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Soil Science Society of America Journal 65:126-132 (2001)
© 2001 Soil Science Society of America

DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Effects of Oxygen on Denitrification Inhibition, Repression, and Derepression in Soil Columns

D.J. McKenneya, C.F. Druryb and S.W. Wanga

a Dep. of Chemistry and Biochemistry, Univ. of Windsor, Windsor, ON, Canada N9B 3P4
b Greenhouse and Processing Crops Research Centre, Agriculture and Agri-Food Canada, Harrow, ON, Canada N0R 1G0

Corresponding author (druryc{at}em.agr.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Although it is known that O2 inhibits and represses denitrification, few studies have examined the effect of O2 on NO production. Our objectives were to measure O2 inhibition, repression, and derepression of NO and N2O production by denitrifying microorganisms in soil columns continuously purged by N2 or various constant (0.074–15%) O2–N2 mixtures. Net rates of NO and N2O production were measured under successive anaerobic, partially aerobic, and anaerobic conditions. Oxygen inhibition was rapid and reversible. Within 5 min after exposure to >5% O2, NO production was reduced to {approx}50 to 58% and N2O rates to {approx}29 to 32% of their maximum anaerobic rates. Maintaining O2 at >=5% in soil without added C or at >10% O2 in C-amended soil decreased (repressed) NO production rates by a factor of {approx}1.5 to 1.8 d-1. Rates of N2O repression remained constant at {approx}0.07 d-1 for all C and O2 treatments. Restoration of anoxic conditions following the aerobic phase reversed inhibition; within 5 min, NO production rates by the nonrepressed denitrifiers increased to 55 to 101% of their respective anaerobic rates and N2O production rates increased to 26 to 62%. The rates of NO and N2O production then increased more slowly (derepression) during this anaerobic period. This research supports previous observations for O2 effects on N2O production and apparently is the first systematic study of O2 inhibition, repression, and derepression of NO production.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
OXYGEN is usually considered to be the most critical proximal regulator of microbial denitrification (Hutchinson, 1995; Tiedje, 1988). The reductive enzymatic steps in the process are most active under anaerobic conditions, but many organisms are known to respire O2 and denitrify simultaneously (Ottow and Benckiser, 1994; Robertson and Kuenen, 1991). Similarly, many species from different genera can nitrify and denitrify simultaneously (Robertson and Kuenen, 1991). Competition for O2 limits aerobic processes in soil and depletion of O2 results in the formation of local anaerobic microsites (Parkin, 1987) that stimulate denitrifier production of gaseous NO, N2O, and N2.

Production and consumption of NO and N2O by different denitrifying bacterial species may exhibit different enzyme kinetic properties (Conrad, 1995). Although the regulation of enzyme synthesis depends mainly on O2 and N substrate concentrations, the patterns of regulation and sensitivity to O2 may differ (Conrad, 1996 and references therein). This may have a major bearing on the NO/N2O/N2 ratio produced with variation in O2 availability.

To obtain a complete description of the function and concentration distribution of O2 within the soil would require an accurate model of soil structure, porosity, O2 diffusion, and rates of major microbial processes. Several attempts to generate useable models of soil anaerobiosis in aggregated soil have been made (Sierra and Renault, 1996). The results emphasize the complexity of the problem and the need to obtain direct quantitative data regarding the effect of O2 on specific steps in processes such as denitrification in soil.

Adding to the complexity, O2 not only inhibits denitrifying enzyme activity, but also represses denitrifying enzyme synthesis (Smith and Tiedje, 1979; Knowles, 1981; Tiedje, 1988; McKenney et al., 1994). Few studies have examined the kinetics of these effects, particularly in relation to specific intermediate reactions in the process. Studies using pure cultures of nitrifying or denitrifying organisms (Kester et al., 1997 and references cited therein) have greatly increased our understanding of O2 effects at the cellular level. How inhibition, repression, and derepression are expressed in soils rather than in pure microbial cultures is of special importance, but it remains difficult to quantitatively relate field NO and N2O emissions to the chemostat investigations (Conrad, 1996).

Studies by Dendooven and Anderson (1994) and Smith and Tiedje (1979) provided considerable insight regarding derepression or de novo synthesis of denitrifying enzymes following O2 depletion in soil. Kramer and Conrad (1991) studied the overall influence of O2 on production and consumption of NO in an agricultural soil containing both nitrifying and denitrifying microbes and in a forest soil that contained only denitrifiers. However, quantitative evaluation of inhibition and repression of net NO and N2O production and derepression of net NO production by denitrification in soils apparently have not yet been examined. Such processes are undoubtedly related to the very large spatial and temporal variability observed in field-scale studies (Parkin, 1993; Groffman, 1991).

Many factors influence O2 availability to soil organisms, and the effect can be rapid and significant. For example, O2–consuming microbial respiration, which is strongly dependent on C availability, is a very important controller of denitrification in partially aerobic soil (Tiedje, 1988). The emissions of NO and N2O from nitrification and denitrification have been shown to respond rapidly to wetting dry soil, due at least partially to the resulting decrease in O2 availability (Kester et al., 1997; Davidson, 1992). De novo synthesis of denitrifying enzymes begins within a few hours after wetting (Rudaz et al., 1991; Dendooven and Anderson, 1994).

In this paper we present a laboratory study using a gas flow system to investigate how O2 effects NO and N2O production via denitrification in Brookston clay loam (fine-loamy, mixed, superactive, mesic Typic Argiaquoll) soil. The specific objectives were to quantitatively measure the rates of O2 inhibition of NO and N2O production and the rate of O2 repression and derepression of these processes. The experimental system allows measurement of the production rates of these gaseous intermediates without resorting to the use of acetylene inhibition and its associated possible complications, particularly in the presence of O2 (Bollmann and Conrad, 1997; McKenney et al., 1997). Also, the rapid sparging of NO and N2O from the soil columns minimizes production of N2 (McKenney et al., 1996). To overcome possible C limitation, some experiments were carried out using soil with added glucose.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The soil used was a field-moist Ap horizon (0–15 cm) Brookston clay loam. The Brookston soil is classified as an Orthic Humic Gleysol in the Canadian classification system, and it has 337 g kg-1 clay, 363 g kg-1 silt, and 300 g kg-1 sand. The soil has 21 g C kg-1 and a pH (1:1) of 6.1. After air drying to a gravimetric moisture content of {approx}100 g H2O kg-1 and sieving to <4 mm, the soil was stored at 4°C prior to the incubation study. Experiments were carried out using a modified version of the gas flow system described by McKenney et al. (1996). A constant flow (372 mL min-1) of humidified N2 (high purity grade [99.995%], Liquid Carbonic, Scarborough, ON) carrier gas containing only trace amounts (<100 µL L-1) of O2 sparged the gases evolved from the soil columns. Net NO and N2O production rates were measured at selected intervals. At 27 h, a similar constant flow of N2 containing various measured quantities (0.074, 0.5, 5, 10, and 15%) of O2 (Liquid Carbonic) was directed through three of the soil columns, while the pure N2 flow was continued through one column. Rates were monitored under these conditions for another 24 h, at which time anaerobic conditions were reestablished in all columns. Rate measurements were continued for a further 21 h, to a total run time of 72 h. Each O2 treatment was replicated three times. All columns were maintained at constant temperature of 20.0 ± 0.1°C.

Samples (100 g 105°C oven-dried basis) of the air-dried Brookston clay loam were amended with KNO3 (100 mg N kg-1) solution and distilled water to achieve a gravimetric moisture content of 200 g H2O kg-1. Duplicate samples containing glucose (500 mg C kg-1) but otherwise identical were also used for most of the O2 levels (0.5–15%).

Flow rates were controlled by adjusting needle valves (Nupro Fine metering, Nupro Co., Willoughby, OH) installed just upstream of the humidifier and soil columns. Flows were measured using calibrated mass flow controllers (Matheson, Whitby, ON), and calibrated Gilmont rotameters (Barnant Company, Barrington, IL). The net production rates of NO and N2O were calculated using the equations and , where [NO] and [N2O] are concentrations in the effluent gas and fc is the total flow rate of gas evolved from the soil column and m is the mass of dry soil.

The gas evolved from the soil was analyzed for NO based on the chemiluminescent reaction of NO with O3 using a NO/NO2/NOx analyzer (Model 42, Thermo Environmental Instruments, Franklin, MA) by flowing the gas stream through a zero gas (N2) constant pressure flask directly into the instrument. Gas samples (1 mL) were extracted by syringe from the effluent gas stream for N2O analysis using a gas chromatograph (Model 5880 A, Hewlett Packard, Avondale, PA) equipped with an electron capture detector and Porapak Q column. The NO and N2O calibrations were routinely made using standard gas mixtures (Liquid Carbonic). At selected time intervals (prior, during, and after the aerobic period) in three experimental runs, soil samples (10 g, fresh weight at 20% gravimetric water content) were taken from the columns, weighed into 300-mL Erlenmeyer flasks, shaken for 1 h with 100 mL of 2 M KCl solution, filtered through Whatman no. 40 filter paper, and stored at 4°C. The extracts were analyzed for NH+4, NO-3 and NO-2 using a TRAACS 800 autoanalyzer (Bran and Luebbe, Buffalo Grove, IL). Analysis for NH+4 was accomplished using the Berthelot reaction. A Cd reduction method was used for analysis of NO-3 plus NO-2 (Tel and Heseltine, 1990). The Cd column was removed for NO-2 analysis, and the NO-3 concentration was then obtained by difference.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
During the first few hours following initiation and regulation of gas flows, the net production rates of NO and N2O gradually increased to approximately steady-state conditions after {approx}20 h (Fig. 1) . At 27 h when a N2–O2 gas mixture replaced the pure N2 carrier gas through three columns, there was a very rapid decrease in net production rates of both NO and N2O (Fig. 1a). In both cases, the rates then slowly changed during the next 24 h. When the columns were returned to anoxic conditions at 51 h, the rates increased, first rapidly, then more slowly, to approximately the same production rates observed in the column maintained under anaerobic conditions over the entire 72 h (Fig. 1b). The changes in net rates of both NO and N2O depended on the amount of O2 in the carrier gas mixtures. As a typical example, Fig. 1 illustrates the pattern observed with addition of 10% O2 with added C. The rapid decrease and subsequent slower decrease during the aerobic period is discussed below as enzymatic inhibition and repression, and the increase in rates on return to anaerobic conditions is discussed as derepression. Nitrogen dioxide (NO2) was not detected in the effluent gas.



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  		Fig. 1. (a) Net rates of NO and N2O production under anaerobic (0.5–27 h), partially aerobic (27–51 h), and restored anaerobic (51–72 h) conditions. Error bars indicate standard errors , and those not shown were smaller than the symbols. (b) Net rates of NO and N2O production in the anaerobic reference column that was maintained under anaerobic conditions for the entire 72 h

 
Soil analyses for extractable NH+4, NO-3, and NO-2 were carried out on the background soils and at 27, 33, 51, and 72 h during the C-amended treatments and the 10 and 15% O2 treatments without added C. With added C, NH+4 levels in the aerobic period (27–51 h) decreased on average from 4.2 to 2.0 mg N kg-1 for the 5, 10, and 15% O2 treatments but increased from 4.6 to 5.0 mg N kg-1 for the 0.5% O2 treatment. Hence, nitrification was probably occurring at concentrations at or above 5% O2, but nitrification may have been negligible at 0.5% O2. Nitrate concentrations decreased slightly (average 0.7 mg N kg-1) in this aerobic period in the 5, 10, and 15% O2 +C treatments. The 10 and 15% O2 –C treatments were similar. With the 0.5% O2 + C treatment there was a greater decrease in NO-3 concentration by {approx}14.6 mg N kg-1. With added C, the NO-2 levels decreased from {approx}22.3 to 19.4 mg N kg-1 or by {approx}2.9 mg N kg-1 in all cases except the 0.5% O2 treatment, which increased from 21.2 to 30.0 , or by {approx}8.8 mg N kg-1. These data show that during the aerobic period denitrification clearly occurred with the 0.5% O2 + C treatment. Soil respiration may have reduced the effective O2 concentration at the cellular level with the 0.5% O2 + C treatment, which enabled anaerobic conditions to occur. With the higher O2 treatments, the data suggest that nitrification occurred along with denitrification.

When anaerobic conditions were restored, NO-3 decreased by an average of 31.4 mg N kg-1 with the +C treatments and by an average of only 8.8 mg N kg-1 with the -C treatments. Nitrite increased by an average of 21.0 mg N kg-1 in the +C treatments and by an average of only 3.0 mg N kg-1 in the -C treatments. In all treatments NH-4 levels were low ({approx}4.0 mg N kg-1) and did not change appreciably, perhaps indicating that mineralization was balanced by nitrification and/or immobilization. These data show that denitrification was considerably stimulated by C addition. However, a similar thorough systematic study showing the temporal changes in these mineral N species would provide more insight into the relative importance of nitrification and denitrification under variable aerobic conditions.

Inhibition
Since it was not possible to completely separate inhibition from the slower repression process, we suggest that the initial rapid decrease during the 5 min after O2 addition is a reasonably valid measure of inhibition. Also, since the rate and magnitude of inhibition of the individual consecutive reactions in the denitrification sequence differ, the intermediate concentrations would abruptly change following O2 addition, and their subsequent production rates would then adjust to new approximate steady-state levels. Hence, NO production often showed a transient increase in rate following the rapid decrease after O2 was added (Fig. 2) . This was particularly evident in trials using soil without the C (glucose) amendment. Except for the 0.074% O2 treatment, the transient increase in rate was then followed by a slow decrease, presumably resulting from the gradual repression of individual denitrifying enzymes.



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  		Fig. 2. Net rates of production of NO after addition of O2 relative to their respective anaerobic rates from 27 to 33 h. Error bars indicate standard errors , and those not shown were smaller than the symbols

 
The transient increase in rate of NO production that followed the rapid decrease on O2 addition (Fig. 2) is similar to the pattern of rise and fall of net rates of NO and N2O observed by McKenney et al. (1994) with addition of small quantities of O2 to anaerobic denitrifying pure cultures of Pseudomonas fluorescens. Not surprisingly, with the pure culture study using aqueous suspensions, the transient changes occurred more quickly (within {approx}5–15 min) because mixing of the gaseous and aqueous phases was more efficient; hence, diffusion effects were minimal compared with the soil system described herein. To explain this transient behavior, they suggested that O2 inhibited reduction of NO-2, NO, and N2O to increasingly greater extents. The results presented here are consistent with that interpretation. Oxygen rapidly and reversibly inhibits all steps in the denitrification process, and the sensitivity to inhibition apparently increases in the following order: NO-3 -> NO-2 < NO-2 -> NO < NO -> N2O < N2O -> N2 (McKenney et al., 1994). However, an alternative explanation for the observed transient effects may be an enhancement of NO production via nitrification concurrent with denitrification inhibition and enzyme repression.

The pronounced rapid decrease in net rates of NO and N2O production as a function of O2 concentration is illustrated in Fig. 3 , which shows measurements taken 5 min after O2 addition. Rates of NO production approach a limiting value of {approx}50 to 57% of the maximum anaerobic rate with addition of >5% O2. The decrease in rates of N2O production is greater, declining to values of {approx}15 to 32% of the maximum anaerobic rate with addition of >=5% O2. Kramer and Conrad (1991) also found that NO release rates were less sensitive to O2 than N2O release rates. The shape of these curves is similar to those obtained by Parkin and Tiedje (1984) for N2O production. They reported decreases in N2O rates in a Spinks sandy loam and a Capac clay loam (see Parkin et al. 1984 for soils description), reaching values <2% of the anaerobic rate with added O2 concentrations >3%. However, their experimental techniques differed from ours. They combined a gas flow soil core method with the acetylene inhibition method. Furthermore, their measurements were taken between 1 and 1.5 h after addition of O2 to the soil, which had first been exposed to air as the recirculating gas, then to Ar for 10 to 15 min just prior to adding O2. Also the presence of acetylene probably ensured that their N2O measurements included the amount that would have been converted to N2. Further, there have been recent reports of other complications associated with the acetylene inhibition technique that influence NO and N2O production (Bollmann and Conrad, 1997; McKenney et al., 1997).



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  		Fig. 3. Effect of O2 on net rates of NO and N2O production expressed as a percentage of the anaerobic rates. Rates were measured 5 min after addition of O2. Error bars indicate standard errors , and those not shown were smaller than the symbols

 
Repression and Derepression
The relatively slow decrease in rates, following the initial more rapid changes (Fig. 1 and 2), probably reflects the declining denitrifying enzyme activity, (i.e., repression) brought about by O2. Thus, the slopes of the linear portion (from {approx}28–33 h) of the plots shown in Fig. 2 presumably provide an indirect measure of the rate of change of enzyme activity. The slopes appear to approach a limiting value, that is, a constant repression rate of {approx}1.5 to 1.8 d-1 for NO with O2 levels in excess of 5% in soil without the C amendment (Fig. 4) . With the C-amended soil, the O2 repression rates for NO show a steady increase with O2 levels up to 10%. The rates with and without added C were virtually identical for the 10 and 15% O2 treatments. The N2O repression rates also remain approximately equal and constant (0.07 d-1) with or without C addition at all O2 treatments. The constant repression rates observed suggest that any contribution to NO and N2O production via nitrification was either insignificant or did not change significantly over the 28- to 33-h period.



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  		Fig. 4. Rate of repression of NO and N2O production as a function of added O2. Note that the more negative the slope of production rates presented in Fig. 2, the greater is the repression rate. Error bars indicate standard errors , and those not shown were smaller than the symbols

 
From 27 to 51 h, with the soil exposed to various partly aerobic conditions, it is clear that denitrification persists at reduced rates compared with the completely anaerobic rates. Removal of O2 rapidly reverses the decrease in rates due to inhibition, and the production rates increase rapidly and then more slowly (Fig. 5) . The relatively rapid increase in rates provides a measure of the in situ active denitrifier population remaining after the aerobic period and is a function of the extent of repression. The slower increase presumably results from derepression. With 0.074% O2, (-C), rates of NO production increased to approximately equal the anaerobic 51-h rate within 15 min followed by a more gradual increase. The O2 treatments >0.074% O2 followed similar patterns and the increasing rates depended on the O2 level (Fig. 5). After 6 h, the >0.074% O2 treatments (-C) were within {approx}92 to 100% of the anaerobic 51-h rate, and the +C treatments were similar or greater than the 51-h anaerobic rate (97–116%). Similar results for N2O were obtained (i.e., a relatively rapid increase in rates when O2 was removed followed by a much more gradual increase; data not shown).



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  		Fig. 5. Net rates of NO production in soil relative to the 51-h anaerobic rate following restoration of anoxic conditions. Error bars indicate standard errors , and those not shown were smaller than the symbols

 
In order to compare approximate estimates of the relative activities of the nonrepressed denitrifiers following the various partial aerobic treatments, NO and N2O production rates were determined within 5 min after anoxic conditions were restored (Fig. 6) . During this brief interval following O2 removal, net NO production rates in the soil column with the 0.074% O2 (-C) treatments increased to {approx}95% of the anaerobic rate at 51 h. The soil columns exposed to 0.5% increased to {approx}78%, and the >=5% O2 (-C) treatments increased to {approx}60% of the 51-h anaerobic rate within 5 min. The C-treated soil showed a similar response except for the 0.5% O2 column, which increased to {approx}100%, while the 5, 10, and 15% treatments increased to {approx}61 to 70% within 5 min.



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  		Fig. 6. Net rates of NO and N2O production 5 min after restoration of anaerobic conditions at 51 h (expressed as a percentage of their respective reference anaerobic rates at 51 h) vs. percentage O2 added. Error bars indicate standard errors , and those not shown were smaller than the symbols

 
These data suggest that repression of net NO production by O2 was affected by O2 consumption resulting from increased microbial activity stimulated by added C. Thus, repression was not evident with 0.074% O2 when no C was added or with 0.5% in the presence of added C, but was clearly evident with the >5% treatments (Fig. 6). Net N2O production rates also indicated the presence of relatively high levels of active denitrifiers. Although the N2O data showed more experimental scatter, a similar, although slower, response to the return to anaerobic conditions was observed (Fig. 6). In this case the increase in rates within 5 min was less than for NO (26–62% for all O2 treatments >=0.5%). These data show that recovery of NO production by denitrifying microbes after exposure to O2 is faster than the recovery of N2O production. This may be partly due to the fact that NO is the precursor of N2O.

Carbon addition appears to decrease repression of NO by O2. Carbon addition would enhance O2 consumption and thereby reduce the effective O2 concentration at the cellular level (Tiedje, 1988). Hence NO repression may not be as great in the presence of added C. It appears that there is no significant effect of added C on N2O repression (Fig. 4 and 6).

Smith and Tiedje (1979) used the acetylene inhibition method to assess soil denitrification response to reduced aeration following wetting. Nitric oxide was not measured in their study. They observed an initial constant N2O production rate lasting 1 to 3 h (Phase 1), which was attributed to existing denitrifying enzymes in the soil, followed by a second linear phase (Phase 2) due to derepression of denitrifying enzyme synthesis. Phase 1 depended on the existing aeration state of the soil. In our study, the initial response to the imposition of anaerobic conditions at 51 h also clearly depended on the preexisting aeration state of the soil (Fig. 6). After Phase 1 ({approx}54 h) we measured rates only at 57.5 and 72 h; hence, we could not unequivocally verify the biphasic pattern observed by Smith and Tiedje (1979). However, in most cases the production rates of NO and N2O appeared to increase more rapidly from 57.5 to 72 h (data not shown).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
It is well established that O2 is a major regulator of denitrification in soil (Dendooven and Anderson, 1994; Smith and Tiedje, 1979; Tiedje, 1988). Most previous studies focused on the effects of O2 on N2O production using the acetylene inhibition method. In this study, we examined the effect on NO production in addition to N2O production without the use of acetylene and the associated complications. Our results showed that when the aeration status of the soil was altered, the change in the NO rates generally parallelled that observed for the N2O rates. Although the overall effects of O2 on NO production follow the same general pattern as we observed for N2O, the rates and magnitude of the effects differed substantially.

Nitrous oxide production was considerably more sensitive than was NO to the initial change from anoxic to oxic conditions (i.e., inhibition of net N2O production was greater). The response during the partially aerobic period could be reasonably explained either by simple repression of denitrifiers or by denitrifier repression combined with some NO production via nitrification. Surprisingly, repression of NO production was substantially greater than that of N2O production (1.5–1.8 d-1 vs. 0.07 d-1). The fact that repression of NO was observed suggests that a substantial amount of NO was produced by denitrifiers and this was evident during the entire aerobic phase.

In our experiments, after 24 h of exposure to O2, more NO was produced than N2O. This can be reasonably explained by the fact that there was less inhibition of NO production and/or that some NO was produced by nitrifiers under the partially aerobic conditions. When the anaerobic conditions were restored, the production patterns observed were consistent with the preceding inhibition and repression changes that had occurred.

Better understanding of how O2 functions in the control of production and release of NO and N2O in natural soil would require knowledge of the composition of the microbial community in soil, the soil environmental factors (e.g., structure, porosity), as well as a knowledge of the process at the molecular level (Conrad, 1996). Nevertheless, as demonstrated in this study, the processes of inhibition, repression, and derepression can be identified and their rates estimated for both NO and N2O during intermittent aeration episodes. To the best of our knowledge, the measurements reported herein are the first direct measurements of the rates of O2 inhibition, repression, and derepression of NO, and repression of N2O production through denitrification in soil. Since repression and derepression are much slower than inhibition, and the latter is also reversible, it follows that emission of NO and N2O from soil would be highly sensitive to changing moisture status and undoubtedly contributes to the high spatial variability reported in the literature.


   ACKNOWLEDGMENTS
 
We greatly acknowledge the contribution from the Greenhouse Gas component of the Canada-Ontario Agriculture Green Plan. We are grateful to Dr. Tom Oloya for his assistance with the NH+4, NO-3, and NO-2 analyses.

Received for publication November 17, 1999.


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





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