188 A.A. Elmi et al. Agriculture, Ecosystems and Environment 79 2000 187–197
province of Quebec Madramootoo et al. 1992 docu- mented NO
3 −
concentrations as high as 40 mg l
− 1
in 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
− 1
are 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 NO
3 −
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 NO
3 −
kg
− 1
in plots which had never been amended with ma- nure or fertilizer, while plots fertilized with 260 kg
N ha
− 1
contained 8.7 mg NO
3 −
kg
− 1
. A significantly higher NO
3 −
concentration 25 mg NO
3 −
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 NO
3 −
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 NO
3 −
concentrations in the unsaturated zone to be higher than in the saturated zone. Gilliam and Skaggs 1986
predicted a 32 decrease in NO
3 −
leaching losses due to controlled drainage. While reduction for the poten-
tial NO
3 −
contamination of surface and ground waters is a positive aspect of denitrification Gilliam, 1994,
emission of N
2
O 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 N
2
O, knowledge of the proportion of denitrifi- cation gases entering the atmosphere as N
2
O relative to N
2
is paramount. In laboratory experiments, Weier et al. 1993 and Maag and Vinther 1996 indicated
that under wet soil conditions N
2
rather than N
2
O 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 NO
3 −
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
120
. A factorial arrangement of treatments was used in a randomized complete block
design. There were three blocks, 120 m wide and 75 m long, and eight plots 15 m wide×75 m length
in each block. Blocks were arranged from east to west, and separated from each other by a 30 m wide
strip of undrained land Fig. 1. To minimize seepage and chemical flow between plots, 1.5 m deep plastic
A.A. Elmi et al. Agriculture, Ecosystems and Environment 79 2000 187–197 189
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
N
200
: first application 23 May
23 23 May
23 N
200
: second application
a
20 June 177
18 June 177
N
120
: first application 23 May
23 23 May
23 N
120
: second application 20 June
97 18 June
97 Herbicide applications
23 May
b
25 June
c a
Ammonium nitrate was applied to reach the treatment N fertilizer level in every second application.
b
Atrazine 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
.
c
Bromoxynil 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 growing seasons. Soil temperature at a depth of 0–0.15 m was measured using a water-resistant
probe Hanna instrument, HI9024HI9025.
2.2. Sampling procedure and analysis SI was initiated in mid July for both years, after all
other field operations were completed. In 1996, soil
190 A.A. Elmi et al. Agriculture, Ecosystems and Environment 79 2000 187–197
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 C
2
H
2
to block further transformation of N
2
O to N
2
, allowing measurement of total denitri- fication as accumulated N
2
O and also inhibit nitrifi- cation process Yoshinari et al., 1977. To represent
field conditions, samples were incubated outdoors overnight.
The concentration of N
2
O 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
63
Ni electron capture detector ECD to measure the concentration
of N
2
O. Values for N
2
O 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 N
2
O in N
2
were also transferred to vacuutainers at that time and they were used for calibration of the
analysis of N
2
O 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 NO
3 −
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. NO
3 −
and NH
4 +
were determined colorimetri- cally using an autoanalyzer Quickchem, Milwaukee,
WI and then converted into kg ha
− 1
using bulk den- sities from respective soil samples.
Significance of main treatment effects on NO
3 −
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
