Agricultural Water Management 98 (2010) 199–212
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Agricultural Water Management
journal homepage: www.elsevier.com/locate/agwat
Distribution of roots and root length density in a maize/soybean strip
intercropping system
Yang Gao a,b , Aiwang Duan a,b,∗ , Xinqiang Qiu a , Zugui Liu a,b , Jingsheng Sun a,b ,
Junpeng Zhang a , Hezhou Wang a
a
b
Key Laboratory for Crop Water Requirement and Its Regulation, Ministry of Agriculture, Xinxiang, Henan 453003, PR China
Farmland Irrigation Research Institute, Chinese Academy of Agricultural Sciences, Xinxiang, Henan 453003, PR China
a r t i c l e
i n f o
Article history:
Received 10 April 2010
Accepted 27 August 2010
Keywords:
Maize/soybean intercropping
Root distribution
Root length density
2D model
a b s t r a c t
In a field experiment in the Yellow River Basin conducted in 2007 and 2008, it was found that, under full
irrigation, the roots of maize not only penetrated deeper than those of soybean but also extended into
soybean stands underneath the space between inner rows of soybean. The roots of soybean, however,
were confined mainly to the zone near the plants. Horizontal growth of the roots of both the crops was
confined mainly to the soil layer 16–22 cm below the surface, a layer that lay above an existing plough
pan. Root length density (RLD) was much higher in the top layer (0–30 cm deep) and in the zone closer
to the plants. The exponential model proved suitable to describe the RLD vertically and horizontally in
both sole cropping and in intercropping.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Many intercropping systems have proved to be better than
sole crops in terms of yield (Li et al., 2001; Tsubo and Walker,
2002; Awal et al., 2006; Zhang et al., 2007) because intercropping makes better use of one or more agricultural resources both
in time and in space (Willey, 1990; Rodrigo et al., 2001). Such
improvements in yield have been attributed almost exclusively
to above-ground interactions between intercropped species, for
example greater interception of sunlight or more efficient conversion of the intercepted radiation. However, yield advantages of
intercropping systems are due to both above- and below-ground
interactions between intercropped species (Li et al., 2006).
Crop growth and final yield of an intercropping system are
closely related to the spread of roots, which determines the uptake
and utilization of water and nutrients. Root extension and distribution can be expressed as root length density (RLD) or root
weight density (RWD) (Adiku et al., 2001). Root length density,
although critical to the uptake of water and nutrients by a crop,
is hard to measure in the field. Box (1996) describes several methods of measuring RLD, but these methods are frequently technically
demanding and costly. A great deal of work related to RLD is based
∗ Corresponding author at: Key Laboratory for Crop Water Requirement and Its
Regulation, Ministry of Agriculture, Xinxiang, Henan 453003, PR China.
Fax: +86 373 3393308.
E-mail addresses: [email protected],
[email protected] (A. Duan).
0378-3774/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.agwat.2010.08.021
on observations in minirhizotrons and therefore biased given the
atypical distribution of roots on the surface of the access tube
(Taylor and Bohm, 1976; Bragg et al., 1983; Chopart and Siband,
1999). Mapping of root impacts on a soil profile is an indirect
method of measuring RLD (Bohm, 1976). Van Noordwijk (1987)
built a model to calculate RLD on the basis of root impacts counted
on a single plane, and Chopart and Siband (1999) improved upon
that work to develop a robust and economical method to quantify
RLD.
Root distribution in space plays a major role in interactions
between species, but only a few studies have investigated root distribution in intercropping systems (Zhang et al., 2002; Zhang and
Huang, 2003; Li et al., 2006). Zhang et al. (2002) indicated that the
growth stages of wheat and faba bean when root weight is maximum did not overlap, which reduced the competition between
the two crops for water and nutrients and resulted in higher yields
of both. Zhang and Huang (2003) reported that root distribution
in a maize/cabbage intercropping system showed a clearly unbalanced distribution, with the roots of maize extending horizontally
to greater distances than those of cabbage, and Adiku et al. (2001)
found that although the roots of maize and cowpea had extended
into the rhizospheres of each other, the ‘encroachment’ on part
of maize was much greater. Li et al. (2006) showed that roots of
maize had not only penetrated deeper than those of the faba bean
but had also spread under the faba bean strip in a maize/faba bean
intercropping system.
The paucity of information on root distribution in intercropping
systems in the past led to oversimplifications, the root systems
being described as either completely mixed (Adiku et al., 1995)
200
Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
Table 1
Physical properties of the soil at the experimental site ( s : saturated water content, f : field capacity).
Soil depth (cm)
0–15
15–30
30–65
65–100
Particle size distribution (%)
Sand
Silt
Clay
Soil texture
20.3
20.5
30.5
74.5
40.5
29.3
42
16.4
29.2
50.2
27.5
9.1
Loamy clay
Clay
Loamy clay
Sandy loam
or fully separate (Kiniry et al., 1992; Kiniry and Williams, 1995).
Recent studies, however, show varying degrees of root mixing in
intercrops depending on availability of soil water (Ozier-Lafontaine
et al., 1998; Adiku et al., 2001; Li et al., 2006): when water is not a
limiting factor, roots grow profusely into all sections of soil; under
water stress, roots are clumped within their ‘own’ zone; under
severe water stress, the roots may not intermingle at all (Adiku
et al., 2001). Thus, the simplistic assumption, namely that roots are
completely mixed or completely separate, is inadequate to describe
the distribution of roots in intercropping systems.
The objectives of this paper were (1) to investigate the horizontal and vertical distribution of crop roots in a maize/soybean
intercropping system, (2) to analyse the interaction between the
two crops, and (3) to establish a 2D model to describe the distribution of crop roots in sole cropping and intercropping.
2. Materials and methods
2.1. Experimental site
The field experiments were conducted at Shangqiu Agroecosystem Experimental Station (34◦ 35 N, 115◦ 34 E, elevation
51.0 m) during the crop-growing season from April to August in
2007 and 2008. The station lies north of Shangqiu City in Henan
province in a semi-humid, temperate and monsoon climate zone.
The annual mean temperature is 13.9 ◦ C, annual accumulated temperature above 10 ◦ C is about 4800.0 ◦ Cd, average annual sunshine
duration is 2510 h, annual frost-free days are 230, mean annual
precipitation is 780 mm, and mean potential evaporation 1735 mm.
The natural conditions and agricultural production levels of the area
around the station are typical of the Huang-Huai-Hai plain.
2.2. Soil characterization
Soil nitrogen was measured using the micro-Kjeldahl procedure,
soil P with vanadomolybdate method, and soil K with flame photometry (Page, 1982). Soil organic matter was measured with the
simplified colorimetric method (Sims and Haby, 1971). Soil total N
of the cultivated horizon (0–30 cm) was 0.78 g kg−1 , soil available
P and K were 10.5, and 52.6 mg kg−1 respectively, and soil organic
matter content was 9.8 g kg−1 .
Particle size analysis was carried out by the hydrometer method,
and soil water retention characteristics ( –) were determined by
using pressure chamber apparatus. Physical properties of the soil
are given in Table 1.
2.3. Experimental design
The experimental site was ploughed and then divided into 12
plots before sowing since the field experiment comprised three
treatments each with four replicates. The three treatments were
sole maize (SM), sole soybean (SSB), and maize/soybean intercropping (I). The intercropping consisted of a row of maize (Zea mays
L. ‘Zhengdan 958’) flanked by three rows of soybean (Glycine max
‘Yudou 22’) on either side. Each replicate comprised a plot 6 m wide
and 10 m long, with the crop rows oriented north–south. In SM, row
Bulk density (g cm−3 )
s (m3 m−3 )
f (m3 m−3 )
1.40
1.54
1.45
1.46
0.334
0.354
0.325
0.291
0.279
0.295
0.277
0.254
spacing and plant spacing were 50 and 30 cm, respectively, whereas
in SSB, both were 30 cm. In I, the distance between maize and
soybean rows was 30 cm; the inter-row spacing for soybean was
30 cm; and the inter-plant spacing in both was also 30 cm (Fig. 1).
Plant density of maize in SM and I was 6.7 and 2.8 plants m−2 ,
respectively, whereas that of soybean in SSB and I was 11.1 and
8.9 plants m−2 , respectively. Both maize and soybean were sown on
16 April in both the years. Irrigation, weeding, application of fertilizers, and other field management practices were kept the same
for all the treatments and replicates. Maize was harvested on 18
August in 2007 and on 20 August in 2008, with soybean following
exactly a week later.
2.4. Measurements
2.4.1. Vertical and horizontal distribution of crop roots
Data on the horizontal and vertical distribution of roots were
obtained by washing off the soil on site (Gregory and Reddy,
1982; Ozier-Lafontaine et al., 1999; Adiku et al., 2001), a time- and
labour-intensive operation carried out only in I, the intercropping
treatment. For this purpose, 1.2 m long, 1.0 m wide, and 1.5 m deep
trenches were dug manually in a representative plot. The length
of each trench was perpendicular to the crop row. The trenches
included a complete strip (Fig. 1).
After digging, the working surface of the soil profile was
smoothened and 5 cm long nails were driven into it every 5 cm
along its length and breadth. The soil layer of 2 cm was then carefully washed off with a sprayer, leaving the roots more or less intact
and the nails helping to retain the roots in place. This procedure was
similar to that used by Adiku et al. (2001).
After washing, the horizontal and vertical distribution was
mapped on a 2 cm × 2 cm grid over the trench wall. Each grid square
was assigned a binary value – in a given square, roots were either
present or absent (Sillon et al., 2000). Roots of maize and soybean
were distinguished based on colour, nodules, and smell: the roots
of maize were white and those of soybean were brown; soybean
roots had nodules on them; and soybean roots also had the characteristic smell common to most legumes (Ozier-Lafontaine et al.,
1999; Adiku et al., 2001; Li et al., 2006).
The pattern of root spread was recorded as described above four
times during the growing season: on 25 May and on 5, 17, and 28
June in 2007 and on 21 May, 7 and 17 June, and 27 July in 2008.
2.4.2. Root length density (RLD)
After mapping the distribution of roots, root samples were collected from the soil profile by pressing sharp-edged iron boxes
vertically into the soil surface. Each box was open at the top and
10 cm long, 5 cm wide, and 10 cm deep, which yielded soil cores of
those dimensions. The cores were also collected from the top layer
at 10 cm intervals along the north–south axis and the east–west
axis. The sampling depth was determined by the actual growth of
crop roots: the minimum was 40 cm and the maximum, 90 cm.
The same process was followed for the sole crops, the only difference being the length of the trenches: in SM, the length was
0.7 m; in SSB, it was 0.5 m.
Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
201
120 cm
Root sampling sites
Fig. 1. Layout of the maize/soybean strip intercropping system and sampling points (the distance between adjacent root sampling sites was 10 cm).
To collect the roots, all the core-blocks were soaked in tap water
for about 4 h, then stirred vigorously, and the soil suspension was
poured through a sieve (mesh size 0.2 mm2 , diameter 25 cm, and
depth 8 cm). The root pieces of maize and soybean were separated
into two classes, namely coarse and fine. Root length was measured with the modified Newman-line-intersect method (Tennant,
1975). Only a part of the roots was measured directly; the rest of
the data were obtained by calculations using the regression of root
length and root weight. The regressions of root length (RL) and root
weight (RW) for all the treatments over the two growing seasons
are shown in Table 2. Root length density (RLD) was calculated from
the volume of the soil cores and the root length for each species.
Each sample had two replicates.
and S, the change in soil water stored in the upper (0–100 cm)
layer of soil (mm). The values of U and Dw were estimated with
Darcy’s law.
Soil water content was measured gravimetrically once a week
for every 10 cm depth of the soil profile up to 100 cm. A few additional measurements were taken before and after irrigation and
after heavy-rain events.
2.4.3. Evapotranspiration (ET)
Evapotranspiration (ET) was estimated from the water balance
equation as follows (Hillel, 1998):
where YMM and YMI are the grain yields (g m−2 ) in SM and I respectively and YSBM and YSBI are the grain yields in SSB and I respectively.
ET = Pe + I + U − DW − R − S
(1)
where Pe is the effective precipitation (mm) determined by the
USDA soil conservation services method (SCS, 1972); I, the irrigation (mm); U, the upward capillary flow into the root zone (mm); R,
the runoff (mm); Dw , the downward draining-out root zone (mm);
Table 2
Relationships between root length (RL) and root weight (RW) in maize and soybean
as sole crops and intercrops.
LER =
YMI
YSBI
+
YMM
YSBM
(2)
2.4.5. Radiation use efficiency
Radiation use efficiency (RUE) was calculated as follows (Tsubo
and Walker, 2002):
RUE =
Ybiomass
I0 F
(3)
where Ybiomass is above-ground biomass (g m−2 ); F, the fraction of
radiation intercepted by crop canopy; and I0 , the flux density of
incident radiation above the crop canopy (MJ m−2 ).
The fraction of PAR intercepted by the top canopy of maize,
FM-upper , was determined as follows (Tsubo and Walker, 2002):
Season
Treatment
Class
Equation
r
2007
SM
Coarse
Fine
RL = 1108 × RW
RL = 2054 × RW
0.86
0.94
SSB
Coarse
Fine
RL = 397 × RW
RL = 2487 × RW
0.91
0.96
Maize
Coarse
Fine
RL = 984 × RW
RL = 1894 × RW
0.89
0.94
Soybean
Coarse
Fine
RL = 405 × RW
RL = 2674 × RW
0.88
0.91
SM
Coarse
Fine
RL = 996 × RW
RL = 2341 × RW
0.89
0.94
SSB
Coarse
Fine
RL = 410 × RW
RL = 2618 × RW
0.93
0.94
FM-lower =
Maize
Coarse
Fine
RL = 1015 × RW
RL = 2144 × RW
0.87
0.86
FB =
Soybean
Coarse
Fine
RL = 384 × RW
RL = 2587 × RW
0.91
0.93
where LM-lower and LB were the LAI of maize and soybean in their
bottom canopy, respectively, and KB was the K of soybean. Assum-
I
2008
I
Plant
2.4.4. Land equivalent ratio
Land equivalent ratio (LER) was calculated as follows
(Vandermeer, 1989):
FM-upper = 1 − exp(−KM LM-upper )
(4)
where LM-upper is leaf area index (LAI) in the upper layer and KM is
the extinction coefficient (K) of maize. K was estimated as follows:
K=
ln (1 − F)
LAI
(5)
Using the equation developed by Keating and Carberry (1993),
the fractions of PAR intercepted by maize and soybean at the bottom, represented by FM-lower and FB , respectively, were estimated
as follows:
KM LM-lower
× [1 − exp(−KM LM-lower − KB LB )] (6)
KM LM-lower + KB LB
KB LB
× [1 − exp(−KM LM-lower − KB LB )]
KM LM-lower + KB LB
(7)
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Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
Table 3
Grain yield (g plant−1 ) and N uptake (mg N plant−1 ) in maize and soybean as sole crops and intercrops.
Crop
Cropping
Maize
Sole-cropped
Intercropped
Significance
Sole-cropped
Intercropped
Significance
Soybean
Grain yield
N uptake
2007
2008
2007
2008
181.7 ± 10.7, n = 55
240.1 ± 11.6, n = 30
*
19.9 ± 6.2, n = 75
15.8 ± 4.6, n = 60
*
175.6 ± 14.1, n = 55
234.6 ± 13.3, n = 30
*
19.4 ± 5.1, n = 75
18.1 ± 6.5, n = 60
*
761.6 ± 24.3, n = 20
910.5 ± 31.1, n = 20
*
48.9 ± 6.3, n = 20
40.2 ± 7.5, n = 20
ns
764.7 ± 20.3, n = 20
885.2 ± 19.7, n = 20
*
46.4 ± 7.3, n = 20
39.3 ± 8.4, n = 20
ns
ns, no significant difference.
ing that the leaves were randomly distributed, LM-upper and LM-lower
were estimated by:
LM-upper =
LM-lower =
h − h M
B
hM
h B
hM
LM
2.4.6. Leaf area
Leaf area was measured manually every 7–10 days after the
emergence of seedlings. For this purpose, 5 plants in SM and 10
plants in SSB were destructively sampled from each plot. In I, 5
plants were taken from the maize strip and 10 plants from the soybean strip from the centre of each plot. Leaf area of each leaf was
determined by multiplying leaf length by leaf width at the widest
point and multiplying the product by 0.70 for maize and 0.75 for
soybean. The coefficient used for calculating the leaf area index
(8)
LM
(9)
where hM and hB were the heights of maize and soybean, respectively, and LM was the total LAI of maize.
45
Tmax S1
Tmin
Rainfall
40
S2
S3
60
S4
(a)
dry wet
50
35
40
25
30
20
15
20
Rainfall (mm)
Temperature (C)
30
10
10
5
0
0
100
125
150
175
200
225
250
Julian day
40
Tmax S1
Tmin
Rainfall
35
(b)
S2 S3 S 4
100
80
25
60
20
15
40
Rainfall (mm)
Temperature (C)
30
10
20
dry wet
5
0
0
100
125
150
175
200
225
250
Julian day
Fig. 2. The maximum temperature (Tmax ), the minimum temperature (Tmin ), and rainfall during the growing season in 2007 (a) and 2008 (b). Si indicates a sampling day.
Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
203
( a) 2007, 0- 30cm
27
( b) 2008,, 0-- 30 cm
3
-3
Soil water content( m m )
27
25
25
23
23
21
19
21
SM
SSB
I
70%
17
19
17
106 121 133 146 159 174 190 202 215 224 237
107 119 132 149 164 180 193 206 217 228 240
( d) 2008, 30- 60 cm
29
27
27
25
25
23
23
21
21
3
-3
Soil water content( m m )
( c) 2007, 30- 60 cm
29
19
19
107 119 132 149 164 180 193 206 217 228 240
106 121 133 146 159 174 190 202 215 224 237
( e) 2007,, 60- 100 cm
( f ) 2008, 60- 100 cm
28
26
26
24
24
22
22
20
20
3
-3
Soil water content( m m )
28
18
106 121 133 146 159 174 190 202 215 224 237
18
107 119 132 149 164 180 193 206 217 228 240
Julian day
Julian day
Fig. 3. Soil water dynamics in maize and soybean as sole crops and intercrops in 2007 ((a), (c), (e)) and 2008 ((b), (d), (f)).
(LAI) was determined based on data from image processing. The
leaf area of a single plant was taken as the sum of leaf areas of all
the leaves. The leaf area calculated thus was then converted into
LAI based on row and plant spacing.
3. Results
3.1. Irrigation and rainfall
2.4.7. Yield and nitrogen uptake
The sampling area was 3 m × 3 m in SM and SSB and 3 m × 4 m
in I. The harvested grain was naturally dried until its moisture content was less than 15%, and then weighed to calculate the final
yield (g plant−1 ). Nitrogen content in the grain and the straw was
estimated using the micro-Kjeldahl procedure (Page, 1982).
In 2007, the plots were irrigated twice in the growing season, on
5 May and on 25 May; the irrigation amounted to 45 mm on the first
occasion and 55 mm on the second. In 2008, adequate rainfall made
irrigation unnecessary (during the month after sowing, the rainfall
was 19.4 mm in 2007 and 75.9 mm in 2008). Total precipitation
during the growing season was 542.5 mm in 2007 and 530.3 mm in
2008 (Fig. 2). Rainfall occurred mainly in July and August; in these
two months, it was 398.9 mm in 2007 (74% of the whole season)
and 356 mm in 2008 (67% of the whole season).
2.5. Statistical analysis
3.2. Soil water dynamics and evapotranspiration
Data were subjected to an ANOVA using the SPSS 13.0 software package, and the Duncan mean separation test procedure was
applied.
Soil water content at the time of sowing was similar in 2007 and
2008 (Fig. 3), and changes in it during the growing season were also
similar across the treatments. The topsoil showed sharper changes
204
Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
in soil water dynamics than the subsoil did. Soil water content in all
the three layers (0–30 cm, 30–60 cm, and 60–100 cm) was greater
than 75% of field capacity throughout all the treatments, indicating
complete absence of water stress. Mean values of evapotranspiration (ET) across the years were 433, 467, and 498 mm in SM, SSB,
and I, respectively.
case of soybean, the grain yield of soybean as an intercrop was significantly lower than that of soybean as a sole crop but the two did
not differ significantly in terms of N uptake. The results indicate
that strip intercropping favoured nutrient uptake and growth of
maize but hampered the growth of soybean significantly. This difference may be due to the faster development and deeper reach of
maize roots or/and a higher N uptake capacity under non-limiting
conditions.
3.3. Leaf area and radiation use efficiency
Leaf area index in SM was significantly less than that in both
SSB and I (data not given) but the difference between the latter two
was not significant. Radiation use efficiency (RUE) of maize was
3.14 g MJ−1 in I, slightly less than that in SM (3.18 g MJ−1 ), whereas
that of soybean was 1.65 g MJ−1 in I, slightly greater than that in
SSB (1.55 g MJ−1 ). However, the differences between the different
cropping patterns were not significant. As plants rarely compete for
light without simultaneously competing for water (Caldwell, 1987;
Cannell and Grace, 1993; Wallace, 1995), it could be concluded that
radiation was not the major factor in producing the competition
results in maize/soybean intercropping system.
3.5. Land equivalent ratio
The land equivalent ratio is believed to be an accurate reflection
of the biological efficiency of an intercropping system. Values of
LER greater than 1 are considered advantageous. The LER of the
intercropping system was 1.19 in 2007 and 1.31 in 2008, pointing to
the considerably greater land-use efficiency of the maize/soybean
strip intercropping system.
3.6. The distribution of crop roots
3.6.1. Root maps
Fig. 4 shows the distribution of roots in 2007. At the time of the
first sampling (25 May 2007), the roots of maize and soybean were
confined to the immediate neighborhood of the respective plants.
3.4. Grain yield and nitrogen uptake
The grain yield and N uptake of maize as an intercrop were significantly greater than those of maize as a sole crop (Table 3). In the
0
0
Depth(cm)
Depth(cm)
10
10
20
20
30
(a)
25 May
30
0
10
20
(b)
5 June
40
30
0
0
0
10
10
20
20
30
40
20
30
30
40
50
50
60
70
10
Distance from maize strip(cm)
Depth(cm)
Depth(cm)
Distance from maize strip(cm)
0
10
20
30
40
50
(d)
28 June
60
(c)
17 June
60
Distance from maize strip(cm)
70
0
10
20
30
40
50
60
Distance from maize strip(cm)
Fig. 4. 2D distribution of roots in maize/soybean intercropping in 2007.
Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
By the second sampling (6 June), the two root systems were intermingled slightly, although maize roots had extended laterally over
a greater distance (22 cm in maize and 14 cm in soybean). By the
third sampling (17 June), maize roots had extended underneath the
space between two adjacent rows of soybean. The maximum lateral spread of maize roots occurred in the 16–22 cm layer of the soil.
Roots of both the crops were distributed mainly in the surface layer
(0–30 cm) – deeper than 30 cm, the quantity of roots fell sharply. By
the fourth sampling (28 June), the maximum lateral distance and
root depth in both maize and soybean had increased further only
slightly (the maximum depth was 68 cm for maize and 52 cm for
205
soybean): the data indicate that only a few maize roots and none
of the soybean roots had grew beyond the clay layer to reach the
sandy layer underneath.
Fig. 5 shows the distribution or roots in 2008. At the time of the
first sampling (21 May 2008), only a few roots of maize and soybean
were in direct contact. By the second sampling (7 June), the intermingling of maize and soybean roots had increased markedly. By
the third sampling (15 June), the roots had reached their maximal
lateral spread, and many of the maize roots had extended to the
space underneath the inner rows of the soybean strip. Vertically,
roots of both the crops lay mainly in the 16–22 cm layer. By the
0
0
10
Depth(cm)
Depth(cm)
10
20
30
20
40
(a)
(b)
21 May
30
0
10
20
7 June
50
30
0
10
20
30
Distance from maize strip(cm)
Distance from maize strip(cm)
0
0
10
10
20
20
30
Depth(cm)
Depth(cm)
30
40
40
50
50
60
60
70
(c)
15 June
70
80
0
10
20
30
40
50
Distance from maize strip(cm)
(d)
24 June
80
60
90
0
10
20
30
40
50
Distance from maize strip(cm)
Fig. 5. 2D distribution of roots in maize/soybean intercropping in 2008.
60
206
Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
fourth sampling (24 June), maize roots had penetrated as deep as
86 cm and those of soybean had penetrated up to 68 cm. The data
showed that both had traversed the clay layer to reach the sandy
layer underneath.
3.7. Root length density (RLD)
In 2007, root length density of maize was greater in sole maize
than in intercrop at most soil depths at two sampling positions,
one directly underneath the maize rows and the other 10 cm away
(Fig. 6). However, by the fourth sampling (28 June), RLD of the intercropped maize, recorded at 10 cm from the maize row was greater
than that of sole maize at all depths except the top layer (0–10 cm):
intercropping had probably forced the roots to penetrate deeper. By
the fourth sampling (28 June), the point of higher RLD was 20 cm
away.
(a)25 May
80
3.7.1. 2D models of RLD
The relatively high RLD of maize and soybean was recorded
closer to the soil surface along the vertical dimension and nearer
60
40
40
20
20
0
0
(b)5 Jun
250
200
200
150
150
100
100
50
50
0
0
(c)17 Jun
350
300
250
250
200
200
150
150
100
100
50
50
(d)28 Jun
500
(f)5 Jun
(g)7 Jun
350
300
0
(e)25 May
80
60
250
Root length densities (cm /(100 cm3))
At the first two samplings in 2008, the RLD of sole maize was
greater than that of intercropped maize at most soil depths directly
under the maize plants (Fig. 7). However, at the third and fourth
samplings, the differences between the RLD of sole and intercropped maize in the 0–40 cm layer directly underneath the plants
were not consistent although the RLD of intercropped maize was
markedly greater in the deeper layers. At the spots 10 and 20 cm
away, the RLD of intercropped maize was greater at most of the
depths, as in 2007.
The RLD of intercropped soybean was greater than that of sole
soybean at most of the depths in 2007 and 2008 (Figs. 8 and 9), indicating that intercropping may encourage root growth in soybean.
0 0 cm
500
(h)28 Jun
400
400
0-10 cm
20-30 cm
10-20 cm
30-40 cm
300
300
50-60 cm
200
40-50 cm
60-70 cm
200
100
100
0
0
0
10
20
0
10
20
30
Distance from maize row (cm)
Fig. 6. Root length densities at different horizontal distances from the rows of maize as a sole crop ((a)–(d)) and as an intercrop with soybean ((e)–(f)) on four sampling dates
in the 2007 season.
Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
(a)21 May
70
60
50
50
40
40
30
30
20
20
10
10
0
0
(b)7 Jun
Root length densities (cm /(100 cm3))
(f) 7 Jun
200
150
150
100
100
50
50
0
0
(c)15 Jun
350
300
250
250
200
200
150
150
100
100
50
50
0
(g)15 Jun
350
300
0
(d)24 Jun
500
400
300
300
200
200
100
100
0
10
20
(h)24 Jun
500
400
0
(e)21 May
70
60
200
207
0
0-10 cm
20-3 0 cm
40-50 cm
60-7 0 cm
80-9 0 cm
0
10
20
10-20 cm
30-40 cm
50-60 cm
70-80 cm
30
Distance from maize row (cm)
Fig. 7. Root length densities at different horizontal distances from the rows of maize as a sole crop ((a)–(d)) and as an intercrop with soybean ((e)–(f)) on four sampling dates
in the 2008 season.
the crop rows along the horizontal dimension. If RLD is written as
an empirical function of the exponential form (Gerwitz and Page,
1974; Gregory and Reddy, 1982), it can be calculated from the equation:
RLD = exp(˛0 + ˛1 x + ˛2 z)
(10)
where ˛0 , ˛1 , and ˛2 are empirical constants which is the function
of DAS (days after sowing), x is the distance from crop row (cm),
and z is the depth (cm).
Using regression analysis, the empirical parameters for 2D distribution of RLD of the sole crops and the intercrops in 2007 are
shown in Table 4. The ˛0 values of the intercropped maize were
greater than those of the sole maize but the difference was not significant (p = 0.865). The difference was not significant (p = 0.651)
for ˛1 either, and although the ˛2 values of the intercropped maize
were lower than those of the sole maize, again the decrease was
not significant (p = 0.800). By comparing the parameters for intercropped maize and sole maize, it could be concluded that RLD
was higher in the pure stand of maize than that in the intercropped stand. The values of ˛0 and ˛2 of intercropped soybean
were greater than those of pure soybean whereas the exact reverse
pattern obtained for the ˛1 values. The difference between pure
and intercropped soybean for any of the parameters was not significant (p > 0.600). The results of fitting the exponential model thus
show a clear effect of intercropping on the root length distribution of maize, whereas such an effect was not demonstrated for
soybean.
Parameters of the 2D distribution model of maize RLD were
highly significant (p < 0.01) and so were those for soybean RLD
except at the time of the first sampling in 2007. These results indi-
208
Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
20
(a) 25 May
15
15
10
10
5
5
0
(e) 25 May
0
150
Root length densities (cm /(100 cm3))
20
(b) 5 Jun
15 0
120
12 0
90
90
60
60
30
30
0
(c) 17 Jun
300
0
300
250
250
200
200
150
150
100
100
50
50
0
0
350
(d) 28 Jun
(f) 5 Jun
(g)17 Jun
( h) 28 Jun
350
300
300
250
250
200
200
150
150
100
100
50
50
0-10 cm
20-3 0 cm
40-50 cm
10-2 0 cm
30-40 cm
50-6 0 cm
0
0
0
0
10
10
20
Distance from soybean row (cm)
Fig. 8. Root length densities at different horizontal distances from the rows of soybean as a sole crop ((a)–(d)) and as an intercrop with maize ((e)–(f)) on four sampling dates
in the 2007 season.
cate that the exponential model can represent the RLD distribution
of maize and soybean both as sole crops and as intercrops.
There was a good relationship between the parameters of the 2D
model of RLD and DAS for each treatment (Table 5). The distribution
of RLD in 2008 could be predicted with the 2D model; Fig. 10 shows
the predicted and measured values of RLD in 2008. For intercropped
maize, the predicted value was greater than the observed value in
the top layer (0–10 cm), but the prediction proved inadequate for
most of the deeper layers, the error being greatest for the 10–20 cm
layer. However, most errors were less than 10 cm/(100 cm3 ) for the
layers below 20 cm. For intercropped soybean, the predicted RLD
was greater than the observed RLD for most soil layers. For the
0–40 cm layer, the errors were more than 13 cm/(100 cm3 ) at points
just under the soybean border rows and less than 6 cm/(100 cm3 )
at points 10 cm away. The differences between the predicted and
measured RLD for both maize and soybean as sole crops were less
than 10 cm/(100 cm3 ) at most soil depths. The results indicate that
the 2D model is suitable to describe the root distribution of roots
in the soil profile.
4. Discussion
Maize roots not only penetrated deeper than soybean roots but
also extended into soybean stands underneath the space between
inner rows of soybean. The roots of soybean, however, were confined mainly to the zone near the plants. RLD was much higher
in the top layer (0–30 cm deep) and in the zone closer to the
plants. The exponential model proved suitable to describe the RLD
vertically and horizontally in both sole cropping and in intercropping.
Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
25
( a) 21 May
Root length densities (cm /(100 cm3))
25
20
20
15
15
10
10
5
5
0
0
140
( b)7 Jun
120
100
100
80
80
60
60
40
40
20
20
0
0
( c) 15 Jun
( e) 21 May
140
120
300
209
( f) 7 Jun
300
250
250
200
200
150
150
100
100
50
50
( g)15 Jun
0
0
400
( d)24 Jun
300
( h)24 Jun
400
300
200
200
0-10 cm
10-20 cm
20-30 cm
30-40 cm
40-50 cm
50-60 cm
60-70 cm
100
100
0
0
0
10
0
10
20
30
Distance from soybean row (cm)
Fig. 9. Root length densities at different horizontal distances from the rows of soybean as a sole crop ((a)–(d)) and as an intercrop with maize ((e)–(f)) on four sampling dates
in the 2008 season.
4.1. RUE
The RUE for SM was greater than that for I, but the difference
was not significance. Awal et al. (2006) and Tsubo et al. (2001) also
indicated that RUE of sole maize was greater than that of compared with intercropped maize. The soybean RUE in SSB was less
than in I, probably because of more efficient portioning to pods
with the reduced radiation received by the intercropped soybean.
Intercropped maize had a similar RUE to sole cropped maize, but
soybean had greater RUE in intercropping than sole cropping. This
could lead to a yield advantage of the intercropping systems in this
region. The finding was similar to the results reported by Tsubo and
Walker (2002) in maize/bean intercropping, and Awal et al. (2006)
in maize/peanut intercropping. Root interactions may play a role in
determining RUE by increasing root uptake of nutrients and water.
4.2. Root distribution
Root distribution could be accurately obtained by washing off
the soil (Ozier-Lafontaine et al., 1999; Adiku et al., 2001). The
method, however, is time- and labour-intensive. In this study,
washing a profile took 5–7 h. Roots of maize and soybean were distributed mainly in the 0–30 cm layer because of (1) a layer of clay
at 15–30 cm depth, which significantly inhibited root growth and
(2) the unbroken plough pan that had formed at a depth of about
20 cm.
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Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
(a)
-50
0
50
100
150
200
250
(b)
0
10
-4 0
0
0
40
80
120
160
200
240
10
20
20
40
50
Si mulated 21-May
60
Depth (cm)
Depth (cm)
30
50
Si mulated 15-J un
60
Si mulated 24-J un
70
90
100
80
-3
-3
RLD(cm 100cm )
(c)
-50
0
50
100
RLD(cm 100cm )
(d)
150
200
0
0
0
10
10
20
50
100
150
200
20
Depth (cm)
Depth (cm)
40
Si mulated 7-Jun
70
80
30
30
40
30
40
50
60
50
70
60
80
70
-3
ROD(cm 100cm-3)
RLD(cm 100cm )
-1 0
0
10
20
30
40
50
-2 0
0
0
10
20
40
60
80
10
20
20
Depth (cm)
Depth (cm)
0
30
40
30
40
50
50
60
60
70
80
70
-3
-1 0
0
0
RLD ( cm 100cm )
10
20
30
-3
40
0
10
Depth (cm)
Depth (cm)
40
40
10
20
30
0
RLD ( cm 100cm )
10
20
30
20
30
40
50
60
70
50
60
Fig. 10. Predicted RLD with the 2D model using parameters estimated with the data from 2007 and the deviation from predicted values at 0 cm ((a)–(d)) and 10 cm ((e)–(h))
from crop rows in the 2008 season (a), (e): maize intercropped with soybean; (b), (f): sole maize; (c), (g): soybean intercropped with maize; (d), (h): sole soybean).
Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
211
Table 4
Parameters estimated for a 2D model of root length density (RLD) in maize and soybean as sole crops and intercrops in the 2007 season.
Treatment
RLD = exp(˛0 + ˛1 × x + ˛2 × z)
Date
˛0
Std. error
p
Std. error
p
Std. error
p
Maize
25 May
5 Jun
17 Jun
28 Jun
5.2782
5.4989
6.0268
6.3959
0.0321
0.0553
0.0514
0.0607
0.0001
0.0001
0.0001
0.0001
−0.1951
−0.2069
−0.1769
−0.1697
0.0055
0.0172
0.0106
0.0117
0.0008
0.0001
0.0001
0.0001
−0.138
−0.0859
−0.095
−0.0934
0.0029
0.0042
0.0041
0.0048
0.0004
0.0001
0.0001
0.0001
Soybean
5 Jun
17 Jun
28 Jun
5.5144
5.9861
5.6614
0.0977
0.0893
0.0334
0.0001
0.0001
0.0001
−0.2618
−0.1825
−0.1672
0.0444
0.0196
0.0081
0.002
0.0001
0.0001
−0.105
−0.0941
−0.0609
0.0081
0.007
0.0021
0.0001
0.0001
0.0001
SM
25 May
5 Jun
17 Jun
28 Jun
5.3896
5.5495
6.0568
6.4521
0.0802
0.056
0.0564
0.0423
0.0002
0.0001
0.0001
0.0001
−0.1928
−0.1991
−0.1608
−0.1725
0.0138
0.0161
0.0107
0.0086
0.0051
0.0001
0.0001
0.0001
−0.1359
−0.0823
−0.0845
−0.0913
0.0073
0.0041
0.0042
0.0033
0.0028
0.0001
0.0001
0.0001
SSB
5 Jun
17 Jun
28 Jun
5.3093
5.7047
5.6579
0.0662
0.1003
0.035
0.0001
0.0001
0.0001
−0.2702
−0.1808
−0.1675
0.0362
0.0258
0.0089
0.0017
0.0002
0.0001
−0.0914
−0.0793
−0.0641
0.0052
0.0073
0.0022
0.0001
0.0001
0.0001
I
˛1
˛2
Table 5
Coefficients of correlation between the parameters of a 2D model for root length density (RLD) and days after sowing (DAS) in maize and soybean as sole crops and intercrops
in 2007.
Parameter
Treatment
˛0
˛1
˛2
Maize
0.03DAS + 0.27
r = 0.99, p = 0.008
0.0001DAS − 0.34
r = 0.82, p = 0.183
0.001DAS − 0.28
r = 0.67, p = 0.325
Soybean
0.02DAS + 2.81
r = 0.71, p = 0.299
0.05DAS − 0.95
r = 0.75, p = 0.267
0.001DAS − 0.26
r = 0.75, p = 0.264
SM
0.03DAS + 0.60
r = 0.99, p = 0.015
0.001DAS − 0.32
r = 0.73, p = 0.267
0.001DAS − 0.28
r = 0.67, p = 0.331
SSB
0.02DAS + 2.70
r = 0.91, p = 0.091
0.05DAS − 9.50
r = 0.74, p = 0.265
0.001DAS − 0.22
r = 0.94, p = 0.047
I
A plough pan 10–15 cm thick at a depth of about 20 cm is common in most arable lands in the Huang-Huai-Hai plain (Lu et al.,
2004), which is unfavourable for the growth of roots (Floyd, 1984;
Barraclough and Weir, 1988). Since about 85% of the RLD of maize
and soybean lay in the 0–30 cm soil layer, the plough pan must
have hindered the growth and spread of their roots. Logsdon et
al. (1987) report that greater mechanical impedance is associated
with decline in root growth. Although a few crop roots could break
through the plough pan, their further development was severely
restricted. In our experiment, the lateral growth of maize and
soybean roots in the intercropped plots occurred mainly in the
16–22 cm layer, or just above the pan. The pan, therefore, may have
been a major factor in such growth because the plough pan blocks
the downward movement of water, forcing it to move sideways
instead.
4.3. Root distribution and interspecific competition
Interspecific facilitation implies a change in the environment
brought about by one crop that favours the growth of the other
crop (Begon et al., 1996). Interspecific interactions lead to increased
yields of both the species that make up an intercropping system. Li
et al. (2006) define this kind of interaction as symmetric interspecific facilitation. However, in the maize/soybean strip intercropping
that we studied, interspecific interactions resulted in higher grain
yield and N uptake in maize but lower grain yield and N uptake
in soybean. Such interactions were defined as asymmetric interspecific facilitation between the intercropped species by Li et
al. (2006), who propose that asymmetric interspecific facilitation
results from greater lateral deployment of roots and increased
RLD of one crop, and that compatible spatial root distribution of
the intercropped species contributes to the symmetric interspecific facilitation observed in faba bean/maize intercropping. Results
of this study indicated that intercropping favored lateral spread
of maize roots – possibly the main reason for the superiority of
maize over soybean in terms of root growth, yield, and N uptake.
Our results support the first part of the hypothesis, namely that
asymmetric interspecific facilitation is due to the greater root proliferation of the higher-yielding species, because maize roots in our
experiment had extended to soil beneath the soybean strip and
had also penetrated deeper. Maize roots had thus tapped a larger
volume of soil, which probably enabled them to absorb greater
quantities of water and nutrients from soil, which ultimately led
to higher grain yield. This ability of maize roots may also have
contributed to the increased grain yield in maize/soybean intercropping observed in an earlier study (Li et al., 2006).
The plough pan may reduce root growth, thereby affecting
uptake of water and nutrients (Olesen and Munkholm, 2007) and
may also have played a role in our study. However, it is difficult to
determine the effect of the plough pan on interspecific competition
from the observations of this study; more experiments are needed
for the purpose.
5. Conclusions
Horizontal and vertical distribution of roots of two intercrops,
maize and soybean, was studied in situ by washing off the soil.
Water was not a limiting factor, and maize roots had spread farther and deeper than those of soybean, extending beneath the inner
rows of the soybean strips; soybean roots, however, were confined
212
Y. Gao et al. / Agricultural Water Management 98 (2010) 199–212
to a limited zone near the rows of soybean. Roots of both the species
had the greatest lateral spread in the 16–22 cm soil layer. The RLD
of maize and soybean roots was distributed mainly in the upper
layer (0–30 cm) underneath the plants. The 2D distribution of RLD
could be mathematically described with an exponential equation.
The study was carried out under full irrigation, but more investigation is needed to study root distribution in intercropping systems
under conditions of water stress.
Acknowledgements
This work was financially supported by the National Basic
Research Program of China (2006CB403406), the National Natural
Science Foundation of China (50679082), and the National HighTech Project of China (863, 2006AA100209 and 2006AA100203).
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