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Journal of Environmental Quality 30:2195-2201 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Wetlands and Aquatic Processes

Methane Production and Oxidation in an Anoxic Rice Soil as Influenced by Inorganic Redox Species

S. Kumaraswamya,b, B. Ramakrishnan*,a and N. Sethunathana,c

a Division of Soil Science and Microbiology, Central Rice Research Institute, Cuttack 753 006, India
b Kerala Forest Research Institute, Peechi 680 653 Kerala, India
c CSIRO Div. of Soils and Cooperative Research Centre for Soil and Land Management, Private Main Bag No. 2, Glen Osmond, Adelaide SA 5064, Australia

* Corresponding author (crrictc{at}ori.nic.in)

Received for publication August 31, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
The effects of addition of inorganic redox substances (species of NO-3, Mn4+, Fe3+, and SO2-4) on methane production and oxidation in anoxic rice (Oryza sativa L.) soil samples were examined. Sulfate was the most inhibitory for methane production followed by Fe3+, NO-3, and Mn4+, in that order. Addition of rice straw at a rate of 1% (w/w) as a carbon source to increase the electron donor to the electron acceptor ratio did not completely alleviate the inhibitory effects of redox species on methane production. Interestingly, laboratory incubation studies showed that addition of MnO2 and K2SO4 enhanced aerobic methane oxidation in soil samples held at 60% water holding capacity. The suspensions of pretreated soil samples with different redox species, when tested for their ability to oxidize methane in soil solution equivalent medium supplemented with respective redox species under aerobic and anaerobic conditions showed differential effects of redox species. Nitrate and Fe3+ stimulated methane oxidation under anaerobic conditions and retarded it under aerobic conditions. Manganese(IV) ion retarded methane oxidation under anaerobic conditions, but enhanced it under aerobic conditions. However, SO2-4 stimulated methane oxidation in soil solution equivalent medium under both aerobic and anaerobic conditions.

Abbreviations: sMMO, soluble methane monooxygenase


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
RICE soils are often flooded as a management practice to enhance their agronomic productivity. In the reduced environments of flooded rice soils, anaerobic mineralization of organic matter occurs, resulting in the gaseous production of N2, N2O, H2, NH3, H2S, CH4, mercaptans, and dimethyl sulfide (Ponnamperuma, 1972). Because of the biogenic methane production, the predominantly anoxic flooded rice soils are considered to be one of the major anthropogenic sources of atmospheric methane (Minami and Neue, 1994). Flooding of a soil does not necessarily result in the development of a uniformly reduced profile. A thin, oxidized surface horizon overlying a deep, reduced horizon is formed due to the dissolved oxygen from the overlying floodwater diffusing across the surface water–soil interface and, in soils planted with rice, the rhizosphere is oxidized because of the delivery of O2 into roots. Thus, these flooded soils can also support the activities of methane-oxidizing bacteria in oxic zones (De Bont et al., 1978; Bosse and Frenzel, 1997; Bodelier and Frenzel, 1999).

Methanogenic bacteria are active only under anoxic, reduced soil conditions where there is a high input of labile organic material. Wang et al. (1993) reported that CH4 formation in flooded soils occurred when the redox potential (Eh) fell below -150 mV. Because most redox reactions are biologically mediated, a sequential ecological succession of microorganisms is expected. However, the different redox reactions in the anoxic environments are not mutually exclusive (Lovely, 1991) or cannot be explained only by the competition for common electron donors by those microorganisms that use the electron acceptor with the highest redox potential (Conrad et al., 1987). The oxidized inorganic species are reported to inhibit CH4 production (Bollag and Czlonkowski, 1973) and exhibit a toxic effect on CH4 formation (Roy and Conrad, 1999). Microbially mediated CH4 consumption in soils may also be regulated by the reduction characteristics. The methane-consuming activity of soil incubated under aerobic conditions decreased with low concentration of O2 (Schnell and King, 1995). Murase and Kimura (1994) demonstrated that CH4 was anaerobically oxidized at less reduced sites in the plow layer than at the sites where CH4 was produced. An association of methane oxidation with sulfate reduction under the anoxic conditions was suggested (Murase and Kimura, 1994). Anaerobic methane oxidation is a poorly understood process because the microorganisms capable of performing this process have not been isolated from flooded rice soils or marine sediments (Kumaraswamy et al., 2000; Valentine and Reeburgh, 2000). Depending on the availability of electron donors and acceptors, the different functional groups of microorganisms can be active during the sequential reduction processes in rice soils. In the present study, we investigated the effects of different inorganic redox species on methane production and oxidation in anoxic rice soil.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Soil Samples
Soil samples were collected from flooded fields (planted to rice) in the experimental farm of the Central Rice Research Institute, Cuttack, Orissa, India (20°N, 86°E; 23 m above mean sea level) during wet season, 1997. The soil is a deltaic alluvium (Typic Haplaquept), with a sandy, clay loam texture (25.9% clay, 21.6% silt, 52.5% sand), pH 6.2, maximum water holding capacity (WHC) 43.7%, cation exchange capacity (CEC) 18 cmolc kg-1, organic carbon 1.12%, total N 0.07%, free Fe2O3 3.96%, easily reducible MnO2 0.05%, and electrical conductivity 0.7 dS m-1. Larger dry lumps were broken manually by slight impounding before passing the samples through a sieve of 2-mm mesh size. Samples were stored in plastic containers.

Methane Production
For methane production studies, air-dried and sieved soil samples in 5-g portions were either unamended or amended with rice straw powder (aboveground parts of the rice plant after harvest, dried and pulverized to pass through a sieve of 0.2 mm, C to N ratio 64.03, at 1% level w/w), placed in presterilized 15-mL B–D vacutainer tubes (125 x 16 mm; Becton, Dickinson and Co., Rutherford, NJ), and flooded at a weight ratio of 1:1.25 in sterile distilled water. Aqueous solutions of different inorganic redox substances (KNO3, MnO2, Fe2O3, and K2SO4 as NO-3, Mn4+, Fe3+, and SO2-4, respectively at 2500 mg kg-1 soil) were added separately, with minimum change to the total moisture content. Soil samples, unamended or amended with rice straw, were supplemented with equal amounts of sterile water to serve as controls. After closing with butyl rubber septa, the headspace of the vacutainer tubes was flushed with high-purity argon (99.98%) to create anoxic conditions rapidly. These soil tubes were then incubated in the dark at 32 ± 2°C. At given time intervals, gas samples were withdrawn from the headspace after vigorously shaking the soil incubation tubes by hand to allow equilibration between the liquid and gas phases. The concentrations of CH4 in the headspace gas samples were measured in a Varian (Palo Alto, CA) 3600 gas chromatograph as previously described (Ramakrishnan et al., 1998). For pH and redox potential measurements, bulk soil samples (50 g in 150-mL screw-capped containers, at a 1:1.25 soil–water ratio) were tested. The pH was measured with an Elico (Hyderabad, India) pH meter and the redox potential with a Barnant-20 digital ORP meter (Barnant Company, Barrington, IL). To estimate carbon dioxide concentrations, 10 mL of the headspace gas was injected into a 0.01 M NaOH trap solution in air-tight serum bottles. The amount of carbon dioxide trapped by NaOH solution was determined by titration (Stotzky et al., 1965) using 0.01 M HCl with phenolphthalein as an indicator. A control was used to determine the atmospheric carbon dioxide concentration, which was subtracted from the samples. The experiment was carried out by preparing in parallel numerous incubation tubes and containers, and all measurements were made on five replicates, sacrificed at each time of sampling.

Methane Oxidation
The incubation vessel for methane oxidation experiments was a 120-mL serum bottle sealed with butyl rubber septum. Soil samples in 10-g portions, placed in presterilized serum bottles, were held at 60% moisture holding capacity and then allowed to equilibrate with ambient air for 24 h in a dark incubator at 30 ± 2°C. As described earlier, the aqueous solutions of different inorganic redox species (as KNO3, MnO2, Fe2O3, and K2SO4 at 2500 mg kg-1 soil) were added separately. Controls included the sealed, autoclaved (121°C for 2 h) soil samples and another with acetylene, an inhibitor of methane oxidation, added at a final headspace concentration of 1%. A time course study for methane consumption was initiated by sealing the serum bottles and injecting the headspace with 10 mL of methane (5%) in argon (approximately 2200 µmol CH4 L-1). The gases in the headspace were in the ratio of 73.04 (nitrogen):18.26 (oxygen):7.89 (argon):0.04 (methane). These incubation bottles were kept in the dark in an incubator at 30 ± 2°C, with intermittent shaking on a rotary shaker for a period of 8 h on each day. At 2-d intervals, the headspace gas (0.2 mL) of the serum bottles of all soil samples was analyzed in a Varian 3600 gas chromatograph. As there was no statistically significant difference in methane concentrations between autoclaved soil and acetylene-treated controls, the decrease in methane concentration in the headspace between two consecutive samplings under the CH4–amended atmosphere was used to estimate methane oxidation (Kumaraswamy et al., 1997). On each sampling day, all determinations were made in a minimum of five incubation vessels for each treatment and the mean values were presented.

Soil Solution Equivalent Medium Experiment
In a follow-up experiment, soil samples in 10-g portions, placed in presterilized serum bottles (120 mL), were held at 60% moisture holding capacity and then treated with the aqueous solutions of different inorganic redox species (as KNO3, MnO2, Fe2O3, and K2SO4 at 2500 mg kg-1 soil) separately. After closing with butyl rubber stoppers, a set of incubation vessels was flushed with nitrogen for 30 min and another set was left with ambient air to create anoxic and aerobic conditions in the headspace, respectively. The headspace of all the serum bottles was injected with 10 mL of methane (5%) in argon. Then, the incubation bottles were kept in the dark in an incubator, with intermittent shaking on a rotary shaker for a period of 8 h on each day, at 30 ± 2°C for 10 d. After 10 d of incubation, 1 g of soil slurry from the respective treatment was inoculated to a 10-mL portion of sterile soil solution equivalent medium (Angle et al., 1991), supplemented with KNO3, MnO2, Fe2O3, and K2SO4 separately to give final concentration of 5 mM in serum bottles. The transfers were made under aseptic conditions using a sterile syringe. The headspace of the culture medium containing incubation vessels was then replaced with a mixture of nitrogen and oxygen (80:20) and pure argon (99.98%) to create aerobic and anoxic conditions, respectively. Incubation bottles were injected with methane (5%) in argon to provide 2500 µmol CH4 L-1 and incubated in the dark with intermittent shaking on a rotary shaker for a period of 8 h on each incubation day. Methane concentration in the headspace of the serum bottles was analyzed on alternate days until 10 to 12 d, in a Varian 3600 gas chromatograph. This experiment was performed twice and the mean values from a minimum of six determinations for each treatment at every sampling period were presented.

Enumeration of Microbial Population
Total aerobic heterotrophic bacterial population of soil samples was estimated by dilution plate technique (Rand et al., 1975). Most probable number (MPN) estimate of total methane oxidizers (TMO) was determined according to Arif et al. (1996). Methane oxidizers with soluble methane monooxygenase (sMMO) activity were enumerated as described by Graham et al. (1992). Dilution plates were incubated under the atmosphere of methane (5%)–air mixture in vacuum desiccators for 30 d at 30 ± 2°C. The headspace was evacuated and replenished with methane–air mixture at 5-d intervals during the incubation period. The colonies that developed a colored complex with naphthalene and O-dianisidine (tetrazotized) were counted positive for methane oxidizers. Methanogens (H2 and CO2 utilizers) were counted by most probable number technique at 10-fold dilution using tubes prepared under N2 and pressurized with a mixture of H2 and CO2 (Kaspar and Tiedje, 1982). The MPN culture tubes, incubated for 30 d at 28 ± 2°C, were examined for the presence of methanogens by detection of methane in the headspace to make rapid observations on population densities.

Statistical Analysis
Data were analyzed by the standard statistical methods using IRRISTAT Version 3/93 (International Rice Research Institute, 1993). The significance of the differences between treatments was assessed by analysis of variance (ANOVA) and subsequently by Duncan's multiple range test (DMRT).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
In the experiment to examine the influence of different inorganic redox species (NO-3, Mn4+, Fe3+, and SO2-4) on methane production from flooded rice soil, different inorganic redox substances added at 2500 mg kg-1 soil distinctly suppressed the production of methane as compared with that in the untreated soil samples (Table 1). The inhibitory effects of all the tested inorganic redox species were more pronounced at 30 d of incubation and the effect of SO2-4 was higher compared with those of other redox species, even at 10 d of incubation. Thus, the net methane production in soil samples under different treatments during the experimental period showed that SO2-4 was the most inhibitory followed by Fe3+, NO2-3, and Mn4+, in that order. Generally, methane is produced in soils following submergence after reduction of NO-3, Mn4+, Fe3+, and SO2-4, as predicted by the thermodynamic theory (i.e., electron acceptors with a higher redox potential will be reduced first) (Ponnamperuma, 1972). Methanogens are strictly anaerobic microorganisms and are inhibited by high redox. Macgregor and Keeney (1973) reported that addition of NO-3 and SO2-4 to a flooded soil maintained the soil in a more oxidized state and thereby inhibited methane production. Methanogenesis was found to be inhibited by the addition of NO-3, Fe3+, and SO2-4 in anoxic paddy soil (Achtnich et al., 1995). The repression of CH4 production may be mediated by redox active compounds, such as Fe3+ to Fe2+ ratios, acting as a signal (Peters and Conrad, 1996). Alternatively, the competition for acetate and H2, the important methanogenic substrates, with nitrate-, iron-, and sulfate-reducing bacteria is one of the important factors for the inhibition of methanogenesis (Achtnich et al., 1995). Roy and Conrad (1999) showed that the inhibition of methanogenesis due to the addition of nitrate was caused by the toxic effects of the denitrification intermediates nitrite, nitric oxide (NO), and N2O, rather than by competition for acetate. Thus, the inhibitory effects of different inorganic redox species are probably due to high redox conditions, the toxic effects, and the competition among different microorganisms for the methanogenic substrates. There is also evidence that methanogenesis is restricted in flooded soils with significant numbers of electron acceptors (Neue et al., 1995).


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  		Table 1. Methane production (µmol CH4 produced kg-1 soil) in flooded rice soil samples, unamended or amended with rice straw (1% w/w), as influenced by different inorganic redox species (added at 2500 mg kg-1 soil) under laboratory incubation. In a column, means followed by the same letter are not significantly different by Duncan's multiple range test.

 
Addition of rice straw to soil samples at a rate of 1% w/w effected a several-fold increase in CH4 production at all treatments relative to that of the respective unamended soil samples (Table 1). Decomposition of rice straw in predominantly anaerobic flooded soil can lead to the accumulation of acetate as a major but transitory intermediate (Rao and Mikkelsen, 1977). Acetate is one of the important substrates for methanogens in flooded soils and this would explain the substantial accumulation of methane in rice straw–amended soil under flooded conditions. Abundant production of methane during anaerobic decomposition of rice straw in flooded soils was reported before (Acharya, 1935). Rice straw as a carbon source can increase the electron donor to electron acceptor ratio, thereby reducing the time period for the reduction of oxidants and intensity of the competition for the electron donors among the various groups of microorganisms. In the present study, the net CH4 production in rice straw–amended soil samples treated with different inorganic redox species, as in unamended soil, was significantly inhibited. Particularly interesting was the increased percentage of inhibition by addition of different inorganic redox species in soil samples amended with rice straw, relative to that of rice straw–amended soil samples without any addition of redox species at 10 d of incubation (Table 1). This strongly suggests that the intensity of competition for electron donors among these microorganisms was severe, despite the fact that addition of rice straw prevented the suppression of CH4 production to some extent, compared with the corresponding unamended soil samples. A similar observation was made before by Kluber and Conrad (1998) that addition of rice straw to reduce competition for electron donors decreased the inhibition period only and did not prevent the inhibition of methanogenesis after addition of nitrate.

In anaerobic mineralization, organic carbon is transformed into methane or carbon dioxide, depending on the availability of electron acceptors. The anaerobic respiration by the nitrate, ferric iron, and sulfate reducers can channel the flow of electrons toward CO2 production. In the present study, the rate of increase in CO2 evolution from the unamended soil sample, without any addition of redox species, was lesser during the incubation period. On the contrary, the evolution of CO2 from soil samples added with different redox species and rice straw–amended soil samples, with or without addition of different inorganic redox species, invariably increased in time (data not provided). Irrespective of the presence or absence of rice straw, soil samples amended with different redox species registered higher redox potentials (-81 to -172 mV) than soil samples not amended with inorganic redox species (-175/-115 and -260 mV in unamended and rice straw–amended samples, respectively; Table 2). According to Wang et al. (1993), CH4 formation in flooded soils can occur when the redox potential (Eh) falls below -150 mV. However, the production of CH4 in Methanosarcina barkeri starts as soon as the redox potential of the medium drops below +50 mV (Fetzer and Conrad, 1993). There are suppositions that only biogeochemical reactions whose oxidation potentials match the redox potential of a given environmental niche can operate. But, the environmental redox potential is only a symptom, as well as a measure of the dominant chemical oxidizing and reducing species present that can undergo the redox reactions (Ehrlich, 1993). Methanogenic population, enumerated by incubating the most probable number (MPN) culture tubes for a brief period of 30 d, was also suppressed by the inorganic redox species in rice straw–amended soil (Table 2). Evidently, under oxidized conditions in soil samples amended with the inorganic redox species, the toxic and/or the competitive effects were not congenial for the activities and proliferation of methanogens.


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  		Table 2. Changes on the redox potential (Eh) and methanogenic (H2 and CO2 utilizers) population in soil samples, unamended or amended with rice straw (1% w/w), as influenced by different inorganic redox species (added at 2500 mg kg-1 soil). Redox potential measurements and methanogenic population density enumeration were done with samples after 30 d of incubation. Most probable number (MPN) estimates of methanogens were obtained from MPN culture tubes after 30 d of incubation.

 
Methane Oxidation
In another experiment, the effect of different inorganic redox species on methane oxidation, measured as the decrease in the headspace concentration of methane in soil samples held at 60% water holding capacity, was examined under aerobic conditions. The headspace concentration of methane decreased rapidly both in the control and in soil samples treated with inorganic redox species (Fig. 1) . Addition of SO2-4 and Mn4+ enhanced the oxidation of methane while NO-3 was slightly inhibitory during the initial days. Jugsujinda et al. (1995) demonstrated that there was no possibility of NO-3–N as an oxygen source for CH4 oxidation, by using methyl fluoride, a CH4 oxidation inhibitor. We also monitored the effect of these redox substances on total aerobic heterotrophic bacteria, total methanotrophs, and methane oxidizing population with sMMO activity in soil samples. Total aerobic bacterial population (colony forming units [cfu] x 107 g-1 air-dried soil) decreased in soil samples treated with Fe2O3 and K2SO4 while it increased in soil samples with KNO3 and MnO2 (Table 3). These differences were probably due to the effects of different redox substances under the atmosphere of methane in the headspace. In another recent study, methane was found to be an electron donor for denitrification in oxygen-limited bioreactors, with concomitant increase in the population of denitrifying bacteria (Costa et al., 2000). Likewise, additions of MnO2 and Fe2O3 to soil samples under the atmosphere of methane resulted in increase of total methanotrophic- and sMMO-bearing methanotrophic populations, respectively (Table 3). The sulfate species had repressing effects on the population of methane oxidizers with sMMO activity as well as total methanotrophs.



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  		Fig. 1. Time course of methane oxidation (mmol of net CH4 oxidized kg-1 air-dried soil) in soil samples treated with different inorganic redox substances and incubated under aerobic conditions. The data are means of triplicate measurements. For clarity, error bars are not shown for the treatments, but typically were in the order of CV = <20%.

 

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  		Table 3. Population of total heterotrophic bacteria, total methanotrophs, and soluble methane monooxygenase (sMMO)–bearing methanotrophs in soil samples treated with different inorganic redox species (added at 2500 mg kg-1 soil). Data represent mean values of three replicate plates for each dilution tested or five most probable number (MPN) tubes for each dilution tested. Analyses were done using soil samples after 13 d of incubation.

 
In a follow-up experiment, soil samples were treated with different inorganic redox substances separately and provided with methane as substrate in the headspace for 10 d. Then, the aliquots of soil slurries from the control and those pretreated with different redox substances were tested for their ability to oxidize methane in a soil solution equivalent (culture) medium, in the presence of respective redox species under both aerobic (ambient) and anaerobic (argon atmosphere) conditions. As in the previous experiment, Mn4+ appeared to enhance methane oxidation in soil solution under aerobic conditions (Fig. 2a) . On the other hand, addition of NO-3 retarded the oxidation of methane in soil solution equivalent medium. Interestingly, methane oxidation proceeded rapidly also under anaerobic conditions (argon atmosphere). But, the effects of different redox species on methane oxidation under anaerobic conditions (Fig. 2b) distinctly differed from that of aerobic conditions (Fig. 2a). Nitrate stimulated methane oxidation under anaerobic conditions and retarded it under aerobic conditions. Manganese(IV) ion retarded methane oxidation under anaerobic conditions, but enhanced it under aerobic conditions. Iron(III) was stimulatory to methane oxidation under anaerobic, but not under aerobic conditions. Interestingly, SO2-4 effected very rapid oxidation of methane in soil solution equivalent medium under both aerobic and anaerobic conditions. Presently, the reasons for differential effects of different redox substances on methane oxidation under aerobic and anaerobic conditions are not clear.



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  		Fig. 2. Methane oxidation (mmol of net CH4 oxidized kg-1 air-dried soil) in a soil solution equivalent medium supplemented with different inorganic redox species separately, inoculated with samples pretreated with respective inorganic redox species and incubated under (a) aerobic and (b) anoxic conditions. The data are means of six replicate measurements. For clarity, error bars are not shown for the treatments, but typically were in the order of CV = <20%.

 
Rapid methane oxidation even under anaerobic conditions merits discussion. There is evidence, but not very conclusive, to suggest that anaerobic methane oxidation can be considerable in marine environments (Alperin and Reeburgh, 1985) and in rice soils, too (Murase and Kimura, 1994). We observed that methane oxidation in soil solution equivalent medium (with or without different redox species) was considerable even under anaerobic conditions (Fig. 2b). In addition, methane oxidation was found to be distinctly stimulated by SO2-4 and Fe3+ in anaerobically incubated soil solution medium. Pure cultures of sulfate reducers could also oxidize methane under anaerobic conditions, but only in the presence of SO2-4 as an additional electron acceptor (Panganiban et al., 1978). Likewise, addition of SO2-4 stimulated anaerobic methane oxidation in enrichment cultures (Iversen and Joergensen, 1985), as noticed in soil solution equivalent medium in the present study. Geochemical evidence suggests that anaerobic methane oxidation is associated with SO2-4, Fe3+, and NO-3 reduction (Miura et al., 1992; Murase and Kimura, 1994). Anaerobic methane oxidation may involve the back reaction of methane to CO2 and H2 in methanogens (Zehnder and Brock, 1979). Lipid biomarker and phylogenetic studies using marine sediments show the involvement of archaea and sulfate-reducing bacteria in anaerobic methane oxidation (Valentine and Reeburgh, 2000). Recently, Hinrichs et al. (1999) and Orphan et al. (2001) provided evidence from studies of anoxic marine sediments that methane is being consumed by archaea that are phylogenetically distinct from known methanogens. Thus, the recognition of methanotrophy also as a mode of growth can provide new perspectives on archaeal physiology and ecology. The results of the present study provide further evidence for the involvement of anaerobic methane oxidation, which can be considerable depending upon the dominant species of inorganic redox substances in flooded rice soils. In summary, we conclude that addition of different inorganic redox substances to soil samples suppressed methane production and had differential effects on aerobic and anaerobic methane oxidation.


   ACKNOWLEDGMENTS
 
We thank the director for permission to publish this work and the Department of Science and Technology, New Delhi for funding this work. We thank the Council of Scientific and Industrial Research, New Delhi for a research fellowship grant to S. Kumaraswamy.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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