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TECHNICAL REPORTS
Ecological Risk Assessment

Environmental and Agronomic Implications of Water Table and Nitrogen Fertilization Management

^a Dep. of Natural Resource Science, Macdonald Campus of McGill University, 21 111 Lakeshore Rd. Ste-Anne-de-Bellevue, QC, Canada H9X 3V9
^b Dep. of Agric. Biosyst. Engineering, Macdonald Campus of McGill University, 21 111 Lakeshore Rd. Ste-Anne-de-Bellevue, QC, Canada H9X 3V9

* Corresponding author (Hamel{at}nrs.mcgill.ca)

Received for publication September 28, 2001.

	ABSTRACT

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

 
Nitrate (NO^-₃) pollution of surface and subsurface waters has become a major problem in agricultural ecosystems. Field trials were conducted from 1996 to 1998 at St-Emmanuel, Quebec, Canada, to investigate the combined effects of water table management (WTM) and nitrogen (N) fertilization on soil NO^-₃ level, denitrification rate, and corn (Zea mays L.) grain yield. Treatments consisted of a combination of two water table treatments: free drainage (FD) with open drains at a 1.0-m depth from the soil surface and subirrigation (SI) with a design water table of 0.6 m below the soil surface, and two N fertilizer (ammonium nitrate) rates: 120 kg N ha^-1 (N₁₂₀) and 200 kg N ha^-1 (N₂₀₀). Compared with FD, SI reduced NO^-₃–N concentrations in the soil profile by 37% in spring 1997 and 2% in spring 1998; and by 45% in fall 1997 and 19% in fall 1998 (1 mg NO^-₃–N L^-1 equals approximately 4.43 mg NO^-₃ L^-1). The higher rate of N fertilization resulted in greater levels of NO^-₃–N in the soil solution. Denitrification rates were higher in SI than in FD plots, but were unaffected by N rate. The N₂₀₀ rate produced higher yields than N₁₂₀ in 1996 and 1997, but not 1998. Corn yields in SI plots were 7% higher than FD plots in 1996 and 3% higher in 1997, but 25% lower in 1998 because the SI system was unable to drain the unusually heavy June rains, resulting in waterlogging. These findings suggest that SI can be used as an economical means of reducing NO^-₃ pollution without compromising crop yields during normal growing seasons.

Abbreviations: FD, free drainage • SI, subirrigation • WTD, water table depth • WTM, water table management

	INTRODUCTION

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

 
NITRATE (NO^-₃) pollution of both surface and ground water has become a widely recognized risk in intensively managed agricultural ecosystems. Agricultural activities are considered to be among the most significant sources of NO^-₃ contamination. Intensive use of nitrogen fertilizer to aid food production may lead to increased NO^-₃ levels in surface water bodies, promoting eutrophication of surface waters by stimulating algae growth (Yeomans et al., 1992). Furthermore, NO^-₃ leaching represents a potential source for the degradation of ground water quality (Prunty and Montgomery, 1991). Human consumption of water containing high NO^-₃ concentrations has been linked to cases of methemoglobinemia, also known as blue baby syndrome, which in extreme cases can result in the death of infants of 4 to 6 mo (Comly, 1945; Gelberg et al., 1999), and can cause other health disorders (Prasad and Power, 1995).

Prevention is preferable to the restoration of polluted aquifers. Nonetheless, in some situations it is already too late for prevention, and costly treatments may become inevitable before water is fit for human consumption or recreational purposes. In many regions of the United States, wells have exhibited NO^-₃ concentrations exceeding the USEPA and Canadian health standards limit of 10 mg NO^-₃–N L^-1 for drinking water (Health Canada, 1996; Randall et al., 1997; Thompson et al., 2000; Weil et al., 1990). In the province of Quebec, Canada, Madramootoo et al. (1992) documented levels of NO^-₃ as high as 40 mg NO^-₃–N L^-1 in subsurface drain flow from a sandy loam field cropped to potato (Solanum tuberosum L.). Increasing public concern about deteriorating water quality has prompted a growing interest for the development of various preventive and remedial management strategies.

Water table management (WTM) including controlled drainage–subirrigation (SI) is one promising technique to help reduce NO^-₃ ground water pollution. Raising the water table by subirrigation increases soil saturation and restricts O₂ diffusion in soil pores, thus creating reducing conditions that promote NO^-₃ losses by denitrification. Drury et al. (1997) reported that a SI system reduced NO^-₃ concentration in tile drainage water by 25% compared with a free drainage (FD) system. Similarly, Jacinthe et al. (1999) estimated a 40% reduction in soil NO^-₃ due to denitrification in SI plots compared with FD plots. Enhanced denitrification reduces NO^-₃ in the soil–water solution and, hence, the risk of NO^-₃ leaching to ground water. However, a possible consequence of this practice is increased nitrous oxide (N₂O) emission to the atmosphere. Nitrous oxide emissions are of serious concern as they contribute to greenhouse effects (Smith, 1990) and participate in the depletion of the ozone layer (Mooney et al., 1987). Fortunately, N₂O is not the only end product of denitrification as it may further be reduced to N₂, which is harmlessly carried into the atmosphere. We are currently conducting field trials to elucidate effects of water table depth on the N₂O to N₂ ratio.

Water table management may also benefit crop yield. The water table elevated by SI provides abundant moisture and helps satisfy plant evapotranspiration requirements. Optimum water table depth (WTD) is a function of crop and soil type. Kalita and Kanwar (1993) have shown that the highest yield of corn grown on sandy loam soils was obtained with a WTD of 0.6 to 0.9 m, and the lowest with a WTD of 0.2 to 0.3 m. Similarly, Tan et al. (1996) reported optimal corn yield at a WTD of 0.6 m, but a 15% yield reduction at a WTD of 0.3 m. These findings agree with those of Wesseling (1974), who found that a too-shallow WTD reduces oxygen supply to roots, reduces nutrient uptake and crop growth, and restricts rooting volume.

The integration of water table management into a N fertilization strategy could further mitigate environmental degradation while optimizing crop yield. Knowledge of interactions between WTM and N fertilization is required to development best management practices. The objectives of this study were to (i) assess water table management effectiveness in removing NO^-₃–N from the soil–water system, (ii) investigate the combined effects of water table depth and N fertilization rate on denitrification rate, and (iii) elucidate the effects of SI and N rates on corn yield.

	MATERIALS AND METHODS

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

 
Field Management
The 4.2-ha research site was a privately owned field located at St-Emmanuel near Côteau-du-Lac, Quebec (74°11'15'' N, 45°2'10'' W). The soil was of sedimentary origin. It was a Soulanges fine sandy loam (fine silty, mixed, non-acid Frigid Humaquept, Gleysol, according to the FAO classification system). The site was under pasture before 1991 and under continuous corn production thereafter. Surface topography was generally flat with an average slope of less than 0.5%. A clay layer at about a 0.5-m depth impeded natural drainage. The field was planted with corn (Pioneer [Des Moines, IA] Hybrid 3905) at a density of 75 000 plants ha^-1 with 0.75- and 0.15-m inter- and intra-row spacings, on 17 May 1996, 23 May 1997, and 8 May 1998. Potassium (muriate of potash, 0–0–60 N–P–K) was broadcast at a rate of 90 kg K₂O ha^-1 roughly one week before planting. In addition, the farmer applied manure (cattle slurry) to the field in spring 1998 at a rate of 20 Mg ha^-1 (wet weight). To control weeds, 1.5 kg a.i. ha^-1 atrazine [6-chloro-N ethyl-N (1-methylethyl)-1,3,5-traizine-2,4-diamine], 0.32 kg a.i. ha^-1 dicamba (3,6-dichloro-2methoxybenzoic acid), 0.32 kg a.i. ha^-1 bromoxynil (3,5-dibromo4-4hydroxybenzonitrile), and 1.92 kg a.i. ha^-1 metolachlor [2-chloro-N-(2-ethyl-6-methyl-phenyl)-N-(2-methoxy-1-methylethyl) acetamide] were applied to the field on 23 May 1996, 25 June 1997, and 13 May 1998. All field operations other than seeding, fertilizer treatment, and herbicide applications were performed by the farmer as part of his normal production practices. Experimental plots were under a conventional tillage system (i.e., moldboard-plowed to 0.20 m in fall and disked in spring, the common practice in the region).

Experimental Design
Schematic representation of the field layout is depicted in Fig. 1 . Briefly, treatments consisted of a factorial combination of two water table management treatments (FD with open drains 1 m in depth from the soil surface and SI with a design water table 0.6 m below the soil surface) and two fertilizer rates (120 kg N ha^-1 [N₁₂₀] and 200 kg N ha^-1 [N₂₀₀]). Diammonium phosphate (18–46–0) was banded at planting to provide approximately 24 kg N ha^-1 and 130 kg P₂O₅ ha^-1. One month later, to reach the desired levels of N fertilization, 97 and 178 kg N ha^-1 were surface-applied as ammonium nitrate (34–0–0) for the N₁₂₀ and N₂₀₀ treatments, respectively. This second application occurred on 18 June 1996, 20 June 1997, and 8 June 1998.

View larger version (45K): [in this window] [in a new window]   Fig. 1. Schematic representation of the experimental layout and treatment arrangements.

Sampling Strategies and Analysis
For total denitrification measurements, soil samples (0- to 0.15-m depth) were collected weekly from late May to July and biweekly from August to October during the 1998 growing season, and approximately biweekly during the 1996 and 1997 growing seasons. In 1996, sampling started in mid-July after the drainage system was switched to SI mode. On each sampling date, aluminum cylinders (50 mm in diameter x 150 mm long) were used to collect undisturbed soil cores. On each occasion, soil cores were taken on different non-wheel-tracked inter rows. The cylinders were perforated along the sides (horizontally and vertically) at 50-mm intervals to enhance acetylene gas diffusion. Sample cylinders were placed in 2-L plastic jars fitted with rubber stoppers for gas sampling. To represent field conditions, samples were incubated outdoors overnight. Before incubation, 100 mL of the headspace in the jars was removed and replaced with 100 mL of acetylene (C₂H₂) to give a 5% (v/v) concentration. Acetylene was supplied to inhibit the enzymatic reduction of N₂O to N₂ and nitrification, so that accumulated N₂O + N₂ from denitrification could be measured as N₂O (Yoshinari et al., 1977).

Total N₂O production was determined following the procedure of Mackenzie et al. (1997). Briefly, jar headspace gas was thoroughly mixed by inserting a syringe and pumping several times before gas sampling. About 4 mL of headspace gas were removed from the jars and injected into a Hewlett-Packard (Palo Alto, CA) 5870 Series II gas chromatograph (GC) equipped with a ⁶³Ni electron capture detector (ECD) with Ar and CH₄ (95:5) as a carrier gas, with oven and detector temperatures adjusted at 70 and 400°C, respectively. Soil moisture was determined by oven-drying soil cores at 105°C for 48 h and was used to compute bulk density and water-filled pore space (WFPS).

Nitrate N concentration in the soil profile was assessed by collecting three sets of soil samples per plot with augers on 29 Apr. (preplanting) and 18 Oct. 1997 (postharvest) and 4 May (preplanting) and 18 Oct. 1998 (postharvest) at three depth increments (0–0.25, 0.25–0.50, and 0.50–0.75 m). Samples were also taken at the 0- to 0.2-m depth for NO^-₃–N analysis in conjunction with denitrification measurements. All soil samples for nitrate analysis were stored at 4°C for 1 to 3 wk. The samples were then thoroughly mixed and moist subsamples of 10 g were shaken with 100 mL of 1 M KCl for 60 min. The soil suspension was filtered through Whatman (Maidstone, UK) #5 filter papers. The filtrates were frozen before NO^-₃–N analysis with a colorimetric autoanalyzer (Lachat [Milwaukee, WI] Quickchem).

Corn grain yield was determined by hand-harvesting individual ears from a subplot consisting of a 2.5-m stretch of the three middle rows of each plot. Grain yield was reported on a dry-weight basis. The field was moldboard-plowed to a depth of 0.20 m in the first week of November, incorporating all corn stover (leaves plus stalks) into the soil.

Analysis of variance (AVOVA) was performed separately on individual sampling dates. Unless otherwise stated, the differences between treatments were declared to be significant at the 0.05 probability level. All statistical analysis were conducted with the general linear model (GLM) procedure of the Statistical Analysis System (SAS Institute, 1996).

	RESULTS AND DISCUSSION

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

 
Climatic Data
Total seasonal (May–October) rainfall in 1996 was 8.6% greater than the 30-yr normal (Table 1). However, the month of August was exceptionally dry, at 43% of normal, whereas July and September were both very wet, with 41 and 55% above-normal precipitation, respectively. These wetter months increased the likelihood of NO^-₃–N leaching with the percolation water. The 1997 growing season was drier, with precipitation at 90% of normal, whereas in 1998 growing season precipitation was 29% above normal (Table 1). About 44% of the growing season precipitation in 1998 occurred in June. This overwhelmed the subirrigation system's ability to drain excess soil water, resulting in poor crop growth and yield. June 1998 was the wettest in 70 yr and the second wettest on record (National Oceanic and Atmospheric Administration, 1998). Air temperatures were 0.4 and 1.3°C higher than normal in 1996 and 1998, respectively, but were 0.4°C below normal in 1997 (Table 1); 1998 was the warmest year since 1879 (National Oceanic and Atmospheric Administration, 1998). Soil temperatures (0- to 0.2-m depth), measured in conjunction with soil sampling for denitrification, followed the same pattern as air temperature and were unaffected by either water table depth or N fertilization treatments (data not presented).

View larger version (50K): [in this window] [in a new window]   Fig. 2. Nitrate nitrogen concentrations in the soil profile under free drainage (FD) and subirrigation (SI) practices in (a) spring 1997, (b) fall 1997, (c) spring 1998, and (d) fall 1998. Vertical bars represent standard error of the mean (n = 9).

In the uppermost soil layer (0–0.2 m), where the effect of water table (controlled at approximately 0.6 m below the soil surface) was expected to be minimal, a substantial reduction of NO^-₃–N was achieved under the SI treatment (Fig. 3) . For example, although soil NO^-₃–N values in 1996 were low, most of the sampling dates showed significantly (P 0.08 to 0.1) greater soil NO^-₃–N under FD than SI (Fig. 3a). In 1997, while all sampling dates but the first (11 July sampling date) tended to show a greater soil NO^-₃–N concentration under FD than SI, the difference was statistically significant only on 3 September. In 1998, NO^-₃–N levels tended to be greater under FD than under SI, except on 23 July and 25 August, when NO^-₃–N measured was identical under the two treatments (Fig. 3c). Statistical significance of the differences was obscured by the high variability in the data. Overall, SI management reduced surface (0–0.0.2 m) soil NO^-₃–N concentrations by 42% in 1996, 16% in 1997, and 28% in 1998, compared with FD. The much greater NO^-₃–N levels in 1998 (Fig. 3c) than 1996 (Fig. 3a) and 1997 (Fig. 3b) cropping seasons may be due to the mineralization of manure applied by the farmer in the spring of 1998. Liang et al. (1995) suggested that a major portion of the manure applied in the spring was mineralized during the subsequent summer.

View larger version (40K): [in this window] [in a new window]   Fig. 3. Soil surface (0–0.2 m) NO-3–N concentration under free drainage (FD) and subirrigation (SI) practices in (a) 1996, (b) 1997, and (c) 1998. Vertical bars represent standard error of the mean (n = 3).

Effects of Nitrogen Rate on Nitrate Nitrogen in the Soil Profile
The NO^-₃–N concentrations in the soil profile under 120 kg N ha^-1 (N₁₂₀) and 200 kg N ha^-1 (N₂₀₀) treatments did not differ significantly (Fig. 4) , except in spring 1998 at the surface (0–0.25 m) and intermediate (0.25–0.5 m) soil depths when NO^-₃–N levels under N₂₀₀ exceeded that under N₁₂₀ (Fig. 4c). However, the clear trend for greater soil NO^-₃–N concentrations under N₂₀₀ indicates that, in the long term, even moderately high rates of N application may lead to accumulation of NO^-₃–N in the soil and, consequently, NO^-₃ leaching into ground water.

View larger version (53K): [in this window] [in a new window]   Fig. 4. Nitrate nitrogen concentrations in the soil profile under 120 kg N ha-1 (N120) and 200 kg N ha-1 (N200) (a) spring 1997, (b) fall 1997, (c) spring 1998, and (d) fall 1998. Vertical bars represent standard error of the mean (n = 9).

View larger version (42K): [in this window] [in a new window]   Fig. 5. Soil surface (0–0.2 m) NO-3–N concentrations under 120 kg N ha-1 (N120) and 200 kg N ha-1 (N200) rates in (a) 1996, (b) 1997, and (c) 1998 growing seasons. Vertical bars represent standard error of the mean (n = 3).

View larger version (56K): [in this window] [in a new window]   Fig. 6. Water-filled pore space (WFPS) as influenced by water table management treatments in (a) 1996, (b) 1997, and (c) 1998. Vertical bars represent standard error of the mean (n = 3).

The decrease in soil NO^-₃–N concentration associated with SI was due to enhanced denitrification. This has created a concern that benefits from reducing NO^-₃–N by WTM techniques may be offset by an increase in atmospheric N₂O pollutant, resulting in a partial trade-off between the two environmental concerns. However, Kliewer and Gilliam (1995) concluded that water table depth had no effect on the percentage of N₂O emitted to the atmosphere via denitrification. They found that N₂O emission accounted for only about 2% of the total amount of N denitrified. Weier et al. (1993) noted that an increase in water-filled pore space led to a strong decrease in N₂O to N₂ ratio. Results from our experimental plots showed that FD produced more N₂O than SI (Elmi et al., 2001). These findings appear to suggest that the ecological effect of N₂O produced during the denitrification process may not be as serious as was previously thought. To confirm this conclusively under natural conditions, field trials are needed to quantify the proportion of N₂O to N₂ ratio evolution.

Although it has been widely reported (MacKenzie et al., 1997; Henault et al., 1998; Ellis et al., 1998) that the application rate of nitrogenous fertilizers has a significant influence on denitrification losses, we were unable to confirm this conclusion as differences between N₁₂₀ and N₂₀₀ treatments were seldom significant over all growing seasons (Tables 2, 3, and 4). One plausible explanation may be that soil mineral N content in both treatments was too high and denitrification was not limited in either one.

Effects of Water Table Depth and Nitrogen Fertilization Rate on Corn Yield
Acceptance of a new technology by farmers largely depends on its effect on crop yield. Corn yields were 7% higher under SI than FD in 1996 and 3% higher in 1997 (Table 5). These differences were nonsignificant. Skaggs et al. (1999) suggested that raising the water table generally increased evapotranspiration and, hence, yield. Tan et al. (1996) made similar observations and concluded that lower corn grain yields on a sandy loam soil with a water table depth of 0.8 m compared with 0.6 m were due to reduced stomatal conductance and transpiration rates caused by water stress. Doty (1980) found that the best water table depth for corn in sands or sandy loams was 0.76 to 0.89 m.

View this table: [in this window] [in a new window]   Table 5. Effect of water table management and nitrogen fertilization rate on corn yield.

Subirrigation is expected to be more beneficial than conventional drainage during drier crop seasons as it supplements rainfall to meet crop evapotranspiration demand. Cooper et al. (1999) recorded a significant yield increase from a SI treatment in 1991, a very dry year, whereas the more favorable growing conditions of 1992 resulted in conventional drainage yields not differing from those obtained under SI. These results suggest that in favorable growing years there is no significant yield advantage for SI systems.

Corn yield was not affected by N rate in 1998. This is an indication that 120 kg N ha^-1 was sufficient to maximize crop yield with WTM and favorable climatic conditions. The higher rates of N application (200 kg N ha^-1) produced significantly higher yields than the lower (120 kg N ha^-1) rate in 1996 and 1997, but not without potential environmental cost, as NO^-₃–N in the soil–water system was increased.

	SUMMARY AND CONCLUSIONS

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

 
Integrating water table management and N input strategies can minimize the risk of NO^-₃ contamination of water resources without compromising crop yields. Similar yields were obtained with SI and FD in 1996 and 1997. Yield reduction (25%) under SI in 1998 was attributed to the insufficient capacity of the controlled drainage–subirrigation system used to drain unusually abundant rainfall. This, however, should not adversely affect farmer's acceptance of SI, as this situation could have been averted with more rigorous management such as automating the system. Averaged across all soil depths, reduction in total soil NO^-₃–N under SI was 37% in the spring of 1997, 2% in the spring of 1998, 45% in the fall of 1997, and 19% in the fall of 1998. The adoption of WTM practices may provide an economical means to offer water quality benefits by enhancing NO^-₃ removal from the soil–water system through denitrification. Concentrations of soil NO^-₃ were greater in plots receiving a high rate of N fertilizer. Denitrification was higher in SI plots than FD plots, but it was seldom significantly influenced by N application rate.

Our results suggest that denitrification can be an important mechanism to remove NO^-₃ from the soil–water system and, therefore, control migration and entry of NO^-₃ into surface and ground water resources. It should, however, be pointed out that the greater denitrification rate under SI may lead to an increase in N₂O production. Whether NO^-₃ removal by denitrification is actually beneficial to the environment without a major tradeoff depends on which denitrification gases are produced. If a major portion of the gases produced is N₂O, adoption of WTM techniques could increase atmospheric N₂O loading. Since N₂O is detrimental to the environment, further research under field conditions is required to define the effects of water table depth on the ratio of N₂O to N₂ evolved through denitrification.

	ACKNOWLEDGMENTS

 
The authors wish to thank Mr. Guy Vincent, the owner of the experimental site, for his collaboration, Dr. Georges Dodds (post-doctoral fellow, Dep. of Agricultural and Biosystems Engineering, McGill University) for his critical reading of the manuscript, and three anonymous reviewers whose comments helped improve the focus of the paper. This research was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) grant.

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

 

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