Field Crops Research 71 (2001) 123±137
Wheat/maize or wheat/soybean strip intercropping
I. Yield advantage and interspeci®c interactions on nutrients
Long Lia,b, Jianhao Sunc, Fusuo Zhanga,*, Xiaolin Lia, Sicun Yangc, Zdenko Rengelb
a
b
Department of Plant Nutrition, China Agricultural University, Beijing 100094, PR China
Department of Soil Science and Plant Nutrition, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
c
Institute of Soils and Fertilizers, Gansu Academy of Agricultural Sciences, Lanzhou 730070, PR China
Received 22 October 2000; received in revised form 28 March 2001; accepted 30 March 2001
Abstract
This study investigated yield advantage of intercropping systems and compared N, P and K uptake by wheat, maize, and
soybean in two ®eld experiments in Gansu province. At Baiyun site the ®eld experiment compared two P levels (0 and
53 kg P ha 1), two planting densities for wheat and maize, and three cropping treatments (wheat/maize intercropping, sole
wheat and sole maize). The design for the wheat/soybean intercropping experiment at Jingtan site was similar, except that
fertilization rates were 0 and 33 kg P ha 1 without plant density treatment. Yield and nutrient acquisitions by intercropped
wheat, maize and soybean were all signi®cantly greater than for sole wheat, maize and soybean with the exception of K
acquisition by maize. Intercropping advantages in yield (40±70% for wheat intercropped with maize and 28±30% for wheat
intercropped with soybean) and in nutrient acquisition by wheat resulted from both the border- and inner-row effects. The
relative contribution to increasing biomass was two-thirds from the border-row effect and one-third from the inner-row effect.
Similar trends were noted for N, P and K accumulation. During the co-growth period, lasting for about 80 days from maize or
soybean emergence to wheat harvesting, yield and nutrient acquisition by intercropped wheat increased signi®cantly while
those by maize or soybean intercropped with wheat decreased signi®cantly. Aggressivities of wheat relative to either maize
(0.26±1.63 of Awm) or soybean (0.35±0.95 of Aws) revealed the greater competitive ability of wheat than either maize or
soybean. The nutrient competitive ratio, 1.09±7.54 for wheat relative to maize and 1.2±8.3 for wheat relative to soybean,
showed that wheat had greater capability to acquire nutrients compared to soybean and maize. Comparison of overall N and K
acquisition by intercropping with weighted means of those of sole cropping revealed interspeci®c facilitation in nutrient
acquisition during co-growth. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Intercropping; Interspeci®c competition; Interspeci®c facilitation; Maize; Nutrient uptake; Soybean; Wheat; Yield advantage
Abbreviations: Awm or Aws, aggressivity of wheat relative maize or soybean; CR, competitive ratio; D1 and D2, planting density (low and
high); DM, dry matter yield; Fa and Fb, the proportion of crop `a' and `b' in intercropping; IM, IS and IW, intercropped maize, soybean and
wheat, respectively; IW/M, wheat intercropped with maize; IW/S, wheat intercropped with soybean; LER, land equivalent ratio; NM, no
maize was planted in the area between two wheat strips; NS, not signi®cant; NU, nutrient uptake; P0, P33 and P53, rates of P fertilization were
0, 33 and 53 kg P ha 1, respectively; SM, SS and SW, sole maize, sole soybean and sole wheat
*
Corresponding author. Tel.: ‡86-10-62892499; fax: ‡86-10-62891016.
E-mail address: [email protected] (F. Zhang).
0378-4290/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 4 2 9 0 ( 0 1 ) 0 0 1 5 6 - 3
124
L. Li et al. / Field Crops Research 71 (2001) 123±137
1. Introduction
Intercropping is practiced in many parts of the
world (Francis, 1986). It is particularly important
and continues to be widely employed not only in
tropical areas (Vandermeer, 1989), but in temperate
areas also, resulting in high grain yield (Li et al.,
1999) and can be environmentally-benign by reducing accumulation of NO3-N in soil pro®le
(Stuelpnagel, 1993) and by reducing N input (Exner
et al., 1999). Both wheat/maize and wheat/soybean
strip intercropping systems are long-established
major grain production systems in northwestern
China, especially in areas with irrigation, but with
only one cropping season annually due to temperature
limitation. The area under wheat/maize intercropping was 75,100 ha in Ningxia in 1995, producing
43% of total grain yield for the area. In Gansu there
is 200,000 ha annually. Wheat/soybean intercropping occupies a smaller area than wheat/maize
intercropping.
Compared with corresponding sole crops, yield
advantages have been recorded in many intercropping systems, including maize/soybean (West and
Grif®th, 1992; Ghaffarzadeh et al., 1994), sorghum/
soybean (Elmore and Jackobs, 1986), maize/cowpea
(Watiki et al., 1993), wheat/mungbean (Chowdhury
and Rosario, 1994), wheat/chickpea (Mandal et al.,
1996), barley/medic (Moynihan et al., 1996), canola/
soybean (Ayisi et al., 1997), maize/lucerne (Smith and
Carter, 1998), maize/faba bean (Li et al., 1999), etc. In
maize/soybean strip intercropping, West and Grif®th
(1992) observed a 26% increase in maize and a 27%
yield reduction in soybean border rows in eight-row
alternating strips in Indiana. Ghaffarzadeh et al.
(1994) found that strip intercropping had 20±24%
greater maize yields and 10±15% smaller soybean
yields in adjacent border rows in the maize/soybean
intercropping in Iowa.
There are close relationships between yield advantage and nutrient uptake by intercropped species
(Morris and Garrity, 1993; Stern, 1993). Almost all
published intercropping combinations with a signi®cant yield advantage were non-legume/legume combinations (Morris and Garrity, 1993). Studies of
nutrient acquisition such as N have also focused on
non-legume/legume combinations in various cereal/
legume associations (reviewed by Stern (1993)), e.g.
barley/pea (Jensen, 1996), maize/beans (Siame et al.,
1998), mucuna (Mucuna pruriens)/maize (Sanginga
et al., 1996), and legume genotypes/maize
(Mandimba, 1995). In addition, P acquisition was
studied in wheat/lupin (Gardner and Boundy, 1983;
Horst and Waschkies, 1987) and pigeon pea/sorghum
associations (Ae et al., 1990).
In many studies on the yield advantage of intercropping, there was a signi®cant increase of the yield
of the border row compared to the inner row (Fortin
et al., 1994; Ghaffarzadeh et al., 1998; Lesoing and
Francis, 1999). These observations focused only on
yield and above-ground biomass with little attention
paid to border-row effects in relation to nutrient
uptake. Interspecies interactions, including aboveground and below-ground competition and facilitation, play an important role in determination of the
structure and dynamics of plant communities in both
agricultural and natural ecosystems (Aerts, 1999;
Callaway, 1999). As for interspecies interactions in
intercropping ecosystems, more has been reported on
interspecies above-ground than below-ground interactions (Willey, 1979; Vandermeer, 1989). In wheat/
clover intercropping, Dauro and Mohamedsaleem
(1995) found that root but not shoot interaction
affected signi®cantly the yields of the two component
crops. The intermingling of clover and wheat roots
increased wheat yield while reducing that of clover. In
oats/faba bean intercropping, intraspecies competition
was more severe than interspecies competition, while
oats was a relatively stronger competitor than faba
bean (Helenius and Jokinen, 1994). Application of
adequate K improved the competitive ability of the
legume in legume/grass mixtures, further enhancing
yield of the intercropping system (Senaratne et al.,
1993). As for wheat/maize intercropping, an important
cereal/cereal intercropping pattern in northwest part of
China, there is little published research on yield
advantage and nutrient uptake by the component
species.
The objectives of this study were to (1) determine yield advantage of the intercropping systems
and the border-row effect on yield and nutrient acquisition by intercropped wheat in cereal/cereal and
cereal/legume intercropping, and (2) elucidate the
relationship between the border-row effect and
the interspecies interactions between intercropped
species.
L. Li et al. / Field Crops Research 71 (2001) 123±137
2. Materials and methods
2.1. Site descriptions
The study was conducted in 1997 and 1998 at the
Baiyun and Jingtan experimental sites of the Institute
of Soils and Fertilizers, Gansu Academy of Agricultural Sciences. The Baiyun site (388370 N, 1028400 E) is
located 15 km north of Wuwei City, Gansu Province at
1504 m a.s.l. Annual mean temperature is 7.78C.
Accumulated temperatures above 0 and 108C are
3646 and 31498C, respectively. The frost-free period
is 170±180 days. Total solar radiation is 5988 MJ m 2
per year. Annual precipitation is 150 mm, and potential evaporation is 2021 mm. The Jingtan site
(378050 N, 1048400 E) is located in Jinyuan county,
Gansu Province, at 1645 m a.s.l. Annual mean temperature is 6.68C and accumulated temperatures above
0 and 108C are 3208 and 26228C, respectively. The
frost-free period is 160±170 days. Total solar radiation
is 6162 MJ m 2 per year. Annual precipitation is
259 mm, and potential evaporation is 2369 mm. This
site is on the edge of a loess plateau. The soils at both
sites are classi®ed as Aridisol.
2.2. Wheat and maize intercropping
Wheat and maize were planted in a west±east
orientation in alternating 1.5 m wide strips, which
included a 0.72 m wide wheat strip (six rows of wheat
with 0.12 m inter-row distance) and a 0.78 m maize
strip of two rows with 0.39 m inter-row distance. The
experimental design was a 2 2 3 split±split±plot
of three replicates, with the main plot treatments being
0 and 53 kg P ha 1, applied as triple superphosphate.
Sub-main-plot treatments were planting density, low
(D1, 7,500,000 plants ha 1 for wheat and 126,000
plants ha 1 for maize) and high density (D2,
11,250,000 plants ha 1 for wheat and 675,000
plants ha 1 for maize). Sub-plot treatments consisted
of sole wheat, sole maize and wheat/maize intercropping. Maize occupied 52% of intercropped area and
wheat 48%. The overall proportional densities of each
crop species was equal in both the monocropping and
intercropping treatments.
Dates of sowing were 20 March for wheat and 16
April for maize. Dates of harvesting were 15 July for
wheat and 24 September for maize. The individual
125
plot area was 4:5 m 7:4 m. All plots were given
identical application of 300 kg N ha 1 as ammonium
nitrate. All P fertilizer and a half of N were evenly
broadcast and incorporated into the top 20 cm of the
soil prior to sowing, and the other half of N fertilizer
was topdressed at the ®rst irrigation for sole wheat, or
was divided into two portions applied at the elongation
stage and the pre-tasseling stage for intercropped and
sole maize. All plots were irrigated during the growing
season to prevent water stress. Six irrigations were
carried out on 25 April, 16 May, 10 June, 2 July, 1
August and 1 September, respectively. Each irrigation
was 75 mm.
2.3. Wheat and soybean intercropping
The experimental design was a split±plot with three
replicates, with the main plot treatments being 0 and
33 kg P ha 1 applied as triple superphosphate. Subplot treatments consisted of sole wheat, sole soybean
and wheat/soybean intercropping. Six rows of wheat
plants were grown in alternating 1.2 m wide strips
with two rows of maize. The row spacing in the
intercropping treatment was 0.15 m for wheat and
0.15 m for soybean. Three strips constituted a plot.
The densities of sole wheat and soybean were
6,000,000 and 330,000 plants ha 1, respectively.
Three-fourths of each intercropped area were occupied by wheat and one-fourth by soybean. The overall
proportional density of each crop species was equal in
both the monocropping and intercropping treatments.
Dates of sowing were 25 March for wheat and 14
April for soybean, Corresponding dates of harvesting
were 25 July and 22 September. The plots were
3:6 m 6 m. All plots were given a basal application
of 225 kg N ha 1 as urea. Both the N and P fertilizers
were evenly broadcast and incorporated into the top
20 cm of the soil prior to sowing. All plots were
irrigated during the growing season as in wheat/maize
intercropping.
2.4. Data collection
Dry matter yields were measured at intervals of
14 days for wheat and maize in wheat/maize intercropping or at intervals of 20 days in the wheat/
soybean experiment. The sampling areas for each
occasion were 0:3 m 0:72 m and 0:3 m 0:9 m
126
L. Li et al. / Field Crops Research 71 (2001) 123±137
for intercropped wheat and 0:3 m 0:48 m and 0:3 m
0:6 m for sole wheat in the wheat/maize and wheat/
soybean intercropping systems, respectively, and 0:3 m
0:3 m for intercropped soybean. Sampling for intercropped maize and sole maize was on plant number,
being 10 at ®rst sampling and 4 for the second sampling through ninth sampling. Grain yield and biomass
were determined by harvesting 1 m of each row in
intercropped and sole wheat at maturity. Nitrogen,
phosphorus and potassium concentrations in the plant
dry matter were determined after wet digestion with
H2SO4 and H2O2. Nitrogen was measured by microKjeldahl procedure, P by vanadomolybdate method,
and K by ¯ame photometry. Nutrient acquisition was
calculated as the product of nutrient concentration and
biomass in above-ground parts of the crops.
2.5. Statistical analysis
2.5.1. Aggressivity
Aggressivity measures the interspecies competition
in intercropping by relating the yield changes of the
two component crops (Willey and Rao, 1980). In the
present paper, we employed the aggressivity concept
to evaluate the difference between the extent to which
intercropped species `a' and `b' vary from their
respective sole cropping yields:
Aab ˆ
Yia
Ysa Fa
Yib
Ysb Fb
(1)
where Yia and Yib are yields of crops `a' and `b' in
intercropping, Ysa and Ysb are yields of crops `a' and
`b' in sole cropping. Fa and Fb are the proportion of the
area occupied by crops `a' and `b' in the intercropping.
When Aab is greater than 0, competitive ability of crop
`a' exceeds that of crop `b' in intercropping.
2.5.2. Nutrient competitive ratio
Competitive ratio, introduced by Willey and Rao
(1980), measures competitive ability of different species. Morris and Garrity (1993) used it as an indicator
of the advantage in nutrient uptake by one species in
intercropping over the other as follows:
NUia
NUib
CRab ˆ
(2)
NUsa Fa
NUsb Fb
where NUia and NUib are nutrient uptakes by species
Ca and Cb in intercropping, NUsa and NUsb are nutrient
uptakes by species Ca and Cb in sole cropping, Fa and
Fb are the proportions of the area occupied by crops `a'
and `b' in the intercropping. When Cab is greater than
1, competitive ability in taking up given nutrient by
crop `a' is greater than crop `b' in intercropping.
2.5.3. Weighted means of nutrient acquisition by
sole crops
Comparing total nutrient uptake by intercropping
with that of sole cropping system can indicate the
effect of interspecies interactions on nutrient acquisition within the intercropping system. As intercropping
comprises at least two crops, the total nutrient acquisition by intercropping can be compared with the
weighted means of two sole crops based on their
proportion in the intercropping. In this paper, we
calculated the weighted means of nutrient acquisition
by sole cropping as follows:
weighted mean ˆ NUsa Fa ‡ NUsb Fb
(3)
where NUsa and NUsb are nutrient uptakes by crops `a'
and `b' in sole cropping. Fa and Fb are the proportions
of the area occupied by the respective crops in the
intercropping.
Statistical signi®cance of differences between treatments in the split±split±plot and split±plot design models was analyzed by analysis of variance (ANOVA) and
multiple comparison (SAS, 1985).
3. Results and discussions
3.1. Signi®cant grain yield advantage of
intercropping
There were signi®cant yield advantages of both the
wheat/maize intercropping and the wheat/soybean
intercropping. The grain yields of wheat were
increased by intercropping regardless of plant density
or P application. Similar results were observed for
maize, with the exception of P0D1 treatment. Phosphorus fertilizer application increased yields of intercropped wheat at density D2 and those of intercropped
maize by 15% at D2 (Table 1). In the wheat/soybean
intercropping, grain yields of wheat and soybean were
generally increased by intercropping, with an exception of soybean for P0 treatment, indicating that yield
advantage of intercropping was affected by P supply.
L. Li et al. / Field Crops Research 71 (2001) 123±137
Table 1
Grain yields (t ha 1) of wheat and maize in wheat/maize
intercroppinga
Rate of
P applied
Planting
density
P0
D1
D2
D1
D2
P53
Wheatb
Maizec
Intercropped
Sole
Intercropped Sole
8.1
7.3
8.4
9.2
5.5
5.2
4.7
5.9
11.8
14.9
12.9
17.1
(3.9)
(3.5)
(4.0)
(4.4)
(6.1)
(7.7)
(6.7)
(8.9)
12.2
8.6
10.8
12.9
a
Values in parentheses are yields based on whole of the
intercropping area, including the areas occupied by both wheat and
maize.
b
LSD0:05 ˆ 0:7.
c
LSD0:05 ˆ 1:7.
Overall yields of soybean (included intercropped and
sole) were enhanced by P application but not for wheat
(Table 2).
High plant density signi®cantly decreased wheat
grain yield in intercropping compared to low plant
density when no P fertilizer was applied. The high
plant density, however, increased the yield signi®cantly at the 0.05 level when P fertilizer was applied
(Table 1), indicating that yield increase with increasing planting density in the intercropped wheat was
dependent on adequate P fertilizer. Furthermore,
yields of intercropped maize were signi®cantly greater
in density D2 than in D1, regardless of P fertilizer
application, indicating a higher optimal plant density
for intercropping than for sole cropping (Table 1).
3.2. Border-row yield effects on intercropped wheat
The yields of wheat in the border row were signi®cantly increased compared with in the inner rows
in the wheat/maize intercropping or in the rows of sole
127
Table 2
Grain yields (t ha 1) in the wheat/soybean intercroppinga
Rate of
P applied
Wheatb
Soybeanc
Intercropped
Sole
Intercropped
Sole
P0
P33
7.4 (5.6)
8.3 (6.2)
5.8
6.4
1.5 (3.8)
1.8 (4.5)
1.5
1.6
a
Values in parentheses are yields based on whole of the
intercropping area, including the areas occupied by both wheat and
soybean.
b
LSD0:05 ˆ 1:2.
c
LSD0:05 ˆ 0:1.
wheat. Compared to sole wheat, yields of intercropped
wheat in border row were higher by 56% (row 1) to
92% (row 6) for intercropping with maize (Fig. 1A and
B) and by 60% (row 1) to 75% (row 6) for intercropping with soybean (Fig. 1C).
As for biological yields (grain ‡ straw), the borderrow wheat yield increase was 125% over respective
sole cropping, which was equivalent to 42% increase
on average for all six rows of intercropped wheat
(including both border and inner rows). The yield
of the inner row in intercropped wheat was also greater
than that of the rows in sole wheat. The increase was
equivalent to 22% on an average for six rows of
intercropped wheat, including both border and inner
rows. Therefore, out of a 64% overall increase in yield
in intercropped wheat, about 33% came from innerrow effects and about 67% came from the border-row
effects.
If the maize was not grown in the area between two
wheat strips in the designed wheat/maize intercropping, i.e. leaving all of above- and below-ground
resource for wheat, the border yield effect on wheat
would not have been enhanced further (data not
Fig. 1. Grain yields of wheat located at various row positions for sole wheat (A), wheat intercropped with maize (B), and with soybean (C).
128
L. Li et al. / Field Crops Research 71 (2001) 123±137
shown), indicating that the border-row yield potential
for wheat intercropped with maize had been achieved.
3.3. Nutrient acquisition by intercropped species
Wheat intercropped with maize acquired more
nitrogen than did sole wheat, regardless of P application and plant density. Compared to low plant density,
high plant density enhanced N acquisition by intercropped wheat with P application but reduced it without P application. Furthermore, N uptake by
intercropped maize was signi®cantly greater than by
sole maize, with the exception of treatment D1 without
P application, suggesting no signi®cant N uptake
advantage at low planting density of maize when P
supply was limited. Overall N uptake by the wheat/
maize intercropping system was also signi®cantly
higher than the weighted mean of sole wheat and
maize at maturity, except for treatment P0D1. However, effects of P application and crop planting density
on N uptake by wheat/maize intercropping were not
signi®cant (Table 3).
Phosphorus acquisition by wheat was signi®cantly
greater in intercropping than in sole crop regardless of
P application or planting density treatments. There
was a signi®cant decrease of P uptake by intercropped
wheat at high plant density, but not at 53 kg ha 1 of P
application. This result revealed that high planting
density of intercropped wheat depends on adequate P
supply. Phosphorus uptake by maize, in general, was
greater in intercropping than in sole crops at high plant
density without P and at low plant density with
53 kg P ha 1 application. Overall phosphorus acquisition by wheat/maize intercropping system was signi®cantly greater than the weighted mean determined
for sole cropping. Phosphorus application enhanced
P acquisition by intercropping only at low plant
densities (Table 3).
Potassium uptake by intercropped wheat was
greater than by sole wheat regardless of P fertilizer
and planting density treatments. There was a signi®cant difference in K uptake by intercropped wheat
between low and high P supply at high plant density,
and between the high and low planting density at high
and low P supply. However, P fertilization and planting density did not in¯uence K uptake by maize and the
effect of intercropping on K acquisition by maize
was not signi®cant. Total K uptake by intercropping,
Table 3
Nutrient uptake by wheat and maize in intercropping and sole cropping at maturity (15 July for wheat and 24 September for maize)
Rate of P applied
Planting density
Wheata
Maizeb
Weighted meanc,d
Intercropped
Sole
Intercropped
Sole
Intercropped
Sole
D1
D2
D1
D2
270
236
284
324
188
193
175
213
395
441
414
410
419
286
349
338
335
343
352
369
308
241
265
278
D1
D2
D1
D2
36
31
38
40
24
23
20
25
61
65
71
58
62
40
55
52
49
49
55
50
44
32
39
39
D1
D2
D1
D2
387
322
362
418
233
208
185
238
697
609
655
667
642
519
622
536
472
472
514
548
446
370
412
393
1
Nitrogen (kg N ha )
P0
P53
Phosphorus (kg P ha 1)
P0
P53
Potassium (kg K ha 1)
P0
P53
a
LSD0:05 ˆ 29 (for nitrogen), 4 (for phosphorous) and 40 (for potassium).
LSD0:05 ˆ 59 (for nitrogen), 10 (for phosphorous) and 139 (for potassium).
c
LSD0:05 ˆ 32 (for nitrogen), 5 (for phosphorous) and 73 (for potassium).
d
Weighted mean was calculated by land area proportion for each crop in the intercropping as weight coef®cient.
b
L. Li et al. / Field Crops Research 71 (2001) 123±137
129
Table 4
Nutrient uptake by wheat and soybean in intercropping and sole cropping at maturity (25 July for wheat and 22 September for soybean)
Wheata
Rate of P applied
Soybeanb
Weighted meanc,d
Intercropped
Sole
Intercropped
Sole
Intercropped
Sole
180
217
137
132
181
172
170
197
180
206
145
148
Phosphorus (kg P ha 1)
P0
P33
31
27
25
27
15
21
13
17
27
26
22
25
Potassium (kg K ha 1)
P0
P33
299
383
211
211
146
178
206
223
260
332
210
214
1
Nitrogen (kg N ha )
P0
P33
a
LSD0:05 ˆ 35 (for nitrogen), 6 (for phosphorous) and 49 (for potassium).
LSD0:05 ˆ 54 (for nitrogen), 4 (for phosphorous) and 29 (for potassium).
c
LSD0:05 ˆ 30 (for nitrogen), 4 (for phosphorous) and 37 (for potassium).
d
Weighted mean was calculated by land area proportion for each crop in the intercropping as weight coef®cient.
b
including wheat and maize, was signi®cantly greater
than that by the weighted mean for sole wheat
and maize with an exception for P0D1 treatment
(Table 3).
Nitrogen acquisition by wheat intercropped with
soybean was signi®cantly increased when compared
with sole wheat, regardless of P fertilization. However,
nitrogen acquisition by soybean was not signi®cantly
affected by intercropping. Furthermore, total N uptake
by intercropped wheat and soybean was signi®cant
greater than that by the weighted mean of sole wheat
and soybean at maturity (Table 4). These results
suggest that wheat bene®ted from intercropping in
terms of N acquisition, whereas soybean did not, but
overall, the system bene®ted in N acquisition from
intercropping. Phosphorus acquisition by wheat
increased by intercropping only without P application.
Phosphorus acquisition by soybean was signi®cantly
increased by P application under intercropping and by
intercropping under P application. Potassium uptake
by wheat was enhanced by P application and intercropping. Potassium uptake by soybean, however,
was signi®cantly decreased by intercropping, and
was not signi®cantly increased by P application in
the sole cropping. There were signi®cant differences
in total K uptake between intercropping and the
weighted mean for sole crops (Table 4). This demonstrated that there was interspecies competition
between intercropped wheat and soybean during the
co-growth stage, with wheat being the dominant
species, and that intercropping could result in more
K uptake from the soil.
3.4. Border-row effect on nutrient acquisition by
intercropped wheat
Nitrogen accumulation by wheat in the border row
was signi®cantly greater than in the inner row or in
sole wheat, regardless of P fertilizer or plant density
(Fig. 2). Furthermore, average N accumulation by
intercropped wheat in inner rows was 14% higher
than that by sole wheat rows, indicating that the
increase of N acquisition by intercropped wheat
resulted not only from the border-row effect but also
from the inner-row effects on N acquisition.
There was a 59±76% increase in P accumulation by
the intercropped wheat in the border row over that at
the inner row (Fig. 2), and 107±129% increase over
sole wheat. Average P accumulation by intercropped
wheat in inner rows exceeded by 30% the average P
accumulation by sole wheat (Fig. 2). When maize was
not grown in the area between two wheat strips, i.e.
when the competitor (maize) was removed (P53D2,
IW/M, NM), there was a greater border-row effect on
P accumulation in the wheat (82±96%) compared to
wheat intercropping with maize. These results suggested that the increase in P accumulation by intercropped wheat might have resulted from both the
130
L. Li et al. / Field Crops Research 71 (2001) 123±137
Fig. 2. Nitrogen (A), phosphorus (B) and potassium (C) acquisition by wheat tops and (D) nitrogen concentration in wheat tops located at
various row positions: ( ) border row (rows 1 and 6); (&) inner row (rows 2±5); ( ) sole cropping row. D1 and D2: planting density (low and
high); IW/M: wheat intercropped with maize; NM: no maize was planted in the area between two wheat strips; NS: not signi®cant; P0 and P53:
rates of P fertilization were 0 and 53 kg P ha 1, respectively; SW: sole wheat.
border- and inner-row effects. Without P fertilization,
phosphorus accumulation by the inner row of intercropped wheat was greater at lower than at higher
planting density, indicating that intraspeci®c competition is density dependent. There was a greater borderrow effect on P accumulation in the intercropped
wheat when P fertilizer was applied, compared to
treatments without P fertilization, but there was no
signi®cant change in P uptake by inner-row wheat
regardless of planting density.
In wheat/soybean intercropping, nitrogen, phosphorus and potassium acquisition by border-row wheat
were also greater than by inner rows of wheat (Table 5).
3.5. Effect of intercropping on yield and nutrient
acquisition by intercropped maize and soybean
during co-growth
The biomass, N, P and K accumulation in intercropped wheat were signi®cantly greater than by sole
wheat from the second sampling (22 May) to the ®fth
sampling (2 July) (Table 6) to maturity (15 July)
(Table 4) due to border- and inner-row effects. However, the biomass accumulation of intercropped maize
and soybean was only 45±78% of sole maize and 39%
of sole soybean when wheat was harvested. Nitrogen,
P and K accumulation in intercropped maize were
L. Li et al. / Field Crops Research 71 (2001) 123±137
131
Table 5
N, P and K uptake by border and inner rows of wheat intercropped with soybeana
Rowb
N (g N m 1 of row)
P (g P m 1 of row)
K (g K m 1 of row)
a
b
1
2
3
4
5
6
4.10 a
0.54 a
5.88 a
2.74 b
0.33 b
4.35 ab
2.81 b
0.35 b
4.48 ab
2.38 b
0.29 b
3.60 b
3.70 ab
0.4 ab
5.85 a
4.16 a
0.54 a
6.44 a
Within each row means followed by the same letter are not signi®cantly different by multiple comparison.
Rows 1 and 6 are border rows. Rows 2±5 are inner rows.
signi®cantly reduced after the third sampling, and
were only 45±66, 40±76 and 50±86%, respectively,
of those by sole maize at the ®fth sampling (2 July).
Similarly, nitrogen, P and K accumulation in the
intercropped soybean during the wheat/soybean
co-growth stage were also signi®cantly less than those
by sole soybean for co-growth stage (Table 7). These
results showed there were positive and negative interspeci®c interactions between intercropped species in
the wheat/maize and wheat/soybean intercropping
systems. So, it is important that the trade-off between
the increase of wheat and decrease of maize or soybean in terms of biomass, N, P and K accumulation is
positive or negative. Namely, whether there is facil-
itation of the wheat by the maize or the soybean, or
that interspecies competition alters the distribution of
limited resources between the intercropped species.
3.6. Nutrient accumulation by intercropping and sole
cropping during the co-growth stage
The N and K acquisitions by wheat/maize intercropping system were greater than the weighted mean
of N and K acquisition by sole wheat and maize at
most sampling dates, as shown for the ®fth sampling
(Table 6). A similar result was found in wheat/soybean
intercropping for P without P application. This reveals
interspeci®c facilitation in N and K acquisition
Table 6
Nutrient uptake by wheat and maize in intercropping and sole cropping at 5th sampling (2 July for wheat and maize)
Rate of P applied
Planting density
Wheata
Maizeb
Weighted meanc,d
Intercropped
Sole
Intercropped
Sole
Intercropped
Sole
D1
D2
D1
D2
358
341
367
453
196
195
264
232
81
100
59
80
76
155
98
168
214
216
207
259
134
174
178
199
D1
D2
D1
D2
36
28
33
43
19
18
25
22
9
12
7
10
10
16
13
26
22
20
20
26
14
17
19
24
D1
D2
D1
D2
353
331
361
484
183
181
166
217
164
287
170
302
97
158
87
163
220
241
219
317
173
237
168
262
1
Nitrogen (kg N ha )
P0
P53
Phosphorus (kg P ha 1)
P0
P53
Potassium (kg K ha 1)
P0
P53
a
LSD0:05 ˆ 59 (for nitrogen), 5 (for phosphorous) and 70 (for potassium).
LSD0:05 ˆ 27 (for nitrogen), 6 (for phosphorous) and 32 (for potassium).
c
LSD0:05 ˆ 27 (for nitrogen), 4 (for phosphorous) and 38 (for potassium).
d
Weighted mean was calculated by land area proportion for each crop in the intercropping as weight coef®cient.
b
132
L. Li et al. / Field Crops Research 71 (2001) 123±137
Table 7
Nutrient uptake by wheat and soybean tops in intercropping and sole cropping at second sampling (16 June for wheat and soybean)
Wheata
Rate of P applied
Soybeanb
Weighted meanc,d
Intercropped
Sole
Intercropped
Sole
Intercropped
Sole
126
147
86
118
17
26
87
108
99
117
86
116
Phosphorus (kg P ha 1)
P0
P33
19
22
12
17
15
18
10
16
Potassium (kg K ha 1)
P0
P33
254
292
243
252
198
229
209
225
1
Nitrogen (kg N ha )
P0
P33
1.4
2.8
7.3
11.1
31
39
106
146
a
LSD0:05 ˆ 39 (for nitrogen), 5 (for phosphorous) and 61 (for potassium).
LSD0:05 ˆ 7 (for nitrogen), 2 (for phosphorous) and 15 (for potassium).
c
LSD0:05 ˆ 29 (for nitrogen), 4 (for phosphorous) and 46 (for potassium).
d
Weighted mean was calculated by land area proportion for each crop in the intercropping as weight coef®cient.
b
3.7. Aggressivity of wheat relative to maize or
soybean
between intercropped wheat and maize, and in P
acquisition between intercropped wheat and soybean
without P fertilizer. However, there was no signi®cant
difference in P acquisition between wheat/maize intercropping and the weighted means of sole wheat and
maize (Table 6) or in N and K acquisition between the
wheat/soybean intercropping and sole wheat and soybean (Table 7). These results suggest that interspeci®c
interaction only changes the distribution of P resource
between wheat and maize and the distribution of N and
K resource between wheat and soybean. Wheat
acquired more and maize and soybean less. At the
same time, these results reveal not only interspecies
competition, but also interspecies facilitation in nutrient acquisition between intercropped species during
the co-growth stage in the wheat/maize and wheat/
soybean intercropping.
During the co-growth period (from maize or soybean emergence to wheat harvesting), wheat was
always the dominant species. Awm and Aws were
always signi®cantly greater than zero (Tables 8 and
9). This further emphasizes that wheat is able to
acquire more resources than that the other species
in the wheat/maize and wheat/soybean intercropping.
3.8. Nutrient competitive ratios of intercropping
species
In wheat/maize intercropping, the N, P and K
nutrient competitive ratios of wheat relative to maize
always exceeded 1 during co-growth of two species
Table 8
Aggressivity (Awm) of wheat relative to maize during co-growth in wheat/maize intercroppinga
Rate of P applied
Planting density
Date of sampling
7 May
P0
D1
D2
D1
D2
P53
a
*
0.26
0.04 NS
0.55*
0.04 NS
22 May
*
0.42
1.63*
1.08*
0.70*
NS: refers to signi®cance of difference more than 0 at no signi®cance.
Refers to signi®cance of difference more than 0 at P < 0:05.
**
Refers to signi®cance of difference more than 0 at P < 0:01.
*
4 June
0.46 NS
0.99*
0.64*
0.49*
18 June
*
0.95
1.15**
1.02*
1.01**
2 July
15 July
0.99 NS
0.92*
1.02**
1.59*
0.69*
0.75*
1.31*
1.17*
L. Li et al. / Field Crops Research 71 (2001) 123±137
133
Table 9
Aggressivity (Aws) of wheat relative to soybean during co-growth in
wheat/soybean intercropping
Table 11
Nutrient competitive ratios of wheat relative to soybean during cogrowth in wheat/soybean intercropping
Rate of P applied
Rate of P applied
Date of sampling
25 May
16 June
**
P0
P33
0.96
0.35**
*
*
0.89
0.82**
0.80
0.72**
Refers to signi®cance of difference more than 0 at P < 0:05.
Refers to signi®cance of difference more than 0 at P < 0:01.
**
(Table 10), suggesting that N, P and K acquisition
ability of wheat was greater than maize during the cogrowth stage. Secondly, there was an effect of planting
density on N, P and K competitive ratios for wheat
relative to maize. Without P application, N, P and K
competitive ratios at high planting density were higher
than those at low planting density (Table 10). However, with P fertilization, the ratios for wheat relative
to maize were not dependent on planting density.
Thirdly, peak competition for N, P and K between
wheat and maize under high planting density appeared
at the early growth stage without P fertilization, and at
the late growth stage under low plant density
(Table 10). In the wheat/soybean intercropping system, N and P competitive ratios of wheat relative to
Table 10
Nutrient competitive ratios of wheat relative to maize during cogrowth in wheat/maize intercropping
Rate of
P applied
Planting
density
Date of sampling
7
May
22
May
4
June
18
June
2
July
15
July
D1
D2
D1
D2
1.2
1.0
1.9
1.2
1.7
3.6
3.0
2.6
1.7
2.8
2.7
2.0
2.7
3.4
3.7
3.5
2.0
2.9
2.3
4.6
2.3
2.5
2.9
4.3
Phosphorus
D1
P0
D2
P53
D1
D2
1.4
1.4
1.7
1.4
2.0
7.5
3.3
3.1
1.6
3.2
1.9
1.5
3.4
4.4
4.9
4.5
2.2
2.1
2.7
4.9
2.1
2.4
2.9
4.8
1.4
1.3
1.7
1.1
1.5
6.7
3.0
2.7
1.4
2.9
2.3
1.4
2.8
3.0
3.3
4.7
3.7
3.5
4.2
4.4
2.0
1.8
1.2
3.9
Nitrogen
P0
P53
Potassium
P0
P53
D1
D2
D1
D2
Average
25 May
16 June
5 July
Nitrogen
P0
P33
2.1
1.2
8.1
5.1
5.6
4.1
5.3
3.5
Phosphorus
P0
P33
1.9
1.2
8.3
5.2
4.9
3.1
5.0
3.2
Potassium
P0
P33
1.8
1.5
3.7
4.4
3.4
2.7
3.0
2.5
5 July
*
Date of sampling
soybean were also greater than 1 during co-growth.
Phosphorus fertilization reduced the N and P competitive ratios of wheat relative to soybean, but did not
in¯uence K competitive ratios (Table 11).
4. Discussion
The trade-off between increasing the yield of the
dominant species and decreasing that of the dominated
species has three possible outcomes for intercropping
systems, i.e. yield advantage (LER > 1), yield disadvantage (LER < 1) and the intermediate result
(LER ˆ 1) (Vandermeer, 1989). The results of present
experiment, however, showed that yields of intercropped wheat, maize and soybean were all increased
by intercropping.
In general, shoot N concentrations in plants are
decreased with increasing shoot dry matter in sole
cropping (Plenet and Lemaire, 1999). However, shoot
N and P concentrations of intercropped wheat in
border rows were not reduced, rather they were
increased compared to the inner row, although dry
matter yield of border row was always greater than
that of inner row (Fig. 2D). This indicated that nutrition did play an important role in yield determination
in intercropping, as did water and light.
In pigeon pea/sorghum intercropping, the optimal
density for intercropped pigeon pea was considerably
higher than for the sole crop (Natarajan and Willey,
1980). This was shown to be true in wheat/maize
intercropping in present study.
134
L. Li et al. / Field Crops Research 71 (2001) 123±137
The greater N acquisition by a non-legume crop
intercropped with a legume is frequently reported in
the literature (Francis, 1986; Vandermeer, 1989; Stern,
1993). In wheat/soybean/legume intercropping, an
increase in N acquisition may be derived in two ways.
First, the difference in competitive abilities of component species may increase N uptake by wheat.
Wheat, with higher competitive ability relative to
soybean, acquired more N from soil in the present
study. This may conversely stimulate nodulation in
legume, as noted for beans intercropped with maize
(Rerkasem et al., 1988). Second, an increase in N
acquisition may also be attributed to N transfer to
wheat from soybean, as in most legume/cereal associations (Brophy et al., 1987). Intercropped maize
derived 30±35% of its N content from the associated
groundnut plants (Senaratne et al., 1993). There are
few reports on an advantage of N uptake by cereal/
cereal intercropping system over respective sole cropping. The explanation of the N increase of wheat/
soybean intercropping was not applicable to wheat/
maize intercropping because that association has no
N2 ®xation. An increase in N by intercropped wheat
has probably arisen from difference in N competitive
ability between wheat and maize, whereas the increase
in N uptake by intercropped maize probably resulted
from ``recovery'' of growth and N uptake by maize
after wheat harvest. These will be discussed in a
subsequent paper.
Suryatana (1976) also observed an increase in P
uptake by maize/rice and sorghum/sun¯ower intercroppings over sole cropping (cited from Morris and
Garrity, 1993). In ®eld trials, intercropping two onion
cultivars with cotton also increased P uptake and seed
cotton yield, compared with cotton in pure stands.
It was suggested that chelating compounds exuded
by onion increasing available P to cotton plants
(Shanmugham, 1988). These results all show that
some legume/non-legume intercropping combinations
could indeed acquire more P from soil and fertilizer.
Several studies have reported an interspeci®c facilitation between intercropped legumes and cereals,
such as in wheat/lupin (Gardner and Boundy, 1983;
Horst and Waschkies, 1987) and pigeon pea/sorghum
(Ae et al., 1990), where legumes increased P uptake by
associated cereals by mobilizing insoluble phosphate
by releasing exudation from roots. However, such
interspeci®c facilitation was not observed here in
wheat/soybean intercropping. Furthermore, there
was no evidence that the increase in P uptake by
intercropped wheat and maize derived from the exudation of component species. In cassava/groundnut
intercropping systems, cassava accumulated 96±99%
of the 32 P applied to cassava and 48±88% when it was
applied to the intercrops depending on whether cassava was planted on paired row-ridges, mounds or ¯at
beds. Groundnut absorbed negligible quantities of 32 P
from the cassava root zone (Ashokan et al., 1988).
These results reveal that differences in interspeci®c
competitive abilities play important role in P acquisition by intercropped species.
In other studies, K uptake by sorghum/soybean and
sorghum/cowpea intercropping declined compared
with the weighted mean of respective sole crops.
Potassium uptake was increased by intercropping in
maize/soybean, maize/rice, maize/pigeon pea, cassava/cowpea, sorghum/soybean, sorgum/sun¯ower,
sorghum/blackgram and sorghum/cowpea combinations (Morris and Garrity, 1993).
Yield advantage of intercropped wheat came from
both the border- and inner-row effects in the wheat/
maize intercropping. The former is in agreement with
the literature. Yield increase in a maize/soybean strip
intercropping system were primarily due to increases
in the border rows of maize adjacent to soybean (West
and Grif®th, 1992; Fortin et al., 1994; Lesoing and
Francis, 1999). In canola and soybean strip intercropping, land equivalent ratios (LERs) were signi®cantly
greater than 1.0, and edible oil and crude protein yields
were up to 170% of the mean of the two sole crops, due
largely to a border-row effect. Canola border-row
yields were 225±590% of those of sole crop rows
and the soybean border-row yield remained unchanged
(Ayisi et al., 1997). However, most researchers have
paid inadequate attention to contributions for yield
increases in inner rows to yield advantage of intercropping. In contrast, no comparison was made of the yield
in inner rows in intercropping to sole cropping rows.
Our results reveal that yield increases of intercropped
wheat are attributed not only to yield increase of border
rows, but also of inner rows in the wheat/maize intercropping. In maize/wheat/soybean strip intercropping,
maize border-row yield was increased at the expense of
yield of border-row soybean (Iragavarapu and Randall,
1996; Ghaffarzadeh et al., 1998). Our results in wheat/
maize and wheat/soybean intercropping showed that
L. Li et al. / Field Crops Research 71 (2001) 123±137
increase in growth and nutrient accumulation of intercropped wheat was gained at the expense of maize or
soybean during the co-growth stage.
Intense interspeci®c interactions occurred between
two intercropped species at the interface of the border
row of each species strip. The result was greater
accumulation of nutrients by the dominant species
and less by the subordinate species. In grass±white
clover associations, competitive abilities of the grass
in complex mixture were positively associated with
dry matter yield (Piano and Annicchiarico, 1995). As
for the inner-row effect, it is probable that interspecies
interactions allowed wheat in border rows to obtain
more resources than did sole wheat, while leaving
more nutrients for inner-row wheat. Consequently, this
alleviates intraspecies competition in the wheat strip,
probably resulting in a greater yield of inner-row
wheat than of sole wheat.
Plant ecologists de®ne interspeci®c competition as
an interaction between two species that reduces the
®tness of one (0, ) or both of them ( , ) (Crawley,
1997). During the co-growth of the two crops in the
wheat/maize and wheat/soybean intercropping, apparent interspeci®c interactions resulted in increasing
wheat growth and decreasing growth of soybean or
maize (‡, ) in terms of both yield and nutrient
acquisition. This is similar to contramensalism (one
species was increased and the other was decreased) in
micro-organism communities (Hodge and Arthur,
1996). However, when two plants grow near one
another, basic physiological principles suggest that
they will almost always compete, whether or not
facilitation is operative (Vandermeer, 1989). The
results also demonstrate that biomass and nutrient
accumulation in intercropped maize or soybean was
decreased signi®cantly during the co-growth stage in
wheat/maize and wheat/soybean intercropping systems. At the same time, an increased biomass and
N and K accumulation in wheat/maize intercropping,
and P acquisition in wheat/soybean intercropping,
without P application over weighted mean of respective sole cropping revealed interspecies facilitation
between the intercropped wheat and maize and
between the wheat and the soybean during the cogrowth stage. This showed that interspecies competition and interspecies facilitation exist together in the
two intercropping systems. The co-existence of positive and negative interactions in the same ecosystem
135
has also been found in forests between Abies lasiocarpa and Pinus albicaulis (Callaway, 1998), in the
shrub Retama sphaerocarpa and herb Marrubium
vulgare community (Pugnaire et al., 1996), in other
ecosystems (Callaway, 1999). Here, there was a positive effect on wheat and a negative effect on soybean
or maize during the co-growth. The interactions were
strongly asymmetrical, so the direction of the stronger
effect is crucial to apparent performance of intercropping as Connell (1990) suggested for natural plant
communities.
The competitive ability of wheat, as indicated by the
aggressivity and the nutrient competitive ratios, was
greater than for maize or soybean. The indicators
correspond to the Crimes' theory of competitive success in which the species with greater capacity for
resource capture will be the superior competitor
(Grace, 1990). Wheat was the superior competitor
during the co-growth stage. Below-ground competitive
ability was correlated with such attributes as density,
surface area, and plasticity either in root growth or in
the properties of enzymes involved in nutrient uptake
(Casper and Jackson, 1997). Moreover, the extent of
below-ground competition experienced by an individual will be a function of both the sizes and number of
neighbors (Casper and Jackson, 1997). Initial crop size
may also in¯uence the competitive ability of species
(Gerry and Wilson, 1995). In the present study, initial
size of wheat exceeded that of maize or soybean due to
earlier emergence. Further work is necessary to investigate the mechanisms underlying the difference
between wheat and the other species.
5. Conclusions
The yields of species at maturity in the two intercropping systems were generally greater than those of
respective sole species. Intercropping also enhanced
nutrient acquisition by the crops. The yield and nutrient increases of intercropped wheat were contributed
to border-row effects (about 67% contribution for
biomass) and inner-row effect (about 33% contribution for biomass). The border-row effects of intercropping on yield and nutrient acquisition were
attributed to the difference in competitive ability for
nutrients between wheat and maize or soybean
because aggressivities of wheat relative to maize or
136
L. Li et al. / Field Crops Research 71 (2001) 123±137
soybean were 0.26±1.63 or 0.35±0.96, respectively.
The nutrient competitive ratios also showed that wheat
had greater competitive ability for N, P and K than did
maize and soybean.
Acknowledgements
We are very grateful to the Major State Basic
Research Development Program of the People's Republic of China (Project number G1999011707), the
National Natural Science Foundation of China (Project
number 39670435) and AusAid for generous ®nancial
support. We are also very grateful to two anonymous
referees for valuable comments on the manuscript.
References
Ae, N., Arihara, J., Okada, K., Yoshihara, T., Johansen, C., 1990.
Phosphorus uptake by pigeon pea and its role in cropping
systems of the Indian subcontinent. Science 248, 477±480.
Aerts, R., 1999. Interspeci®c competition in natural plant
communities: mechanisms, trade-offs and plant±soil feedbacks.
J. Exp. Bot. 50, 29±37.
Ashokan, P.K., Wahid, P.A., Sreedharan, C., 1988. Relative uptake
of 32 P by cassava, banana, elephant foot yam and groundnut in
intercropping systems. Plant and Soil 109, 1±30.
Ayisi, K.K., Putman, D.H., Vance, C.P., Russelle, M.P., Allan, D.L.,
1997. Strip intercropping and nitrogen effects on seed, oil, and
protein yields of canola and soybean. Agron. J. 89, 23±29.
Brophy, L.S., Heichel, G.H., Russelle, M.P., 1987. Nitrogen
transfer from forage legumes to grass in a systematic planting
design. Crop Sci. 27, 753±758.
Callaway, R.M., 1998. Competition and facilitation on elevation
gradients in subalpine forests of the northern Rocky Mountains,
USA. Oikos 82, 561±573.
Callaway, R.M., 1999. Facilitation in plant communities. In:
Pugnaire, F.I., Valladares, F. (Eds.), Handbook of Functional
Plant Ecology. Marcel Dekker, New York, pp. 623±648.
Casper, B.B., Jackson, R.B., 1997. Plant competition underground.
Ann. Rev. Ecol. Systematics 28, 545±570.
Chowdhury, M.K., Rosario, E.L., 1994. Comparison of nitrogen,
phosphorus and potassium utilization ef®ciency in maize/
mungbean intercropping. J. Agric. Sci. Camb. 122, 193±199.
Connell, J.H., 1990. Apparent versus ``real'' competition in plants.
In: Grace, J.B., Tilman, D. (Eds.), Perspective on Plant
Competition. Academic Press, San Diego, pp. 9±26.
Crawley, M.J., 1997. Plant Ecology. Blackwell Scienti®c Publications, Cambridge.
Dauro, D., Mohamedsaleem, M.A., 1995. Shoot and root interactions in intercropped wheat and clover. Trop. Agric. 72 (2),
170±172.
Elmore, R.W., Jackobs, J.A., 1986. Yield and nitrogen yield of
sorghum intercropped with nodulating and non-nodulating
soybeans. Agron. J. 78, 780±782.
Exner, D.N., Davidson, D.G., Ghaffarzadeh, M., Cruse, R.M.,
1999. Yields and returns from strip intercropping on six Iowa
farms. Am. J. Alternative Agric. 14, 69±77.
Fortin, M.C., Culley, J., Edwards, M., 1994. Soil water, plant
growth, and yield of strip intercropped corn. J. Prod. Agric.
7, 63±69.
Francis, C.A. (Ed.), 1986. Multiple Cropping Systems. Macmillan,
New York.
Gardner, W.K., Boundy, K.A., 1983. The acquisition of phosphorus
by Lupinus albus L. IV. The effect of interplanting wheat and
white lupin on the growth and mineral composition of the two
species. Plant and Soil 70, 391±402.
Gerry, A.K., Wilson, S.D., 1995. The in¯uence of initial size on the
competitive responses of six plant species. Ecology 76, 272±279.
Ghaffarzadeh, M., Prechac, F.G., Cruse, R.M., 1994. Grain yield
response of corn, soybean, and oat grown in a strip
intercropping system. Am. J. Alternative Agric. 9, 171±177.
Ghaffarzadeh, M., Prechac, F.G., Cruse, R.M., Harbur, M.M., 1998.
Fertilizer and soil nitrogen use by corn and border crops in a
strip intercropping system. Agron. J. 90, 758±762.
Grace, J., 1990. On the relationship between plant traits and
competitive ability. In: Grace, J.B., Tilman, D. (Eds.),
Perspective on Plant Competition. Academic Press, San Diego,
pp. 51±65.
Helenius, J., Jokinen, K., 1994. Yield advantage and competition in
intercropped oats (Avena sativa L.) and faba bean (Vicia faba
L.) Ð application of the hyperbolic yield density model. Field
Crops Res. 37, 85±94.
Hodge, S., Arthur, W., 1996. Contramensal interactions between
species. Oikos 77, 371±375.
Horst, W.J., Waschkies, C., 1987. Phosphorus nutrition of spring
wheat (Triticum aestivum L.) in mixed culture with white lupin
(Lupinus albus L.). Zeitschrift fur P¯anzenernahrung und
Bodenkunde 150, 1±8 (in German with English abstract).
Iragavarapu, T.K., Randall, G.W., 1996. Border effects on yields in
a strip intercropped soybean, corn, and wheat production
system. J. Prod. Agric. 9, 101±107.
Jensen, E.S., 1996. Grain yield, symbiotic N2 ®xation and
interspeci®c competition for inorganic N in pea±barley
intercrops. Plant and Soil 182 (1), 25±38.
Lesoing, G.W., Francis, C.A., 1999. Strip intercropping effects on
yield and yield components of corn, grain sorghum, and
soybean. Agron. J. 91, 807±813.
Li, L., Yang, S.C., Li, X.L., Zhang, F.S., Christie, P., 1999.
Interspeci®c complementary and competitive interactions
between intercropped maize and faba bean. Plant and Soil
212 (2), 105±114.
Mandal, B.K., Das, D., Saha, A., Mohasin, M., 1996. Yield
advantage of wheat (Triticum aestivum) and chickpea (Cicer
arietinum) under different spatial arrangements in intercropping. Indian J. Agron. 41 (1), 17±21.
Mandimba, G.R., 1995. Contribution of nodulated legumes on the
growth of Zea mays under various cropping systems. Symbiosis
19, 213±222.
L. Li et al. / Field Crops Research 71 (2001) 123±137
Morris, R.A., Garrity, D.P., 1993. Resource capture and utilization
in intercropping: non-nitrogen nutrients. Field Crops Res. 34,
319±334.
Moynihan, J.M., Simmons, S.R., Sheaffer, C.C., 1996. Intercropping annual medic with conventional height and semidwarf
barley grown for grain. Agron. J. 88, 823±828.
Natarajan, M., Willey, R.W., 1980. Sorghum±pigeon pea intercropping and the effects of plant population density. 1. Growth
and yield. J. Agric. Sci. Camb. 95, 1±58.
Piano, E., Annicchiarico, P., 1995. Interference effects in grass
varieties grown as pure stand, complex mixture and binary
mixture with white clover. J. Agron. Crop Sci. (Zeitschrift fur
Acker und P¯anzenbau) 174, 301±308.
Plenet, D., Lemaire, G., 1999. Relationships between dynamics of
nitrogen uptake and dry matter accumulation in maize crops.
Plant and Soil 216, 65±82.
Pugnaire, F.I., Haase, P., Puigdefabregas, J., 1996. Facilitation
between higher plant species in a semiarid environment.
Ecology 77, 1420±1426.
Rerkasem, B., Rerkasem, K., Peoples, M.B., Herridge, D.F.,
Bergersen, F.J., 1988. Measurement of N2 ®xation in maize
(Zea mays L.)±ricebean (Vigna umbellata [Thumb.] Ohwi and
Ohashi) intercrops. Plant and Soil 108, 125±135.
Sanginga, N., Ibewiro, B., Houngnandan, P., Vanlauwe, B.,
Okogun, J.A., Akobundu, I.O., Versteeg, M., 1996. Evaluation
of symbiotic properties and nitrogen contribution of mucuna to
maize grown in the derived savanna of west Africa. Plant and
Soil 179, 119±129.
137
SAS Institute, 1985. SAS User's Guide: Statistics, Ver 5. SAS
Institute, Cary, NC, USA.
Senaratne, R., Liyanage, N.D.L., Ratnasinghe, D.S., 1993. Effect of
K on nitrogen ®xation of intercrop groundnut and the competition
between intercrop groundnut and maize. Fertil. Res. 34, 9±14.
Shanmugham, K., 1988. Effect of onion and greengram intercrops
on phosphorus release and its uptake by cotton. Curr. Sci. 57,
1128±1130.
Siame, J., Willey, R.W., Morse, S., 1998. The response of maize/
phaseolus intercropping to applied nitrogen on oxisols in
northern Zambia. Field Crops Res. 55, 73±81.
Smith, M.A., Carter, P.R., 1998. Strip intercropping corn and
alfalfa. J. Prod. Agric. 11 (3), 345±353.
Stern, W.R., 1993. Nitrogen ®xation and transfer in intercrop
systems. Field Crops Res. 34, 335±356.
Stuelpnagel, R., 1993, Intercropping of faba beans (Vicia faba L.)
with oats or spring wheat. In: Proceedings of the International
Crop Science Congress, 14±44 July 1992, Iowa State
University, Ames, IA.
Vandermeer, J., 1989. The Ecology of Intercropping. Cambridge
University Press, New York.
West, T.D., Grif®th, D.R., 1992. Effect of strip intercropping corn
and soybean on yield and pro®t. J. Prod. Agric. 5, 107±110.
Willey, R.W., 1979. Intercropping Ð its importance and research
needs. Part 1. Competition and yield advantages. Field Crop
Abstr. 32, 1±10.
Willey, R.W., Rao, M.R., 1980. A competitive ratio for quantifying
competition between intercrops. Exp. Agric. 16, 117±125.