fire results in an increase in net mineralization and nitrification in boreal and temperate forest ecosystems (Tamm, 1991; Stark and Hart, 1997; Kaye and Hart, 1998; Neary et al., 1999; DeLuca and Zouhar, 2000; Choromanska and DeLuca, 2001; DeLuca et al., 2002). Surprisingly, those factors that control nitrification and the activity of nitrifying organisms in forest ecosystems have remained largely enigmatic in spite of the known importance of nitrification in the N economy of forest soils. Charcoal, one of few legacies of fire found in soil, has the capacity to adsorb organic compounds (Zackrisson et al., 1996) that might otherwise induce net immobilization of NO3– (Fierer et al., 2001) or interfere with nitrification (Lodhi and Killingbeck, 1980; White, 1994). And although charcoal has great longevity in the environment (Seiler and Crutzen, 1980; Massiello and Druffel, 1998; Dickens et al., 2004), it may lose its sorptive capacity after only a century (Zackrisson et al., 1996). Recent studies in boreal ecosystems demonstrate that the addition of activated C to forest humus significantly increases net nitrification in the presence of added glycine (DeLuca et al., 2002; Berglund et al., 2004). It is not clear, however, how natural wildfire-produced charcoal might influence N transformations in mineral soils of temperate coniferous forest ecosystems.

The purpose of the work reported was to determine whether natural field collected charcoal has the capacity to enhance net nitrification, nitrification potential, and gross nitrification in mineral soils of ponderosa pine forest ecosystems of western Montana. Additionally, we examined whether sorption of phenolic compounds may be one possible mechanism by which charcoal alters the nitrifying environment in temperate forest ecosystems.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Soils used in these studies were collected from a number of locations in western Montana on National Forest lands that had been exposed to fire between 2 and 130 yr (Table 1) before collection (MacKenzie et al., 2004). Soils were collected from up to one to three different stands within each site, each of which represented a different time since last fire. Thus up to 14 soils may appear in any individual laboratory incubation. All soils were collected between the elevations of 1155 and 1700 m above sea level and all forest soils (pH 4.5-6) are classified as Typic Dystrocryepts. A grassland soil (pH 7.5), used for comparative analysis, is classified as a Typic Haplocryolls. The charcoal used in the studies described below was formed predominantly from ponderosa pine and Douglas-fir [Pseudotsuga menziesii (Mirbel) Franco var. glauca] wood and collected from a wildfire-exposed forest in Western Montana while avoiding collection from any area where fire retardant may have been applied. The charcoal was homogenized, ground coarsely, sieved to 2-mm sieve (creating a range of particle sizes), and analyzed for total C and N, pH, and extractable inorganic N and P. The grinding of charcoal clearly increases its total surface area compared with intact, large pieces of charred wood, however, charcoal particles found in the forest floor and mineral soil often exist in this size range (Zackrisson et al., 1996) and it is this condition that we attempt to reconstruct in these laboratory studies. The charcoal used in this study had a pH of 6.7, a total C content of 740 g kg–1, and contained 9.0 and 7.6 µg g–1 of 2 M KCl extractable NH4+ and NO3– respectively and 13.6 µg g–1 of 0.01 M CaCl2 soluble PO4–3.


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  		Table 1. Site and soil characteristics for soils collected from four forest and one grassland in western Montana.

 
Laboratory Incubations
A laboratory experiment was conducted to assess the direct influence of charcoal on net nitrification. Five replicate 300-g samples of charcoal free subsoil samples from a ponderosa pine forest not exposed to fire for 94 yr (Willow Creek, Table 1) were incubated with the addition of NH4+ and soluble P to enhance net nitrification potential with or without the addition of 1000 mg charcoal kg–1 soil. Soil samples were moistened to 80% water holding capacity (WHC) with a nutrient solution (0.75 mM (NH4)2SO4, 0.3 mM KH2PO4, and 0.7 mM K2HPO4) and allowed to stand in a constant temperature chamber at 25°C for 0, 1, 3, 7, 14, and 21 d. At each time intervals, 30-g subsamples were removed and extracted with 2 M KCl (2:1 extract/soil), and analyzed for NO3––N (Mulvaney, 1996) using an Auto Analyzer III (Bran Luebbe, Chicago, IL).

In a second laboratory incubation, nutrient solution and plant litter extracts were added to evaluate the influence of charcoal on nitrification in the presence of phenol rich plant litter. Arctostaphylos uva-ursi (L.) is a common understory shrub in western Montana known to be particularly rich in phenolic compounds (Almendros et al., 2000). Four replicate 300-g charcoal-free subsoil samples from a ponderosa pine forest site not exposed to fire for 94 yr (Willow Creek, Table 1) were treated with a factorial combination of A. uva-ursi leaf-litter extract (0 or 45 mg phenol kg–1soil) and charcoal (0 or 1000 mg charcoal kg–1soil). The samples were moistened to 60% WHC with nutrient solution (described above) and incubated at 25°C for 14 d, then extracted with 2 M KCl, and analyzed for NO3––N (Mulvaney, 1996) as described above. We used 60% WHC in this experiment to avoid the formation of anaerobic microsites given the addition of a C rich substrate (leaf extracts) to soils.

To evaluate the ability of the field collected charcoal to adsorb phenolic compounds produced by the common dwarf shrub A. uva-ursi, leaf extracts were analyzed for total phenols with or without the addition of charcoal. One hundred grams of fresh leaf litter was chopped to approximately 2 mm and extracted in 1000 mL of deionized water by shaking for 24 h and filtering through a 1-µm glass fiber filter. Five replicate samples of 50 mL of leaf extract were then amended with 0 or 0.5 g of field collected charcoal, shaken for 24 h and analyzed for total polyphenols using the Prussian blue method (Stern et al., 1996).

Slurry Assay for Nitrification Potential
We tested the effect of charcoal on the activity of nitrifying organisms in both forest and grassland soils (Table 1) by using aerated slurry assay (Hart et al., 1994) to eliminate microsites and provide a short-term evaluation of nitrification potential. We used soils collected from a fire chronosequence in Western Montana (MacKenzie et al., 2004) and soils collected from three intermountain grasslands in Western Montana (Table 1). Soil samples (30 g) were placed in an Erlenmeyer flask with 100 mL of nutrient solution (0.75 mM (NH4)2SO4, 0.3 mM KH2PO4, and 0.7 mM K2HPO4) with the pH adjusted to 7.2, and the suspensions shaken to maintain aeration. Four replicates of each soil were amended with field-collected charcoal (described above) at a rate of 0.75 g (2.5% w/w of soil mass added) and four replicates received no charcoal. Aliquots of 2 mL of soil suspension were removed using a wide mouth pipette at 0, 1, 2, 23, and 24 h, analyzed for NO3––N and the change in NO3––N content used to generate a line equation the slope of which is equal to nitrification potential in µg NO3––N g–1 h–1.

Gross Nitrification as Estimated by 15N Pool Dilution
We used the pool isotope dilution technique (Hart et al., 1994; Stark and Hart, 1997) to evaluate the influence of charcoal on the gross nitrification rates at one fire excluded site (89 yr since last fire). Briefly, three replicate samples of 100 g of fire excluded soil from a stand at Lost Horse that had not been exposed to fire for 89 yr (Table 1) was amended with 1000 mg charcoal kg–1 soil and then treated with 0.9 mg K15NO3. At time zero and time 24 h, 2 M KCl extracts of the soils were then analyzed for 14NO3– and 15NO3– following the use of micro-diffusion to isolate NO3– isotopes on acid treated filter paper disks on a mass spectrometer (Europa Integra Sercon Ltd., Cheshire, UK, University of California Davis Stable Isotopes Facility). Gross nitrification rates were then calculated using the assumptions of Kirkham and Bartholomew as described elsewhere (Hart et al., 1994).

Statistical Analysis
Data from the incubation experiments were analyzed with repeated measures ANOVA. Gross nitrification in forest soils with and without charcoal and the charcoal adsorption of phenols from leaf litter extracts were analyzed by using a Student t test. Comparisons of NO3––N concentration and nitrifier activity in forest and grassland soils were made with ANOVA. The mean separation of ANOVA results was determined using Tukey's Honest multiple range test. All data conformed to or were log transformed to conform to the assumptions of the general linear model and were analyzed with SYSTAT version 9.0 for windows (Wilkinson, 1999).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
The addition of field-collected charcoal to soil readily stimulated net nitrification in this static, aerobic 21-d incubation (Fig. 1 ). Nitrate concentrations were almost double in the presence of charcoal when compared with the untreated control. In this study, soils were amended with NH4+ to eliminate any possibility of substrate limitation of nitrification as previous studies had demonstrated that the addition of NH4+ alone did not significantly increase nitrification. It is possible that the charcoal addition results in the sorption of compounds that are otherwise inhibitory to nitrification (White, 1994). Alternatively, charcoal may have resulted in adsorption of N poor organic compounds (such as phenolic acids) resulting in reduced immobilization and increased net ammonification and nitrification. This concept is supported by previous studies demonstrating that phenol rich litter additions to soil result in rapid immobilization of inorganic N (e.g., Schimel et al., 1996; Castells et al., 2003).


Figure 1
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  		Fig. 1. Nitrate concentrations during a 21-d aerobic incubation of fire excluded soil with and without 1% charcoal (w/w). Soils were moistened with a nutrient solution containing (NH4)2SO4 and KH2PO4 and replicated samples removed at 0, 3, 7, 14, and 21 d and analyzed for extractable NO3––N. Error bars represent ± one standard error of the mean (n = 5).

 
There was no effect of charcoal on soil pH (data not shown), thus it is unlikely that a shift in soil base saturation status (Bélanger et al., 2004) is a mechanism for observed increases in nitrification. It is possible that the observation of increased base saturation in forest soils associated with higher charcoal concentrations (Bélanger et al., 2004) was a function of ash deposition with the charcoal. In our incubations we used charcoal with no addition of ash, and thus base status of the system was not altered. However, it should be noted that all incubations were conducted with the addition of a nutrient solution containing NaH2PO4 with the pH adjusted to 7.0 using 1.0 M NaOH. Although this addition may have increased nitrifier activity in the control compared with no nutrient solution, the charcoal addition greatly increased the nitrifier activity compared with the control.

Charcoal adsorption of organic compounds is one possible mechanism by which charcoal influences nitrification (DeLuca et al., 2002; Berglund et al., 2004). By adsorbing organic compounds charcoal may reduce the presence of factors inhibitory to nitrification (e.g., Lodhi and Killingbeck, 1980; White, 1994) or reduce the presence of C compounds that might otherwise stimulate immobilization (Fierer et al., 2001; Castells et al., 2003). Net nitrate accumulation after 14 d was significantly reduced in the Willow Creek forest soil following the addition of NH4+ and leaf extracts of A. uva-ursi (Fig. 2 ). Given that the leaves of A. uva-ursi have a C/N of 46 and a relatively high concentration of total phenols (Alemendros et al., 2000) it is possible that the extract amendments simply resulted in net immobilization of inorganic N and a reduction in net nitrification (e.g., Schimel et al., 1996). The addition of charcoal with the extracts resulted in a significant increase in nitrification compared with the extract addition alone and charcoal added alone resulted in an increase in NO3– accumulation significantly greater than the control with no extracts or charcoal added. These results demonstrate that charcoal has the potential to enhance nitrification in the presence of a substrate that would otherwise induce net immobilization of inorganic N. The addition of this same charcoal directly to A. uva-ursi leaf litter extracts resulted in reduction (P < 0.01) in total phenol concentrations from 450 to 25 mg L–1.


Figure 2
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  		Fig. 2. Nitrate concentration in subsurface soil (charcoal free Willow Creek soil, 89 yr since last fire) treated with 2 mL of a nutrient solution containing 1.5 mM (NH4)2SO4 and 1 mM KH2PO4 in factorial combination with charcoal (1% w/w) and phenol-rich A. uva-ursi litter extract (45 g phenolic compounds kg–1 soil) and incubated for 14 d. Error bars represent ± one standard error of the mean (n = 5). Bars with similar letters adjacent to them indicate a not significantly different at P < 0.05 as determined using a Tukey's multiple range test.

 
Nitrification potential as estimated using an aerated slurry assay (Hart et al., 1994) was performed on soils collected from sites of varied time since last fire (MacKenzie et al., 2004) allowing us to evaluate the immediate influence of charcoal on nitrification potential in fire excluded as well as fire exposed soils. Charcoal significantly increased nitrification potential in soils collected from across the chronosequence (Fig. 3 ) with the most dramatic effects being observed in early to mid-succession. There was, however, no effect of charcoal on the soil with the most recent charcoal accumulation, which may simply relate to the recent deposition of charcoal at this site. To determine how much NO3– might be contributed by the charcoal alone, we added charcoal to the nutrient solution (without soil) and found no difference between charcoal and no charcoal (data not shown) indicating that there is little or no contribution of NO3– or nitrifying organisms from the charcoal. Interestingly, we did observe a small increase in nitrification potential in a sterile soil amended with 1% sterile charcoal (significant at P < 0.10), but this increase was significantly less than in the non-sterile soil with non-sterile charcoal (data not shown). It is possible that charcoal releases a small quantity of NO3– during the 24-h incubation or that the charcoal stimulates some quantity of auto-oxidation. The addition of 0.75 g of charcoal to 30 g of soil (2.5% w/w) in a slurry assay would potentially result in the release of 0.0041 mM NO3––N (7.6 µg KCl extractable NO3––N/g charcoal) with little increase with time.


Figure 3
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  		Fig. 3. Nitrification potential in soils collected from 10 sites along a fire chronosequence in western Montana as influenced by the addition of charcoal (1% w/w). Error bars represent ± one standard error of the mean (n = 4). Bars with similar letters adjacent to them (for each pair of burned and unburned) are not significant different at P < 0.05 as determined using a Tukey's multiple range test.

 
To determine whether charcoal would increase nitrification potential in soils with naturally high rates of net nitrification, we repeated the aerated slurry assays on grassland soils (that exhibit net nitrification in laboratory incubations) with and without the addition of charcoal. In these soils, charcoal had no effect on the nitrification potential (Fig. 4 ). It is important to note that nitrification potentials in the unamended grassland soils was almost 10 times that of the unamended forest soils before charcoal additions indicating that grassland soils had an active nitrifying bacterial community that were not "released" by the addition of charcoal. This would suggest that the addition of charcoal alone to productive agricultural soil might have little or no effect on nitrification. There is generally little NO3– accumulation in native grassland soils in spite of high rates of gross nitrification as a result of rapid microbial and plant uptake of NO3– (DeLuca and Keeney, 1995). This is also thought to be true in forest ecosystems (Stark and Hart, 1997), but in the case of these fire excluded pine forests, nitrification potential is notably low, but are brought up to a level somewhat similar to the grassland soils by the addition of charcoal.


Figure 4
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  		Fig. 4. Nitrification potential in soils collected from three grassland sites in western Montana and three forest sites in western Montana as influenced by the addition of charcoal (1% w/w). Error bars represent ± one standard error of the mean (n = 3). Bars with similar letters adjacent to them are not significant different at P < 0.05 as determined using a Tukey's multiple range test.

 
Charcoal amendment of one forest soil not exposed to fire for 89 yr (Lost Horse Creek) resulted in a significant (P < 0.05, n = 5) increase in gross nitrification as measured using pool isotope dilution (Hart et al., 1994; Stark and Hart, 1997). Gross nitrification averaged 0.61 µg g–1 d–1 for charcoal amended soil compared with < 0.14 µg g–1 d–1 for the unamended soil. Gross nitrification soil provides an assessment of the rate of NO3– formation in soil without the confounding effect of NO3– losses to immobilization or denitrification. These results strongly suggest that the charcoal induced increase in net nitrification (Fig. 1) is not a factor of reduced microbial immobilization, but rather a result of greater activity of the nitrifying community.

These findings support laboratory and field studies where activated C added to boreal forest soils resulted in a significant increase in NO3– accumulation and a significant decline in soluble phenolic compounds (DeLuca et al., 2002; Berglund et al., 2004). These previous studies also indicated that ammonification, in fire excluded boreal forest soils, is substrate limited whereas nitrification is limited by rapid immobilization or some inhibitory factor. This is illustrated by the observed decrease in net nitrification with the addition of A. uva-ursi litter and its release in the presence of charcoal and litter extracts (Fig. 2). Although nitrification is not thought to be inhibited by tannins (Fierer et al., 2001), polyphenols may cause net immobilization (Schimel et al., 1996; Castells et al., 2003) or formation of polyphenol-N complexes (Northup et al., 1995; Hättenschwiler and Vitousek, 2000) and evidence does exist to suggest that secondary metabolites including some polyphenols (Lodhi and Killingbeck, 1980) and monoterpenes (White, 1994) may cause inhibition of nitrification. Our results indicate that charcoal readily stimulates nitrification, however, the mechanism for this stimulation is not clear. The increase in gross nitrification with charcoal addition would suggest that charcoal alters the activity of the nitrifying community rather than simply reducing the rate of immobilization. And although charcoal clearly has the capacity to adsorb secondary plant metabolites, herein we provide only indirect evidence that charcoal enhances nitrification via elimination of inhibitory compounds.

The dramatic effect of charcoal on nitrification and nitrifier activity has significant ecological implications. Historically, charcoal has been considered to be a biologically inert material in the soil environment and primarily influences physical processes such as increased absorption of radiant energy. Our work indicates that charcoal may remove an impediment or open a pathway to nitrification rather than simply reducing NO3– immobilization potential. Although plants are generally not dependent on NO3– as an N source, specific plants or communities may prefer NO3– over NH4+ or amino acid N (Persson et al., 2003) as a result of the great mobility of NO3– in the soil ecosystem compared with the latter forms of N. The ability of roots to access NO3– by mass flow rather than root interception or diffusion may be of particular importance in early secondary succession where root systems are more limited and mycorrhizal associations are still forming.

Fire is the dominant form of disturbance in boreal and western forest ecosystems, and charcoal is one of the few byproducts of this disturbance. Although the actual disturbance associated with fire is short lived, charcoal functions as a long-term driver of ecosystem processes and specifically N cycling. Active fire suppression policies and the exclusion of fire as a result of road building and timber harvest could potentially reduce charcoal deposition rates and thus alter nutrient cycling processes at the landscape scale. If charcoal does indeed lose its biotic activity after 100 yr (Zackrisson et al., 1996) in boreal forests and potentially less time in western forests (with a shorter fire return interval) then most forest ecosystems of the Inland Northwest are just now entering a period where the effect of fire exclusion on reducing charcoal deposition and altering nutrient cycles will be realized. Further studies, however, are still required to elucidate the specific mechanism by which charcoal stimulates nitrification and further clarify the role of charcoal in soils of fire adapted ecosystems.


   ACKNOWLEDGMENTS
 
The Authors thank Valerie Kurth and Tricia Burgoyne for their assistance in the field and laboratory and David Wardle and Olle Zackrisson for their helpful comments on a previous version of the manuscript. This work was supported by a grant from NSF-DEB-03171108.

Received for publication March 29, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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