soil nitrate distribution under grain and silage corn using three tillage practices on a loamy sand...

Soil nitrate distribution under grain and silage corn using three

tillage practices on a loamy sand in southwestern Quebec

B. Mehdi, C.A. Madramootoo*

Department of Agricultural and Biosystems Engineering, Macdonald Campus of McGill University,

21 111 Lakeshore Rd., Ste. Anne de Bellevue, Que., H9X 2C9 Canada

Received 5 May 1998; received in revised form 21 December 1998; accepted 24 March 1999

Abstract

Nitrate leaching is a signi®cant non-point pollution source in intensive agriculture. Appropriate agronomic management

practices which reduce nitrate leaching need to be investigated. A two-year ®eld study was conducted in southwestern Quebec

on a 2.4 ha site of Typic Endoaquent planted to corn (Zea mays L.). Three types of tillage practices [conventional tillage (CT),

reduced tillage (RT), and no-till (NT)] were combined with two residue levels [with (�R) and without (ÿR)] in a randomized

complete block design. The effects of residue, tillage, depth and wheel tracks were examined on soil NOÿ3 ÿN distribution.

Soil samples were collected at nine sampling times between November 1995 to November 1997, from 0±15, 15±25, and 25±

50 cm depth, except in July and August of 1996, where samples were collected from 0±10, 10±20 and 20±30 cm depth. In

spring and fall, a tractor-borne hydraulic auger was used to sample the soil. During the growing season soil cylinders and

T-samplers were used. Soil samples taken a few weeks after the application of fertilizer had highly variable NOÿ3 ÿN

concentrations in the surface layers (2.5±311.5 kg NOÿ3 ÿN haÿ1), whereas samples obtained before fertilizer application, in

spring, ranged from 7.3 to 11.3 kg NOÿ3 ÿN haÿ1, and fall samples ranged from 4.5 to 13.1 kg NOÿ3 ÿN haÿ1: In spring and

fall, NOÿ3 ÿN was higher at 25±50 cm than at 0±15 cm, which may have been an indication of leaching during the non-

growing season. July and August samples had less NOÿ3 ÿN at lower depths compared to the spring and fall, despite the fact

that surface layers in these months had up to 70 times more NOÿ3 ÿN than in spring or fall. In May 1996 residues were found to

have a signi®cant effect on decreasing soil NOÿ3 ÿN at 0±15 cm depth in all tillage treatments. In July 1996 wheel tracks were

found to have higher NOÿ3 ÿN in CT�R (77.0 kg haÿ1) and NT�R (39.6 kg haÿ1), compared to RT�R (13.0 kg haÿ1). Two

weeks later signi®cantly lower NOÿ3 ÿN concentrations were measured in CT�R (52.7 kg haÿ1) and NT�R (7.4 kg haÿ1), but

RT�R had slightly higher NOÿ3 ÿN concentrations (20.9 kg haÿ1). When ammonium nitrate fertilizer was applied, no

difference in soil NOÿ3 ÿN concentrations between the treatments was observed. Implementing conservation tillage practices,

such as reduced tillage and no-till, and residue management practices were found to be an ef®cient way of reducing NOÿ3 ÿN

levels in the soil pro®le when urea fertilizer is applied. # 1999 Elsevier Science B.V. All rights reserved.

Keywords: Nitrate distribution; Residues; Grain corn; Silage corn; Conventional tillage; Reduced tillage; No-till; Soil pro®le

Soil & Tillage Research 51 (1999) 81±90

*Corresponding author. Tel.: +1-514-398-7778; fax: +1-514-398-8387.

E-mail address: [email protected] (C.A. Madramootoo)

0167-1987/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 0 3 4 - 3

1. Introduction

For the past 15 years, corn has been the second most

widely grown crop in Quebec and Ontario (Statistics

Canada, 1991, 1994; Bureau de la statistique du

Quebec, 1995). The nutrient requirements (especially

N) of corn are very demanding causing most growers

to apply excessive N fertilizer. These generous appli-

cations year after year, sometimes in excess of

180 kg N haÿ1 (MinisteÁre de l'Agriculture et de l'Ali-

mentation, 1992), are a potential threat to the envir-

onment. The excess N may leach into groundwater

supplies, or move into water bodies. Excessive nutri-

ents in water bodies encourages eutrophication, or

may lead to harmful chemical reactions, which could

pose potential human health risks. For example, con-

centrations greater than 10 mg NOÿ3 ÿNLÿ1 may lead

to methaemoglobinaemia in human infants (Health

and Welfare Canada, 1987).

Alternative farming practices are one way of redu-

cing the risk of agrochemical pollution. No-till, ridge

tillage, strip cropping, contour cropping and terracing

are a few examples of the different farming methods

implemented to reduce chemical contamination. The

decrease in soil disturbance and the maintenance of

residues in these practices provide for an environment

which encourages microbial activity and reduces soil

and water erosion.

Residues can immobilize N and thereby reduce

excess NOÿ3 leaching (Christensen, 1986). Corn stalks

have a particularly high C:N ratio, favouring immo-

bilization when decomposing (Blevins et al., 1984).

Substantial N immobilization occurs after initial

incorporation or surface placement of residues with

high C:N ratios such as wheat (Triticum sp.) and corn.

Some soil nitrate-N depression may initially occur due

to this N immobilization (White, 1984).

Although conservation tillage practices reduce sedi-

ment transport and chemical laden water runoff, there

is still some uncertainty as to whether these practices

encourage chemical transport via alternative path-

ways, such as pore transport or seepage (Evans et

al., 1994 and Granovsky et al., 1993).

Regular soil disturbance causes a decline in soil N

(McCarthy et al., 1995) due to the mineralization of

organic matter. Cultivating exposes organic matter

previously not accessible to microbial attack. There-

fore most N losses occur during the ®rst few years

after cultivation (Stevenson, 1965). Due to the

decrease in soil disturbance in no-till, the N miner-

alization rate is much slower. Lower mineralization,

higher leaching and higher denitri®cation (due to

higher surface water content) in no-till (NT) tend to

lower the available N, particularly in spring (Thomas

and Frye, 1984). However, in the presence of a crop

cover, NT plots were found to contain more biologi-

cally mineralizable N than in the absence of a cover

crop (Dalal, 1989). Without a crop cover, the potential

for nitrate leaching was greater in no-till (Bandel et al.,

1975), especially in the spring and early summer in

temperate climates before the crop canopy emerges

(Blevins et al., 1984). Tillage was found to in¯uence

both the amount and the distribution of soil N

(McCarthy et al., 1995).

Research on the effects of tillage practices on

NOÿ3 ÿN distribution has so far been inconclusive

with regards to soil NOÿ3 ÿN distribution. Results vary

depending on the type of crop grown, the amount of

fertilizer applied, the soil type, and on the time of the

year when soil samples were obtained. The objectives

of this study were (1) to examine the effects of

conventional tillage, reduced tillage, and no-till on

NOÿ3 ÿN distribution, and (2) to determine the effects

of residue and wheel compaction on NOÿ3 ÿN distri-

bution.

2. Materials and methods

2.1. Experimental design

The study was conducted on a 2.4 ha level area of

loamy sand or sandy loam (mean thickness of 28 cm)

overlying marine clay (mean thickness 18 cm) on the

Macdonald Campus Research Farm, located in south-

western Quebec (428250 N lat., 758560 W long.). The

soil was mostly a St. Amable loamy sand to shallow

loamy sand, with some Courval sandy loam (both

Humic Gleysols: Typic Endoaquent). In May 1991,

the site was cultivated, limed (6±8 Mg haÿ1) and

planted to corn (Zea mays L.). Prior to the corn, the

site was planted with alfalfa (Medicago sativa L.).

Each plot had a subsurface drain (mean depth 1.2 m),

and was separated by a 2 m grass buffer strip.

The experimental design consisted of a factorial

arrangement of three tillage types and two residue

82 B. Mehdi, C.A. Madramootoo / Soil & Tillage Research 51 (1999) 81±90

levels (grain corn and silage corn). The layout con-

sisted of three blocks, with six plots (each measuring

approximately 15 m�80 m) in each block, assigned in

a randomized complete block design. The treatments

were: conventional tillage with residue (CT�R) and

no residue (CTÿR); reduced tillage with residue

(RT�R) and no residue (RTÿR); and no-till with

residue (NT�R) and no residue (NTÿR). The non

residue plots simulated silage corn cropping practices

and the residue plots represented grain corn plots.

Corn (Funk 4120 hybrid) was seeded at 76 000

plants haÿ1, with 40 kg N haÿ1 applied as diammo-

nium phosphate banded 5 cm below and 5 cm beside

the seed. Three weeks after planting, 140 kg N haÿ1

was applied as urea in 1996, care was taken to

incorporate the urea 5±10 cm deep using a double

disk opener in all plots (including NT). In 1997,

140 kg N haÿ1 was applied as ammonium nitrate.

The no-till plots were not tilled (except for the urea

incorporation by disking); the reduced tillage plots

were disked to a depth of 10 cm in the spring (before

planting), and in the fall (after harvest); the conven-

tional tillage plots were moldboard plowed to a

depth of 20 cm after harvest and disked before plant-

ing. At harvest, the grain in the residue plots (grain

corn) was removed with a combine and the residues

(cobs, leaves and stalks) were chopped. The residues

were (i) left on the surface as in no-till, (ii) partially

incorporated by disking in reduced tillage, or (iii)

completely buried by the moldboard plow, as in con-

ventional tillage. No residue (silage corn) plots had the

whole plant (grain, cob, leaves and stalk) removed at

harvest.

2.2. Soil sampling

Soil samples were obtained in May, July, August

and October or November of each year (Table 1). A

tractor-borne hydraulic corer (Giddings Machine Co.,

Ft. Collins, Colorado) was used in May and October or

November of each year to penetrate the soil to 50 cm,

at 0±15, 15±25, and 25±50 cm depth increments. Five

soil samples were obtained in each plot, and compos-

ited according to depth. In July and August wheel

track rows covered 27% of the total area in each plot.

To accurately represent treatment average NOÿ3 ÿN

concentrations, two wheel tracked and two non

tracked rows were randomly chosen in each plot for

sampling in July and August, and weighted average

NOÿ3 ÿN concentrations were calculated. In July and

August 1996, aluminum cores were used to sample the

soil to 30 cm, at 10 cm increments, and the samples

were not composited according to depth. In July and

August 1997, T-samplers were used to collect soil

samples to 50 cm depth (0±15, 15±25, and 25±50 cm),

these were composited according to depth, which

corresponded to the fall and spring sampling depths.

Soil samples were stored in glass jars at 48C until

further analysis.

The soil samples were weighed and dried to con-

stant weight at 1058C, to determine gravimetric soil

moisture content. Bulk density was also measured in

May 1996, July 1996, and August 1996 from the soil

cores obtained.

The analysis of NOÿ3 ÿN from the soil extracts was

carried out using a modi®ed Keeney and Nelson

(1982) method, using a Quickchem AE Automated

Table 1

Soil sampling data for 1996 and 1997 obtained from study site in southwestern Quebec

Date sampled Days in

between

sampling

Cumulative days

from second N

fertilizer to sampling

Sampled with Total depth

sampled

November 14 ± ± Tractor auger 50 cm

May 14 ± ÿ18 Tractor auger 50 cm

July 3 49 30 Bulk cylinders 30 cm

August 8 36 66 Bulk cylinders 30 cm

October 29 82 148 Tractor auger 50 cm

May 22 ± ÿ21 Tractor auger 50 cm

July 8 47 27 T-sampler 50 cm

August 14 37 64 T-sampler 50 cm

November 6 84 148 Tractor auger 50 cm

B. Mehdi, C.A. Madramootoo / Soil & Tillage Research 51 (1999) 81±90 83

Analyzer (Lachat Instruments Milwaukee, WI), all

NOÿ3 ÿN values were corrected for their respective

bulk density at each depth.

2.3. Calculations and statistical analysis

Mean treatment NOÿ3 ÿN values in July and August,

in both years, were presented as weighted averages

(corrected for bulk density) from the two samples

taken in the wheel tracked, and the two samples taken

in the non-wheel tracked rows.

All NOÿ3 ÿN data was corrected for bulk density

and analysed using repeated measures ANOVA

(Dutilleul, 1998a, b). The data analysis was carried

out with the SAS (Statistical Analysis Systems

Institute) GLM procedure. Depth was used as one

spatial repeated measure factor for spring and

fall samples. Depth and wheel were used as two

repeated measure factors for July and August samples;

depth was considered a repeated measure within the

`̀ wheel track'' and `̀ no-wheel track'' repeated mea-

sures. Due to the high coef®cients of variation

observed with the raw NOÿ3 ÿN data (up to 247%),

all outliers (values exceeding three interquartile

ranges) were replaced by the mean value of the data

set excluding the outliers. The maximum number of

outliers replaced by the mean value for any given

sample date was three. In addition, a logarithmic

transformation was carried out on the nitrate-N data.

This transformation improved the normality of the

data. All analyses used plot means rather than sub-

sample values.

Main effects and interactions that proved to be

signi®cant (p�0.05) were graphed. Non signi®cant

effects or interactions were not graphed in the interest

of brevity. Details on ANOVA tables and polynomial

contrasts are given in Mehdi (1998).

3. Results and discussion

3.1. Meterological data

There was little variation in the amount of precipi-

tation received during the two growing seasons. There

was also little difference between temperature and the

growing degree days (Table 2).

3.2. NOÿ3ÿN levels

From 1995 to 1997, May and October or November

NOÿ3 ÿN levels in the 0±15 cm soil layer were much

lower than those observed during July and August.

May concentrations varied from 7.3 to 11.3 kg

NOÿ3 ÿN haÿ1; and fall concentrations ranged from

4.5 to 13.1 kg NOÿ3 ÿN haÿ1 whereas July and August

samples ranged from 2.5 to 311.5 kg NOÿ3 ÿN haÿ1:The samples obtained during the growing season

demonstrate high variability due to the application

of fertilizer a few weeks prior to sampling.

Nitrate values in both July 1996 and 1997 ¯uctuated

highly after fertilizer application, since the soil was

sampled 30 and 27 days after the second fertilizer

application in 1996 and 1997, respectively (Table 1),

Table 2

Meterological conditions between sampling dates from November 1995 to November 1997 (Monthly Meteorological Summaries 1995±1997,

Environment Canada; Atmospheric Environmental Service; Montreal International Airport)

Sampling days Number of

days between

sampling dates

Average mean

daily temperature

(8C)

Total precipitation

received (mm)

Total growing

degree days

(Base 58C)

1995±1996 November 14±May 14 182 ÿ1.80 554.0 134.7

May 14±July 3 50 18.39 99.0 617.0

July 3±August 8 36 21.19 111.5 558.5

August 8±October 29 82 14.74 198.6 782.0

1996±1997 October 29±May 22 205 0.1 634.0 178.4

May 22±July 8 47 18.1 160.5 654.1

July 8±August 14 37 20.33 122.5 568.8

August 14±November 6 84 12.6 253.7 657.4


the standard errors for the treatments were corre-

spondingly high in July (and August). The treatments

with the highest mean were found to have the highest

standard errors.

In July 1997, higher NOÿ3 ÿN concentrations were

observed than in July 1996 (Fig. 1), probably because

in 1996, 140 kg N haÿ1 was applied as urea (26-0-13),

2.5 weeks after planting. In 1997, the same amount of

N was applied as ammonium nitrate (26-0-13) three

weeks after planting. The higher soil NOÿ3 ÿN levels at

0±25 cm exhibited in July 1997 are probably due to

the fact that ammonium nitrate is converted to nitrate

more rapidly than urea (Recous et al., 1995). Accord-

ing to Recous et al. (1995), ammonium is preferen-

tially immobilized as compared to nitrate when

ammonium nitrate is applied.

In general, for samples taken in July and August,

lower NOÿ3 ÿN contents were exhibited deeper in

the soil pro®le (15±25 cm) than at the surface, prob-

ably because the fertilizer effect was diluted, and

because corn was utilizing N (Fig. 1). The signi®cant

decrease in NOÿ3 ÿN concentration observed in May

and October/November from the soil surface to 25 cm

may be due to cooler soil temperatures and moister

soils in spring and fall which encourage denitri®ca-

tion.

Fig. 1. Weighted NOÿ3ÿN concentrations (kg haÿ1) for each treatment in three soil layers, to 50 cm, with respective standard error bars.�Weighted NOÿ3ÿN concentrations calculated from two samples taken from wheel track rows (representing 27% of the plot) and two samples

from non wheel tracked rows (representing 73% of the plot). NT�no-till; RT�reduced tillage; CT�conventional tillage; �R�residue;

ÿR�no residue; Nov�November; Aug�August; Oct�October.


During the fall, winter, and spring when no crop was

present; May and October or November, NOÿ3 ÿN

levels were higher at 25±50 cm, than at the surface

(Fig. 1). The increase in NOÿ3 ÿN deeper in the soil in

spring and fall would mainly be due to leaching, as fall

conditions are not favourable for immobilization (or

mineralization) to occur due to colder temperatures,

with a concurrent decline in microbial activity.

Furthermore, when the crop is harvested in fall, any

subsequent accumulation in N would move through

the pro®le, reaching deeper depths, unhindered by

crop uptake.

A small NOÿ3 ÿN decrease was observed from

October 1996 to May 1997 in most treatments

(Fig. 1), possibly due to higher precipitation received

during the non-growing season of 1997 than in 1996;

634.0 mm versus 554.0 mm, respectively (Table 2). A

decline in NOÿ3 ÿN from fall to spring each year was

also observed by Jokela and Randall (1989) and Pesek

(1964). Liang et al. (1991) and Roth and Fox (1990)

reported that treatment differences become smaller

over-winter with increasing fall N concentrations; the

greater the amount of N present in the soil in fall, the

greater the losses by spring.

Nitrate leaching in fall has the potential to be

signi®cant because of (a) a lack of crop N utilization,

(b) low evapotranspiration levels, and (c) less micro-

bial activity. Therefore, the amount of NOÿ3 ÿN in

the soil remaining in fall, after the crop has been

harvested, is a good indicator of the potential amount

of leaching that may take place.

3.3. Wheel effect

Whenever samples were obtained from wheel

tracks and from non wheel tracks, (i.e., July 1996,

August 1996, July 1997, August 1997), the wheel

tracks showed signi®cantly higher NOÿ3 ÿN concen-

trations (Fig. 2). The different methods of sampling

during the growing season did not reduce the NOÿ3 ÿN

variability observed (Fig. 1). A signi®cant wheel-

�depth interaction was found (Fig. 3) which was

attributed to the method of fertilizer application.

The large NOÿ3 ÿN variation during both growing

seasons was attributed to the uneven distribution of

fertilizer, which was applied as 40 kg N haÿ1, 5 cm

below and 5 cm to the side of the seed (i.e., in wheel

tracked rows). This was applied at the beginning of

June, and three weeks later as urea in 1996, and as

ammonium nitrate in 1997 which was applied on

wheel tracked rows in 45 cm width bands (Peter Kirby,

personal communication). Thus, it was found that

wheel tracked rows had up to 20 times as much total

NOÿ3 ÿN as the non wheel tracks. Despite NOÿ3 ÿN

concentrations being so much higher on the wheel

tracked rows, levels declined at 20±30 cm and 25±

50 cm to levels similar to those of the non wheel tracks

(Fig. 3). These similarities between the wheel and non

wheel tracked rows at deeper depths suggest that

during the growing season, NOÿ3 ÿN leaching does

not take place, due to corn roots absorbing N deeper in

the soil pro®le.

3.4. Residue versus no residue plots

Non residue (i.e., silage) plots were referred to as

having no surface residue cover, however, they did

have minimal residue cover (Fig. 4), e.g., CT-R had

1.3% surface residue cover. Even though the whole

plants (stalks and cobs) were removed, harvesting

caused some of the corn leaves and stalks to fall on

the surface during combining. This should be taken

into consideration, when interpreting the results, in

that no treatment was completely free of residue. Corn

roots formed part of the belowground soil biomass,

and contributed to a source of residue in silage plots,

however, they were not included in any analysis.

Fig. 2. Wheel effect for mean NOÿ3ÿN concentrations (kg haÿ1),

averaged over the three tillage practices and two levels of corn

residue at 0±30 cm for 1996 and 0±50 for 1997 (significant at the

p�0.05 level). Means with the same letter are within a sampling

date are not significantly (p�0.05) different according to the LSD

test.


Residue cover was found to signi®cantly lower

NOÿ3 ÿN concentrations at 0±15 cm, than at the other

depths in May 1996 (data not shown). This ®nding

corroborates with our expectations of treatments with

higher amounts of residue cover (i.e., NT�R and

RT�R) to have greater amounts of immobilization.

At 25±50 cm, theÿR plots had less NOÿ3 ÿN than�R

plots, which may be an indication of �R plots con-

tributing to greater N movement deeper into the soil

pro®le through pore continuity ¯ow, or surface residue

typing up NOÿ3 ÿN in the upper soil layers.

Signi®cant wheel�tillage�residue interactions

were observed in July 1996 (Fig. 5) and August

1996 (Fig. 5) (higher ¯uctuating NOÿ3 ÿN levels in

1997 may have masked any effect in 1997). The

presence of residues on the wheel tracked rows in

July 1996 signi®cantly contributed to a decline in

NOÿ3 ÿN in RT possibly by immobilization. However,

residue signi®cantly increased NOÿ3 ÿN in CT and in

NT, compared to the wheel tracked rows with no

residue. The greater soil aeration caused by plowing

in CT, and the greater microbial activity in NT may

encourage mineralization in these treatments.

In August 1996, residues had little in¯uence on

NOÿ3 ÿN levels in CT, compared to in July 1996, but

once again the wheel tracked rows with residue had

lower levels of NOÿ3 ÿN in RT, than the wheel tracked

rows without residue (Fig. 5). No-till with residue

demonstrated a pronounced decrease in NOÿ3 ÿN

compared to NT-R (7.4 kg NOÿ3 ÿN haÿ1 versus

86.7 kg NOÿ3 ÿN haÿ1). This decrease is most likely

attributed to immobilization.

The NOÿ3 ÿN increase observed in NT-R and CT-R

from July 1996 to August 1996 (Fig. 5), can be

Fig. 3. Wheel�depth interactions for NOÿ3ÿN concentrations (kg haÿ1), averaged over three tillage practices and two levels of corn residue

for each date (significant at the p�0.05 level).


attributed to urea nitri®cation. Urea undergoes two

transformations in the soil: (i) hydrolysis; whereby

urea forms ammonium and carbon dioxide; and (ii)

nitri®cation; where the ammonium is oxidized to

nitrite and then to nitrate by microbial processes

(Hignett, 1985). Ammonium nitrate is more readily

available to the plant than urea. However, it is also

more prone to leaching than other ammoniacal pro-

ducts (Hignett, 1985). Similarly, the decrease in

NOÿ3 ÿN in NT�R and CT�R from July 1996 to

August 1996 could have been due to N being incor-

porated (by immobilization) into the residues. A slight

increase in NOÿ3 ÿN was noticeable from July 1996 to

August 1996 in RT�R because the immobilized N in

the RT�R plots in July may have been undergoing

mineralization (Fig. 5). The lower NOÿ3 ÿN in RT�R,

compared to NT�R and CT�R, in July 1996 is

evident throughout the soil pro®le (Fig. 6). Perhaps

the composition of the microbial biota is changed in

RT, which causes it to have a greater immobilization

than CT or NT, especially earlier in the growing

season.

Fig. 4. Amounts of corn residue (Mg haÿ1) (stalks and leaves)

incorporated after the 1996 harvest, and after cultivating. The

incorporated biomass does not include corn roots. The above-

ground biomass includes standing stubble. NT�no-till; RT�re-

duced tillage; CT�conventional tillage; �R�residue; ÿR�no

residue.

Fig. 5. Wheel�tillage�residue interactions for NOÿ3ÿN concen-

trations (kg haÿ1) in July 1996 and August 1996, averaged over 0±

30 cm (significant at the p�0.05 level). NT�no-till; RT�reduced

tillage; CT�conventional tillage.

Fig. 6. Depth�tillage�residue interactions for NOÿ3ÿN concen-

trations (kg haÿ1) in July 1996 (significant at the p�0.05 level).

NT�no-till; RT�reduced tillage; CT�conventional tillage.


4. Conclusions

The type of tillage practice, as well as the type of

residue practice implemented in¯uenced the amount

of soil nitrate distribution. Residue cover signi®cantly

decreased soil NOÿ3 ÿN concentrations in reduced

tillage four weeks after the application of urea ferti-

lizer, and signi®cantly decreased NOÿ3 ÿN in RT and

NT approximately 6±7 weeks after application.

In spite of high NOÿ3 ÿN concentrations at 0±15 cm,

especially on the wheel tracked rows, nitrate leaching

was not a concern during the growing season, as corn

roots appeared to be taking up NOÿ3 ÿN below 10±

15 cm depths. However, in the absence of crop growth,

soil NOÿ3 ÿN concentrations were higher at 25±50 cm

soil depth than at 0±25 cm. The presence of residue

may have minimized the amount of spring leaching

observed, as was noticed in May 1996.

The presence of residue was bene®cial in reducing

soil NOÿ3 ÿN levels during the growing season in RT

when urea was applied. In CT, the presence of residue

cover encouraged higher soil NOÿ3 ÿN concentrations

shortly after fertilizer application (four weeks). Little

difference between NOÿ3 ÿN concentrations was

found in CT�R and CT-R six weeks after urea appli-

cation. In NT, residue signi®cantly decreased NOÿ3 ÿN

concentrations, especially six weeks after fertilizer

application.

When ammonium nitrate fertilizer was applied, no

difference in soil NOÿ3 ÿN concentrations between the

treatments was observed. Implementing conservation

tillage practices, such as reduced tillage and no-till,

and residue management practices is an ef®cient way

to reduce NOÿ3 ÿN levels in the soil pro®le when urea

fertilizer is applied. Reducing NOÿ3 ÿN leaching may

have signi®cant impacts on improving groundwater

quality. This will have further implications on ame-

liorating drinking water and health quality.

Acknowledgements

The authors would like to thank the Fonds pour la

Formation de Cherchers et l'Aide aÁ la Recherche

(FCAR) for ®nancial support of this project. We also

thank Mr. Peter Kirby of the Natural Resource Science

Department, of McGill University, for his assistance

with the ®eld work, as well as A. Grifferty and S.

Gadoury for summer ®eld assistance.

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