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Relationships among light distribution, radiation use efficiency and land equivalent ratio in maize-soybean strip intercropping
Field Crops Research 224 (2018) 91–101
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Relationships among light distribution, radiation use efficiency and land equivalent ratio in maize-soybean strip intercropping
T
Xin Liua,b, Tanzeelur Rahmana, Chun Songc,d, Feng Yanga,d, Benying Sua, Liang Cuia, ⁎ Weizhao Bua, Wenyu Yanga,d, a
College of Agronomy, Sichuan Agricultural University, Chengdu, 611130, China College of Agronomy, Shandong Agricultural University, Taian, 271018, China c Institute of Ecological and Environmental Sciences, Sichuan Agricultural University, Chengdu, 611130, China d Key Laboratory of Crop Ecophysiology and Farming Systems in the Southwest, Ministry of Agriculture, Chengdu, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Intercropping Land equivalent ratio Light distribution Photosynthesis
Maize (Zea mays)-soybean (Glycine max) intercropping is popular in many developing countries because of its high land equivalent ratio (LER). However, very few studies have explored the reason of its high LER, and the relationships between light distribution and the variations in radiation use efficiency (RUE) and LER in different intercropping arrangements. In this study, we conducted field experiments with different row arrangements of intercropping patterns from 2013 to 2015. The three different strip intercropping (SI) row arrangements were 0.2 m, 0.4 m, and 0.7 m (SI1); 0.4 m, 0.4 m, and 0.6 m (SI2); and 0.6 m, 0.4 m, and 0.5 m (SI3) for maize row distance, soybean row distance, distance between maize and soybean rows, respectively. The results showed that, as compared to single row intercropping, the strip intercropping increased the PAR at top of soybean canopy by 1.42 (SI3), 1.67 (SI2) and 1.93 (SI1) times, and increased the PAR at maize leaves close to the ear by 1.02 (SI3), 1.11 (SI2) and 1.12 (SI1) times. Moreover, the increased PAR at crucial positions in SI potentially improved the photosynthetic rate (Pn) for maize leaves close to the ear and radiation use efficiency (RUE) of maize by 1.08 and 1.09 times (averaged by SI1, SI2 and SI3), respectively, and improved the Pn of leaves at top of canopy and intercepted PAR of soybean by 1.75 and 1.36 times (averaged by SI1, SI2 and SI3), respectively. Compared to monoculture, SI also enhanced the RUE of intercropped maize (by 1.18 times) and soybean (by 1.51 times), which compensated for the partial yield loss caused by decreased crop intercepted PAR. Overall, in SI, intercropped maize achieved 90% of the monoculture yield, and intercropped soybean achieved 47% of the monoculture yield. With the expanding gap width for growing soybeans under a fixed bandwidth (2 m), the increasing intercepted PAR of intercropped soybean alleviated the interspecific competition disadvantage of soybean, while the reduction of maize row width decreased the dominant interspecific competition of maize. By adjusting the distances, we suggest that the optimal gap width for growing soybeans is 1.6 m-1.8 m, and the best maize row distance is 0.4 m. The SI2 achieved LER of 1.42, representing the leading level in the world.
1. Introduction
et al., 2015; Yang et al., 2015; Rahman et al., 2016; Du et al., 2018). The main recent improvements are as follows: first, narrow the distance between maize rows in order to increase the distance between maize and soybean rows; second, ensure the same plant densities for maize and soybean in intercrops as in sole crops (with optimal density) by using a smaller plant distance within each row in intercropping, which compensates for the reduced row number. In the recent five years, this approach has been practiced in several Provinces of China, such as Sichuan, Shandong, Guizhou, Henan, Ningxia, etc (Du et al., 2018). High crop yields in each Province were used for comparison to calculate LER, e.g., 9000 kg ha−1 for maize and 3000 kg ha−1 for soybean in
Intercropping has been adopted worldwide since it can improve the radiation use efficiency (RUE) and land use efficiency (Oseni, 2010; Echarte et al., 2011; Mahallati et al., 2014; Yang et al., 2015). However, for maize-soybean intercropping, the LER rarely reaches 1.4 when the crops are grown at their optimal density (Oseni, 2010; Echarte et al., 2011; Lv et al., 2014; Mahallati et al., 2014; Yu et al. 2015), which limits the application of intercropping in practice. The relay intercropping of maize and soybean is the main planting pattern in southwest China and has been studied extensively (Gong
⁎
Corresponding author. E-mail address: [email protected] (W. Yang).
https://doi.org/10.1016/j.fcr.2018.05.010 Received 19 July 2017; Received in revised form 11 May 2018; Accepted 11 May 2018 0378-4290/ © 2018 Elsevier B.V. All rights reserved.
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2.2. Experimental design
Shandong province. This approach achieved LER of more than 1.8 in relay strip intercropping, and more than 1.4 in strip intercropping, which are much higher than the averaged values of LER (1.18 for intercropping; up to 1.39 for relay intercropping) summarized by 3313 publications (Yu et al., 2015). In this study, we aim to investigate the underlying causes of achieving this high LER, which can promote its application and further improvement to increase crop yield. The intercepted photosynthetic active radiation (PAR) and RUE in relay intercropping has been reported (Zhang et al., 2008; Gou et al., 2017b; Liu et al., 2017b). The PAR assimilation mainly includes two steps: the interception of PAR by leaf area of the intercrops and the assimilation of the intercepted PAR by intercrops to produce dry matter. The intercepted PAR can be calculated by models (Wang et al., 2015; Liu et al., 2017b). The HHLA model and ERCRT model were used to calculate the intercepted PAR of single row intercropping and strip intercropping, respectively (Liu et al., 2017b). The intercepted PAR in a strip intercropping system representing the sum of intercropped maize and soybean was about 1.1 times of that in monoculture. The RUE for strip intercropped maize and soybean were about 1.2 and 1.5 times of those in monoculture (Liu et al., 2017b), respectively. However, the reasons for high RUE in strip intercropping need further investigation. The dynamic changes of PAR at top of soybean canopy in maizesoybean intercropping has been described in our previous study (Liu et al., 2017a). Compared to single row intercropping, strip intercropping increased the PAR at top of soybean canopy (Liu et al., 2017a). Also, strip intercropping increased the PAR at maize leaves close to the ear (Gao et al., 2010), which contributed more to the yield compared to other leaves. Few studies have thoroughly described the PAR distribution within canopy in different intercropping systems, including single row intercropping and strip intercropping with different row arrangements. Also, the relationships among PAR distribution, net photosynthetic rate (Pn) of leaves, intercepted PAR and RUE are rarely analyzed. How the spatial distribution of PAR (especially for some key positions within mixed canopy) potentially relate to Pn and RUE for intercropped soybean and maize remains to be answered. The row arrangement of intercropping can alter the light distribution, leading to changes in intercepted PAR (Liu et al., 2017b). The intercepted PAR of intercrops differs in different arrangements, causing changes of interspecific competitive relationship. Moreover, the change of light distribution leads to variations of crop yield and LER in different cropping systems. However, how the competitive relationships change with the alterations in row arrangements and light distribution in strip intercropping has not been clearly explained. Taken together, considering the effects of PAR distribution and leaf photosynthesis, our study has three aims: (i) to investigate the causes for high RUE and LER in strip intercropping considering the PAR distribution; (ii) to hypothesize how the PAR distribution affects intercepted PAR and RUE; (iii) to find the optimal intercropping arrangement resulting in the highest LER.
The field experiments consisted of six arrangements with triplicates, which were randomly organized (Fig. 2). The arrangements included (Table 1) sole soybean (SS, row distance was 0.5 m), sole maize (SM, row distance was 0.7 m), maize-soybean single row intercropping (RI, 1 row of maize intercropped with 1 row of soybean, the distance between maize rows and soybean rows was 0.5 m), and maize-soybean strip intercropping (2 rows of maize intercropped with 2 rows of soybean) with three different row arrangements: SI1, maize row distance was 0.2 m, soybean row distance was 0.4 m, distance between maize row and soybean row was 0.7 m; SI2, maize row distance was 0.4 m, soybean row distance was 0.4 m, distance between maize row and soybean row was 0.6 m; SI3, maize row distance was 0.6 m, soybean row distance was 0.4 m, distance between maize row and soybean row was 0.5 m. Three adjacent bands of maize plus soybean formed a plot. The size of each experimental plot was 6 m by 6 m. The distance between maize plants within each row (MD) was 0.14 m in intercropping, and 0.2 m in monoculture. The distance between soybean plants within each row (SD) was 0.07 m in intercropping, and 0.14 m in monoculture. The maize density was 70,500 plants per hectare (commonly used in production of sole maize) for both intercropping and monoculture. The soybean density was 141,000 plants per hectare (commonly used in production of sole soybean) for both intercropping and monoculture. The row orientation was east-west. The soybean and maize used in the study were local cultivars 'Hedou19' and 'Xundan26', respectively. The previous crop was winter wheat. The seeds were manually sowed in rows at depths of 0.04 m for soybean and 0.05 m for maize. The organic fertilizer containing 4% organic matter was applied at 30 ton ha−1 before sowing. The fertilizer was applied to maize at jointing stage (both intercropped and sole maize) by spreading it in maize rows, with the dosages of 150 kg N ha-1, 100 kg P2O5 ha-1 and 100 kg K2O ha1 . No fertilizer was applied to soybean (both intercropped and sole soybean) in order to prevent overly long and thin stems. Both soybean and maize were sown on June 10th–14th from 2013 to 2015, and harvested on September 26th–28th. 2.3. Experimental measurements Six LI-191SA quantum sensors (LI-COR Inc., Lincoln, NE, USA) with a LI-1400 data logger were used to measure PAR in this study. The radiation profile on a cross section through the canopy was measured using light sensors. The cross section was 2 m wide (cross-row) and up to 2.4 m high (depending on crop height), and measurements were made on a grid with a mesh size of 20 × 20 cm (Fig. 2), resulting in x×y measurement positions. The measurements at each position were made using six sensors that were mounted on a stick. Measurements by the six sensors were averaged. The stick was mounted in two scaffolds placed in parallel to the crop rows (Fig. 3). The stick with the sensors was moved manually by an observer with approximately 5 s. residence time per position. Data were recorded by a second observer using a logger. The PAR spatial distribution in all arrangements were measured manually from 11:30am to 12:30pm on a sunny day, which was repeated three times between August 15th and 25th (pod filling stage for soybean; grain filling stage for maize) in 2014 and 2015. Based on these data, the PAR intensity at top of soybean canopy (hollow squares), maize outer (hollow circle) and inner (solid circle) leaves close to the ear were calculated (Fig. 2). The net photosynthetic rate (Pn) was measured on the same day as PAR in 2014 and 2015. Measurements were taken from the third leaf from the top of soybean (upper canopy leaf) and the three leaves close to the ear of maize during 10:30 AM to 11:30 AM with a LI-6400XT (LiCor Inc., USA). In strip intercropping, the outer leaves close to the ear (the leaves extended to the gap width for growing soybeans) and inner leaves close to the ear (the leaves within maize rows) of maize were measured separately at positions shown in Fig. 2. The natural light
2. Materials and methods 2.1. Sites description Field experiments were conducted from 2013 to 2015 at an experimental station in the Crop Breeding Farm (115°25′05″E, 35°15′09″N), located in Heze City, Shandong Province, China. It has temperate continental monsoon climate. The annual average air temperature is 14.7 °C, and the frost-free period is 210 days (data from local meteorological bureau). The daily radiation and temperature data during the growing seasons from 2013 to 2015 are shown in Fig. 1. The clayey soil is formed by Yellow River alluvial silt. At the beginning of the study, the pH of surface soil was 7.6; the available N, P and K contents of soil were 101, 34, and 187 mg⋅kg−1, respectively; and the soil organic matter content was 18.26 g⋅kg−1. Enough irrigation was provided to each arrangement by surface irrigation system. 92
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Fig. 1. Daily temperature (A for 2013, B for 2014, C for 2015) and radiation (D for 2013, E for 2014, F for 2015) during the growth stage of intercrops at the experiment site.
Fig. 2. Row arrangement, plant spacing in rows and plant density of each treatment in the field experiment from 2013 to 2015. SI: strip intercropping; SM: sole maize; SS: sole soybean; RI: single row intercropping. MD: maize spacing in rows; SD: soybean spacing in rows. The grid was used for measuring PAR spatial distribution. The circles represent the measured points of Pn for maize inner leaves close to the ear (solid circle) and maize outer leaves close to the ear (hollow circle). The hollow squares represent the measured points of Pn for soybean leaves at top of canopy.
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Table 1 Row arrangement, plant spacing in rows and plant density of each treatment in the field experiment from 2013 to 2015. Arrangement
Row ratio (maize: soybean)
Row arrangement Band width
Maize row distance
Distance between maize row and soybean row
Soybean row distance
gap width for growing soybeans
m SI1 SI2 SI3 RI SS SM
2:2 2:2 2:2 1:1 Sole soybean Sole maize
2 2 2 1 0.5 0.7
0.2 0.4 0.6 1
0.7 0.6 0.5 0.5
0.4 0.4 0.4
1.8 1.6 1.4
Maize
Maize
0.14 0.14 0.14 0.14
0.5 0.7
0.20
Soybean
CRm =
CRs =
103 ha−1
(1)
Yim LERm = Ym
(2)
0.07 0.07 0.07 0.07 0.14
70.5 70.5 70.5 70.5
141 141 141 141 141
70.5
LERm Pis × LERs Pim
LERs Pim × LERm Pis
(4)
(5)
The proportions of land occupancy of maize and soybean in intercropping are represented as Pim and Pis, respectively. For this calculation, the area between the maize (or soybean) rows was completely occupied by maize (or soybean); the area between the maize and soybean rows was assigned equally to maize and soybean. The data of intercrops in Fig. 7 were divided by those of monoculture, and the data of intercepted PAR and RUE were cited from our previous study (Liu et al. 2017a,b).
The land equivalent ratio (LER) reflects the yield of two species in intercropping compared with monoculture. The LER is calculated as below: LER = LERm + LERs
Soybean
by total land area. The yields of intercropped maize and monoculture maize are represented as Yim and Ym, respectively, and the yields of intercropped soybean and monoculture soybean are represented as Yis and Ys, respectively. Competition between different species can be represented by the competitive ratio (CR). A higher CR value indicates a competitive advantage of the dominant species in an intercropping system (Willey and Rao, 1980). The CR is calculated as below:
2.4. Calculations
Yis Ys
Plant density
m
source was used to measure the actual Pn. Pn was measured at the middle of the leaves for both maize and soybean, while the leaves were manually kept horizontal to expose them to the incoming radiation. The height of measured maize leaves close to the ear were selected at 100 cm (lower ear leaf), 120 cm (ear leaf) and 140 cm (upper leaves close to the ear) in order to match their PAR values. The height of the top of soybean canopy was 100 cm. Dry matter and yield were measured during the growth periods in 2013, 2014 and 2015. Ten soybean plants and five maize plants were selected for yield measurement in each plot. These plants were continuously sampled in each plot, avoiding 0.5 m gaps from previous samples and plot boundary. Sample from each plot was weighed after air drying.
LERs =
Plant spacing in rows
2.5. Data analysis (3)
The experimental data were represented as the mean value from three replicates. Statistical calculations were performed with SPSS statistical software (version 19.0, SPSS Inc., Chicago, USA).
The partial LER of maize and soybean are represented as LERm and LERs, respectively. Partial LER is not calculated by occupied area, but
Fig. 3. The device for the measurement of light distribution, including four vertical and two horizontal poles with the scale of 20 cm. 94
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Fig. 4. The PAR distribution for SI1 (A), SI2 (B), SI3 (C), RI (D), SM (E), and SS (F). The green label indicates maize position, and the white label indicates soybean position. The black line represents the position of the top of soybean canopy. The squares represent the positions of maize outer (black) and inner (white) leaves close to the ear. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
from 1.33 to 1.42, and SI2 achieved the highest LER (1.42).
Comparisons among different arrangements were conducted using one way ANOVA and Duncan’s multiple range test (DMRT). P < 0.05 was considered statistically significant. The heatmaps of light distribution (Fig. 4) were generated by Sigmaplot 10.0 from three column input data: the column 1 was the horizontal distance from starting point (X axis); the column 2 was the vertical height (Y axis); the column 3 was the PAR data at corresponding point. In Fig. 4, the PAR data are shown as changing colors in plane diagrams.
3.2. The PAR distribution and photosynthetic rate The incident PAR of maize leaves close to the ear in different arrangements showed the trend of SI1＞SI2＞SI3＞RI＞SM (Figs. 4, 5). The received PAR of maize leaves close to the ear in strip intercropping and single row intercropping were 1.38 and 1.27 times of that in monoculture, respectively. In intercropping arrangements, the PAR intensity for maize outer leaves close to the ear was higher than that for inner leaves close to the ear. Moreover, the maize outer leaves close to the ear received more PAR intensity in the strip intercropping arrangements with larger gap width for growing soybeans, while the maize inner leaves close to the ear showed the opposite correlation. The PAR intensity at soybean canopy became significantly higher as the gap width for growing soybeans increased with different arrangements. The PAR intensity at soybean canopy in strip intercropping and single row intercropping were 55% and 33% of that in soybean monoculture, respectively. The net photosynthetic rate (Pn) of maize leaves close to the ear showed the trend of SI1＞SI2＞SI3＞RI＞SM (Figs. 4, 5); however, the differences were not significant in spite of the different Pn of maize inner leaves and outer leaves in strip intercropping. The Pn of maize leaves close to the ear in intercropping were 1.49 times of that in maize monoculture. The Pn of soybean leaf at top of canopy showed the trend of SS＞SI1＞SI2＞SI3＞RI, and all the differences were significant. The Pn of soybean leaf in strip intercropping and single row intercropping were 80% and 46% of that in soybean monoculture.
3. Results 3.1. Yield and land equivalent ratio (LER) Different planting arrangements led to differences in dry matter, harvest index, yields and partial LER of maize and soybean (Table 2). Averaging the data of three years, we found that the dry matter of maize in intercropping arrangements was 80%–87% of that in monoculture; but the maize yield of intercropping arrangements was 81%98% of that in monoculture (except SI1), mainly because of the higher harvest index (HI). The dry matter and yield of maize in strip intercropping (averaged by SI1, SI2 and SI3) were 0.95 times of those in single row intercropping. With the reduced distance between adjacent maize rows, the dry matter, harvest index, yield and partial LER of maize also decreased. The dry matter and harvest index of soybean in intercropping arrangements were 40%–59% and 88% of those in monoculture, respectively. The yield of intercropped soybean was 35%-53% of that in monoculture. The dry matter and yield of soybean in strip intercropping (averaged by SI1, SI2 and SI3) were 1.4 times of that in single row intercropping (RI). With the increased gap width for growing soybeans in strip intercropping, the dry matter, yield and partial LER of soybean significantly decreased. The LER in intercropping arrangements ranged
3.3. The relationships between row arrangement, PAR and Pn The wide gap width for growing soybeans (in strip intercropping) enhanced the PAR at maize leaves close to the ear compared to 95
96
Mean
2015
RI SS SM SI1 SI2 SI3 RI SS SM SI1 SI2 SI3 RI SS SM
SI1 SI2 SI3 RI SS SM SI1 SI2 SI3
2013
2014
Arrangement
Year
22.71 ± 0.46d 23.25 ± 0.23 cd 23.72 ± 0.39bc 24.45 ± 0.52b – 28.64 ± 0.44a 27.24 ± 0.39d 28.26 ± 0.54c 29.33 ± 0.35 b 29.72 ± 0.69b – 33.29. ± 0.56a 22.34 ± 0.28d 23.83 ± 0.26b 23.26 ± 0.29c 24.05 ± 0.37b – 28.16 ± 0.66a 24.10 ± 2.73b 25.11 ± 2.74ab 25.43 ± 3.38ab 26.07 ± 3.17ab – 30.03 ± 2.83a 0.41 – 0.37 0.38 0.41 0.43 0.42 – 0.37 0.38 0.41 0.43 0.42 – 0.37
0.37 0.42 0.43 0.44 – 0.38 0.38 0.41 0.42 0.06b 0.13b 0.06a 0.09a
± ± ± ±
0.12b 0.06b 0.06a 0.06a 0.15a
± ± ± ± ± ± 0.06b
0.06b 0.09b 0.11a 0.13a 0.15a
± ± ± ± ±
± 0.12a
0.16b 0.11a 0.15a 0.06a
± ± ± ±
12.29 ± 0.17ab – 12.41 ± 0.26a 8.49 ± 0.31c 9.77 ± 0.16b 10.00 ± 0.20ab 10.10 ± 0.17ab – 10.42 ± 0.25a 9.12 ± 1.16a 10.41 ± 1.12a 10.80 ± 1.12a 11.05 ± 1.12a – 11.24 ± 1.04a
8.40 ± 0.15c 9.77 ± 0.11b 10.20 ± 0.22ab 10.76 ± 0.25a – 10.88 ± 0.29a 10.46 ± 0.25c 11.71 ± 0.29b 12.20 ± 0.17ab 0.99 – – 0.81 0.94 0.96 0.97 – – 0.81 0.93 0.96 0.98 – –
0.77 0.90 0.94 0.99 – – 0.84 0.94 0.98
0.02c 0.04b 0.02ab 0.03a
± ± ± ±
± ± ± ±
0.04c 0.02b 0.02ab 0.01a
0.02b 0.01a 0.01a 0.02a
± 0.04a
± 0.03b ± 0.02a ± 0.02a
± ± ± ±
LERm
2.98 7.53 – 4.60 4.30 3.69 3.22 7.74 – 4.48 4.12 3.42 3.05 7.57 –
4.37 4.02 3.16 2.95 7.44 – 4.47 4.05 3.41
0.20b 0.14b 0.16c 0.18c 0.23a
± ± ± ± ±
± ± ± ± ±
0.11b 0.15c 0.27d 0.15e 0.16a
0.30b 0.22b 0.24bc 0.27c 0.15a
± 0.14d ± 1.43a
± 0.25b ± 0.17bc ± 0.22 cd
± ± ± ± ±
DM [103 kg/hm2]
Yield [103 kg/hm2]
DM [103 kg/hm2] HI
Soybean
Maize
0.37 0.20 – 0.37 0.37 0.36 0.36 0.43 – 0.38 0.38 0.37 0.36 0.42 –
0.38 0.38 0.37 0.36 0.41 – 0.39 0.38 0.37
HI
0.01b 0.01b 0.01b 0.02b 0.02a
± ± ± ± ±
± ± ± ± ±
0.01b 0.01b 0.01bc 0.01c 0.02a
0.01b 0.01b 0.01b 0.01b 0.02a
± 0.01b ± 0.01a
± 0.02b ± 0.01b ± 0.01b
± ± ± ± ±
1.10 3.16 – 1.70 1.59 1.33 1.16 3.33 – 1.70 1.55 1.25 1.11 3.18 –
1.66 1.53 1.17 1.06 3.05 – 1.74 1.54 1.26
00.10b 0.08b 0.09c 0.06c 0.09a
± ± ± ± ±
± ± ± ± ±
0.04b 0.0.3c 0.08d 0.05e 0.14a
0.08b 0.13bc 0.11 cd 0.05d 0.13a
± 0.14c ± 0.05a
± 0.014b ± 0.10bc ± 0.10c
± ± ± ± ±
Yield [103 kg/hm2]
0.35 – – 0.51 0.48 0.40 0.35 – – 0.53 0.49 0.39 0.35 – –
0.54 0.50 0.38 0.35 – – 0.55 0.49 0.40
LERs
0.01a 0.01b 0.02c 0.01c
± ± ± ±
± ± ± ±
0.02a 0.01b 0.01c 0.00d
0.02a 0.01b 0.01c 0.01d
± 0.01d
± 0.02a ± 0.01b ± 0.02c
± ± ± ±
1.34 – – 1.32 1.42 1.36 1.32 – – 1.34 1.42 1.35 1.33 – –
1.32 1.40 1.32 1.34 – – 1.39 1.43 1.38
LER
0.01b 0.02a 0.01b 0.01b
± ± ± ±
± ± ± ±
0.04b 0.02a 0.03b 0.01b
0.00c 0.01a 0.01b 0.01c
± 0.02c
± 0.01b ± 0.01a ± 0.00b
± ± ± ±
Table 2 The dry matter (DM), harvest index (HI), yield and partial LER of maize (LERm) and soybean (LERs) in 2013–2015. Both the mean values ( ± SD) of each year (n = 3 repeats) and the mean values ( ± SD) of three years are shown (n = 3 years) here. Lowercase letters indicate significant differences among different arrangements based on Duncan’s new multiple range test (P ≤ 0.05).
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Fig. 5. The PAR for maize outer and inner leaves close to the ear (A), and for all maize leaves close to the ear (B). The PAR for soybean leaves at top of canopy (C). The PAR values were selected in Fig. 4. The Pn for maize outer and inner leaves close to the ear (D), and for all maize leaves close to the ear (E). The Pn for soybean leaves at top of canopy (F).
monoculture (Fig. 6A), resulting in high Pn of intercropped maize (Fig. 6C). In addition, it also increased the PAR at top of soybean canopy (Fig. 6B), leading to high Pn of intercropped soybean (Fig. 6D). The data from 2014 and 2015 showed the same trend.
dominance of maize under these crop arrangements. The value of CR for maize was larger when the maize rows were expanded, indicating that the positive benefit for maize was more significant with wider maize rows (or narrower gap width for growing soybeans and soybean strip). The highest value of CRm was found in SI3, which suggested that the intercropped maize in SI3 had greater advantage. The value of CR for soybean was larger when the gap width for growing soybeans was expanded, indicating that the positive benefit for soybean was more significant with a wider gap width. The highest value
3.4. Competitive indices The CR of intercropped maize was more than 1 and the CR of intercropped soybean was less than 1 (Table 3), demonstrating the 97
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Fig. 6. The relationships between gap width for growing soybeans and PAR at maize leaves close to the ear (A), and PAR at top of soybean canopy (B). The relationships of Pn and PAR at maize leaves close to the ear (C), and PAR at top of soybean canopy (D). Empty and solid circles showed the data from 2014 and 2015, respectively.
high densities (the same as monoculture) brought increased LAI and dry matter by nearly 1.60 and 1.39 times of monoculture (Fig. 7). The border row advantage was beneficial to both two rows of maize within a band in SI. Although the intercepted PAR of strip intercropped maize was 77% of that in monoculture (Liu et al., 2017b), the maize yields in SI2 and SI3 were nearly equal to monoculture (Fig. 7). The border row advantage was shown as the enhanced PAR at maize leaves, especially for leaves close to the ear which contribute more to maize yield (Awal et al., 2006; Gao et al. 2010). Due to the border row advantage, the reduced plant distance within rows had little effect on dry matter of the individual maize plants. The other study (Gou et al., 2018) reported the same as our results that the leaves close to the ear of intercropped maize had higher Pn, which provides at least partial explanation for the high RUE of intercropped maize found in our previous work (Liu et al., 2017a,b). The spatial complementary advantage in SI was due to the large gap width for growing soybeans. The PAR at soybean canopy was regarded
of CRs was found in SI1, which suggested that the intercropped soybean in SI1 suffered less from competition with maize than soybean in the two other configurations. 4. Discussion 4.1. The causes of high RUE and LER in strip intercropping As compared to many of previous studies (Yu et al., 2015), the increased LER in our strip intercropping arrangements (SI) were mainly due to the density-adding advantage and configuration advantage. The configuration advantage included border row advantage and spatial complementary advantage. The density of both intercropped maize and intercropped soybean in SI, which were equal to their monoculture, were much higher than the other studies (Gao et al., 2010; Oseni, 2010; Echarte et al., 2011; Mahallati et al., 2014). The combined planting of two crops in relatively
Table 3 Competitive ratio (CR) of each arrangement in 2013–2015. Both the mean values ( ± SD) of each year (n = 3 repeats) and the mean values ( ± SD) of three years are shown (n = 3 years) here. Lowercase letters indicate significant differences among different arrangements based on Duncan’s new multiple range test (P ≤ 0.05). Arrangement
2013
2014
Maize CRm SI1 SI2 SI3 RI
1.16 1.79 2.99 2.84
± ± ± ±
Soybean CRs 0.05d 0.04c 0.08a 0.02b
0.86 0.56 0.33 0.35
± ± ± ±
2015
Maize CRm 0.04a 0.01b 0.02c 0.00c
1.25 1.94 3.01 2.84
± ± ± ±
Soybean CRs 0.09d 0.07c 0.06a 0.04b
0.80 0.52 0.33 0.35
± ± ± ±
Mean
Maize CRm 0.03a 0.02b 0.01c 0.01c
98
1.31 1.95 2.93 2.77
± ± ± ±
Soybean CRs 0.04d 0.05c 0.06a 0.05b
0.77 0.51 0.34 0.36
± ± ± ±
0.02a 0.03b 0.01c 0.00c
Maize CRm
Soybean CRs
1.24 ± 0.08d 1.89 ± 0.09c 2.980.04a 2.82 ± 0.04b
0.81 0.53 0.33 0.35
± ± ± ±
0.05a 0.03b 0.01c 0.01c
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Fig. 7. The analysis of high LER in strip intercropping. The percentages were calculated by dividing values of intercropping [SI1 (A), SI2 (B), SI3 (C) and RI (D)] by those of monoculture. CR: competitive ratio; LAI: leaf area index; LER: land equivalent ratio; PAR: photosynthetic active radiation; Pn: net photosynthetic rate; RUE: radiation use efficiency.
potential relationship to the change of the intercepted PAR, Pn, RUE, interspecific competition and dry matter partitioning of intercrops in different intercropping arrangements (Fig. 7). The total intercepted PAR was nearly the same for all intercropping systems (107–110%). Therefore, the intercepted PAR showed an inverse regular pattern for intercropped maize (67-85%) and soybean (41%-25%) (Fig. 7). From correlation analysis, we found that the intercepted PAR had significant positive correlation with competitive ratio in SI (Fig. 8). Therefore, the yield of intercropped maize and soybean also had inverse correlations (Fig. 7). The PAR at top of soybean canopy positively affected the intercepted PAR, Pn and yield of intercropped soybean, but had no obvious relationship with RUE (Fig. 7). The wide gap width for growing soybeans (Fig. 7C→A) enhanced the PAR at top of soybean canopy, and led to increased Pn and intercepted PAR of intercropped soybean. The wide gap width for growing soybeans increased competitive ability of soybean (Fig. 8), and finally enhanced the yield of soybean. However, high
as the incident PAR, which was directly related to Pn and intercepted PAR. Compared to other studies of intercropping (Oseni, 2010, Echarte et al., 2011, Mahallati et al., 2014), the gap width for growing soybeans in SI was expanded so that the intercropped soybean could receive more PAR and increase their Pn (Fig. 7). Compared to monoculture, the decreased incident PAR intensity at top of canopy of intercropped soybean reduced its intercepted PAR, which then enhanced its RUE (Wang et al., 2015, Gou et al., 2017a, Liu et al., 2017a; Li et al., 2008). The intercepted PAR of soybean in SI was 32% of that in monoculture, and was higher than other studies (Awal et al., 2006; Gao et al., 2010). Moreover, the yield was nearly half (48%) of that in monoculture, which mainly resulted from its high RUE. 4.2. The relationships among PAR, Pn, RUE, competitive ratio and LER The row arrangement of intercropping can modify the PAR distribution. The variation of PAR distribution might have direct or 99
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Fig. 8. Relationship between intercepted PAR and competitive ratio in strip intercropping arrangements. The data of intercepted PAR were used from our previous paper (Liu et al., 2017a,b).
our study), in order to increase the intercepted PAR of soybean. However, due to the increase of the gap width for growing soybeans, the interspecific competition of maize was decreased when the maize row distance was reduced from 0.6 m to 0.2 m, especially from 0.4 m to 0.2 m. In order to achieve high yield of both maize and soybean, the suitble gap width for growing soybeans was regarded as 1.6 m (SI2)1.8 m (SI3), which can provide enough space for soybean and border row advantage for maize, and the best maize row distance was considered as 0.4 m, which ensures the maize yield. Overall, the LER of SI2 (distance between maize rows of 0.4 m, gap width for growing soybeans of 1.6 m) was more than 1.40, which was the leading level in cereal and legume intercropping compared with the literature reports (Lv et al., 2014; Mahallati et al., 2014; Yogesh et al., 2014; Oseni, 2010; Echarte et al., 2011; Yu et al. 2015). Furthermore, the selection of appropriate cultivars of maize and soybean is another efficient method to increase crop yield.
Pn did not directly improve its RUE, which might be related to light absorption and transformation and need further investigation. The decease of maize row distance led to reduction of intercepted PAR of intercropped maize (Fig. 7C→A), and the weakened competitive ability resulted in reduced dry matter and yield of intercropped maize (Fig. 8). However, the wide gap width for growing soybeans (Fig. 7D→A) in SI enhanced the PAR and Pn of maize leaves close to the ear, and potentially increased the RUE of intercropped maize, which could partially compensate for the yield loss caused by decrease of intercepted PAR. In our study, the spatial distribution of PAR was measured from about 65DAS (days after sowing) to 75DAS. This stage can represent the trend from 45DAS to harvesting due to the little change of LAI (Liu et al., 2017a,b). The differences of intercepted PAR, RUE and dry matter accumulation from different arrangements mainly came from this stage (from 45DAS to maturity). Our study showed many important data within this time frame (Fig. 7), and proposed the potential positive or negative relationships among different parameters. More future studies are needed in order to build data-based relationship models among those parameters, such as Pn and RUE.
5. Conclusions The high LER of strip intercropping was a result of their high crop RUE (118% for maize and 149% for soybean). In strip intercropping with a enough gap width for growing soybeans, the improved PAR at maize leaves close to the ear increased their Pn, and potentially increased the maize RUE; the increased PAR at top of soybean canopy enhanced the Pn and intercepted PAR of soybean, which then improved its dry matter and yield. Moreover, the density-adding advantage, which ensured an increased population, LAI and dry matter, was the foundation for high yield. With the increase of gap width for growing soybeans in strip intercropping systems, the yield of maize and soybean showed an inverse
4.3. The optimal arrangement for application Improving the competitive position of soybean is crucial for obtaining a high LER of intercropping system. However, in previous studies, the gap width for growing soybeans was no more than 0.52 m (Lv et al., 2014; Oseni, 2010; Echarte et al., 2011), which resulted in the negative impact of maize on the PAR intensity at top of soybean canopy. An effective way to relieve the competitive disadvantage of soybean is to expand the gap width for growing soybeans (1.4–1.8 m in 100
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regular pattern due to the interspecific competition of system intercepted PAR. By comparison, SI2 (distance between maize rows of 0.4 m, gap width for growing soybeans of 1.6 m) is our recommended configuration, which showed the highest LER (1.42) among all the arrangements.
Gou, F., van Ittersum, M.K., Couëdel, A., Zhang, Y., Wang, Y.J., van der Putten, P.E.L., Zhang, L.Z., van der Werf, W., 2018. Intercropping with wheat lowers nutrient uptake and biomass accumulation of maize, but increases photosynthetic rate of the ear leaf. AOB Plants. http://dx.doi.org/10.1093/aobpla/ply010. Li, F., Meng, P., Fu, D., Wang, B.P., 2008. Light distribution, photosynthetic rate and yield in a paulownia-wheat intercropping system in china. Agroforest. Sys. 74 (2), 163–172. Liu, X., Rahman, T., Song, C., Su, B.Y., Yang, F., Yong, T.W., Wu, Y.S., Zhang, C.Y., Yang, W.Y., 2017a. Changes in light environment, morphology, growth and yield of soybean in maize-soybean intercropping systems. Field Crops Res. 200, 38–46. Liu, X., Rahman, T., Yang, F., Song, C., Yong, T.W., Liu, J., Zhang, C.Y., Yang, W.Y., 2017b. Light interception and utilization in different maize and soybean intercropping patterns. Plos One 12 (1), e0169218. Lv, Y., Francis, C., Wu, P., Chen, X., Zhao, X., 2014. Maize-soybean intercropping interactions above and below ground. Crop Sci. 54 (3), 914–922. Mahallati, M.N., Koocheki, A., Mondani, F., Feizi, H., Amirmoradi, S., 2014. Determination of optimal strip width in strip intercropping of maize (Zea mays L.) and bean (Phaseolus vulgaris L.) in Northeast Iran. J. Clean Prod. 106 (3), 390–404. Oseni, T.O., 2010. Evaluation of sorghum-cowpea intercrop productivity in savanna agroecology using competition indices. J. Agr. Sci. 2 (3), 229–234. Rahman, T., Ye, L., Liu, X., Iqbal, N., Junbo Du, J.B., Gao, R.C., Liu, W.G., Yang, F., Yang, W.Y., 2016. Water use efficiency and water distribution response to different planting patterns in maize–soybean relay strip intercropping systems. Exp. Agric. 53, 1–19. Wang, Z.K., Zhao, X.N., Wu, P.T., He, J.Q., Chen, X.L., Gao, Y., Cao, X.C., 2015. Radiation interception and utilization by wheat/maize strip intercropping systems. Agr. For. Meteorol 204, 58–66. Yang, F., Wang, X.C., Liao, D.P., Lu, F.Z., Gao, R.C., Liu, W.G., Yong, T.W., Wu, X.L., Du, J.B., Liu, J., Yang, W.Y., 2015. Yield response to different planting geometries in maize-soybean relay strip intercropping systems. Agron. J. 107 (1), 296–304. Yogesh, S., Halikatti, S.I., Hiremath, S.M., Potdar, M.P., Harlapur, S.I., Venkatesh, H., 2014. Light use efficiency, productivity and profitability of maize and soybean intercropping as influenced by planting geometry and row proportion. Pharmacoeconomics 11 (Suppl. 1(1)), 24–34. Yu, Y., Stomph, T.J., Makowski, D., van der Werf, W., 2015. Temporal niche differentiation increases the land equivalent ratio of annual intercrops: a meta-analysis. Field Crops Res. 184, 133–144. Zhang, L.Z., van der Werf, W., Bastiaans, L., Zhang, S., Li, B., Spiertz, J.H.J., 2008. Light interception and utilization in relay intercrops of wheat and cotton. Field Crops Res. 107 (1), 29–42.
Acknowledgments This study was supported by the National Key Research and Development Program of China (grant numbers 2016YFD0300209, 2016YFD0300602), the National Nature Science Foundation (grant number 31571615), and Program on Industrial Technology System of National Soybean (grant number CARS-04-PS19). References Awal, M.A., Koshi, H., Ikeda, T., 2006. Radiation interception and use by maize/peanut intercrop canopy. Agr. For. Meteorol. 139 (1), 74–83. Echarte, L., Della Maggiora, A., Cerrudo, D., Gonzalez, V.H., Abbate, P., Cerrudo, Cerrudo.A., Sadras, V.O., Calviño, P., 2011. Yield response to plant density of maize and sunflower intercropped with soybean. Field Crops Res. 121 (3), 423–429. Du, J.B., Han, T.F., Gai, J.Y., Yong, T.W., Sun, X., Wang, X.C., Yang, F., Liu, J., Su, K., Liu, W.G., Yang, W.Y., 2018. Maize-soybean strip intercropping: achieved a balance between high productivity and sustainability. J. Integr. Agr 17 (4), 747–754. Gao, Y., Duan, A.W., Qiu, X.Q., Liu, Z.G., Sun, J.S., Zhang, J.P., Wang, H.Z., 2010. Distribution and use efficiency of photosynthetically active radiation in strip intercropping of maize and soybean. Agron. J. 102 (4), 1149–1157. Gong, W.Z., Jiang, C.D., Wu, Y.S., Chen, H.H., Liu, W.Y., Yang, W.Y., 2015. Tolerance vs. avoidance: two strategies of soybean (Glycine max) seedlings in response to shade in intercropping. Photosynthetica 53 (2), 259–268. Gou, F., van Ittersum, M.K., Simon, E., Leffelaar, P.A., van der Putten, P.E.L., Zhang, L.Z., van der Werf, W., 2017a. Intercropping wheat and maize increases total radiation interception and wheat RUE but lowers maize RUE. Eur. J. Agron 84, 125–139. Gou, F., van Ittersum, M.K., van der Werf, W., 2017b. Simulating potential growth in a relay-strip intercropping system: model description, calibration and testing. Field Crops Res. 200, 122–142.
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Relationships among light distribution, radiation use efficiency and land equivalent ratio in maize-soybean strip intercropping
T
Xin Liua,b, Tanzeelur Rahmana, Chun Songc,d, Feng Yanga,d, Benying Sua, Liang Cuia, ⁎ Weizhao Bua, Wenyu Yanga,d, a
College of Agronomy, Sichuan Agricultural University, Chengdu, 611130, China College of Agronomy, Shandong Agricultural University, Taian, 271018, China c Institute of Ecological and Environmental Sciences, Sichuan Agricultural University, Chengdu, 611130, China d Key Laboratory of Crop Ecophysiology and Farming Systems in the Southwest, Ministry of Agriculture, Chengdu, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Intercropping Land equivalent ratio Light distribution Photosynthesis
Maize (Zea mays)-soybean (Glycine max) intercropping is popular in many developing countries because of its high land equivalent ratio (LER). However, very few studies have explored the reason of its high LER, and the relationships between light distribution and the variations in radiation use efficiency (RUE) and LER in different intercropping arrangements. In this study, we conducted field experiments with different row arrangements of intercropping patterns from 2013 to 2015. The three different strip intercropping (SI) row arrangements were 0.2 m, 0.4 m, and 0.7 m (SI1); 0.4 m, 0.4 m, and 0.6 m (SI2); and 0.6 m, 0.4 m, and 0.5 m (SI3) for maize row distance, soybean row distance, distance between maize and soybean rows, respectively. The results showed that, as compared to single row intercropping, the strip intercropping increased the PAR at top of soybean canopy by 1.42 (SI3), 1.67 (SI2) and 1.93 (SI1) times, and increased the PAR at maize leaves close to the ear by 1.02 (SI3), 1.11 (SI2) and 1.12 (SI1) times. Moreover, the increased PAR at crucial positions in SI potentially improved the photosynthetic rate (Pn) for maize leaves close to the ear and radiation use efficiency (RUE) of maize by 1.08 and 1.09 times (averaged by SI1, SI2 and SI3), respectively, and improved the Pn of leaves at top of canopy and intercepted PAR of soybean by 1.75 and 1.36 times (averaged by SI1, SI2 and SI3), respectively. Compared to monoculture, SI also enhanced the RUE of intercropped maize (by 1.18 times) and soybean (by 1.51 times), which compensated for the partial yield loss caused by decreased crop intercepted PAR. Overall, in SI, intercropped maize achieved 90% of the monoculture yield, and intercropped soybean achieved 47% of the monoculture yield. With the expanding gap width for growing soybeans under a fixed bandwidth (2 m), the increasing intercepted PAR of intercropped soybean alleviated the interspecific competition disadvantage of soybean, while the reduction of maize row width decreased the dominant interspecific competition of maize. By adjusting the distances, we suggest that the optimal gap width for growing soybeans is 1.6 m-1.8 m, and the best maize row distance is 0.4 m. The SI2 achieved LER of 1.42, representing the leading level in the world.
1. Introduction
et al., 2015; Yang et al., 2015; Rahman et al., 2016; Du et al., 2018). The main recent improvements are as follows: first, narrow the distance between maize rows in order to increase the distance between maize and soybean rows; second, ensure the same plant densities for maize and soybean in intercrops as in sole crops (with optimal density) by using a smaller plant distance within each row in intercropping, which compensates for the reduced row number. In the recent five years, this approach has been practiced in several Provinces of China, such as Sichuan, Shandong, Guizhou, Henan, Ningxia, etc (Du et al., 2018). High crop yields in each Province were used for comparison to calculate LER, e.g., 9000 kg ha−1 for maize and 3000 kg ha−1 for soybean in
Intercropping has been adopted worldwide since it can improve the radiation use efficiency (RUE) and land use efficiency (Oseni, 2010; Echarte et al., 2011; Mahallati et al., 2014; Yang et al., 2015). However, for maize-soybean intercropping, the LER rarely reaches 1.4 when the crops are grown at their optimal density (Oseni, 2010; Echarte et al., 2011; Lv et al., 2014; Mahallati et al., 2014; Yu et al. 2015), which limits the application of intercropping in practice. The relay intercropping of maize and soybean is the main planting pattern in southwest China and has been studied extensively (Gong
⁎
Corresponding author. E-mail address: [email protected] (W. Yang).
https://doi.org/10.1016/j.fcr.2018.05.010 Received 19 July 2017; Received in revised form 11 May 2018; Accepted 11 May 2018 0378-4290/ © 2018 Elsevier B.V. All rights reserved.
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2.2. Experimental design
Shandong province. This approach achieved LER of more than 1.8 in relay strip intercropping, and more than 1.4 in strip intercropping, which are much higher than the averaged values of LER (1.18 for intercropping; up to 1.39 for relay intercropping) summarized by 3313 publications (Yu et al., 2015). In this study, we aim to investigate the underlying causes of achieving this high LER, which can promote its application and further improvement to increase crop yield. The intercepted photosynthetic active radiation (PAR) and RUE in relay intercropping has been reported (Zhang et al., 2008; Gou et al., 2017b; Liu et al., 2017b). The PAR assimilation mainly includes two steps: the interception of PAR by leaf area of the intercrops and the assimilation of the intercepted PAR by intercrops to produce dry matter. The intercepted PAR can be calculated by models (Wang et al., 2015; Liu et al., 2017b). The HHLA model and ERCRT model were used to calculate the intercepted PAR of single row intercropping and strip intercropping, respectively (Liu et al., 2017b). The intercepted PAR in a strip intercropping system representing the sum of intercropped maize and soybean was about 1.1 times of that in monoculture. The RUE for strip intercropped maize and soybean were about 1.2 and 1.5 times of those in monoculture (Liu et al., 2017b), respectively. However, the reasons for high RUE in strip intercropping need further investigation. The dynamic changes of PAR at top of soybean canopy in maizesoybean intercropping has been described in our previous study (Liu et al., 2017a). Compared to single row intercropping, strip intercropping increased the PAR at top of soybean canopy (Liu et al., 2017a). Also, strip intercropping increased the PAR at maize leaves close to the ear (Gao et al., 2010), which contributed more to the yield compared to other leaves. Few studies have thoroughly described the PAR distribution within canopy in different intercropping systems, including single row intercropping and strip intercropping with different row arrangements. Also, the relationships among PAR distribution, net photosynthetic rate (Pn) of leaves, intercepted PAR and RUE are rarely analyzed. How the spatial distribution of PAR (especially for some key positions within mixed canopy) potentially relate to Pn and RUE for intercropped soybean and maize remains to be answered. The row arrangement of intercropping can alter the light distribution, leading to changes in intercepted PAR (Liu et al., 2017b). The intercepted PAR of intercrops differs in different arrangements, causing changes of interspecific competitive relationship. Moreover, the change of light distribution leads to variations of crop yield and LER in different cropping systems. However, how the competitive relationships change with the alterations in row arrangements and light distribution in strip intercropping has not been clearly explained. Taken together, considering the effects of PAR distribution and leaf photosynthesis, our study has three aims: (i) to investigate the causes for high RUE and LER in strip intercropping considering the PAR distribution; (ii) to hypothesize how the PAR distribution affects intercepted PAR and RUE; (iii) to find the optimal intercropping arrangement resulting in the highest LER.
The field experiments consisted of six arrangements with triplicates, which were randomly organized (Fig. 2). The arrangements included (Table 1) sole soybean (SS, row distance was 0.5 m), sole maize (SM, row distance was 0.7 m), maize-soybean single row intercropping (RI, 1 row of maize intercropped with 1 row of soybean, the distance between maize rows and soybean rows was 0.5 m), and maize-soybean strip intercropping (2 rows of maize intercropped with 2 rows of soybean) with three different row arrangements: SI1, maize row distance was 0.2 m, soybean row distance was 0.4 m, distance between maize row and soybean row was 0.7 m; SI2, maize row distance was 0.4 m, soybean row distance was 0.4 m, distance between maize row and soybean row was 0.6 m; SI3, maize row distance was 0.6 m, soybean row distance was 0.4 m, distance between maize row and soybean row was 0.5 m. Three adjacent bands of maize plus soybean formed a plot. The size of each experimental plot was 6 m by 6 m. The distance between maize plants within each row (MD) was 0.14 m in intercropping, and 0.2 m in monoculture. The distance between soybean plants within each row (SD) was 0.07 m in intercropping, and 0.14 m in monoculture. The maize density was 70,500 plants per hectare (commonly used in production of sole maize) for both intercropping and monoculture. The soybean density was 141,000 plants per hectare (commonly used in production of sole soybean) for both intercropping and monoculture. The row orientation was east-west. The soybean and maize used in the study were local cultivars 'Hedou19' and 'Xundan26', respectively. The previous crop was winter wheat. The seeds were manually sowed in rows at depths of 0.04 m for soybean and 0.05 m for maize. The organic fertilizer containing 4% organic matter was applied at 30 ton ha−1 before sowing. The fertilizer was applied to maize at jointing stage (both intercropped and sole maize) by spreading it in maize rows, with the dosages of 150 kg N ha-1, 100 kg P2O5 ha-1 and 100 kg K2O ha1 . No fertilizer was applied to soybean (both intercropped and sole soybean) in order to prevent overly long and thin stems. Both soybean and maize were sown on June 10th–14th from 2013 to 2015, and harvested on September 26th–28th. 2.3. Experimental measurements Six LI-191SA quantum sensors (LI-COR Inc., Lincoln, NE, USA) with a LI-1400 data logger were used to measure PAR in this study. The radiation profile on a cross section through the canopy was measured using light sensors. The cross section was 2 m wide (cross-row) and up to 2.4 m high (depending on crop height), and measurements were made on a grid with a mesh size of 20 × 20 cm (Fig. 2), resulting in x×y measurement positions. The measurements at each position were made using six sensors that were mounted on a stick. Measurements by the six sensors were averaged. The stick was mounted in two scaffolds placed in parallel to the crop rows (Fig. 3). The stick with the sensors was moved manually by an observer with approximately 5 s. residence time per position. Data were recorded by a second observer using a logger. The PAR spatial distribution in all arrangements were measured manually from 11:30am to 12:30pm on a sunny day, which was repeated three times between August 15th and 25th (pod filling stage for soybean; grain filling stage for maize) in 2014 and 2015. Based on these data, the PAR intensity at top of soybean canopy (hollow squares), maize outer (hollow circle) and inner (solid circle) leaves close to the ear were calculated (Fig. 2). The net photosynthetic rate (Pn) was measured on the same day as PAR in 2014 and 2015. Measurements were taken from the third leaf from the top of soybean (upper canopy leaf) and the three leaves close to the ear of maize during 10:30 AM to 11:30 AM with a LI-6400XT (LiCor Inc., USA). In strip intercropping, the outer leaves close to the ear (the leaves extended to the gap width for growing soybeans) and inner leaves close to the ear (the leaves within maize rows) of maize were measured separately at positions shown in Fig. 2. The natural light
2. Materials and methods 2.1. Sites description Field experiments were conducted from 2013 to 2015 at an experimental station in the Crop Breeding Farm (115°25′05″E, 35°15′09″N), located in Heze City, Shandong Province, China. It has temperate continental monsoon climate. The annual average air temperature is 14.7 °C, and the frost-free period is 210 days (data from local meteorological bureau). The daily radiation and temperature data during the growing seasons from 2013 to 2015 are shown in Fig. 1. The clayey soil is formed by Yellow River alluvial silt. At the beginning of the study, the pH of surface soil was 7.6; the available N, P and K contents of soil were 101, 34, and 187 mg⋅kg−1, respectively; and the soil organic matter content was 18.26 g⋅kg−1. Enough irrigation was provided to each arrangement by surface irrigation system. 92
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Fig. 1. Daily temperature (A for 2013, B for 2014, C for 2015) and radiation (D for 2013, E for 2014, F for 2015) during the growth stage of intercrops at the experiment site.
Fig. 2. Row arrangement, plant spacing in rows and plant density of each treatment in the field experiment from 2013 to 2015. SI: strip intercropping; SM: sole maize; SS: sole soybean; RI: single row intercropping. MD: maize spacing in rows; SD: soybean spacing in rows. The grid was used for measuring PAR spatial distribution. The circles represent the measured points of Pn for maize inner leaves close to the ear (solid circle) and maize outer leaves close to the ear (hollow circle). The hollow squares represent the measured points of Pn for soybean leaves at top of canopy.
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Table 1 Row arrangement, plant spacing in rows and plant density of each treatment in the field experiment from 2013 to 2015. Arrangement
Row ratio (maize: soybean)
Row arrangement Band width
Maize row distance
Distance between maize row and soybean row
Soybean row distance
gap width for growing soybeans
m SI1 SI2 SI3 RI SS SM
2:2 2:2 2:2 1:1 Sole soybean Sole maize
2 2 2 1 0.5 0.7
0.2 0.4 0.6 1
0.7 0.6 0.5 0.5
0.4 0.4 0.4
1.8 1.6 1.4
Maize
Maize
0.14 0.14 0.14 0.14
0.5 0.7
0.20
Soybean
CRm =
CRs =
103 ha−1
(1)
Yim LERm = Ym
(2)
0.07 0.07 0.07 0.07 0.14
70.5 70.5 70.5 70.5
141 141 141 141 141
70.5
LERm Pis × LERs Pim
LERs Pim × LERm Pis
(4)
(5)
The proportions of land occupancy of maize and soybean in intercropping are represented as Pim and Pis, respectively. For this calculation, the area between the maize (or soybean) rows was completely occupied by maize (or soybean); the area between the maize and soybean rows was assigned equally to maize and soybean. The data of intercrops in Fig. 7 were divided by those of monoculture, and the data of intercepted PAR and RUE were cited from our previous study (Liu et al. 2017a,b).
The land equivalent ratio (LER) reflects the yield of two species in intercropping compared with monoculture. The LER is calculated as below: LER = LERm + LERs
Soybean
by total land area. The yields of intercropped maize and monoculture maize are represented as Yim and Ym, respectively, and the yields of intercropped soybean and monoculture soybean are represented as Yis and Ys, respectively. Competition between different species can be represented by the competitive ratio (CR). A higher CR value indicates a competitive advantage of the dominant species in an intercropping system (Willey and Rao, 1980). The CR is calculated as below:
2.4. Calculations
Yis Ys
Plant density
m
source was used to measure the actual Pn. Pn was measured at the middle of the leaves for both maize and soybean, while the leaves were manually kept horizontal to expose them to the incoming radiation. The height of measured maize leaves close to the ear were selected at 100 cm (lower ear leaf), 120 cm (ear leaf) and 140 cm (upper leaves close to the ear) in order to match their PAR values. The height of the top of soybean canopy was 100 cm. Dry matter and yield were measured during the growth periods in 2013, 2014 and 2015. Ten soybean plants and five maize plants were selected for yield measurement in each plot. These plants were continuously sampled in each plot, avoiding 0.5 m gaps from previous samples and plot boundary. Sample from each plot was weighed after air drying.
LERs =
Plant spacing in rows
2.5. Data analysis (3)
The experimental data were represented as the mean value from three replicates. Statistical calculations were performed with SPSS statistical software (version 19.0, SPSS Inc., Chicago, USA).
The partial LER of maize and soybean are represented as LERm and LERs, respectively. Partial LER is not calculated by occupied area, but
Fig. 3. The device for the measurement of light distribution, including four vertical and two horizontal poles with the scale of 20 cm. 94
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Fig. 4. The PAR distribution for SI1 (A), SI2 (B), SI3 (C), RI (D), SM (E), and SS (F). The green label indicates maize position, and the white label indicates soybean position. The black line represents the position of the top of soybean canopy. The squares represent the positions of maize outer (black) and inner (white) leaves close to the ear. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
from 1.33 to 1.42, and SI2 achieved the highest LER (1.42).
Comparisons among different arrangements were conducted using one way ANOVA and Duncan’s multiple range test (DMRT). P < 0.05 was considered statistically significant. The heatmaps of light distribution (Fig. 4) were generated by Sigmaplot 10.0 from three column input data: the column 1 was the horizontal distance from starting point (X axis); the column 2 was the vertical height (Y axis); the column 3 was the PAR data at corresponding point. In Fig. 4, the PAR data are shown as changing colors in plane diagrams.
3.2. The PAR distribution and photosynthetic rate The incident PAR of maize leaves close to the ear in different arrangements showed the trend of SI1＞SI2＞SI3＞RI＞SM (Figs. 4, 5). The received PAR of maize leaves close to the ear in strip intercropping and single row intercropping were 1.38 and 1.27 times of that in monoculture, respectively. In intercropping arrangements, the PAR intensity for maize outer leaves close to the ear was higher than that for inner leaves close to the ear. Moreover, the maize outer leaves close to the ear received more PAR intensity in the strip intercropping arrangements with larger gap width for growing soybeans, while the maize inner leaves close to the ear showed the opposite correlation. The PAR intensity at soybean canopy became significantly higher as the gap width for growing soybeans increased with different arrangements. The PAR intensity at soybean canopy in strip intercropping and single row intercropping were 55% and 33% of that in soybean monoculture, respectively. The net photosynthetic rate (Pn) of maize leaves close to the ear showed the trend of SI1＞SI2＞SI3＞RI＞SM (Figs. 4, 5); however, the differences were not significant in spite of the different Pn of maize inner leaves and outer leaves in strip intercropping. The Pn of maize leaves close to the ear in intercropping were 1.49 times of that in maize monoculture. The Pn of soybean leaf at top of canopy showed the trend of SS＞SI1＞SI2＞SI3＞RI, and all the differences were significant. The Pn of soybean leaf in strip intercropping and single row intercropping were 80% and 46% of that in soybean monoculture.
3. Results 3.1. Yield and land equivalent ratio (LER) Different planting arrangements led to differences in dry matter, harvest index, yields and partial LER of maize and soybean (Table 2). Averaging the data of three years, we found that the dry matter of maize in intercropping arrangements was 80%–87% of that in monoculture; but the maize yield of intercropping arrangements was 81%98% of that in monoculture (except SI1), mainly because of the higher harvest index (HI). The dry matter and yield of maize in strip intercropping (averaged by SI1, SI2 and SI3) were 0.95 times of those in single row intercropping. With the reduced distance between adjacent maize rows, the dry matter, harvest index, yield and partial LER of maize also decreased. The dry matter and harvest index of soybean in intercropping arrangements were 40%–59% and 88% of those in monoculture, respectively. The yield of intercropped soybean was 35%-53% of that in monoculture. The dry matter and yield of soybean in strip intercropping (averaged by SI1, SI2 and SI3) were 1.4 times of that in single row intercropping (RI). With the increased gap width for growing soybeans in strip intercropping, the dry matter, yield and partial LER of soybean significantly decreased. The LER in intercropping arrangements ranged
3.3. The relationships between row arrangement, PAR and Pn The wide gap width for growing soybeans (in strip intercropping) enhanced the PAR at maize leaves close to the ear compared to 95
96
Mean
2015
RI SS SM SI1 SI2 SI3 RI SS SM SI1 SI2 SI3 RI SS SM
SI1 SI2 SI3 RI SS SM SI1 SI2 SI3
2013
2014
Arrangement
Year
22.71 ± 0.46d 23.25 ± 0.23 cd 23.72 ± 0.39bc 24.45 ± 0.52b – 28.64 ± 0.44a 27.24 ± 0.39d 28.26 ± 0.54c 29.33 ± 0.35 b 29.72 ± 0.69b – 33.29. ± 0.56a 22.34 ± 0.28d 23.83 ± 0.26b 23.26 ± 0.29c 24.05 ± 0.37b – 28.16 ± 0.66a 24.10 ± 2.73b 25.11 ± 2.74ab 25.43 ± 3.38ab 26.07 ± 3.17ab – 30.03 ± 2.83a 0.41 – 0.37 0.38 0.41 0.43 0.42 – 0.37 0.38 0.41 0.43 0.42 – 0.37
0.37 0.42 0.43 0.44 – 0.38 0.38 0.41 0.42 0.06b 0.13b 0.06a 0.09a
± ± ± ±
0.12b 0.06b 0.06a 0.06a 0.15a
± ± ± ± ± ± 0.06b
0.06b 0.09b 0.11a 0.13a 0.15a
± ± ± ± ±
± 0.12a
0.16b 0.11a 0.15a 0.06a
± ± ± ±
12.29 ± 0.17ab – 12.41 ± 0.26a 8.49 ± 0.31c 9.77 ± 0.16b 10.00 ± 0.20ab 10.10 ± 0.17ab – 10.42 ± 0.25a 9.12 ± 1.16a 10.41 ± 1.12a 10.80 ± 1.12a 11.05 ± 1.12a – 11.24 ± 1.04a
8.40 ± 0.15c 9.77 ± 0.11b 10.20 ± 0.22ab 10.76 ± 0.25a – 10.88 ± 0.29a 10.46 ± 0.25c 11.71 ± 0.29b 12.20 ± 0.17ab 0.99 – – 0.81 0.94 0.96 0.97 – – 0.81 0.93 0.96 0.98 – –
0.77 0.90 0.94 0.99 – – 0.84 0.94 0.98
0.02c 0.04b 0.02ab 0.03a
± ± ± ±
± ± ± ±
0.04c 0.02b 0.02ab 0.01a
0.02b 0.01a 0.01a 0.02a
± 0.04a
± 0.03b ± 0.02a ± 0.02a
± ± ± ±
LERm
2.98 7.53 – 4.60 4.30 3.69 3.22 7.74 – 4.48 4.12 3.42 3.05 7.57 –
4.37 4.02 3.16 2.95 7.44 – 4.47 4.05 3.41
0.20b 0.14b 0.16c 0.18c 0.23a
± ± ± ± ±
± ± ± ± ±
0.11b 0.15c 0.27d 0.15e 0.16a
0.30b 0.22b 0.24bc 0.27c 0.15a
± 0.14d ± 1.43a
± 0.25b ± 0.17bc ± 0.22 cd
± ± ± ± ±
DM [103 kg/hm2]
Yield [103 kg/hm2]
DM [103 kg/hm2] HI
Soybean
Maize
0.37 0.20 – 0.37 0.37 0.36 0.36 0.43 – 0.38 0.38 0.37 0.36 0.42 –
0.38 0.38 0.37 0.36 0.41 – 0.39 0.38 0.37
HI
0.01b 0.01b 0.01b 0.02b 0.02a
± ± ± ± ±
± ± ± ± ±
0.01b 0.01b 0.01bc 0.01c 0.02a
0.01b 0.01b 0.01b 0.01b 0.02a
± 0.01b ± 0.01a
± 0.02b ± 0.01b ± 0.01b
± ± ± ± ±
1.10 3.16 – 1.70 1.59 1.33 1.16 3.33 – 1.70 1.55 1.25 1.11 3.18 –
1.66 1.53 1.17 1.06 3.05 – 1.74 1.54 1.26
00.10b 0.08b 0.09c 0.06c 0.09a
± ± ± ± ±
± ± ± ± ±
0.04b 0.0.3c 0.08d 0.05e 0.14a
0.08b 0.13bc 0.11 cd 0.05d 0.13a
± 0.14c ± 0.05a
± 0.014b ± 0.10bc ± 0.10c
± ± ± ± ±
Yield [103 kg/hm2]
0.35 – – 0.51 0.48 0.40 0.35 – – 0.53 0.49 0.39 0.35 – –
0.54 0.50 0.38 0.35 – – 0.55 0.49 0.40
LERs
0.01a 0.01b 0.02c 0.01c
± ± ± ±
± ± ± ±
0.02a 0.01b 0.01c 0.00d
0.02a 0.01b 0.01c 0.01d
± 0.01d
± 0.02a ± 0.01b ± 0.02c
± ± ± ±
1.34 – – 1.32 1.42 1.36 1.32 – – 1.34 1.42 1.35 1.33 – –
1.32 1.40 1.32 1.34 – – 1.39 1.43 1.38
LER
0.01b 0.02a 0.01b 0.01b
± ± ± ±
± ± ± ±
0.04b 0.02a 0.03b 0.01b
0.00c 0.01a 0.01b 0.01c
± 0.02c
± 0.01b ± 0.01a ± 0.00b
± ± ± ±
Table 2 The dry matter (DM), harvest index (HI), yield and partial LER of maize (LERm) and soybean (LERs) in 2013–2015. Both the mean values ( ± SD) of each year (n = 3 repeats) and the mean values ( ± SD) of three years are shown (n = 3 years) here. Lowercase letters indicate significant differences among different arrangements based on Duncan’s new multiple range test (P ≤ 0.05).
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Fig. 5. The PAR for maize outer and inner leaves close to the ear (A), and for all maize leaves close to the ear (B). The PAR for soybean leaves at top of canopy (C). The PAR values were selected in Fig. 4. The Pn for maize outer and inner leaves close to the ear (D), and for all maize leaves close to the ear (E). The Pn for soybean leaves at top of canopy (F).
monoculture (Fig. 6A), resulting in high Pn of intercropped maize (Fig. 6C). In addition, it also increased the PAR at top of soybean canopy (Fig. 6B), leading to high Pn of intercropped soybean (Fig. 6D). The data from 2014 and 2015 showed the same trend.
dominance of maize under these crop arrangements. The value of CR for maize was larger when the maize rows were expanded, indicating that the positive benefit for maize was more significant with wider maize rows (or narrower gap width for growing soybeans and soybean strip). The highest value of CRm was found in SI3, which suggested that the intercropped maize in SI3 had greater advantage. The value of CR for soybean was larger when the gap width for growing soybeans was expanded, indicating that the positive benefit for soybean was more significant with a wider gap width. The highest value
3.4. Competitive indices The CR of intercropped maize was more than 1 and the CR of intercropped soybean was less than 1 (Table 3), demonstrating the 97
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Fig. 6. The relationships between gap width for growing soybeans and PAR at maize leaves close to the ear (A), and PAR at top of soybean canopy (B). The relationships of Pn and PAR at maize leaves close to the ear (C), and PAR at top of soybean canopy (D). Empty and solid circles showed the data from 2014 and 2015, respectively.
high densities (the same as monoculture) brought increased LAI and dry matter by nearly 1.60 and 1.39 times of monoculture (Fig. 7). The border row advantage was beneficial to both two rows of maize within a band in SI. Although the intercepted PAR of strip intercropped maize was 77% of that in monoculture (Liu et al., 2017b), the maize yields in SI2 and SI3 were nearly equal to monoculture (Fig. 7). The border row advantage was shown as the enhanced PAR at maize leaves, especially for leaves close to the ear which contribute more to maize yield (Awal et al., 2006; Gao et al. 2010). Due to the border row advantage, the reduced plant distance within rows had little effect on dry matter of the individual maize plants. The other study (Gou et al., 2018) reported the same as our results that the leaves close to the ear of intercropped maize had higher Pn, which provides at least partial explanation for the high RUE of intercropped maize found in our previous work (Liu et al., 2017a,b). The spatial complementary advantage in SI was due to the large gap width for growing soybeans. The PAR at soybean canopy was regarded
of CRs was found in SI1, which suggested that the intercropped soybean in SI1 suffered less from competition with maize than soybean in the two other configurations. 4. Discussion 4.1. The causes of high RUE and LER in strip intercropping As compared to many of previous studies (Yu et al., 2015), the increased LER in our strip intercropping arrangements (SI) were mainly due to the density-adding advantage and configuration advantage. The configuration advantage included border row advantage and spatial complementary advantage. The density of both intercropped maize and intercropped soybean in SI, which were equal to their monoculture, were much higher than the other studies (Gao et al., 2010; Oseni, 2010; Echarte et al., 2011; Mahallati et al., 2014). The combined planting of two crops in relatively
Table 3 Competitive ratio (CR) of each arrangement in 2013–2015. Both the mean values ( ± SD) of each year (n = 3 repeats) and the mean values ( ± SD) of three years are shown (n = 3 years) here. Lowercase letters indicate significant differences among different arrangements based on Duncan’s new multiple range test (P ≤ 0.05). Arrangement
2013
2014
Maize CRm SI1 SI2 SI3 RI
1.16 1.79 2.99 2.84
± ± ± ±
Soybean CRs 0.05d 0.04c 0.08a 0.02b
0.86 0.56 0.33 0.35
± ± ± ±
2015
Maize CRm 0.04a 0.01b 0.02c 0.00c
1.25 1.94 3.01 2.84
± ± ± ±
Soybean CRs 0.09d 0.07c 0.06a 0.04b
0.80 0.52 0.33 0.35
± ± ± ±
Mean
Maize CRm 0.03a 0.02b 0.01c 0.01c
98
1.31 1.95 2.93 2.77
± ± ± ±
Soybean CRs 0.04d 0.05c 0.06a 0.05b
0.77 0.51 0.34 0.36
± ± ± ±
0.02a 0.03b 0.01c 0.00c
Maize CRm
Soybean CRs
1.24 ± 0.08d 1.89 ± 0.09c 2.980.04a 2.82 ± 0.04b
0.81 0.53 0.33 0.35
± ± ± ±
0.05a 0.03b 0.01c 0.01c
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Fig. 7. The analysis of high LER in strip intercropping. The percentages were calculated by dividing values of intercropping [SI1 (A), SI2 (B), SI3 (C) and RI (D)] by those of monoculture. CR: competitive ratio; LAI: leaf area index; LER: land equivalent ratio; PAR: photosynthetic active radiation; Pn: net photosynthetic rate; RUE: radiation use efficiency.
potential relationship to the change of the intercepted PAR, Pn, RUE, interspecific competition and dry matter partitioning of intercrops in different intercropping arrangements (Fig. 7). The total intercepted PAR was nearly the same for all intercropping systems (107–110%). Therefore, the intercepted PAR showed an inverse regular pattern for intercropped maize (67-85%) and soybean (41%-25%) (Fig. 7). From correlation analysis, we found that the intercepted PAR had significant positive correlation with competitive ratio in SI (Fig. 8). Therefore, the yield of intercropped maize and soybean also had inverse correlations (Fig. 7). The PAR at top of soybean canopy positively affected the intercepted PAR, Pn and yield of intercropped soybean, but had no obvious relationship with RUE (Fig. 7). The wide gap width for growing soybeans (Fig. 7C→A) enhanced the PAR at top of soybean canopy, and led to increased Pn and intercepted PAR of intercropped soybean. The wide gap width for growing soybeans increased competitive ability of soybean (Fig. 8), and finally enhanced the yield of soybean. However, high
as the incident PAR, which was directly related to Pn and intercepted PAR. Compared to other studies of intercropping (Oseni, 2010, Echarte et al., 2011, Mahallati et al., 2014), the gap width for growing soybeans in SI was expanded so that the intercropped soybean could receive more PAR and increase their Pn (Fig. 7). Compared to monoculture, the decreased incident PAR intensity at top of canopy of intercropped soybean reduced its intercepted PAR, which then enhanced its RUE (Wang et al., 2015, Gou et al., 2017a, Liu et al., 2017a; Li et al., 2008). The intercepted PAR of soybean in SI was 32% of that in monoculture, and was higher than other studies (Awal et al., 2006; Gao et al., 2010). Moreover, the yield was nearly half (48%) of that in monoculture, which mainly resulted from its high RUE. 4.2. The relationships among PAR, Pn, RUE, competitive ratio and LER The row arrangement of intercropping can modify the PAR distribution. The variation of PAR distribution might have direct or 99
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Fig. 8. Relationship between intercepted PAR and competitive ratio in strip intercropping arrangements. The data of intercepted PAR were used from our previous paper (Liu et al., 2017a,b).
our study), in order to increase the intercepted PAR of soybean. However, due to the increase of the gap width for growing soybeans, the interspecific competition of maize was decreased when the maize row distance was reduced from 0.6 m to 0.2 m, especially from 0.4 m to 0.2 m. In order to achieve high yield of both maize and soybean, the suitble gap width for growing soybeans was regarded as 1.6 m (SI2)1.8 m (SI3), which can provide enough space for soybean and border row advantage for maize, and the best maize row distance was considered as 0.4 m, which ensures the maize yield. Overall, the LER of SI2 (distance between maize rows of 0.4 m, gap width for growing soybeans of 1.6 m) was more than 1.40, which was the leading level in cereal and legume intercropping compared with the literature reports (Lv et al., 2014; Mahallati et al., 2014; Yogesh et al., 2014; Oseni, 2010; Echarte et al., 2011; Yu et al. 2015). Furthermore, the selection of appropriate cultivars of maize and soybean is another efficient method to increase crop yield.
Pn did not directly improve its RUE, which might be related to light absorption and transformation and need further investigation. The decease of maize row distance led to reduction of intercepted PAR of intercropped maize (Fig. 7C→A), and the weakened competitive ability resulted in reduced dry matter and yield of intercropped maize (Fig. 8). However, the wide gap width for growing soybeans (Fig. 7D→A) in SI enhanced the PAR and Pn of maize leaves close to the ear, and potentially increased the RUE of intercropped maize, which could partially compensate for the yield loss caused by decrease of intercepted PAR. In our study, the spatial distribution of PAR was measured from about 65DAS (days after sowing) to 75DAS. This stage can represent the trend from 45DAS to harvesting due to the little change of LAI (Liu et al., 2017a,b). The differences of intercepted PAR, RUE and dry matter accumulation from different arrangements mainly came from this stage (from 45DAS to maturity). Our study showed many important data within this time frame (Fig. 7), and proposed the potential positive or negative relationships among different parameters. More future studies are needed in order to build data-based relationship models among those parameters, such as Pn and RUE.
5. Conclusions The high LER of strip intercropping was a result of their high crop RUE (118% for maize and 149% for soybean). In strip intercropping with a enough gap width for growing soybeans, the improved PAR at maize leaves close to the ear increased their Pn, and potentially increased the maize RUE; the increased PAR at top of soybean canopy enhanced the Pn and intercepted PAR of soybean, which then improved its dry matter and yield. Moreover, the density-adding advantage, which ensured an increased population, LAI and dry matter, was the foundation for high yield. With the increase of gap width for growing soybeans in strip intercropping systems, the yield of maize and soybean showed an inverse
4.3. The optimal arrangement for application Improving the competitive position of soybean is crucial for obtaining a high LER of intercropping system. However, in previous studies, the gap width for growing soybeans was no more than 0.52 m (Lv et al., 2014; Oseni, 2010; Echarte et al., 2011), which resulted in the negative impact of maize on the PAR intensity at top of soybean canopy. An effective way to relieve the competitive disadvantage of soybean is to expand the gap width for growing soybeans (1.4–1.8 m in 100
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regular pattern due to the interspecific competition of system intercepted PAR. By comparison, SI2 (distance between maize rows of 0.4 m, gap width for growing soybeans of 1.6 m) is our recommended configuration, which showed the highest LER (1.42) among all the arrangements.
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Acknowledgments This study was supported by the National Key Research and Development Program of China (grant numbers 2016YFD0300209, 2016YFD0300602), the National Nature Science Foundation (grant number 31571615), and Program on Industrial Technology System of National Soybean (grant number CARS-04-PS19). References Awal, M.A., Koshi, H., Ikeda, T., 2006. Radiation interception and use by maize/peanut intercrop canopy. Agr. For. Meteorol. 139 (1), 74–83. Echarte, L., Della Maggiora, A., Cerrudo, D., Gonzalez, V.H., Abbate, P., Cerrudo, Cerrudo.A., Sadras, V.O., Calviño, P., 2011. Yield response to plant density of maize and sunflower intercropped with soybean. Field Crops Res. 121 (3), 423–429. Du, J.B., Han, T.F., Gai, J.Y., Yong, T.W., Sun, X., Wang, X.C., Yang, F., Liu, J., Su, K., Liu, W.G., Yang, W.Y., 2018. Maize-soybean strip intercropping: achieved a balance between high productivity and sustainability. J. Integr. Agr 17 (4), 747–754. Gao, Y., Duan, A.W., Qiu, X.Q., Liu, Z.G., Sun, J.S., Zhang, J.P., Wang, H.Z., 2010. Distribution and use efficiency of photosynthetically active radiation in strip intercropping of maize and soybean. Agron. J. 102 (4), 1149–1157. Gong, W.Z., Jiang, C.D., Wu, Y.S., Chen, H.H., Liu, W.Y., Yang, W.Y., 2015. Tolerance vs. avoidance: two strategies of soybean (Glycine max) seedlings in response to shade in intercropping. Photosynthetica 53 (2), 259–268. Gou, F., van Ittersum, M.K., Simon, E., Leffelaar, P.A., van der Putten, P.E.L., Zhang, L.Z., van der Werf, W., 2017a. Intercropping wheat and maize increases total radiation interception and wheat RUE but lowers maize RUE. Eur. J. Agron 84, 125–139. Gou, F., van Ittersum, M.K., van der Werf, W., 2017b. Simulating potential growth in a relay-strip intercropping system: model description, calibration and testing. Field Crops Res. 200, 122–142.
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