Influence of water table and nitrogen management on residual
soil NO
3
−
and denitrification rate under corn production
in sandy loam soil in Quebec
Abdirashid A. Elmi
∗, C. Madramootoo, C. Hamel
Department of Natural Resource Science, Macdonald Campus of McGill University, 21 111 Lakeshore Road, Ste-Anne-de-Bellevue, Que., Canada, H9X 3V9
Received 9 February 1999; received in revised form 23 September 1999; accepted 3 December 1999
Abstract
Nitrate-N (NO3−) effluents from agricultural ecosystems contributing to the degradation of water quality has become a
serious environmental problem. A field experiment was conducted in 1996 and 1997 at St. Emmanuel, Que., Canada, to investigate the combined effects of water table management (WTM) and N fertilization on soil NO3−level and denitrification
rates in the top soil layer (0–0.15 m). The field was planted to corn (Zea mays L.) in both years. Treatments consisted of a factorial combination of two water table treatments, free drainage (FD) at about 1.0 m and subirrigation (SI) at 0.6 m below the soil surface, and two N fertilizer rates, 200 kg ha−1(N
200) and 120 kg ha−1(N120). SI reduced NO3−concentration in the
top soil layer by 42 and 16% in 1996 and 1997, respectively. Nitrate levels in soil were 50% lower in N120plots in 1996, and
20% in 1997 compared to the N200plots. Denitrification was higher in SI compared to FD, but not influenced by N rate. As
a consequence, WTM practices have implications for both water quality and greenhouse gas emissions. Climatic conditions played a large role in regulating N dynamics in the soil. Due to drier and cooler conditions in 1997, denitrification rates were lower than in 1996, leaving higher residual NO3−in the soil profile following corn harvest. © 2000 Elsevier Science B.V. All
rights reserved.
Keywords: Denitrification rate; Emissions; Nitrate; Subirrigation; Water quality
1. Introduction
Nitrogen (N) is a key element in plant nutrition. High yielding crops, such as corn (Zea mays L.), re-quire large amounts of N fertilizer to ensure optimum yield. Corn has become a major crop in the province of Quebec because of its high potential productivity.
∗Corresponding author. Tel.:+1-514-398-7759; fax:+1-514-398-7990.
E-mail address: [email protected] (A.A. Elmi)
Liang et al. (1992) reported a maximum grain corn yield of 15.2 Mg ha−1 resulting from the best com-binations of hybrid, population density, fertilizer rate and irrigation. In an attempt to reach such an optimal yield, high rates of N fertilizer are often applied. Con-sequently, significant quantities of nitrate (NO3−) may be lost via leaching and eventually reach groundwater (Prunty and Montgomery, 1991). In many agricultural areas in the US, NO3− levels have already exceeded USEPA safety limit for drinking water, 10 mg l−1 (Hubbard and Sheridan, 1989). Similarly, in the
province of Quebec Madramootoo et al. (1992) docu-mented NO3−concentrations as high as 40 mg l−1in subsurface drain flow from a sandy loam field cropped to potato (Solanum tuberosum L.), a value far exceed-ing the present Canadian health standard of 10 mg l−1. Nitrate levels higher than 10 mg l−1are linked to cases of methemoglobinemia (also known as blue baby syndrome) which can ultimately result in the death of infants of up to 6 months (Gelberg et al., 1999).
The amount of leachable NO3− in the soil pro-file generally increases with fertilizer application rate (Angle et al., 1993; Errebhi et al., 1998). For exam-ple, Angle et al. (1993) measured 2.5 mg NO3−kg−1 in plots which had never been amended with ma-nure or fertilizer, while plots fertilized with 260 kg N ha−1contained 8.7 mg NO
3−kg−1. A significantly higher NO3− concentration (25 mg NO3−kg−1) was observed when corn was fertilized with excessive N (Angle et al., 1993). Thus, a major challenge, now, for agricultural scientists is to develop management strategies which will minimize the adverse impacts of N fertilizers on the environment and water resources, without concomitant reductions in crop yield.
Water table management (WTM) systems, includ-ing controlled drainage and subirrigation (SI), have been identified as beneficial practices for reducing NO3− content in groundwater by enhancing denitri-fication in the water saturated zone (Gilliam and Sk-aggs, 1986; Wright et al., 1992). Kalita and Kanwar (1993) and Madramootoo et al. (1993) found NO3− concentrations in the unsaturated zone to be higher than in the saturated zone. Gilliam and Skaggs (1986) predicted a 32% decrease in NO3−leaching losses due to controlled drainage. While reduction for the poten-tial NO3−contamination of surface and ground waters is a positive aspect of denitrification (Gilliam, 1994), emission of N2O is a serious environmental concern. It contributes to the greenhouse effect and participates in the depletion of ozone (Mooney et al., 1987). In order to properly assess ecological impacts associated with N2O, knowledge of the proportion of denitrifi-cation gases entering the atmosphere as N2O relative to N2is paramount. In laboratory experiments, Weier et al. (1993) and Maag and Vinther (1996) indicated that under wet soil conditions N2rather than N2O is the dominant end-product of denitrification.
The integration of WTM into a N fertilization strat-egy could further reduce environmental degradation in
crop production systems. Knowledge of interactions between WTM and N fertilizer is essential for the de-velopment of best management practices. The objec-tives of this study were to investigate the combined impacts of water table depths and N fertilization rate on (1) the quantity of potentially leachable nitrates in the upper soil layer, and (2) the relationship between denitrification rate and the reduction of NO3− concen-trations in the soil profile of a corn field.
2. Materials and methods
2.1. Field description and experimental design
A field experiment was conducted during the 1996 and 1997 growing seasons near Coteau du Lac, Que. Most of the soil above the bedrock is of sedimen-tary origin. The soil is a Soulanges fine sandy loam (fine silty, mixed, nonacid, grigid Humaquept; FAO, Glesol) overlying a sandy clay loam in the mid layer (0.25–0.3 m) and finally a clay parent material (0.5–1 m). Surface topography was generally flat with an average slope of less than 0.5% (Kaluli, 1996).
The field was planted (17 May for 1996 and 23 May for 1997) with corn, Hybrid Pioneer 3905, at a plant-ing density of 80 000 plants ha−1 (0.75 m between rows and 0.15 m within rows). Fertilizer (diammo-nium phosphate, 18-46-0) was broadcast at the time of seeding at a rate of 150 kg ha−1. Potassium (Muriate of potash, 0-0-60) was also applied at 90 kg ha−1. To reach N fertilizer treatment level, ammonium nitrate (34-0-0) was surface applied after planting. Weeds were controlled with atrazine, dicamba, bromoxynil, and metolachlor. Details of N and herbicide applica-tions are summarized in Table 1.
Treatments consisted of a combination of two wa-ter management treatments, free drainage (FD) at about 1.0 m and SI at 0.6 m below the soil surface, and two N fertilizer rates, 200 kg N ha−1 (N
200) and 120 kg N ha−1 (N
Table 1
Timing, rate, and chemical form of N applications and weed managements strategy
Operation 1996 1997
Date Amount (kg ha−1) Date Amount (kg ha−1)
N200: first application 23 May 23 23 May 23
N200: second applicationa 20 June 177 18 June 177
N120: first application 23 May 23 23 May 23
N120: second application 20 June 97 18 June 97
Herbicide applications 23 May b 25 June c
aAmmonium nitrate was applied to reach the treatment N fertilizer level in every second application.
bAtrazine at 1.5 kg active ingredient (ai) ha−1, metolachlor at 1.92 kg ai ha−1, and Dicamba at 0.31 kg ai ha−1. cBromoxynil at 0.31 kg ai ha−1.
curtains were installed between plots. Each plot with water table control at 0.6 m had two buffer plots on either side also with water table control at 0.6 m (Fig. 1). The purpose of the buffer plots was to help main-tain the water table constant. In the middle of each plot, 75 mm diameter subsurface drain pipes were installed, at a depth of 1.0 m, with a slope of 0.3%. A building was located between Blocks A and B, and between Blocks B and C. All drains entered one of the two buildings. Tipping buckets were located at the outlet of each subsurface drain to monitor drain dis-charge. Piezometers were installed in duplicate in the
Fig. 1. Schematic representation of the field and treatment arrangements.
middle of each treatment and buffer plots, to a depth of 1.5 m. The piezometers were capped to prevent rainfall from entering. A graduated rod with a water sensor was used to monitor the water table levels dur-ing both growdur-ing seasons. Soil temperature at a depth of 0–0.15 m was measured using a water-resistant probe (Hanna instrument, HI9024/HI9025).
2.2. Sampling procedure and analysis
sampling began on 15 July in conjunction with SI. In 1997, however, the soil sampling procedure was slightly modified and started immediately after plant-ing. Denitrification rates were measured bi-weekly during the two growing seasons. On each sampling date, aluminum cylinders (50 mm id×150 mm long) were used to collect undisturbed soil cores in dupli-cate from randomly selected locations in the center between the two middle rows of each plot. The cylin-ders used were perforated along the sides (horizontal and vertical) at 50 mm intervals to enhance acetylene gas diffusion. Samples were placed in 2 l plastic jars fitted with rubber stoppers for gas sampling with 5% of acetylene (C2H2) to block further transformation of N2O to N2, allowing measurement of total denitri-fication as accumulated N2O and also inhibit nitrifi-cation process (Yoshinari et al., 1977). To represent field conditions, samples were incubated outdoors overnight.
The concentration of N2O produced through deni-trification was determined following the procedure of Liang and Mackenzie (1997). Briefly, before gas sam-pling, the air in the plastic jars was thoroughly mixed by inserting a syringe and pumping several times. About 4 ml of gas were removed from the jars and injected into a gas chromatograph [GC, (5870 series II Hewlett Packard)] equipped with a 63Ni electron capture detector (ECD) to measure the concentration of N2O. Values for N2O emissions by denitrification were calculated on a per area basis (g N ha−1). In 1997, there was a problem with the GC and the gas samples could not be analyzed immediately after the incubation period. Therefore, 7 ml of head space gas were with-drawn from incubating jars after the 24 h of incubation
Table 2
Monthly precipitation (mm) in the growing seasons of 1996 and 1997 as compared to long term (1961–1990) averages measured at Côteau-du-Lac weather station
Month 1961–1991 1996 1997
Rain (mm) Rain (mm) Deviation (%) Rain (mm) Deviation (%)
May 76.3 103.8 36 64.8 −15.1
June 90.1 81.8 −9.2 98 8.8
July 94.6 133.9 41.5 97 2.5
August 93.9 40.8 −56.6 86.3 −8.1
September 90.6 140.6 55.2 81.4 −10.2
October 76.7 66 −13.9 41.4 −46
Total 522.2 566.9 8.6 468.9 −10.2
period and stored in vacuutainers (Vacuutainers Brand, Beckon Dickson company, Rutherford, NJ). Standards of N2O in N2 were also transferred to vacuutainers at that time and they were used for calibration of the analysis of N2O at each sampling date. After denitrifi-cation measurement, soil cores were dried at 65◦C for
3 days and the soil then ground. Soil moisture content to depth of 0.15 m was determined gravimetrically.
To monitor NO3− levels in the soil, triplicate soil samples were collected up to a 0.20 m depth on the same sampling dates as for denitrification. The soil samples were thoroughly mixed, then 10 g moist sub-sample was taken and shaken with 100 ml of 1M KCl for 60 min. The extracted solution was filtered through Whatman #5 filter papers, and then frozen until anal-ysis. NO3− and NH4+were determined colorimetri-cally using an autoanalyzer (Quickchem, Milwaukee, WI) and then converted into kg ha−1using bulk den-sities from respective soil samples.
Significance of main treatment effects on NO3− and denitrification rates in the soil and their interac-tions were investigated using General Linear Models (GLM) procedure of the Statistical Analysis System (SAS Institute, Version 6.12).
3. Results and discussion
3.1. Climatic data
8.6% greater than the long term average. However, the month of August was exceptionally dry, with 56% less precipitation than average, whereas July and Septem-ber were both very wet, with rainfalls of 41 and 55% above average, respectively (Table 2) increasing the risk of NO3−leaching.
In comparison, 1997 rainfall differed from that of 1996 in two main respects. First, deviation of monthly rainfall from the long term (30 years) average was gen-erally smaller, and therefore rainfall distribution was relatively more uniform. Secondly, in spite of rainfall in June and July being slightly above normal (Table 2), the total 1997 growing season rainfall was 10.2% below average. Variations in the amount and distribu-tion of rainfall have a strong influence on water table fluctuations, and consequently on soil moisture levels and N dynamics in the soil profile.
Soil temperature was on average higher in 1996 than 1997 (Fig. 2). During the experimental period, average soil temperature in 1996 was 20.5◦C, whereas it was
17.9◦C in 1997. Soil temperature directly influences
microbial activity in the soil, which is responsible for denitrification (Bergstrom and Beauchamp, 1993; Granli and Bøckman, 1994; MacKenzie et al., 1997).
3.2. Water table depth (WTD)
Water table level fluctuated throughout both grow-ing seasons, respondgrow-ing primarily to rainfall events.
Fig. 2. Mean soil temperature (◦
C) at 0–0.20 m depth at the time of soil sampling.
For example, in 1996, the shallowest WTD was ob-served on 15 July. In that month, high amounts of rainfall (Table 2) saturated the soil resulting in rise of water table. In contrast, WTD dropped significantly on 22 August and 3 September compared to previous sampling dates (Fig. 3). This decrease had two main reasons. First, in spite of the fact that 1996 was a wet year, August was extremely dry with rainfall 56% be-low the long term monthly average. Second, although September was very wet, 55% above average, only 1.4 mm of rainfall occurred between the 22 August and 3 September sampling dates. As a result, WTD in SI plots were as deep as in FD plots (Fig. 3). Overall, as shown in Fig. 3, average WTDs in SI plots were deeper in the drier season of 1997 (0.8 m) than 1996 (0.7 m).
3.3. Water table and soil NO3−
Freely draining plots had higher soil NO3− lev-els than subirrigated plots. This trend was consistent over the two seasons of this study except for 11 July 1997 (Fig. 4). Due to the large experimental error (Fig. 4a), 22 August was the only sampling date when the effect of the water table was not statistically sig-nificant (p<0.05) during the 1996 growing season.
Fig. 3. Mean water table depth (m) fluctuations in (a) 1996 and (b) 1997 as influenced by free drainage (FD ) and subirrigation (SI) treatments.
(Fig. 4b). Overall NO3−concentrations were reduced by 42 and 16% in 1996 and 1997, respectively, in the SI treatment compared to the FD treatment. This is probably due to the shallower water table enhancing denitrification in SI plots. This is an indication that maintaining a shallow water table depth could be a useful tool in reducing NO3−pollution to the ground-water. Similarly, Fogiel and Belcher (1991) found that controlled drainage/subirrigation reduced NO3− loading through drainage by 25–59% over a 2-year period compared with conventional drainage. Gilliam and Skaggs (1986) predicted a 32% decrease in NO3−
Fig. 4. Soil NO3− concentration differences between free drainage and subirrigated plots during (a) 1996 and (b) 1997 growing seasons. Vertical bars represent standard errors.
be as serious as was previously thought. To confirm this under natural conditions, field trials are needed to quantify the proportion of N2O:N2ratio evolution.
3.4. Nitrogen rate effect on soil NO3− level and
denitrification rate
The effect of N fertilization rate on the level of NO3−in the soil was evident (Fig. 5). Because of the higher rate of N fertilizer, soil NO3− concentrations were higher in the N200 treatment than N120 during both growing seasons; the 11 and 26 June sampling dates in 1997 being the only exceptions (Fig. 5).
Fig. 5. Soil NO3− concentration differences between 120 and 200 kg ha−1 of N fertilizer rate in (a) 1996 and (b) 1997 growing seasons. Vertical bars are standard errors.
One plausible explanation could be that mineral N content was not the main limiting factor of soil N2O emission (Henault et al., 1998).
One important feature that must be emphasized when comparing N200and N120treatments in 1997 is the higher NO3−in the latter treatment on 11 and 26 June sampling dates. These high NO3−concentrations were unexpected, and may be erroneous values. It may be due to NO3− influx from recharge areas or seep-age through confining plots. The higher cumulative soil NO3−in 1997 than 1996 (34.8 and 9.58 kg ha−1, respectively), may be due to the relatively dry
con-ditions in 1997 during which denitrification was not enhanced. This speculative relationship is consistent with the lower denitrification rate in 1997 than 1996 (Tables 3 and 4).
3.5. Denitrification and water table management
Table 3
Denitrification rates (g N ha−1 per day) as influenced by water table depth and N fertilization rate and analysis of variance during 1996 growing season
Treatments Sampling dates
15 July 06 August 22 August 03 September 20 September 05 October
FDa 184.7 31.36 27.3 5.5 14.32 4.64
SIb 225 112.95 21.8 30.51 25.78 15.3
N120c 124 62.5 15.74 20.75 26.95 9.47
N200d 285 82 33.38 15.27 13.3 10.47
Mean 204 72.25 24.56 18 20.2 10
Summary of analysis of variance
WTMe nsf ∗∗
ns ∗
ns ∗
N-rate ∗∗
ns ns ∗∗
ns ns
WTM×N ns ns ns ns ns ns
aFree drainage. bControlled/subirrigation. c120 kg N ha−1. d200 kg N ha−1.
eWater table management. fns=not significant at 5%. ∗,∗∗
=statistically significant at 5 and 1% probability level, respectively.
dry month), denitrification rates were always higher in SI than FD treatments (Tables 3 and 4). Higher deni-trification losses were associated with higher moisture content in SI treatment plots as compared to FD
treat-Table 4
Denitrification rates (g N ha−1 per day) as influenced by water table depth and N fertilization rate and analysis of variance during 1997 growing season
Treatments Sampling days
28 May 11 June 26 June 11 July 23 July 06 August 18 August 03 September 17 September 03 October
Fda nig ni ni 36.1 6.7 6.77 1.1 4.84 7.84 4.93
SIb ni ni ni 38.1 14.15 7.2 8.78 12 20 11.15
N120c 36.76 150 143 34.83 11.45 9.1 7 10.62 17.8 9.81
N200d 25.36 64 140 39.45 9.36 4.87 2.87 6.2 10.64 6.27
Mean 31.54 107.2 141.5 37.14 10.41 7 4.94 8.42 14.2 8.02
Summary of analysis of variance
WTMe ni ni ni nsh ∗
ns ∗∗ ∗∗ ∗∗ ∗
N-rate ns ∗
ns ns ns ns ns ns ns ns
WTM×N naf na na ns ns ns ns ns ns ns
aFree drainage. bControlled/subirrigation. c120 kg N ha−1. d200 kg N ha−1.
eWater table management. fna=not applicable. gni=not initiated. hns=not significant. ∗,∗∗
=significant at 5 and 1% probability level, respectively.
Tiedje (1990), Beauchamp et al. (1996) and Fan et al. (1997) concluded that these peaks were due to warm-ing of saturated soils, and enhanced microbial activity. Denitrification rates during both growing seasons appeared to be regulated largely by climatic factors such as soil temperature, and amount and distribution of rainfall. In relatively dry periods, for example in the 1997 growing season, water table dropped sharply and denitrification N loss was not promoted resulting in NO3− accumulation in the soil profile. In rainy periods, on the other hand, the water table rose and denitrification losses were promoted. In 1996, for example, the highest average denitrification flux was measured on the 11 July sampling date (Table 3). Heavy rainfall occurring at the beginning of this month saturated the soil and caused the rise of the WTD to about 0.45 m below the soil surface (Fig. 3). In 1997, denitrification peaked on the 26 June sampling date (Table 4). Four days before this sampling (22 June), the highest amount of daily rainfall during that season was measured (daily rainfall data not shown) creating an anaerobic environment favourable for denitrifica-tion process.
3.6. Seasonal variability and denitrification
Consistently higher denitrification losses were measured in 1996 in all treatments, compared to 1997. Several environmental and field management practices may be responsible for the large differences in seasonal denitrification. This seasonal differences may be attributable to soil temperatures which were generally higher in 1996 than 1997 (Fig. 2). Tem-perature is considered as one of the most influential factors on the magnitude of denitrification (Granli and Bøckmam, 1994). Bergstrom and Beauchamp (1993) and Liang and Mackenzie (1997) asserted that lower temperatures can result in a reduction in the denitrification rate. Rainfall events in May and July 1996 (36 and 41.5% above average, respectively), fol-lowing the first and second N fertilizer applications, respectively, might have increased moisture content beyond the saturation limit, and hence enhanced denitrification. Additionally, considerable amount of NO3− might have been leached from the surface layer to deeper depths, leaving less NO3− in the soil surface.
As shown in Table 1, herbicide application in 1997 was somewhat later. As a result, tremendous weed growth was observed in all plots. It is therefore reason-able to assume that significant amount of NO3−which would have been lost as denitrification might have been taken up by weeds. This, however, contradicts the larger amount of NO3− remaining in the soil in 1997 than 1996 (34.8 and 9.58 kg ha−1, respectively). Therefore, another plausible explanation could be the enrichment of NO3−through mineralization from the previous hot and wet growing season. Nitrogen min-eralization is an important source of NO3− and can supply from 30 to 100% of N nutritional needs and increases with precipitation (Douglas et al., 1998).
Since denitrification measurements in 1996 started mid July, it is likely that denitrification peak was not captured. Therefore, one should be cautious when comparing the two seasons. Similarly, since measure-ments of denitrification rate were carried out only during the growing season, cumulative denitrification losses reported in Tables 3 and 4 should not be as-sumed as being annual losses. If annual losses were to be estimated, additional sampling must be contin-uing during spring thaw when denitrification may be vigorous (Christensen and Tiedje, 1990; Ellis et al., 1998; Henault et al., 1998).
4. Conclusions
Acknowledgements
Authors would like to thank Mr. Guy Vincet, the owner of the experimental site, Mr. Peter Kirby, for field operations, Dr. Georges Dodds for his critical reading, and two anonymous reviewers whose com-ments helped improve the focus of the paper. This research was supported by Natural Sciences and En-gineering Research Council of Canada (NSERC).
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