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Journal of Environmental Quality 31:1071-1078 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Atmospheric Pollutants and Trace Gases

Nitrous Oxide Emissions from an Ultisol of the Humid Tropics under Maize–Groundnut Rotation

M.I. Khalil*,a, A.B. Rosenanib, O. Van Cleemputc, C.I. Fauziahb and J. Shamshuddinb

a Soil Science Division, Bangladesh Institute of Nuclear Agriculture, Mymensingh 2200, Bangladesh
b Dep. Land Management, Univ. Putra Malaysia, 43400 UPM Serdang, Selangor D.E., Malaysia
c Faculty of Agricultural and Applied Biological Sciences, Ghent Univ., Coupure Links 653, B-9000 Ghent, Belgium

* Corresponding author (khalilmi{at}bttb.net.bd)

Received for publication July 9, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nitrous oxide (N2O) contributes to global climate change and agricultural soils seem to be the major source. Lack of information led to this study on the influence of different amounts and sources of nitrogen on N2O emission from a maize (Zea mays L.)–groundnut (Arachis hypogae L.) crop rotation in an Ultisol of the humid tropics. The treatments were: inorganic N + crop residues (NC), inorganic N only (RN), and half of inorganic N + crop residues + chicken manure (NCM). The corresponding amount of N applied was 322, 180, and 400 kg ha-1 yr-1, respectively. The N2O emissions depended on the amounts and types of N. A maximum peak (9889 ± 2106 µg N2O-N m-2 d-1) was detected at 2 wk before maize sowing amended with chicken manure, showing a persistent influence on N transformations and N2O release. The mineral N from either applied source became low by 2 to 4 wk, coinciding with the small N2O fluxes or its consumption to a few isolated instances. The N2O flux significantly correlated with the mineral N and water-filled pore spaces. The direct annual N2O emission was 3.94 ± 0.23, 1.90 ± 0.08, and 1.41 ± 0.07 kg N2O-N ha-1 from the NCM, NC, and RN treatments, respectively. The corresponding N2O-N loss of the applied N plus N fixed by groundnut was 0.83, 0.49, and 0.59%. Overestimations of direct annual N2O emission using the Intergovernmental Panel on Climate Change (IPCC) methodology suggest a location-specific emission factor for variable N sources to be considered.

Abbreviations: IPCC, Intergovernmental Panel on Climate Change • NC, recommended inorganic N + crop residues treatment • NCM, a half-dose of recommended inorganic N + crop residues + chicken manure treatment • RN, recommended inorganic N only treatment • WFPS, water-filled pore space • WSOC, water-soluble organic carbon


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
NITROUS OXIDE (N2O) is formed in soils during nitrification and denitrification (Davidson, 1992). It contributes to global warming and stratospheric ozone depletion (Cicerone, 1987; Warneck, 1988). The N2O release depends on the indigenous soil characteristics (e.g., pH and organic matter content), addition of organic residues and fertilizers, land use patterns, and various environmental factors, namely moisture, temperature, and aeration (Eichner, 1990; Martín-Olmedo and Rees, 1999). In isolated instances, a small N2O emission or consumption has been observed at low moisture content (Duxbury and Mosier, 1993; Teira-Esmatges et al., 1998). Under upland conditions, the N2O production and diffusion out of the soil are considerably influenced by irrigation and/or rainfall events through changing soil physico–chemical conditions or by affecting the soil gas diffusivity and microbial activity and subsequent N gas production and efflux (Delgado and Mosier, 1996). In general, N2O emissions from agricultural land vary from 0.03 to 2.7% (Eichner, 1990) and a maximum emission of 5.8% from the applied N has also been reported (Dobbie et al., 1999).

In Asia, the use of synthetic N fertilizers to agricultural fields has been increased from 2.1 Tg N in 1961 to 40.2 in 1994 and the corresponding increase of N2O emission has been estimated to be from about 0.8 to about 2.1 Tg N2O-N (Mosier and Zhaoliang, 2000). In the humid tropics, the increasing use of mineral and organic fertilizers, the warm and humid climate, and frequent rainfall events may importantly influence the N2O emissions. However, there is only limited information with regard to the utilization of crop residues and organic manures and/or wastes bound N from upland cropping systems of the tropics. Therefore, a field experiment was carried out with a maize–groundnut crop rotation to study the influence of amounts and sources of nitrogen on the N2O emission and relevant soil properties, and to compare the observed and estimated (as per Intergovernmental Panel on Climate Change; Mosier et al., 1998) direct annual contribution of N2O from the cropping system to the atmosphere.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The study was conducted at the Universiti Putra Malaysia experimental farm (101°42' E, 3°02' N), which has a slope of 9°. The soil is well drained and belongs to the Bungor Series (loamy, kaolinitic, isohyperthermic family of Typic Paleudults, Ultisols). The pHH2O is low (5.3) and the organic C content and cation exchange capacity (CEC) are 1.25% and 6.86 cmolc kg-1, respectively. The daily pattern of rainfall and air temperature during the study period is shown in Fig. 1 . The air humidity during the study period ranged from 72 to 100% with an average of about 90%. Generally, the rainfall was evenly distributed throughout the year. Total rainfall during the investigation period (365 d, 1998–1999) was 2293 mm. The highest rainfall was recorded in March, July, September, and October (>200 mm) and the lowest in April (24 mm). More than 100 mm rainfall was recorded for the rest of the months. The minimum and maximum air temperature was 19.5 and 34.5°C with a yearly average of 20.7 and 31.9°C, respectively.



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  		Fig. 1. Daily minimum and maximum air temperature and rainfall during the groundnut–fallow–maize–fallow period (Day 1 corresponds to 19 November 1998).

 
The current study was carried out from November 1998 to November 1999 with a maize–groundnut crop rotation in order to cover a 1-yr period. A brief history (time table) of the experiment is presented in Table 1 . The treatments were as follows: NC, recommended inorganic N + crop residues; RN, recommended inorganic N only; and NCM, a half-dose of recommended inorganic N + crop residues + chicken manure. The experiment was conducted in a randomized complete block design with four replications for each treatment. The size of each plot was 20 x 8 m with a total area of 3000 m2. The chemical fertilizers were applied in furrows for both the crops. Rhizobium-inoculated groundnut seeds were sown in furrows after application of the fertilizers on 18 Nov. 1998 and harvested on 19 Feb. 1999. Maize seeds were sown on 29 May 1999 and harvested on 1 Sept. 1999.


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  		Table 1. A brief history of the experiment and the treatments that dealt with different management practices during a maize-groundnut crop rotation.

 
Gas samples were collected using a closed chamber (cross section = 184 cm2, height = 8 cm) that fitted with a vented perspex lid containing a rubber septum at the middle. The chambers were placed in between the plants. The number of chambers per plot and treatment were two and eight, respectively. During the fallow periods, the same number of chambers was reinstalled at the same locations. Gas samples were taken in the morning (0900–1100 hours) through the use of a double-sided needle in 10-mL vacutainers (Beckton Dickinson, Franklin Lakes, NJ) and each vacutainer was allowed to fill for 3 min. Gas samples were collected at 0, 15, and 30 min after closing the chambers. After sampling, the chambers were kept open until the next gas collection. Collection of gas samples started the day following fertilizer application–sowing of groundnut seeds. The samples were analyzed by gas chromatography (Model HP 6890; Hewlett Packard, Palo Alto, CA) using 63Ni electron capture detector. The N2O flux was calculated according to Flessa et al. (1998).

Composite soil samples were collected at each day of gas sampling to a depth of 15 cm to analyze for NH+4-N, NO-3-N, and NO-2-N from an extract at a soil to KCl (1 M) solution ratio of 1:2.5. Soil pH and water-soluble organic carbon (WSOC) (Nelson and Sommers, 1982) at a soil to water ratio of 1:2.5 was determined as well. Soil temperature to a depth of 10 cm was measured and the soil water content was measured gravimetrically at each gas sampling time. The corresponding water-filled pore spaces (WFPS) were calculated as follows: WFPS = (gravimetric water content x soil bulk density)/total soil porosity, where total soil porosity = (1 - soil bulk density)/2.65, with 2.65 being the assumed particle density of the soil. The bulk density was measured following a core-sampling method three times in a cropping period. Statistical analyses, namely analysis of variance (ANOVA), F test, and simple and multiple linear regression, were performed using the statistical package SAS (SAS Institute, 1989). A probability level of 5% was used to test the significance of treatment effects. The relationships between the dependent variable (daily N2O flux) and the independent variables (NH+4-N, NO-3-N, and NO-2-N, pH [H2O], WSOC, WFPS, and soil temperature) were developed. Total N2O flux was calculated by integrating (step-wise) over a specific time interval under the N2O flux curve. In addition, the theoretical direct N2O emission was calculated for each treatment using the IPCC methodology (Mosier et al., 1998) as follows:

where FSN (synthetic N added, kg ha-1 yr-1) = amount of N fertilizer x (1 - 0.1 [volatilization factor]); FAW (animal waste N used, kg ha-1 yr-1) = amount of N as chicken manure x (1 - 0.2 [volatilization factor]); FBN (N fixed, kg ha-1 yr-1) = 2 x seed yield of groundnut x 0.03; FCR (N added as crop residues, kg ha-1 yr-1) = amount of maize residue x 0.015 + amount of groundnut residue x 0.03; and EF1 (emission factor) = 0.0125. For FBN, the factor 2 is used to represent the total crop yield to achieve the amount of biological N fixation and for FCR, factor 2 is omitted to represent the added crop residues only.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Nitrous Oxide Fluxes
Groundnut Growth Period (Day 1 to 90)
The groundnut crop was harvested at Day 90 after sowing. The N and/or C sources added immediately before sowing influenced N2O fluxes considerably. The highest emissions were detected within 2 wk after N fertilizer application (Fig. 2A) . In the treatment where N fertilizer and crop residues were applied previously (NC), emissions were higher at Day 4 (983 ± 309 µg N2O-N m-2 d-1) than other treatments. It was closely followed by the treatment receiving half of the amount of recommended N fertilizer at this stage plus crop residues and chicken manure previously (NCM), depicting two large peaks. There was a clear indication of the influence of chicken manure applied during the previous cropping periods. Indeed, addition of organic N and extra carbon influenced the N transformations more than the inorganic N application only (Fig. 2B,C). The WFPS ranged between 50 and 60%, allowing both nitrification and denitrification to occur (Fig. 2D). A good correlation (adjusted R2 = 0.20, significant at the 0.001 probability level; n = 120) between N2O fluxes and NO-3–N coupled with NO-2–N and water-soluble organic carbon (WSOC) during groundnut growth period supports the above findings. This is in line with Abbasi and Adams (2000a)(b) and Wolf and Russow (2000). They mentioned that nitrification and denitrification might occur simultaneously to produce N2O. Two weeks after fertilization, the amount of mineral N decreased considerably, resulting in a smaller N2O emission. MacKenzie et al. (1997) also observed similar results from soybean [Glycine max (L.) Merr.] and alfalfa (Medicago sativa L.) plots.



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  		Fig. 2. Changes in the N2O flux, soil mineral N, and water-filled pore space (WFPS) with time during the groundnut–fallow–maize–fallow period as influenced by different amounts and sources of nitrogen. (A) N2O flux, (B) NH+4–N, (C) NO-3–N, and (D) WFPS (NC = recommended N + crop residue, RN = recommended N only and NCM = half of the recommended N + crop residue + chicken manure). Day 1 corresponds to 19 Nov. 1998. Arrows indicate the day of management practices as in Fig. 2A.

 
Fallow Period after Groundnut (Day 90 to 177)
The fallow period after groundnut was from Day 90 to 177 and groundnut residue was spread at Day 118. In comparison with the RN treatment, a significant N2O release (P <= 0.001) was observed upon the supply of groundnut residue in the treatment NCM (652 ± 15 µg N2O-N m-2 d-1) and NC (614 ± 75 µg N2O-N m-2 d-1) at Day 120 (Fig. 2A). Both the treatments receiving crop residue continued to emit N2O at higher rates up to 2 wk. The increased N2O flux applied with groundnut residue matched with the increase of mineral N under favorable moisture conditions (>60% WFPS), conducive to nitrification and marginal denitrification (Fig. 2B–D). The finding is in line with Flessa and Beese (1995). The amount of N applied as groundnut residue during the fallow period was higher than the mineral N applied at groundnut sowing. However, among the highest peaks, the N2O fluxes were smaller with former than with the latter. It was probably due to accumulation of acetate in the presence of nitrate pointed out to the occurrence of dissimilatory nitrate reduction to NH+4 and/or high denitrification in the presence of residues having low C to N ratio (Paul and Beauchamp, 1989a). Nevertheless, a multiregression study using the data of fallow period after groundnut indicated that the contents of mineral N, WSOC, and WFPS had a significant contribution to N2O emission (adjusted R2 = 0.40, significant at the 0.001 probability level; n = 72).

Maize Growth Period (Day 177 to 285)
At Day 177, chicken manure was applied on the NCM plots only before maize crop cultivation. Its application showed a significant (P <= 0.01) increase of N2O flux within 1 wk, ranging from 3820 ± 834 to 9889 ± 2106 µg N2O-N m-2 d-1 (Fig. 2A). At the same time, the RN treatment showed either the lowest (10 µg N2O-N m-2 d-1) emission or a small sink for N2O (47 µg N2O-N m-2 d-1). The maximum N2O flux in the plot amended with chicken manure coincided probably with the rapid decrease of NH+4–N and the resultant large nitrification (Fig. 2B,C), which is in agreement with Cates and Keeney (1987). The chicken manure, with a low C to N ratio, increases the respiratory activity and mineralizes quickly compared with other types of litter (Paul and Beauchamp, 1989a) and coupled nitrification–denitrification (Paul and Beauchamp, 1989b) with coinciding N2O emissions. An important NO-2 formation (data not shown) was observed, but its linkage to N2O emission could not been envisaged in the current study.

Maize seeds were sown and fertilized at Day 190. The N2O flux increased 2 wk after N fertilizer application (Fig. 2A), as in Skiba et al. (1996). Application of a high amount (100 kg N ha-1) of (NH4)2SO4 in the NC and RN treatments showed only a relatively modest increase in N2O flux. Until Day 201, nitrification was probably the dominant process for N2O emission due to the low WFPS value (approximately 50%). Afterward, a sharp release of N2O was detected due to the increased nitrate disappearance under favorable WFPS (>70%) conditions (Fig. 2B,D). Accordingly, the highest peak of N2O flux was observed at Day 205, mainly from the NCM treatment (4043 ± 1526 µg N2O-N m-2 d-1), though lower than as emitted immediately after application of chicken manure. This shows the contribution of the chicken manure applied 2 wk before maize sowing. Lessard et al. (1996) found a similar N2O flux after manure application. Arcara et al. (1999) observed a higher N2O emission from the combined application of slurry and urea under maize cover than from single application and found the greatest losses of N2O-N during the first month after fertilizer application. Two weeks after fertilization, the influence of fallow-amended crop residue to N2O emission indicated probable denitrification, which could have been favored by a higher WFPS (>60%), pH (approximately 6.0), and availability of WSOC (data not shown). As such the determining soil factors, influencing N2O release to a great extent during the maize growth period, were mineral N and WFPS (adjusted R2 = 0.56, significant at the 0.001 probability level; n = 180). The emissions from the different treatments are of the same order as those obtained by many other researchers (Flessa et al., 1998; Huang et al., 1995; McKenney et al., 1993; Mosier et al., 1996; Paul and Beauchamp, 1989b; Wagner-Riddle et al., 1997). At the later stages of the maize growth, the N2O fluxes did not vary significantly among the treatments. Application of the remaining one-third of the N fertilizer increased the N2O emission only to a lesser extent.

Fallow Periods after Maize (Day 285 to 365)
Addition of maize residue exhibited a considerable influence on the N2O flux with the highest peak from NC (1265 ± 202 µg N2O-N m-2 d-1) and NCM (1247 ± 113 µg N2O-N m-2 d-1) treatments at Day 328. The latter emitted the same amount of N2O at Day 321. At the end of the fallow period after maize, the N2O release was small. It was comparatively lower from the plots receiving N fertilizer only, which showed even a sink of N2O (58 µg N2O-N m-2 d-1). During that period, the WFPS was also low (<50%). During the 1-yr period, the small N2O emission or its consumption coincided with either low substrate availability or low moisture conditions (WFPS < 50%) to a few isolated instances. Teira-Esmatges et al. (1998) also found that the potential for gaseous N losses at WFPS < 40% was low. Hénault et al. (1998a)(b) also detected low negative fluxes, indicating that the soil was able to immobilize atmospheric N2O, but with a very weak efficiency as supported by Granli and Bøckman (1994). A multiple regression analysis depicted that WFPS coupled with soil temperature, NH+4–N, and soil pH (adjusted R2 = 0.27, significant at the 0.001 probability level; n = 108) influenced N2O fluxes during this period. The fallow period after maize showed higher N2O fluxes than the fallow period after groundnut. This may be attributed to the contribution of a comparatively higher amount of residual N remaining during the fallow period after maize (Mulvaney et al., 1997).

Annual Nitrous Oxide Emission and Total Nitrogen Loss
In Malaysia, temperature fluctuations are minimal throughout the year. Agricultural practices and moisture regime could be the major factors affecting the variations of N2O fluxes. Therefore, the total N2O emission was estimated over the total crop growth and fallow periods, with or without amendment of crop residues and/or chicken manure. Both the growing and fallow periods and the different treatments had a significant (P < 0.001) influence on the total N2O emission (Fig. 3) . It was significantly higher during the maize growth (108 d) period. The NCM treatment that received half of the recommended amount of N fertilizer along with chicken manure and crop residues emitted the highest total N2O (2.67 ± 0.24 kg N2O-N ha-1). The RN and NC treatments (0.75–0.77 kg N2O-N ha-1), which received larger amounts of N fertilizer during the maize growth period, followed it. However, those were insignificantly higher than NC and NCM treatments (0.58–0.62 kg N2O-N ha-1) calculated from the fallow period after maize. The RN treatment, in general, emitted lower N2O than other treatments, indicating the influence of a small amount of organic substrates applied before groundnut and maize cultivation.



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  		Fig. 3. Total N2O fluxes as influenced by the different amounts and sources of nitrogen during groundnut–fallow–maize–fallow periods (NC = recommended N + crop residue, RN = recommended N only and NCM = half of the recommended N + crop residue + chicken manure). The bar(s) having common letter(s) do not differ significantly at the 5% level of Duncan's Multiple Range Test.

 
The magnitude of N2O emissions during both crop growth and fallow periods was mostly influenced by the availability of mineral N under favorable WFPS. Similarly, Hénault et al. (1998a)(b) reported the same seasonal pattern of N2O emission, which appeared to be regulated by rainfall and fertilizer application, indicating the important regulating effects of WFPS and NO-3 concentrations. The trend of N2O emission under maize cover was similar to what has been reported elsewhere (Burton et al., 1997; Flessa et al., 1998; Teira-Esmatges et al., 1998). For temperate regions, seasonal changes in soil temperature exert a strong control over the N2O emission (Skiba et al., 1996) along with mineral N and moisture. However, this may not be true for the humid tropics with relatively stable air temperatures but with a varying intensity of rainfall. A multiple linear regression analysis showed that the N2O fluxes could be best described by mineral N content and WFPS of the soil (Lemke et al., 1998). The correlation was highly significant with a good predictive value of 50%, though the confidence level for WFPS was only 64% (Table 2) .


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  		Table 2. Empirical model obtained as a result of the multiple (step-wise) regression analysis for the groundnut-fallow-maized-fallow period.

 
The observed annual N2O fluxes varied significantly (P < 0.0001) with the highest emission of 3.94 ± 0.23 kg N2O-N ha-1 yr-1 from the NCM treatment (Table 3) . The NC (1.90 ± 0.08 kg N2O-N ha-1 yr-1) followed it and the lowest emission was from the RN treatment (1.41 ± 0.07 kg N2O-N ha-1 yr-1). Addition of N in the form of manure or crop residue had more effect on N2O emissions than fertilizer N application since the RN and NC received the same amount of inorganic N. These findings are in agreement with Chang et al. (1998) and Wagner-Riddle et al. (1997). The increase of total N2O emission from the NCM and NC treatments compared with the RN treatment was 180 and 35%, respectively. The relative N2O-N loss of the added N (including the estimated N fixed by groundnut), considering background emission negligible, varied significantly (P < 0.01). The NCM treatment receiving a half-dose of N fertilizer, chicken manure, and crop residue depicted the highest N2O-N loss, amounting to 0.83%. The relative loss was 0.59% from the RN treatment, followed by the NC treatment (0.49%). The corresponding N2O-N loss of the applied N only (without considering the amount of N fixed by groundnut) was 0.99, 0.78, and 0.59%, respectively. This also confirms the marked influence of chicken manure on N2O emission compared with the addition of N fertilizer alone or coupled with crop residue. MacKenzie et al. (1997) and Zeng et al. (1995) also reported a similar loss of N2O from different upland cropping systems. The yearly estimations of fertilizer-induced N2O emission were of the same order of the 1.25% N2O-N as supported by Bouwman (1996) with an uncertainty range of 0.25 to 2.25%. These results indicate that the addition of chicken manure or in combination with other N sources could be an important potential source of N2O.


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  		Table 3. Observed and estimated direct annual N2O emission using the Intergovernmental Panel on Climate Change (IPCC) methodology (Mosier et al., 1998) and apparent N2O-N loss from the added N sources.

 
A calculation for over- and underestimation was made by subtracting the estimated value from the observed one, and expressed in percent increase (Table 3). There were overestimations of the direct annual N2O emission from the maize–groundnut cropping system, calculated according to the IPCC methodology (Mosier et al., 1998). The plots supplied with a half-dose of inorganic N in combination with crop residues and chicken manure (NCM) showed a small overestimation (42%). It was larger from both RN (94%) and NC (153%) treatments. Van Moortel et al. (2000) have also reported an overestimation of the N2O emission from agriculture by 45% using IPCC methodology. To some extent, the overestimations may be attributed to the lower N2O production from the soil amended with ammonium sulfate than with urea (Khalil et al., 1999) and unaccounted for gaseous loss from the crop residues that kept outside the plots for a certain period before application to the field. However, results suggest a modification of IPCC methodology with respect to emission factor for variable N sources applied to crop fields in similar soils of humid tropics.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The N2O emissions from the maize–groundnut rotation mostly depended on the types and amounts of N fertilizers rather than on the crop and fallow periods, appearing proportional to the presence of mineral N in the soil and favorable moisture conditions for N transformations. Addition of the chicken manure to a crop field, in particular, could persistently influence N transformation processes and thereby larger N2O release in comparison with the applied crop residues and/or inorganic N fertilizer. However, mineral N applied or mineralized from organic sources could have been retained in this sloppy land up to 2 to 4 wk. High temperature and intensive rainfall could have been accelerated the decomposition of organic amendments and N transformations, causing runoff and leaching losses of the applied N. As such, the annual N2O emissions were small relative to N inputs and apparently 99% or more of the applied N remained in the crop–soil–water system, indicating the loss of N2O was insignificant from agronomic standpoint. The immediate large release of N2O due to chicken manure suggests modifying the existing time and method of application for better N utilization by crops. The overestimations using the IPCC methodology suggest a lower emission factor to be considered for estimation of direct N2O emission from similar upland cropping systems of humid tropics.


   ACKNOWLEDGMENTS
 
This research work was funded jointly by Ghent University, Belgium and Universiti Putra Malaysia under a UPM-Ghent University Ph.D. Twinning Programme. The authors wish to thank Professor G. Stoops and Dr. Pascal Boeckx, Ghent University for their help and suggestions, and the Belgian and Malaysian governments for financial assistance.


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





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