Journal of Environmental Quality 30:11-18 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
BIOREMEDIATION AND BIODEGRADATION
Bioremediation of Residual Fertilizer Nitrate
II. Soil Redox Potential and Soluble Iron as Indicators of Soil Health During Treatment
Benjamin U. Ugwuegbu,
Shiv O. Prasher,
Darakhshan Ahmad and
Pierre Dutilleul
INRS-Santé, Université du Québec, 245 Boulevard Hymus, QC, Canada H9R 1G6
Corresponding author (darakhshan_ahmad{at}inrs-iaf.uquebec.ca)
Received for publication September 30, 1999.
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ABSTRACT
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The prospect of using wastewater containing high loads of soluble organic matter (OM) for removing residual agricultural chemicals (fertilizer, pesticide, or herbicide) in farm soil, although promising, could have adverse effects on soil agricultural quality as a result of development of redoximorphic features in the soil profile. In this study, the effect of organic carbon supplement for bioremediation of residual fertilizer nitrate on soil properties, redox potential (Eh), pH, and metal ion mobilization was studied using sandy soils packed in columns. The study was included in a general project, described elsewhere (Ugwuegbu et al., 2000), undertaken to evaluate use of controlled water table management (WTM) systems to supply organic carbon for creating a reduced environment conducive to denitrification of residual fertilizer nitrate leaching from the farm to subsurface water. The columns were subjected to subirrigation with water containing soluble organic carbon in the form of glucose. The work was carried out in two experimental setups and the long-term effect of a range of glucose concentrations on the Eh, pH, and soluble levels of Fe and Mn was investigated. From the results obtained, it could be concluded that excessive organic carbon supplement to soil can have adverse effects on soil quality and that Eh and soluble Fe are the two most practical parameters for monitoring soil health during treatment of farm chemicals.
Abbreviations: Eh, redox potential • OM, organic matter • WTM, water table management
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INTRODUCTION
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AGRICULTURAL chemicals have significantly increased the production and protection of food, feed, and fiber worldwide. However, the fact that pesticide and fertilizer residues can leach from farms and pollute the surface and ground water resources is of great concern. Often the problem arises because excessive loads of contaminants cause microbial bioremediation (biodegradation, biotransformation, or biodetoxification) to become electron donor– or acceptor–limited. In such cases, addition of a suitable electron donor or acceptor is a viable option for efficient bioremediation. Amendment of soil with the soluble or readily available form of limiting nutrients has been successfully used for several in situ soil decontamination endeavors, especially dealing with organic pollutants (Al-Hadhrami et al., 1997; Fava and Gioia, 1998). If such a strategy is applied for remediation of pesticides, herbicides, and fertilizers from agricultural soils, it is of utmost importance to evaluate first its effects on the soil properties that are important to later agricultural use and sustainable development (Banerji et al., 1997; Haynes and Naidu, 1998; Sort and Alcaniz, 1999a,b).
Fertilizer nitrate that remains in the unsaturated zone at the end of the farming season is the most common contaminant of subsurface ground water (Zhou et al., 1997). Several studies have shown that addition of readily available organic carbon has the potential of controlling nitrate contamination of soil and ground water (Firestone, 1982; Knowles, 1982; Hasselblad and Hallin, 1998; Verstraete and Philips, 1998). The readily available organic carbon serves as an electron donor, and creates a reduced environment that stimulates denitrification. The work presented here is part of a study, described in the preceding article (Ugwuegbu et al., 2000), undertaken to evaluate the use of controlled WTM (subsurface drainage–irrigation) systems (Madramootoo et al., 1993) that are used in humid regions of North America and some parts of western Europe. These systems supply readily available organic carbon and to create a reduced environment conducive to denitrification of residual fertilizer nitrate leaching from the farm to subsurface water.
The reduced soil environment is more pronounced below the water table, where oxygen supply is low and biological oxygen demand is high (Buol et al., 1989), thus enhancing degradation of fertilizer nitrate by indigenous denitrifying bacteria (Knowles, 1982; Korom, 1992). However, a reduced environment also causes reduction of redoximorphic compounds, such as iron (Fe) and manganese (Mn) hydroxides (Cate, 1964; Lidster and Ford, 1981). Iron compounds are reduced to the ferrous form, which is known to be highly mobile. The ferrous form then is lost from the system if there is a net downward or upward and outward movement of ground water. If the water is then removed from the soil, precipitation or deposition of the dissolved compounds will not occur in the profile. However, if the water table is raised, the reduced mobile forms of Fe and Mn are transported upward in the soil and this movement of Fe and Mn may result in the formation of redoximorphic features in the soil profile (Anonymous, 1992). During subsequent desaturation, the reoxidation and precipitation of these compounds in the soil profile results in the development of Fe and Mn coatings on mineral surfaces, and soft masses or hard concretions or nodules are formed. Furthermore, if the reduced Fe remains in the system in the presence of a relatively high level of OM, it can react to form sulfides and related compounds (Bloomfield, 1952; Jeffery, 1960; Ford, 1971, 1974, 1975). In this case, subirrigation water containing a high amount of dissolved organic carbon might lead to mobilization, transportation, and precipitation of Fe and Mn compounds, consequently affecting the agricultural quality of the soil. The objective of this study was, therefore, to investigate the effect of glucose C supplement by monitoring the Eh, pH, and Mn and Fe levels in the soil solution.
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MATERIALS AND METHODS
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Field Site, Soil Used, Column Fabrication, Subirrigation of Soil, Nitrate Application and Leaching, and Experimental Setup
This study was part of a general project undertaken to develop a method for denitrifying residual fertilizer nitrate. A detailed description of the field site, test soil, column design, and subirrigation system used in this study are described elsewhere (Ugwuegbu et al., 2000). To simulate a normal farming practice, each column received surface applications of calcium nitrate [Ca(NO3)2] as fertilizer (NO3–N) and several rainfall events were simulated to transfer the nitrate from the upper soil profile to the lower depths (Ugwuegbu et al., 2000; Smith et al., 1992). Soil columns were subirrigated with different concentrations of glucose solution (used merely for simulating subirrigation water loaded with known amounts of soluble organic carbon). Table 1 indicates the experimental parameters, their stages and durations, OM content of soil used, the amount of nitrate applied to the columns, the soil columns used in each stage identified by numbers, and the concentration and average volume of glucose solution (soluble organic carbon concentration in the subirrigation water). The study included two experimental setups. Experiment I used Soil 1 and Experiment II used Soil 2.
Experiment I
Stage 1—Setup with High Glucose C in Subirrigation Water (55-Day Duration)
In Stage 1 of Experiment I, the effect of high glucose C level (970 mg L-1) in subirrigation water was followed. Subirrigation commenced 6 d after fertilizer application. Soil solutions were obtained from the sampling ports before and after the leaching events and analyzed for Eh. After 55 d, the columns were drained completely for Stage 2.
Stage 2—Setup with Low Glucose C in Subirrigation Water (13-Day Duration)
In Stage 2, the effect of low glucose C level (120 mg L-1) was followed by subjecting columns, previously used in Stage 1, to subirrigation. Soil solutions were sampled at different times and depths, and analyzed for Eh.
Experiment II
The second experiment was set up to study the long-term (96 d) effect of addition of a range of glucose levels (0, 20, 70, 150, and 300 mg L-1) in order to determine the glucose C concentration with the least adverse effect on the soil properties. In this experiment a soil with lower OM (1.6%), Soil 2, was used so that the interference from the soluble organic C produced from the hydrolysis of soil OM could be minimized during the long duration of this experiment. The changes in Eh in soil solution were monitored at different times and depths. On Day 96, soil solutions were sampled from all columns and analyzed for changes in Fe, Mn, and pH.
Sampling and Analytical Methods
Soil solutions were collected from below the saturated zone of the columns, 9 h before and after every leaching, at depths of 40, 60, and 85 cm as described elsewhere (Ugwuegbu et al., 2000). During leaching, 20 to 30 mL of effluent was collected for Eh measurements that were done immediately after the sampling using a Hanna Scientific platinum Eh electrode (Springfield Scientific, Springfield, OR). Iron and Mn analyses were carried out on samples filtered through 0.45-µm pore size filters by using an atomic absorption spectrophotometer (PerkinElmer [Wellesley, MA] 2380). Quantification was done by an external standard calibration curve.
Experimental Design
The effect of adding different levels of glucose to the soil by subirrigation was assessed by monitoring changes in Eh of the soil solution at different depths. The experimental design was a three-way factorial design with spatio–temporal repeated measures and one treatment factor. There were three levels of the spatial repetition factor (depth), and a different number of levels for the temporal repetition factor (time duration), depending on the stage after which each stage of the study was commenced. In Experiment I, one group of three soil columns received the treatment and another three were used as controls. In Experiment II, four levels of treatment were considered to estimate the optimal glucose C concentration that would have no adverse effect on the soil, resulting in no significant difference.
Statistical Analysis
Data analyses included pairwise comparisons between the treatments and the control (Zolman, 1993) using the CONTRAST statement of the GLM (General Linear Model) procedure of SAS (SAS Institute, 1997) and the repeated measure analysis of variance (ANOVA) using the REPEATED statement of the same SAS procedure. The univariate approach to repeated-measure ANOVA was adjusted for the heteroscedasticity (inequality of variances) and autocorrelation (lack of independence) of the data, by applying a correction to the number of degrees of freedom of the F-statistics involving either of the repetition factors (time and depth). Details of the method can be found in Dutilleul (1998). The theoretical value of the correction factor, known as epsilon (
), ranges between 1/(r - 1) and 1, with r being equal to the number of repeated measures (temporal, spatial, or spatio–temporal). The effect of is to deflate the number of degrees of freedom according to the size and magnitude of heteroscedasticity and autocorrelation. The significance level was set at 5%.
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RESULTS AND DISCUSSION
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Effect of High (970 mg L-1) Glucose C in Subirrigation Water on Redox Potential (Experiment I, Stage 1, 55-Day Duration)
Figure 1
shows redox changes in the soil solution at different depths in both the treatment and control columns. Generally, the Eh in all control columns at all depths did not show much variation. The values for the treatment columns were significantly lower than the control columns at all depths, and especially at lower depths. In the control columns, the Eh was stable, around 300 mV at all depths throughout the experimental period, but in the treatment columns it varied between -520 to +200 mV on most sampling days at all depths. Although the Eh values for the treatment columns fluctuated greatly, and thus the standard deviations (SD) between sampling data were high, the values were significantly affected more by depth than time (Fig. 1). The adjusted probabilities of significance showed that the three-factor interaction depth x time x carbon was highly significant, indicating that time and depths interacted differently among depths (Table 2).
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Fig. 1. Changes in redox potential (Eh) with high glucose C (970 mg L-1) in the subirrigation water at the (a) 40-cm, (b) 60-cm, and (c) 85-cm depths (Experiment I, Stage 1). Error bars show standard deviations
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Table 2. Repeated measures analysis of variance for redox potential (Eh) in the soil solution, with depth and time duration as repetition factors (Experiment 1, Stage 1, 55-d duration)
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Saturated conditions can limit diffusion of gaseous oxygen in waterlogged soils and microbial respiration may further reduce the oxygen content. The addition of extra organic carbon (glucose C) to the waterlogged soil accelerated the depletion of oxygen and thus established a more reduced state, which was more pronounced at the bottom of the column. The high Eh at the 40-cm depth in the treatment columns (Fig. 1a) could have been due to the diffusion of air from the surface and the dissolved oxygen in the simulated rain water. Also, since the carbon content of the subirrigation water decreased due to consumption by microorganisms as the glucose solution passed upward through the soil profile to the 40-cm depth, it may have received less carbon. The soil profile around the 85-cm depth zone was most reduced and sensitive to the influx of carbon because that zone was in contact with the highest level of inflowing organic carbon (Fig. 1c). However, on Days 13 and 29, the 85-cm depth had a high Eh, which was not expected. This outcome could have been due to the percolating rainfall water, which may have displaced resident soil solution and redox components via draining. In the control columns, the Eh remained about 300 mV on all sampling days because the energy source from the soil OM was small and not readily available.
The Eh of ground water gives an indication of the type of reductant present, for example, soil OM and redox couples (NO-3–NO-2, MnO2–Mn2+, Fe3+–Fe2+, etc.) (Anonymous, 1994). The treatment columns (as opposed to controls) exuded an odor of decomposing OM and a greenish hue, probably due to Fe2+. When exposed to air in the laboratory, effluent from the treatment columns produced a brownish precipitate (probably Fe3+). Dissolution of Fe suggests a reduced state (low Eh). In the treated columns, the Eh level at or below zero indicates that an oxygen-free status existed in the soil (Tate, 1995). The Eh of the soil solution also may be an indicator of the soil component acting as an electron acceptor during a prolonged period of anaerobic conditions. Soil Eh measurement in this study monitored the general effect of adding extra organic carbon to a saturated soil.
Effect of Subirrigation with a High Glucose Level on pH, Iron, and Manganese (Experiment I, Stage 1, 55-Day Duration)
Redox potential and pH could be used to predict Fe or Mn speciation in soil solution. During Experiment I, Stage 1, the pH at the sampling ports for the control and treatment columns stayed at average values of 6.1 and 5.8 (SD ±0.2), respectively. Superimposing the Eh and pH data from this study on a stability field (plot of Eh vs. pH) for Fe and Mn (Collins and Buol, 1970), the form of Fe and Mn was shown to fall within the Fe2+ and Mn2+ species for the treatment columns, while in control, Fe3+ and Mn4+ prevailed. This result affirmed the observation that the glucose C concentration in the subirrigation water might have caused more Fe to come into solution. On exposure to air in the laboratory, the reduced Fe was reoxidized to produce brownish precipitates that, according to the stability diagram, is Fe(OH)3. If these mobile forms of Fe and Mn are transported upward by a rising water table and subsequently exposed to more oxidizing conditions as the soil desaturates, coatings of Fe(OH)3 will form on the mineral surfaces in the desaturated zone. Such alternation between reducing and oxidizing conditions might lead to the formation of mottles and concretions in the soil under subirrigation. Courchesne et al. (1996) hypothesized that such coatings could act as a barrier, limiting contact between the soil solution and fresh mineral surfaces.
While wastewaters containing readily available carbon may be used for subirrigation (Banerji et al., 1997), the adverse effect of low negative Eh should be anticipated. A flooded soil might not be oxygen free, but the depletion of oxygen under such conditions might result in an anaerobic situation that induces facultative metabolism (e.g., fermentation), anoxic metabolism (e.g., denitrification), and strictly anaerobic processes (e.g., methanogenesis) (Tate, 1995). Production and accumulation of organic acid fermentation products in the presence of high organic carbon may result in a pH drop and subsequent mineral dissolution. The low pH and accumulated organic acids also will affect the composition of the microbial community.
These results suggest some adverse effect in the soil subjected to prolonged addition of 970 mg L-1 glucose C. The Eh data also helped to determine when to reduce the level of organic carbon added to the treatment columns so that metal mobilization in soil is not affected.
Effect of Low (120 mg L-1) Glucose C in Subirrigation Water on Soil Profile Soil Redox Potential (Experiment I, Stage 2, 13-Day Duration)
Figure 2
shows changes in Eh at different depths in columns treated with a low glucose C concentration (120 mg L-1). Redox potential data measured over a 13-d period of Stage 2 did not differ from each other at all depths and remained positive (above zero) throughout the experimental period. However, the Eh values at corresponding depths in the treatment and control columns were significantly different (at the 5% level) on most days, being consistently lower in the treatment columns. The adjusted probabilities of significance showed that the depth x carbon interaction was significant, indicating that the carbon effects differed among depths (Table 3). The average Eh values were 269, 156, and 67 mV for the treatment columns and 312, 328, and 273 mV for the controls at the 40-, 60-, and 85-cm depths, respectively (Fig. 2). Thus, the redox trend at all depths in Stage 2 was similar to that observed in Stage 1, with Eh increasing as the depth decreased, except that the values were higher in Stage 2. This result is explained by a glucose C concentration reduction from 970 to 120 mg L-1. The statistically significant differences in Eh values observed between Stages 1 and 2 suggest that there could be a glucose concentration range in subirrigation water that will not necessarily result in a big drop in Eh potential. Higher Eh in the treatment columns, after reducing the glucose level in Stage 2, also suggested the importance of Eh in the design and monitoring of a carbon-rich wastewater irrigation strategy. If the higher optimal range is not exceeded, this would drastically avoid a drop in the Eh, maintaining low Fe and Mn concentrations in the soil solution. To achieve this favorable Eh range, an optimum glucose C concentration range was further investigated in the next stage.
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Fig. 2. Changes in redox potential (Eh) with low glucose C (120 mg L-1) in the subirrigation water at the (a) 40-cm, (b) 60-cm, and (c) 85-cm depths (Experiment I, Stage 2). Error bars show standard deviations
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Table 3. Repeated measures analysis of variance for redox potential (Eh) in the soil solution, with depth and time duration as repetition factors (Experiment 1, Stage 2, 13-d duration)
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Organic Carbon Level in Subirrigation Water with the Least Adverse Effect (Experiment II, 96-Day Duration)
Prolonged addition of glucose to the soil columns in Experiment I, Stage 1 of the study caused a dramatic decline in soil solution Eh. An attempt was made in Experiment II to assess the optimum glucose C concentration range that will not affect the soil solution Fe and Mn levels. It was therefore necessary to quantitatively compare the amount of Fe and Mn mobilized in relation to the different glucose treatments during leaching, when there was neither input of fresh water via rainfall simulation nor loss of Fe and Mn during draining of the columns. It also tested the long-term effect of continued glucose C addition to a packed soil column undergoing subirrigation. Thus, Fe and Mn content of the soil solution at different depths was determined on Day 96, 45 d after the last leaching event.
Saturation of a soil decreases oxygen diffusion into the soil. As a result, microorganisms in the process of decomposing OM switch to alternate electron acceptors. Although the O2 that diffuses into soils and sediments that are not saturated is still preferred and easily reduced (Patrick and Jugsujinda, 1992), the order of preference for alternate electron acceptors and their sequence for reduction is: NO-3, Mn4+, Fe3+, SO2-4, and CO2. Nitrate is the constituent more likely to be used as an electron acceptor by microorganisms, because the energy yield is greater. However, the reducing conditions after O2 depletion are established rapidly, overlapping in the reduction (utilization) of electron acceptors that also has been reported to occur (Patrick and Jugsujinda, 1992). The switch from one electron acceptor to another depends, among other factors, on the level of electron donor present. High levels of electron donor may cause a significant drop in the Eh of saturated soils and may lead to the formation of redoximorphic features. Thus, the extent to which different levels of decomposing organic carbon can cause the Eh to drop was assessed by specifically determining the Eh and amount of soluble Mn2+ and Fe2+ formed after subjecting Soil 2 (a soil of low OM content used so that the interference from the readily available organic carbon released from the hydrolysis of soil OM is minimal) to 96 d of subirrigation with water containing various concentrations of glucose C.
Effect on Soil Redox Potential
Soil Eh values on Day 96 at the 40-, 60-, and 85-cm depths for all treatments are shown in Fig. 3
. At 40 cm, the redox mean values for the 70, 150, and 300 mg L-1 glucose C treatments, varying between -70 mV and -45 mV, were significantly different (at the 5% level) from that of the control, which was above zero. For the 20 mg L-1 glucose C treatment and the control the values were about 12 mV and -60 mV, respectively (Fig. 3a), which are higher compared with the values for all other glucose level treatments (70, 150, and 300 mg L-1 glucose C). This suggests that the redox level attained in each of the treatments may be substantially influenced, negatively, by the increasing concentration of organic carbon supplement.
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Fig. 3. Redox potential (Eh) after 96 d at various glucose C concentrations and depths of (a) 40 cm, (b) 60 cm, and (c) 85 cm (Experiment II). Error bars show standard deviations
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Effect on Iron and Manganese
Interpretation of the redox result is often qualitative (Anonymous, 1994). Therefore, the significance of the Eh values on Day 96 was further assessed by measuring the Fe and Mn content of soil solutions. Figures 4a and 4b
show the levels Mn and Fe in the soil solution at depths of 40, 60, and 85 cm. The control and treatment columns that received 20 mg L-1 glucose C had the least Mn and Fe content in their solutions as compared with the treatment columns with higher glucose C levels, and the levels were not significantly different (at the 5% level) at all depths. Columns that received the 70, 150, and 300 mg L-1 glucose C levels had a higher Mn and Fe dissolution at the 60- and 85-cm depths than at 40 cm, with the highest Mn and Fe dissolution observed at 85 cm. Thus, the concentration of Mn (varying between 0.1 to 2.8 mg L-1 with SD values between 0.02 to 0.42) and Fe (varying between 0.05 to 44.6 mg L-1 with SD values between 0.05 to 5.25) increased in the soil solution as the depth and the concentration level of glucose C increased. The amount of Fe dissolution suggests that Fe is a more sensitive marker than Mn in responding to the addition of carbon to soil. The decrease of Fe concentration at lower depths indicates that the glucose C was gradually used before reaching a depth of 40 cm. This was indicated by lower concentrations of Fe at depths of 40 and 60 cm compared with the concentration at the 85-cm depth. These results clearly show that the sensitivity of soil to subirrigation with water excessively loaded with organic carbon can be monitored by the Eh and Fe dissolution in soil receiving carbon supplement. Thus, these factors can be the two most useful parameters in monitoring and implementing a management strategy for subirrigation with wastewater laden with soluble OM.
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Fig. 4. Iron (a) and Mn (b) concentrations and pH (c) of soil solution on Day 96 at various glucose C concentrations and depths (Experiment II). Error bars show standard deviations
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According to Tate (1995), the Eh remains close to a specific level until the electron acceptor associated with that potential finishes. Subsequently, the Eh drops, allowing use of the next available terminal electron acceptor. Patrick and Jugsujinda (1992) also observed that no overlap occurs in the oxidation or reduction of nitrate and Mn, while little overlap was found between Mn and Fe ions. It also was noted that an overlap might occur between nitrate and Mn if the soil was reduced enough to support Mn reduction before all the nitrate was reduced. Tate (1995) further pointed out that when nitrate or sulfate is used as the final electron acceptor, the organic carbon source of energy is completely oxidized to CO2 and water. Therefore, by-products of the carbon source, other than carbon dioxide, will not pose a threat to water resources if nitrate, continuously leached by rainfall, is serving as the final electron acceptor. Data presented in Fig. 4a and 4b indicate that dissolution of Fe and Mn compounds, which plays a role in contaminant mobility in the soil, increases at low Eh and is determined by the level of glucose added to the soil; the higher the level of glucose added to the soil, the greater the adverse effect of the resultant Fe and Mn levels on the soil ecosystem.
Iron and Mn, although essential elements, are included in the secondary drinking water standard as they become toxic in large doses (Csuros, 1994). The limit recommended for Mn in water by the USEPA and the Canadian Water Quality Guidelines is 0.05 mg L-1, while the combined Mn and Fe limit is 0.30 mg L-1 when based upon aesthetics and taste. The levels obtained in both the control and the 20 mg L-1 glucose C treatment were higher than the stipulated limit of 0.30 mg L-1, primarily because of the high OM content of the soil. The high Fe concentrations, ranging from 1.3 to 44.6 mg L-1 at 300 mg L-1 glucose C was confirmed by the experimental observation (Stage 1) that 970 mg L-1 glucose C produced a solution of greenish hue and formed a brownish precipitate on air exposure, and by the extrapolation from the stability diagram of Eh and pH that indicated that Fe2+ ions were excessively making their way into solution due to treatment with glucose.
The measured pH values of the solutions, 96 d after the experiment had started, did not vary much between treatments and control (Fig. 4c), ranging between 6 and 7. Thus, the pH parameter was not sensitive in discriminating between the different carbon levels. Generally, Fe and Mn are more soluble at a low pH, and a decreasing pH due to high organic carbon in soil solution has been shown to decrease some soil biological activity (e.g., denitrification rate) (Waring and Gilliam, 1983; Tate, 1995). The additive effect of the lower pH and excessive dissolution of Mn and Fe compounds might result in increased degradation of soil agricultural quality.
Evaluating the results obtained in this study, a desirable readily available carbon level was defined as that resulting in maximum denitrification (Ugwuegbu et al., 2000) that produces Eh values and soluble Fe and Mn levels similar or close to those of the control soil solution after subjecting the soil to prolonged subirrigation. Thus, measuring the Eh of the soil solution can be used as a valuable and practical monitoring tool that can be applied in designing or predicting strategy for use of wastewater for decontamination of agricultural fields, since the Eh of the soil system is sensitive to carbon utilization and the cost of measurement is very low. Furthermore, measurement of Eh does not require any extraordinarily special expertise on the part of the farmer. Although the measurement of Eh was considered to be a less expensive, indirect method of estimating the total redox couples contributing to the Eh of the soil solution, Fe might be the most sensitive marker of soil degradation due to wastewater application to farms.
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CONCLUSIONS
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Using wastewater containing readily available organic carbon for residual fertilizer nitrate bioremediation tasks can alter the soil properties if too much readily available carbon is supplied to the soil by subirrigation. The treatment vs. control column studies have shown that the soil solution Eh and levels of soluble Fe and Mn contents will not change much if the amount of carbon applied does not exceed the optimal range, thus reducing the risks of redoximorphic features and maintaining the soil agricultural quality.
Thus, the changes in Fe and Mn concentrations in the soil solution, and their suggested influence on soil properties and microbial activity relating to redoximorphic features, must be carefully considered when contemplating the use of wastewater loaded with high OM to subirrigate farms. To further assess the statistical significance of the treatment levels and the control, Fe and Mn levels in soil solution were measured to discriminate between the treatment levels, especially as they related to a possible adverse effect on the soil and the receiving surface water environment. Finally, the results indicated that changes in Eh and Fe content of the soil solution may be the two most sensitive markers of the changes in soil status due to the application of wastewater to the farms.
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NOTES
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The study was supported in part by an INRS-Santé graduate scholarship to B.U. Ugwuegbu.
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B. U. Ugwuegbu, S. O. Prasher, and D. Ahmad
Bioremediation of Residual Fertilizer Nitrate: I. Laboratory Demonstration of an On-Farm In Situ Pollution Control System
J. Environ. Qual.,
January 1, 2001;
30(1):
1 - 10.
[Abstract]
[Full Text]
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ABSTRACT
INTRODUCTION
