Vol.3, No.6, 854-864 (2012) Agricultural Sciences
http://dx.doi.org/10.4236/as.2012.36104
Effects of broad-leaf crop frequency in various
rotations on soil organic C and N, and inorganic
N in a Dark Brown soil
Sukhdev S. Malhi1*, R. L. Lemke2, S. A. Brandt3
1Agriculture and Agri-Food Canada, Melfort, Canada; *Corresponding Author:
2Agriculture and Agri-Food Canada, Saskatoon, Canada
3Agriculture and Agri-Food Canada, Scott, Canada
Received 23 June 2012; revised 24 July 2012; accepted 2 August 2012
ABSTRACT
The objective of this study was to determine the
impact of frequency of broad-leaf crops canola
and pea in various crop rotations on pH, total
organic C (TOC), total organic N (TON), light
fraction organic C (LFOC) and light fraction or-
ganic N (LFON) in the 0 - 7.5 and 7.5 - 15 cm soil
depths in autumn 2009 after 12 years (1998-2009)
on a Dark Brown Chernozem (Typic Boroll) loam
at Scott, Saskatchewan, Canada. The field ex-
periment contained monoculture canola (herbi-
cide tolerant and blackleg resistant hybrid) and
monoculture pea compared with rotations that
contained these crops every 2-, 3-, and 4-yr with
wheat. There was no effect of crop rotation du-
ration and crop phase on soil pH. Mass of TOC
and TON in the 0 - 15 cm soil was greater in ca-
nola phase than pea phase in the 1-yr (mono-
culture) and 2-yr crop rotations, while the oppo-
site was true in the 3-yr and 4-yr crop rotations.
Mass of TOC and TON (averaged across crop
phases) in soil generally increased with in-
creasing crop rotation duration, with the maxi-
mum in the 4-yr rotation while no difference in
the 1-yr and 2-yr rotations. Mass of LFOC and
LFON in soil was greater in canola phase than
pea phase in the 1-yr, 2-yr and 3-yr rot ations, but
the opposite w as true in the 4-yr rotation. There
was no consistent effect of crop rotation dura-
tion on mass of LFOC and LFON. The N balance
sheet over the 1998 to 2009 period indicated
large amounts of unaccounted N for monocul-
ture pea, suggesting a great potential for N loss
from the soil-plant system in this treatment
through nitrate leaching and/or deni trification. In
conclusion, the findings suggest that the quan-
tity of organic C and N can be maximized by in-
creasing duration of crop rotation and by in-
cluding hybrid canola in the rotation.
Keywords: Broad -Leaf Crops; Canola; Frequency;
Light Fraction Organi c C ; Light Fraction Organic N;
Pea; Total Organic C; Total Organic N
1. INTRODUCTION
Crop rotation (the growing of different crops in the
same field in a planned sequence) is often practiced to
mitigate pests that often become unmanageable in mono-
cultures (cultivation of the same crop year after year on
the same field) [1,2]. In western Canada, research has
indicated that rotations balanced between broad-leaf
crops (canola and pea) with cereals (wheat and barley)
tended to have less pest problems and lower production
risk than rotations that were heavily cereal or broad-leaf
based [3]. In many years, canola and/or pea provide the
best economic return to producers compared to other
field crops grown in western Canada. For this reason
production of canola or pea is often intensive, meaning it
is grown more than once every four years on the same
field.
Research has shown that organic C in soil can be
maintained or enhanced by combining reduced tillage or
no-tillage with proper crop rotations that increase input
of crop residues to soil and/or decrease their decomposi-
tion [4-8]. However, the magnitude of change in organic
C in soil may vary with crop type/species/diversity/in-
tensity, rooting characteristics (layer/volume/mass) of
each crop, soil type and climatic conditions [9,10]. Our
previous paper has discussed the impacts of frequency of
broad-leaf crops canola and pea in various crop rotations
on accumulation and distribution of nitrate-N and ex-
tractable P in the soil profile after 8 years [11]. The ob-
jective of this study was to determine the impact of fre-
quency of broad-leaf crops canola and pea in various
crop rotations (grown in monoculture or in rotation with

S. S. Malhi et al. / Agricultural Sciences 3 (2012) 854-864 855
wheat) on total organic C (TOC), total organic N (TON),
light fraction organic C (LFOC), light fraction organic N
(LFON) and pH in the 0 - 7.5 and 7.5 - 15 cm soil depths
in autumn 2009 after 12 years (1998-2009) on a Dark
Brown Chernozem (Typic Boroll) loam at Scott, Sas-
katchewan, Canada.
2. MATERIALS AND METHODS
A 12-yr field experiment was conducted from 1998 to
2009 on a Dark Brown Chernozem (Typic Boroll) loam
at Scott, Saskatchewan. The field experiment was de-
signed as split-plot of 12 crop rotations (main plots) and
two fungicide (treated and untreated) treatments applied
to sub-plots (Table 1), with all phases of each rotation
present every year in four replications. For this area, the
long-term average precipitation in the growing season
(from May to August) is about 210 mm. Growing season
precipitation was substantially below average in 1998,
2001 and 2008, fairly below average in 2002, 2003 and
2004, slightly below average in 2000, 2006 and 2009,
above average in 1999 and 2007, and very wet in 2005.
All crops were seeded with a Versatile Noble hoe drill set
at 23-cm row spacing. Seeding rate was 245 kg·ha−1 for
pea, 7 - 9 kg·ha−1 for canola, 100 kg·ha−1 for wheat and
50 - 62 kg·ha−1 for flax. All plots received monoammo-
nium phosphate seed placed to supply adequate amounts
of P (15 kg·P·ha−1 plus 7 kg·N·ha−1) and nitrogen (N) as
46-0-0 midrow banded at the time of seeding at rates
based on soil test recommendations (on average ranged
from 11 to 60 kg·N·ha−1, with no N fertilizer to pea) for
optimum crop growth and yield. Appropriate herbicides
were applied to control annual weeds. The crops were
harvested for seed yield every year and straw was re-
turned to each corresponding plot/treatment.
In the autumn of 2009, soil samples in selected treat-
ments were obtained by taking 10 cores (about 2.4 cm
diameter) from the 0 - 7.5, 7.5 - 15 and 15 - 20 cm layers.
Bulk density of soil was determined by the core method
using soil weight and core volume [12]. The soil samples
were air dried at room temperature after removing coarse
roots and easily detectable crop residues, and ground to
pass a 2-mm sieve. Sub-samples were pulverized in a
vibrating-ball mill (Retsch, Type MM2, Brinkman In-
struments Co., Toronto, Ontario) for determination of
TOC, TON, LFOC and LFON in soil. Soil samples used
for organic C and N analyses were tested for the presence
of inorganic C (carbonates) using dilute HCl, and none
was detected in any soil sample. Therefore, C in soil as-
sociated with each fraction was considered to be of or-
ganic origin. Total organic C in soil was measured by
Dumas combustion using a Carlo Erba instrument
(Model NA 1500, Carlo Erba Strumentazione, Italy), and
Technicon Industrial Systems [13] method was used to
determine TON in the soil. Light fraction organic matter
(LFOM) was separated using a NaI solution of 1.7
Mg·m−3 specific gravity, as described by Janzen et al. [6]
and modified by Izaurralde et al. [14]. The C and N in
LFOM (LFOC, LFON) were measured by Dumas com-
bustion. Soil samples (ground to pass a 2-mm sieve)
from the 0 - 7.5 and 7.5 - 15 cm layers were also moni-
tored for pH in 0.01 M CaCl2 solution with a pH meter.
In autumn 2009, soil samples were also obtained by
taking 4 cores (using 4-cm diameter coring tube) from
the 0 - 15, 15 - 30 and 30 - 60 cm layers. The bulk den-
sity of soil was determined by the core method using soil
weight and core volume [12]. The soil samples were air
dried at room temperature, ground to pass a 2-mm sieve,
and then analyzed for ammonium-N [15] and nitrate-N
[16] by extracting soil in a 1:5 ratio of soil and 2 M KCl
solution.
The cumulative amounts of crop residue (CR) input
from 1998 to 2009 growing seasons were estimated as:
above-ground residue (AGR) + belowground residue
(BGR) returned to soil. The AGR was determined from
the straw yield of each crop from 1998 to 2009 growing
seasons. The BGR was estimated from grain dry weight
Table 1. Description of crop rotations in a field experiment from 1998 to 2009 at Scott Saskatchewan.
Crop rotation name (duration) Crop rotation Input of C from crop residue in 12 years
(kg·C·ha−1)
Monoculture (1-yr) Monoculture hybrid canola 20,847
Monoculture (1-yr) Monoculture pea 9930
2-year rotation (2-yr) Hybrid canola-wheat 26,697
2-year rotation (2-yr) Pea-wheat 18,482
3-year rotation (3-yr) Pea-hybrid canola-wheat 20,724
3-year rotation (3-yr) Pea-hybrid canola-wheat 22,705
4-year rotation (4-yr) Hybrid canola-wheat-pea-wheat 24,854
4-year rotation (4-yr) Hybrid canola-wheat-pea-wheat 23,910

S. S. Malhi et al. / Agricultural Sciences 3 (2012) 854-864
856
(GDW) and AGR, using the formula: BGR = a (GDW +
AGR). The value of the constant “a” was 0.24 for wheat
and 0.25 for canola [17], and 0.25 for pea. The amounts
of crop residue C input were estimated by multiplying
the concentration of total C by the amount of crop resi-
due input in various rotation treatments. The estimated C
concentration was 45% for wheat, 42% for canola [18],
and 40% for pea [19]. The cumulative amounts of crop
residue C were calculated as: CR-C = AGR-C + BGR-C;
for 1998 to 2009 (12 years). The estimated amounts of N
balance and unaccounted N over the 1998 to 2009 period
for the 8 treatments [two crop phases (canola and pea)
and four crop rotation durations (1-yr, 2-yr, 3-yr and
4-yr)]; Table 7 was calculated as the difference between
total of N applied as fertilizers in 12 years + N fixed by
pea in rotations with pea + N added in seed at seeding in
12 years minus N removed in seed in 12 years (for N
balance) + nitrate-N recovered in soil in autumn 2009
(for unaccounted N).
The data for each parameter were subjected to analysis
of variance (ANOVA) using procedures as outlined in
SAS [20]. Significant (P ≤ 0.05) differences between
treatments were determined using LSmeans (Proc GLM,
SAS 6.1 for windows). The least significant difference
(LSD0.05) test was used to compare treatment means for
various parameters. Correlations between mass of or-
ganic C or N in soil in autumn 2009 and the amount of
crop residue C input from 1998 to 2009 growing seasons
were calculated using the linear (REG) procedure.
3. RESULTS AND DISCUSSION
3.1. Soil pH
There was no significant effect of crop rotation dura-
tion and crop phase on pH in the 0 - 7.5 and 7.5 - 15 cm
soil layers (data not shown). The soil pH ranged from 4.9
to 5.5 in the 0 - 7.5 cm layer and from 4.7 to 5.6 in the
7.5 to 15 cm layer among different treatments. Similar to
a previous study at this site, there was no consistent ef-
fect of crop diversification/rotation on soil pH [21]. Be-
cause soil at this site is already fairly acid, acidification
of soil from application of N fertilizer at moderate rates
or from including pea legume in the rotation is not ex-
pected to be a serious problem in this soil site.
3.2. Organic C and N Fraction in Soil
The mass of TOC and TON in soil varied with the
crop rotation duration and/or frequency of canola or pea
in the rotation, but the crop rotation duration × crop
phase interaction effect was significant only for TOC (P
≤ 0.10) and TON (P ≤ 0.05) in the 7.5 - 15 cm soil layer
(Table 2). For example, mass of TOC and TON in the 0 -
15 cm soil tended to be greater in canola phase than pea
phase in the 1-yr (monoculture) and 2-yr crop rotations,
while the opposite was true in the 3-yr and 4-yr crop ro-
tations. Averaged across crop phases, mass of TOC and
TON in soil generally increased with increasing crop
rotation duration (but significant only for TOC (P ≤ 0.06)
and TON (P ≤ 0.05) in the 7.5 - 15 cm soil layer, with the
maximum in the 4-yr crop rotation while no difference in
the 1-yr and 2-yr rotations. When ANOVA was con-
ducted separately for canola and pea phases, the ANOVA
for canola phase did not indicate any significant effect of
crop rotation duration and/or frequency of canola in the
rotation on TOC and TON. However, the ANOVA for
pea phase showed significant effect of crop rotation du-
ration and/or frequency of pea in the rotation on TOC
and TON in the 7.5 - 15 (P ≤ 0.05) soil layer and in the
total 0 - 15 (P ≤ 0.14) cm soil depth. Mass of TOC and
TON (averaged across crop rotation duration) tended to
be greater (but not significant) in canola phase than pea
phase.
Of interest, despite markedly lower C inputs, TOC
under the continuous pea treatment was not greatly dis-
similar to other treatments. Similar results have been
reported by other workers. In a long-term study on a
Brown Chernozem, Campbell et al. [22] reported lower
C inputs but similar SOC status for a wheat-lentil com-
pared to a continuous wheat treatment. In an incubation
study using 13C-CO2 to label C inputs from growing
crops, Comeau [23] estimated significantly lower C in-
puts to soil under pea compared to canola, but similar
amounts of 13C “stabilized” in the soil at the end of two
growing cycles. These results support the hypothesis that
pea residues are more efficiently stabilized as SOC than
wheat and particularly canola.
The mass of LFOC and LFON in soil varied greatly
with the duration/frequency of canola/pea in the crop
rotations, but the crop rotation duration × crop phase
interaction effect was significant only for TOC (P ≤ 0.12)
in the 0 - 7.5 cm soil layer (Table 3). Like TOC and TON,
mass of LFOC and LFON in soil tended to be greater in
canola phase than pea phase in the 1-yr and 2-yr rota-
tions, but the opposite was true in the 3-yr and 4-yr rota-
tions. Compared to 1-yr rotation, mass of LFOC and
LFON (averaged across crop phases) in soil decreased
with 2-yr and 3-yr rotations, and then increased, with
greater LFOC and LFON in soil for 4-yr than 1-yr rota-
tions in the 0 - 7.5 cm layer, but again the effect was sig-
nificant only for LFOC (P ≤ 0.12) in the 0 - 7.5 cm soil
layer. When ANOVA was conducted separately for ca-
nola and pea phases, there was no significant effect of
crop rotation duration and/or frequency of canola in the
rotation on LFOC and LFON for canola phase. However,
for pea phase, the ANOVA showed significant effect of
crop rotation duration and/or frequency of canola in the
rotation on LFOC in the 0 - 7.5 cm soil layer (P ≤ 0.06)

S. S. Malhi et al. / Agricultural Sciences 3 (2012) 854-864 857
Tabl e 2. Effect of broad-leaf crop phase and frequency over 12 years from 1998 to 2009 on mass of total organic C (TOC) and total
organic N (TON) in soil in autumn 2009 at Scott, Saskatchewan, Canada.
Treatment TOC mass (Mg·C·ha−1) in soil
layers (cm)
TON mass (Mg·N·ha−1) in soil
layers (cm)
Crop rotation (duration) Crop phase 0 - 7.5 7.5 - 15 0 - 15 0 - 7.5 7.5 - 15 0 - 15
Monoculture (1-year) Canola 24.77 21.57 46.34 2.198 1.965 4.163
Monoculture (1-year) Pea 22.33 21.95 44.28 2.007 1.960 3.967
Canola-wheat (2-year) Canola 26.08 23.99 50.07 2.253 2.166 4.419
Pea-wheat (2-year) Pea 21.19 20.00 41.19 1.858 1.787 3.645
Pea-canola-wheat (3-year) Canola 23.55 24.03 47.58 2.045 2.103 4.148
Pea-canola-wheat (3-year) Pea 23.14 26.81 49.95 2.086 2.388 4.474
Canola-wheat-pea-wheat (4-year) Canola 24.75 24.95 49.70 2.152 2.208 4.360
Canola-wheat-pea-wheat (4-year) Pea 26.00 25.56 51.56 2.222 2.256 4.478
LSD0.05 ns 4.74 ns ns 0.387 ns
SEM (significance) 1.589ns 1.612 2.712ns 0.1348ns 0.1316 0.2330ns
Crop rotation duration
1-year 23.55 21.76 45.31 2.103 1.962 4.065
2-year 23.64 21.99 45.63 2.055 1.977 4.032
3-year 23.35 25.42 48.76 2.066 2.246 4.312
4-year 25.37 25.25 50.63 2.187 2.232 4.419
LSD0.05 ns 3.44 ns ns 0.291 ns
SEM (significance) 1.151ns 1.178 2.015ns 0.0965 ns 0.0997 0.1701 ns
Crop phase
Canola 24.79 23.63 48.42 2.162 2.111 4.273
Pea 23.17 23.58 46.75 2.044 2.098 4.141
LSD0.05 ns ns ns ns ns ns
SEM (significance) 0.814ns 0.833ns 1.425ns 0.0682 ns 0.0705 ns 0.1203 ns
Additional statistical comparisons
Significance of four crop rotation durations for canola phase only
LSD0.05 ns ns ns ns ns ns
SEM (significance) 1.109ns 1.670ns 2.515ns 0.0829ns 0.1458ns 0.2108ns
Significance of four crop rotation durations for pea phase only
LSD0.05 ns 5.00 10.18 ns 0.398 0.832
SEM (significance) 2.135ns 1.564 3.183 0.1856ns 0.1244* 0.2599
, * and ns refer to significant treatment effects in ANOVA at P ≤ 0.1, P ≤ 0.05 and not significant, respectively.

S. S. Malhi et al. / Agricultural Sciences 3 (2012) 854-864
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Table 3. Effect of broad-leaf crop phase and frequency over 12 years from 1998 to 2009 on mass of light fraction organic C (LFOC)
and light fraction organic N (LFON) in soil in autumn 2009 at Scott, Saskatchewan, Canada.
Treatment LFOC mass (kg·C·ha−1) in soil
layers (cm)
LFON mass (kg·N·ha−1) in soil
layers (cm)
Crop rotation (duration) Crop phase 0 - 7.5 7.5 - 15 0 - 15 0 - 7.5 7.5 - 15 0 - 15
Monoculture (1-year) Canola 3283 889 4172 212 48 260
Monoculture (1-year) Pea 2520 694 3214 168 42 210
Canola-wheat (2-year) Canola 2733 831 3564 171 46 217
Pea-wheat (2-year) Pea 2317 814 3131 154 49 203
Pea-canola-wheat (3-year) Canola 2193 816 3009 150 47 197
Pea-canola-wheat (3-year) Pea 2723 962 3685 176 54 230
Canola-wheat-pea-wheat (4-year) Canola 3081 923 4004 186 51 237
Canola-wheat-pea-wheat (4-year) Pea 3348 855 4203 201 48 249
LSD0.05 918 ns ns ns ns ns
SEM (significance) 312.0 110.5ns 365.9ns 18.5ns 6.3ns 22.7ns
Crop rotation duration
1-year 2901 792 3693 190 45 235
2-year 2525 822 3347 162 47 209
3-year 2458 889 3347 163 50 213
4-year 3215 889 4014 193 49 242
LSD0.05 677 ns ns ns ns ns
SEM (significance) 231.8 77.2ns 273.1ns 13.4ns 4.3ns 16.2ns
Crop phase
Canola 2823 865 3688 180 48 228
Pea 2727 831 3558 175 48 223
LSD0.05 ns ns ns ns ns ns
SEM (significance) 163.9ns 54.6ns 193.1ns 9.5ns 3.0ns 11.5ns
Additional statistical comparisons
Significance of four crop rotation durations for canola phase only
LSD0.05 ns ns ns ns ns ns
SEM (significance) 334.5ns 125.2ns 430.8ns 21.9ns 7.2ns 27.1ns
Significance of four crop rotation durations for pea phase only
LSD0.05 769 ns 818 ns ns ns
SEM (significance)
240.3 112.4ns 255.8* 13.2ns 6.3ns 17.6ns
* and ns refer to significant treatment effects in ANOVA at P ≤ 0.1, P ≤ 0.05 and not significant, respectively. ,

S. S. Malhi et al. / Agricultural Sciences 3 (2012) 854-864 859
and in the total 0 - 15 cm soil depth (P ≤ 0.05), but for
LFON it was significant at P ≤ 0.16 in the 0 - 7.5 soil
layer. Mass of LFOC and LFON (averaged across crop
rotation duration) in soil tended to be greater in canola
phase than pea phase in most cases.
There were strong and highly significant positive cor-
relation coefficients between TOC and TON, and be-
tween LFOC and LFON fractions in soil (Ta ble 4). The
correlations between TOC and LFOC (significant at P ≤
0.12), and between TON and LFON (significant at P ≤
0.10) were moderate. The crop residue C or N input over
12 growing seasons (Table 1) had significant correlation
coefficients with TOC (significant at P ≤ 0.05) and TON
(significant at P ≤ 0.06). For linear regressions between
crop residue C input and TOC, TON, LFOC or LFON,
the R2 values were significant (significant at P ≤ 0.05) for
TOC and TON (Table 5).
In an 8-yr study with 4-yr rotations on a Black Cher-
nozem soil in northeastern Saskatchewan, previous re-
search has shown no effect of broad-leaf frequency (1, 2
or 3 broad-leaf crops in 4-yr rotations) on organic C and
N in soil [10]. In another study on a Brown Chernozem
soil in southern Saskatchewan, annually cropped rota-
tions stored more organic C in soil than crop rotations
with bare summer fallow, but there was no influence of
crop phase on soil organic C under continuously cropped
rotations [9]. However in our present 12-yr study, TOC,
TON, LFOC and LFON all increased (although signifi-
cant only in a few cases) with increasing duration of crop
rotation, with the maximum soil organic C and N in the
4-yr rotations and also greater organic C and N in soil in
canola phase than pea phase. It is possible that inclusion
of other crops, such as wheat with high lignin content in
straw which is relatively slow to decompose, may have
resulted in this slow build-up of organic C and N in soil
in the longer duration rotations, especially 4-year rota-
tion with two wheat crops.
Because of relatively much smaller amounts of annual
inputs of crop residue C or N compared to the amounts
of organic C or N stocks in soil, there is generally a slow
build-up of organic C or N in soil. This makes it very
difficult to detect significant increase in storage of or-
ganic C or N in soil due to management practices, espe-
cially from crop rotations where crop residues (roots and
Tab le 4 . Relationships among organic C or N fractions (TOC, TON, LFOC, LFON) in the 0 - 15 cm soil, or
between crop residue input from 1998 to 2009 growing seasons and organic C or N stored in the 0 - 15 cm
soil sampled in autumn 2009 at Scott, Saskatchewan, Canada.
Correlation coefficients (r)
Parameter TOC TON LFOC LFON
Relationships among soil organic C or N fractions
TOC 0.986*** 0.598 0.463ns
TON 0.626 0.516ns
LFOC 0.973***
LFON
Relationships between crop residue C input and soi l o rgan ic C or N fractions
Crop residue C input 0.717* 0.678 0.511ns 0.359ns
, *, *** and ns refer to significant treatment effects in ANOVA at P ≤ 0.1, P ≤ 0.05, P ≤ 0.001 and not significant, respectively.
Table 5. Linear regressions for relationships between crop residue C input from 1998 to 2009 growing sea-
sons and organic C or N (TOC, TON, LFOC, LFON) stored in the 0 - 15 cm soil sampled in autumn 2009 at
Scott, Saskatchewan, Canada.
Crop parameter (X) Soil C or N parameter (Y)zLinear regression (Y = a + bX) R2
Crop residue C input TOC Y = 37.42 + 0.0005X 0.514*
TON Y = 3.408 + 0.00004X 0.458*
LFOC Y = 2640 + 0.047X 0.261ns
LFON Y = 192.4 + 0.002X 0.131ns
zY = Soil organic C or N fraction (TOC and TON as Mg C or N·ha−1; and LFOC, LFON as kg C or N·ha−1; a = Intercept on Y, ori-
gin of the line; b = Regression coefficient of Y on X, slope of line; X = Crop residue and/or swine manure C input (Mg·ha−1); * and
ns refer to significant treatment effects in ANOVA at P ≤ 0.05 and not significant, respectively.

S. S. Malhi et al. / Agricultural Sciences 3 (2012) 854-864
860
straw) are returned to soil in all crop phases. Similarly, in
our study there was an increase in storage of TOC or
TON in soil with increasing length of crop rotation from
monoculture pea (1-year) to canola-wheat-pea-wheat ro-
tation (4-year) but the differences were significant only
in a few cases even after 12 years.
Previous research has shown that dynamic organic C
or N fractions are more responsive to management prac-
tices than total organic C or N under annual crops [24-
27]. Similarly, in our study the changes in LFOC and
LFON were more pronounced than TOC and TON, al-
though the increases in LFOC and LFON in soil due to
crop rotations were not significant.
3.3. Residual Nitrate-N and Ammonium-N
in Soil
The crop rotation duration × crop phase interaction effect
was significant in most soil layers (Table 6). This was
due to usually greater amounts of nitrate-N in pea phase
than canola phase, particularly in the 1-yr rotation. The
mean effect of crop rotation length/duration on soil ni-
trate-N was significant (at P ≤ 0.1) only in the total 0 - 60
cm depth, but the amounts of nitrate-N were usually
highest in all soil layers for monocultures, with the low-
est nitrate-N in the 4-yr rotation. When ANOVA was
conducted separately for canola and pea phases, there
was no significant effect of crop rotation duration and/or
frequency of canola in the rotation on soil nitrate-N.
However, for pea phase there was a significant (at P ≤
0.1) effect of crop rotation duration and/or frequency of
pea in the rotation on nitrate-N in the 15 - 30 and 30 - 60
cm soil layers. The amount of nitrate-N (averaged across
crop rotation duration) was significantly greater in pea
phase than canola phase in all soil layers. The generally
higher soil nitrate-N in most layers with monoculture
was probably due to relatively lower cumulative crop
yield and total N uptake in seed in 1-yr rotation com-
pared to longer rotations [11]. In another study in Sas-
katchewan, nitrate-N in soil was also higher in 6-yr con-
tinuous rotations with low crop diversity compared to
rotations with high diversity of annual grain crops [21].
Earlier research has also suggested that residual N in soil
can be decreased with efficient cropping systems [28].
Similarly, in our study soil nitrate-N in most soil layers
was usually lowest in the 3-yr and 4-yr rotations with pea
and 4-yr rotation with canola, suggesting the importance
of 3- or 4-year rotations in reducing residual nitrate-N in
the soil profile.
The mean effect of crop phase on ammonium-N in soil
was not significant in any soil layer, but the crop rotation
duration × crop phase interaction effect was significant in
the 0 - 60 cm depth most likely due to greater amounts of
ammonium-N in soil in canola phase than pea phase
(Table 6). When ANOVA was conducted separately for
canola and pea phases, there was no significant effect of
crop rotation duration and/or frequency of pea in the ro-
tation on soil ammonium-N. But, there was a significant
effect of crop rotation duration and/or frequency of ca-
nola in the rotation on ammonium-N in the 15 - 30 and
30 - 60 cm soil layers. The tendency of greater amounts
of ammonium-N (averaged across crop phases) in 1-yr or
2-yr rotation than 3-yr or 4-yr rotations in the 30 - 60 cm
soil layer or 0 - 60 cm depth were most likely due to con-
tribution through ammonification of N of recently added
residue from crops with relatively longer taproots at
deeper depth, particularly canola.
3.4. Amounts of N Uptake in Seed, Residual
Nitrate-N in Soil and N Balance
The N balance over the 1998 to 2009 period for the 8
treatments (4 rotation durations × 2 crop phases) in-
cluded amount of inorganic N applied as fertilizers, N
added in seed at seeding time, estimated biologically
fixed N (BFN) by pea when it was grown, amount of N
recovered in seed over 12 years and mineral N (nitrate-N
+ ammonium-N) recovered in the 0 - 60 cm soil in au-
tumn 2009 after 12 growing seasons, and the estimated
amount of unaccounted N (Table 7). The estimated
amount of N recovered in seed (which was removed
from the land/field) plus the amount of mineral-N recov-
ered in soil in various treatments ranged from 658 to
1103 kg·N·ha−1. The corresponding values of N applied
as inorganic fertilizers, plus N added in seed and BFN
during the 12-year experimental period ranged between
556 and 1522 kg·N·ha−1. The amounts of N that could
not be accounted for ranged from 441 to −296 kg·N·ha−1.
The amounts of unaccounted N were positive with
monoculture pea, and the unaccounted N was much
greater with pea than canola in the monoculture and 2-yr
rotations, while the differences were small between the
canola and pea phases in the 3-yr and 4-yr rotations. The
positive unaccounted N reflects an excess of N that was
applied to and/or fixed by pea compared to the N recov-
ered in crop seed yield plus nitrate-N recovered in soil in
the 1-yr rotation. The results of our study do not suggest
any over-application of N, because little N was applied to
pea and annual rates of applied N to other crops were
moderate, and in canola phase the N balance was nega-
tive. It is possible that a portion of the fixed/applied N in
1-yr rotation in pea phase may have leached down below
the 60 cm soil depth, as evidenced by greater amounts of
nitrate-N recovered in the 30 - 60 cm layer in this study
(Table 6), and also in the 60 - 90 cm layer in some treat-
ments in the same experiment in autumn 2005 in our
previous report [11]. Other researchers have reported an
increase in the concentration f residual soil nitrate-N at o

S. S. Malhi et al. / Agricultural Sciences 3 (2012) 854-864 861
Ta b l e 6 . Effect of broad-leaf crop phase and frequency over 12 years from 1998 to 2009 on nitrate-N and ammonium-N in soil in
autumn 2009 at Scott, Saskatchewan, Canada.
Treatment Nitrate-N (kg·N·ha−1) in soil layers (cm) Ammonium-N (kg·N·ha−1) in soil layers (cm)
Crop rotation (duration) Crop phase 0 - 15 15 - 3030 - 600 - 60 0 - 15 15 - 30 30 - 600 - 60
Monoculture (1-year) Canola 12.7 0.0 0.0 12.7 6.9 6.8 14.0 27.7
Monoculture (1-year) Pea 23.7 5.6 11.5 40.8 6.1 5.0 7.6 18.7
Canola-wheat (2-year) Canola 12.7 1.0 0.5 14.2 4.8 4.2 9.1 18.1
Pea-wheat (2-year) Pea 13.5 2.9 6.8 23.2 7.3 6.2 11.8 25.3
Pea-canola-wheat (3-year) Canola 12.2 1.3 2.2 15.7 4.5 2.8 7.5 14.8
Pea-canola-wheat (3-year) Pea 14.0 2.0 1.8 17.8 4.2 5.8 8.7 18.7
Canola-wheat-pea-wheat (4-year) Canola 8.1 0.5 0.2 8.8 5.7 4.8 9.0 19.5
Canola-wheat-pea-wheat (4-year) Pea 15.5 2.2 1.5 19.2 3.6 3.1 6.2 12.9
LSD0.05 ns 2.1 6.7 14.8 ns ns ns 10.4
SEM (significance) 3.39ns 0.72*** 2.26* 5.04** 1.20ns 1.13ns 2.00ns 3.54
Crop rotation duration
1-year 18.2 2.8 5.6 26.6 6.5 5.9 10.8 23.2
2-year 13.1 2.0 3.7 18.8 6.1 5.2 10.5 21.8
3-year 13.1 1.7 2.0 16.8 4.4 4.3 8.1 16.8
4-year 11.8 1.4 0.9 14.1 4.7 4.0 7.6 16.3
LSD0.05 ns ns ns 11.2 ns ns ns ns
SEM (significance) 2.39ns 0.61ns 1.77ns 3.86 0.86 ns 0.87 ns 1.51 ns 2.72 ns
Crop phase
Canola 11.4 0.7 0.7 12.8 5.5 4.7 9.9 20.1
Pea 16.7 3.2 5.4 25.3 5.3 5.0 8.6 18.9
LSD0.05 4.9 1.3 3.7 8.0 ns ns ns ns
SEM (significance) 1.69* 0.43*** 1.25* 2.73** 0.61 ns 0.61ns 1.06 ns 1.93ns
Additional statistical comparisons
Significance of four crop rotation durations for
canola phase only
LSD0.05 ns ns ns ns ns 3.5 4.6 9.6
SEM (significance) 3.50ns 0.47ns 1.00ns 3.89ns 1.09ns 1.09 1.45* 3.00
Significance of four crop rotation durations for pea
phase only
LSD0.05 ns 2.8 9.3 ns ns ns ns ns
SEM (significance) 3.71ns 0.88 2.92 6.33ns 1.36ns 1.14ns 1.90ns 3.39ns
, *and ns refer to significant treatment effects in ANOVA at P ≤ 0.1, P ≤ 0.05 and not significant, respectively.

S. S. Malhi et al. / Agricultural Sciences 3 (2012) 854-864
862
Table 7. Balance sheets of broad-leaf crop phase and frequency over 12 years from 1998 to 2009 at Scott, Saskatchewan, Canada.
Crop rotation duration
1-yr (monoculture)2-yr 3-yr 4-yr Parameters
Canola Pea Canola Pea Canola Pea Canola Pea
Mineral-N recovered in soil (0 - 60 cm) after 12 years in
autumn 2009 (kg·N·ha−1) 40 60 32 49 31 37 28 32
N recovered in seed in 12 years (kg·N·ha−1) 618 1021 820 1054 954 954 972 972
N recovered in soil after 12 years + N recovered in seed in
12 years (kg·N·ha−1) 658 1081 852 1103 985 991 1000 1004
Inorganic N applied in fertilizers to non-legume crops in
12 years (kg·N·ha−1) 552 84 534 300 384 384 477 477
Organic N added in seed in 12 years (kg·N·ha−1) 4 78 22 59 41 41 41 41
Organic N fixed by pea in 12 years (kg·N·ha−1) 0 1460 0 730 487 487 365 365
Total N added in fertilizers + in seed + fixed by pea in
12 years (kg·N·ha−1) 556 1522 556 1089 912 912 883 883
N balance (N applied in fertilizers/seed/fixed – N
recovered in seed) (kg·N·ha−1) −62 501 −264 35 −42 −42 −89 −89
Unaccounted N (N applied in fertilizers/seed/fixed – N
recovered in soil + seed) (kg·N·ha−1) −102 441 −296 −14 −73 −79 −117 −121
high N fertilizer rates [28-30] and nitrate leaching in the
90 - 240 cm soil profile [21], suggesting the need for
deep soil sampling below the 60 or 90 cm depth in future
in this long-term study. Soil nitrate-N below the effective
root zone of crops is susceptible to leaching, and the loss
of nitrate-N through leaching can result in N contamina-
tion of groundwater, and thus represents a potential risk
to groundwater quality and soil health [31-32]. Further-
more, a portion of the applied N in these treatments may
have been immobilized in soil organic N, as evidenced
by large amounts of soil N in LFON in this study (Tables
3 and 4). In addition, a small portion of the applied N
may have been lost from the soil-plant system through
denitrification (e.g., nitrous oxide and other N gases) due
to wet soil conditions which temporarily exist in the pre-
sent study in some years after occasional heavy rainfall
during summer and/or autumn [33,34]. The negative
amounts of N balance and unaccounted N, especially in
canola phase, suggest that N became available to the
crop through mineralization of organic matter in the
growing season, and possibly soil may be gaining N
from wet deposition and/or non-symbiotic N fixation but
this needs further research to verify any contribution of
N from precipitation and non-symbiotic N fixation.
4. CONCLUSION
The findings suggest that the quantity of organic C and
N can be maximized by increasing duration of crop rota-
tion and by including hybrid canola in the rotation.
5. ACKNOWLEDGEMENTS
The authors are grateful to Brett Mollison, Colleen Kirkham, Kara
Lengyel, Kimberly Martin, Don Gerein and Larry Sproule for technical
support, Darwin Leach for preparing the poster, and Erin Cadieu for
printing the poster.
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