maize (Zea mays L.) strip intercropping

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Yield, yield attributes and photosynthetic physiological characteristics of dryland wheat (Triticum aestivum L.)/maize (Zea mays L.) strip intercropping Yinjuan Lia,b,1, Longshuai Maa,b,1, Pute Wua,b,c,*, Xining Zhaoa,b,c,*, Xiaoli Chenb, Xiaodong Gaoa,b a

Institute of Soil and Water Conservation, Northwest A & F University, Yangling, 712100, China Institute of Water Saving Agriculture in Arid Regions of China, Northwest A & F University, Yangling, 712100, China c College of Water Resources and Architectural Engineering, Northwest A & F University, Yangling, 712100, China b

ARTICLE INFO

ABSTRACT

Keywords: Relay strip intercropping Border effect SPAD Photosynthesis Land equivalent ratio WUELeaf

Intercropping has been widely adopted by farmers for higher yield compared to monoculture, and the border effect was responsible for the yield advantage in irrigated areas. However, few studies have investigated the border effect based on photosynthesis, especially under rainfed conditions. Here, we evaluated the yield in rainfed wheat/maize strip relay intercropping, and explored the associated differences in yield components and physiological process compared to sole crops. A two-year field experiments was conducted including three treatments (sole wheat, sole maize and wheat/maize intercropping) in Yangling, located in the semi-humid region of northwest China. Grain yield, yield components, chlorophyll relative content (SPAD) and photosynthetic parameters in different rows were measured for wheat and maize. Results showed that wheat/maize intercropping increased the land use efficiency and total yield of wheat and maize under rainfed conditions. The yield of intercropped wheat was significantly (p < 0.05) increased by 35% on average over two years, resulting not only from the first border rows but also from the second border rows. The yield improvement in the first border rows relative to sole wheat was attributed to the increase of ear number (56%, p < 0.05), kernel number per ear (14%, p < 0.05) and thousand kernel weight (12%, p < 0.05), but the yield improvement in the second border rows was only attributed to the ear number (22%, p < 0.05). The yield of intercropped maize was not significantly (p > 0.05) decreased (6%), mainly attributed to the border rows (19% lower than in sole maize), in which the kernel number per cob and thousand kernel weight were 14% and 8% (p < 0.05) lower than that in sole maize. This was because that the SPAD and photosynthetic rate of maize in intercropping was suppressed during the co-growth period (about 62 days). Although the two had a partly growth recovery after wheat harvest, the growth recovery was not complete, and this was responsible for the reduction of maize yield in intercropping. This study demonstrates that the importance of yield components and the photosynthesis basis for yield advantage in intercropping systems, and reducing the adverse effect of dominate crop on subordinate crops could better exert the advantages of intercropping.

1. Introduction Global population growth (UN, 2017), climate change (COP24, 2018) and water shortage (Famiglietti, 2014;Wang et al., 2017a,b) are three great challenges facing the world. Grain food is needed to increase more than 70% or even double to satisfy the increasing population demand (Alexandratos, 2009; Godfray et al., 2010; Tilman et al., 2001). Rainfed agriculture is considered to be an important way to save water, and it is also an important source of food, providing 60% of the total

grain production (UNESCO, 2009). However, owing to climate change, rainfed agriculture is susceptible to yield loss (Hernandez-Ochoaa et al., 2018). A possible way of increasing productivity would be through multiple cropping system like intercropping system, which has been proved to obtain higher and stable yield compared to sole crops (Gou et al., 2017b; Mahallati et al., 2015; Rao and Willey, 1980; Raseduzzaman and Jensen, 2017). Intercropping is referred to the cultivation method of two or more crop species simultaneously in the same land (Vandermeer, 1989).

Corresponding authors at: Institute of Water Saving Agriculture in Arid Regions of China, Northwest A & F University, No. 22 Xinong Road, Yangling, 712100, Shaanxi Province, China. E-mail addresses: [email protected] (P. Wu), [email protected] (X. Zhao). 1 These authors contributed equally to this work. ⁎

https://doi.org/10.1016/j.fcr.2019.107656 Received 25 January 2019; Received in revised form 12 September 2019; Accepted 9 October 2019 0378-4290/ © 2019 Published by Elsevier B.V.

Please cite this article as: Yinjuan Li, et al., Field Crops Research, https://doi.org/10.1016/j.fcr.2019.107656

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Fig. 1. Daily mean temperature, daily precipitation and frost period during the 2014–2015 and 2015–2016 growing seasons at the experimental site.

effect of intercropped wheat, further studies should be needed. In addition, previous studies focus on the border effect on the dominate crop wheat, but the border effect on the subordinate crop maize is less studied (Gou et al., 2016). Intercropping usually alters the canopy structure, which in turn changes the ventilation, light transmission and interception of the crop (Wang et al., 2015b; Gou et al., 2017a), especially in the border rows. The wheat border rows intercept more light than inner rows (Wang et al., 2017a,b), but the maize border rows had a lower light interception in wheat/maize intercropping system (Gou et al., 2017a). Changes in light interception can influence chlorophyll content, stomatal conductance and photosynthetic rates (Franco et al., 2018; Makoi et al., 2010). For instance, the photosynthetic rate of maize is increased in maize/soybean strip intercropping (Liu et al., 2018). Photosynthesis is an important process in the production of dry matter. Grain yield is directly related to the photosynthesis ability of leaves, and any increase in the dry matter production comes from assimilation products of photosynthesis (Zelitch, 1982; Parry et al., 2011). However, to our knowledge, there have been few studies attempt to reveal the mechanism of yield advantage in wheat/maize intercropping at leaf level based on the photosynthesis. The objectives of this study are to 1) assess the yield performance and land use efficiency of the intercropping system under rainfed conditions; 2) evaluate the border row effect and inner row effect in intercrops; 3) investigate the response of the photosynthetic physiological characteristics both in border rows and center rows to intercropping. Information on these subjects is essential for a better understanding of the intercropping system that will help management decisions.

According to the statistics, intercropping covers the 3% of the land in the world, not only in the irrigated areas (Li et al., 2001a; Gao et al., 2014), but also in the rainfed regions (Scalise et al., 2015; Sharma et al., 2017). Intercropping could greatly improve the crop yield under full irrigation (Li et al., 2001a; Wang et al., 2015a). For example, wheat yield in intercropping was increased by 40–70% in wheat/maize intercropping under well-watered environment (Li et al., 2001a). Previous studies have also showed that the intercropping requires a large amount of water (Gao et al., 2009; Coll et al., 2012). Water is the main limiting factor for agricultural productivity under rainfed conditions. However, apart from the studies of Aliza et al. (2016); Sharma et al. (2017) and Ma et al. (2018), there is few information on the yield performance of intercropping under rainfed conditions, especially in cereal/cereal intercropping. Furthermore, previous studies focused on strip intercropping, the study on relay strip intercropping was relatively scarce (Gao et al., 2009; Mao et al., 2016; Zhang et al., 2007). Wheat and maize are two of the major staple crops providing energy and protein for humanity. Wheat (Triticum aestivum L.)/maize (Zea mays L.) strip intercropping has been popularly practiced in Northwest China (Chai et al., 2014; Fan et al., 2013; Hu et al., 2017; Li et al., 2001a,b). A border effect, defined as the difference in the performance between external plants and internal plants in one plot (Gomez and De Datta, 1971), usually occurs in the intercropping. The yield advantage of wheat/maize intercropping is always attributed to the border effect, due to the border row could capture more resources such as soil water, nutrient and radiation (Gao et al., 2014; Gou et al., 2016; Wang et al., 2017a,b). However, Li et al. (2001a) suggests the yield advantage of intercropping derives from both the border row and inner row effect, contributing 2/3 and 1/3 to the yield increase of intercropping, respectively. Therefore, there is a gap on the understanding on the border

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2. Materials and methods

Huang et al., 2018; Wang et al., 2017a,b). In the intercropped plot, the distance between adjacent wheat and maize rows was 30 cm. The experimental layout was shown in Fig. 2. The previous crop in first experimental year was spring maize. Crops were planted in the orientation from north to south. Winter wheat was sown on October 18, 2014 and October 9, 2015, and spring maize was sown on April 12, 2015 and April 15, 2016. Winter wheat was harvest on June 15, 2015 and June 15, 2016, and spring maize was harvest on August 18, 2015 and August 22, 2016. The co-growth period was 64 days and 61 days in 2015 and 2016, respectively. Three phases of the relay strip intercropping were shown in Fig. 3. The amount of fertilizer applied to wheat and maize is based on the local farmers' fertilization standards. The nitrogen application rates of wheat and maize were 150 and 235 kg ha−1, respectively. The amount of phosphorus and potassium applied in all plots were 180 kg ha−1 and 39 kg ha−1, respectively. The types of nitrogen, phosphate and potassium are urea (46% N), calcium phosphate (16% P2O5) and potassium sulfate (52% K2O), respectively. For wheat (monoculture and intercropping), nitrogen, phosphate and potassium were all used as base fertilizers and evenly incorporated into the plot. For maize (monoculture and intercropping), the full dose of P and K were used as the base fertilizer, 50% of the nitrogen fertilizer was used as the base fertilizer, and the remaining 50% N was applied equally at the V6 and VT stages. No irrigation was applied for both monoculture and intercropped crops during the growing seasons.

2.1. Site description Field experiments were conducted during 2014–2016 at the Institute of Water Saving Agriculture in Arid Areas of China (34°18′N, 108°24′E; 506 m above sea level), located in Yangling, Shaanxi Province, China. This area is semi-humid prone to drought climate. The annual mean temperature is 13.4 °C. The annual precipitation with a mean of 585 mm (55 years), and 60–70% of the precipitation falls from July to September. The soil texture was loam with a field capacity of 24% (mass water content). Other chemical parameters of the 0–30 cm soil layer before planting in 2014 were as follow: total N 0.87 g kg−1; available nitrogen of 51.02 mg kg−1; available phosphorous of 13.57 mg kg−1; available potassium content: 95.32 mg kg−1; organic matter content 11.82 g kg−1; and soil pH 8.13. The daily precipitation and temperature data at the experimental site in the 2014–2015, and 2015–2016 growing seasons are shown in Fig. 1. 2.2. Experimental design A two-year experiment was conducted by randomized complete block design with three treatments and three replicates. The three treatments were: sole wheat (SW), sole maize (SM) and the wheat/ maize relay strip intercropping (WM). The size of each experimental plot was 10.5 m by 9 m and there was 1 m buffer zone between adjacent plots. In relay strip intercropping system, three complete wheat/maize intercropping strips formed a plot. Each strip consisted of eight rows of wheat plants (strip 1.6 m wide) and four rows of maize plants (strip 1.9 m wide), shown in Fig. 2. Thus, 45.7% of land area in each intercropped plot was occupied by wheat and 54.3% by maize, respectively. The wheat and maize varieties used in the study were 'Triticum aestivum L. cv. Xiaoyan22′ and 'Zea mays L. cv. Zhengdan958′, respectively, which were commonly used by local farmers. Crop planting density was also based on local production practices. Seeding rate of wheat was 180 kg ha−1 with a 20 cm inter-row spacing both for intercropping and monoculture. The maize density was 66,667 plants per hectare with a 50 cm inter-row spacing and 30 cm intra-row spacing, same spacing both for intercropping and monoculture. The spacing was designed to represent typical planting practices in the region (Gao et al., 2010;

2.3. Measurements 2.3.1. Yield measurement Grain yield, and yield components were measured at wheat and maize maturity in both seasons. For both sole wheat and intercropped wheat, the sample area was 4.8 m2 (three meters length times eight rows) per plot. For both sole maize and intercropped maize, the sample area was 6 m2 (three meters length times four rows) for each plot. After the plants in quadrat was measured, all the crops in the full plot area was also harvested, but just for reference. Furthermore, the intercropped wheat and intercropped maize was separately harvested in each row of the strip. The grains were sun-dried and weighed after threshing by hand. The ear number per meter (EN) in each row was counted. Kernel number per ear (KN) was counted from the

Fig. 2. Diagrammatic representation of the field experiment: (A) sole wheat, (B) sole maize, (C) wheat/maize intercropping (Unit: cm). 3

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Fig. 3. Conceptual representation of the cross-row profile of a wheat/maize relay strip intercropping. Crop phases: (1) wheat phase from October to middle of Apri; (2) wheat/maize intercropping phase from middle of April to middle of June; (3) maize phase from middle of June to end of August.

representative ears/cobs (40 for wheat and 20 for maize) per row. The thousand kernel weight (TKW) was determined by 10 replications per row. Furthermore, Row1 and Row8 were defined as the first border rows, Row2 and Row7 were defined as the second border rows, Row3 and Row6 were defined as the third border rows, Row4 and Row5 were defined as the center rows in the intercropped wheat. Row1 and Row4 were defined as the first border rows, Row2 and Row3 were defined as the center rows in the intercropped maize. Different from the intercropping system, the border rows account for a small proportion in sole crops. Especially in the production practice, the planting area of sole crop is very large, and even hundreds of acres of contiguous planting, the border row effect can be completely ignored. The measurement sample for yield is usually selected in the middle of the plot rather than on the boundary of the plot, avoiding overestimation of crop yield.

photosynthetic parameter measurements. The measurements avoided major veins and were conducted in the field between 9:00 and 11:00 am.

2.3.2. Photosynthetic parameters Leaf gas exchange parameters, including net photosynthesis rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) were measured using a LI-Cor LI-6400XT Portable Photosynthesis System (Licoln, Nebraska, USA) equipped with a LED leaf chamber. Leaf water use efficiency (WUEleaf) was calculated based on the following equation:

2.3.5. Land equivalent ratio The total yield was calculated as the weighed means of wheat and maize grain yield both for intercropping and monocropping as follows:

WUEleaf =

Pn Tr

2.3.4. Soil water storage Soil water content was measured using Diviner 2000 (Sentek Pty Ltd., Australia) during the critical stages of wheat (jointing, heading, grain filling and maturity stages) and maize (V3, V6, VT and R2 stages). Soil water content was sampled every 10 cm interval in 0–160 cm profile, and the sampling location was referred to Ma et al. (2018). SWS = SWC×H

(2)

Where SWC (cm3 cm−3) is the volumetric water content, H (cm) is the soil height.

Total yield of intercropping = YW , I ZW + YM , I ZM

(3)

Total yield of monocropping = YW , S ZW + YM , S ZM

(4)

Where YW,I and YM,I are the yields of wheat and maize in the intercrop, respectively; and YW,S and YM,S are the yields of wheat and maize in the monocrop, respectively; ZW and ZM are the planting ratio in the intercropped plot. The advantage of intercropping was evaluated using the land equivalent ratio (LER) calculated as follows (Rao and Willey, 1980):

(1)

Measurements were taken on sunny days in the morning (9:00–11:00 am) to avoid potential stomatal closure during the middle of the day. For wheat, before flag leaf stage, the fully expanded top leaves was measured, and after that period, flag leaves were measured. For maize, before VT stage, the youngest fully expanded leaf was measured, and after that period, the ear leaves were measured. On each measurement date, 5–10 leaves in border rows and center rows in each plot were analyzed, respectively. Parameters were measured at jointing, anthesis, grain filling and maturity period for wheat, and V3, V6, V12, R1, R2, and R5 for maize (Ritchie et al., 1993).

LER = PLERW + PLERM =

ZW YW , I ZM YM , I + YW , S YM , S

(5)

Where PLERW and PLERM are partial LERs for wheat and maize. YW,I and YM,I are the yields of wheat and maize in the intercrop, respectively; and YW,S and YM,S are the yields of wheat and maize in the monocrop, respectively; ZW and ZM are the planting ratio in the intercropped plot. An LER greater than 1.0 indicates that intercropping favors the growth and yield of the crops, whereas values lower than 1.0 indicate intercropping negatively affects the growth and yield of the crops.

2.3.3. Chlorophyll relative content (SPAD) During the critical growth stage, the chlorophyll relative content of wheat and maize leaves was recorded using a hand-held dual-wavelength chlorophyll meter (SPAD 502, Minolta Camera Co., Ltd., Japan). Each time, ten leaves of the border rows and the center rows of each plot were measured at the same leaf position. Plants that were damaged or had dead leaves or senescing ear leaves were not included. The measurement date and the leaves were consistent with the

2.3.6. Competition indices The advantage of intercropping and the competition effects in wheat/maize intercropping were calculated using different competition indices. The wheat (or maize) equivalent yield was computed by converting yield of intercrop into the yield of wheat (or maize) on the basis 4

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of prevailing market prices of individual crops (Khonde et al., 2018). Aggressivity (A) is an index used to indicate the competitive relationship between two crops in intercropping (Dhima et al., 2007). Relative crowding coefficient (K) is a measure of the relative dominance of one species over the other in intercropping (Ghosh, 2004). The competition ratio (CR) was another indicator to evaluate the degree with which one crop competes with the other in intercropping (Zhang et al., 2011). The area-time equivalent ratio (ATER) provides more realistic comparison of the yield advantage of intercropping over monocropping in terms of time taken by component crops in the intercropping systems (Hauggaard-Nielsen et al., 2001). The calculation process in detail was presented in the supplementary material (Appendix 1 Method S1).

Table 2 Yield components for wheat and maize in different cropping systems in two years.

PLERm

LER

2015

Monocropping Intercropping Monocropping Intercropping

5.94b 8.27a 6.01b 7.99a NS *** NS

9.51a 9.07a 8.29a 7.71b *** ** NS

7.88b 8.70a 7.25b 7.84a

– 0.64 – 0.61

– 0.52 – 0.51

– 1.15 – 1.12

2016

Year Cropping system Year × Cropping system

Sole wheat Intercropped Sole wheat Intercropped Sole maize Intercropped Sole maize Intercropped

435b 525a 455b 545a 6.7a 6.7a 6.7a 6.7a

36.9b 40.7a 40.0a 38.9a 507a 485b 486a 458b

38.5a 39.4a 36.0b 37.8a 289.5a 283.5b 268.5a 261.8b

wheat wheat maize maize

Wheat equivalent yield (WEY) was higher in intercropping than the yield in sole wheat and the total intercrops yield in two years. In contrast, maize equivalent yield (MEY) was lower in intercropping than in sole maize in two years. The ATER values were lesser than 1 indicating there was a disadvantage for wheat/maize intercropping. In two years, AW value was positive and AM value was positive, this indicated that wheat was the dominant species in wheat/maize intercropping system. The K value was higher than 1 in both years, indicating that wheat/maize intercropping showed yield advantage. The competitive ratios of wheat (CR) in both years was higher than 1, indicating that wheat had greater competitive intensity relative to maize in wheat/maize intercropping (Table 3). 3.3. Yield components of different rows for wheat and maize The wheat yield per meter row (Ym) was 116% higher in the first border rows and 29% higher in the second border rows than in SW in two years (p < 0.05). There was no significant difference in Ym between the third border rows and the center rows in intercropped wheat and SW in both years (Fig. 4G and H). The contribution ratio for the first border rows to the intercropped wheat yield was the highest in both years, 21% for 2015 and 19% for 2016, respectively (Fig. 5). The variation of the EN was consistent with the Ym in two years. In both years, KN and TKW in the first border rows were significantly higher than that of SW and other intercropped rows (Fig. 4C–F). However, there was no significant difference in EN, KN, TKW and Ym at different rows in SW (Appendix 1 Fig. S1 A–H). Significantly positive correlations between EN, KN, TKW and Ym were observed in first border rows (Table 4). For maize, KN, TKW and grain yield per plant (Yp) were significantly lower in the border rows (Row1 and Row4) than in SM in both years. No significant difference in KN and TKW were observed

Table 1 Yields of wheat and maize, the total yield, and land equivalent ratio (LER) for wheat/maize intercropping in two years (t ha−1). PLERw

2015

3.2. Equivalent yield, area time equivalent ratio, aggressivity, relative crowding coefficient and competition ratio

The yield of intercropped wheat on average of two years was 8.13 t ha−1, 36% higher than that of SW (5.98 t ha−1). The yield of intercropped maize on average of two years was 8.39 t ha−1, 6% less than that of SM (8.93 t ha−1). However, the weighted means of yield in intercropping was 10% higher than that of monocropping in 2015 and 8% higher in 2016. The land equivalent ratio (LER) was 1.15 and 1.12 respectively in two years, which was greater than 1. The PLERw was 0.64 and 0.61, and PLERM was 0.52 and 0.51 in two years, respectively (Table 1). The EN, KN and TKW in intercropped wheat was significantly lower than sole wheat, but the opposite pattern was observed in maize except for EN (Table 2). The yield of wheat was not significantly influenced by year. By contrast, the yield of maize was significantly influenced by year. Cropping system had significant effect on the yield of wheat and maize. Interaction of cropping system by year had no influence on the yield of wheat and maize (Table 1).

Total yield

TKW (g)

Note: EN, KN and TKW represent the ear (or cob) number per square meter, kernel number per ear (or cob) and thousand kernel weight, respectively. Values followed by the same lowercase letters are not significantly different between different cropping system for the same crop in the same year at the 5% level by LSD (the same column).

3.1. Yield and land use efficiency

Maize

KN (# ear−1/ # cob−1)

2016

3. Results

Wheat

EN (# m−2)

2015

Statistical analyses were performed using SPSS 20.0 (SPSS Inc., Somers, NY, USA). The data for grain yield of wheat and maize were analyzed by one-way ANOVA at a 0.05 probability level. Correlation analyses were conducted between yield components and grain yield per meter row for wheat and maize. And correlation analyses were conducted between yield components and Pn at different growth stages. Means were compared by using Duncan’s multiple range tests at p < 0.05.

Cropping system

Cropping system

2016

2.4. Statistical analysis

Year

Year

Table 3 Equivalent yield, area time equivalent ratio (ATER), aggressivity (A), relative crowding coefficient (K) and competition ratio (CR) for wheat/maize intercropping in two years. Year

Note: The values for the intercropped wheat and maize are on an equivalent basis of comparable land area of the sole crops. The total yield was calculated as the weighted means of wheat and maize in monocropping and intercropping, respectively. Land equivalent ratios are calculated from the averaged grain yields. Values followed by the same lowercase letters are not significantly different between different cropping system for the same crop in the same year at the 5% level by LSD (the same column). *, p < 0.05; **, p < 0.01; ***, p < 0.001.

2015 2016

Equivalent yield (t ha−1)

A

WEY

MEY

AW

AM

A

KW

KM

K

9.11 9.03

8.42 6.97

1.39 1.31

0.95 0.93

0.44 0.38

2.08 1.78

0.90 0.86

1.88 1.53

K

ATER

CR

0.72 0.68

1.46 1.41

Note: WEY and MEY represent wheat equivalent yield and maize equivalent yield, respectively. 5

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Fig. 4. The Ear number per meter row (EN), kernel number per ear (KN), thousand kernel weight (TKW) and grain yield per meter row (Ym) at different rows in intercropped wheat (R1-R8) and sole wheat (SW) in 2015 (A, C, E and G) and 2016 (B, D, F, and H); the kernel number per ear (KN), thousand kernel weight (TKW) and grain yield per plant (Yp) at different rows in intercropped maize (Row1-Row4) and sole maize (SM) in 2015 (I, K, and M) and 2016 (J, L, N). Different lowercase above bars indicates significant difference (p < 0.05) among different rows. Table 4 Correlation coefficients between yield components and grain yield per meter row in border rows, center rows and monoculture for wheat and maize. Crop species

Treatment

EN

KN

TKW

wheat

Border rows Center rows SW Border rows Center rows SM

0.729** 0.716** 0.630** – – –

0.651** 0.383 0.195 0.432* 0.815** 0.803**

0.404* 0.298 −0.255 0.641** 0.745** 0.349

maize

Note: SW, sole wheat; SM, sole maize. EN, ear number per meter row; KN, kernel number per ear; TKW, thousand kernel weight. **: Significance at a P level of 0.01. *: Significance at a p level of 0.05.

Furthermore, the contribution ratio of center rows was 1.3 times that of the border rows in 2015, and 1.2 times in 2016 (Fig. 5).

Fig. 5. The contribution ratio of grain yield per meter row (Ym) at different rows for intercropped wheat yield and the contribution ratio of grain yield per plant (Yp) at different rows for intercropped maize yield.

3.4. Rate of photosynthesis, stomatal conductance, transpiration and leaf water use efficiency

between the center rows (Row2 and Row3) and SM, but the grain yield per plant (Yp) in the center rows was significantly higher than in SM in both years (Fig. 4I–N). However, there was no significant difference in KN, TKW and Yp at different rows in SM (Appendix 1 Fig. S1 I–N).

3.4.1. Wheat The Pn both in the first border rows and the center rows were significantly (p < 0.05) higher than that of SW in two years. The average 6

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Fig. 6. The net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and leaf water use efficiency (WUEleaf) at the border rows, and center rows in intercropping and monocropping for wheat (A–H) and maize (I–P) at different stages. Different lowercase above bars indicates significant difference (p < 0.05) among different positions during the same growth stages.

Pn of the first border rows and the center rows were 26% and 15% (p < 0.05) higher than that of SW during the whole growing stages in two years. The pattern of Gs and Tr were consistent with Pn. Except for the jointing stage in 2015, the WUEleaf of the first border rows was significantly higher than that of SW. And the WUEleaf on average two years was 15% higher in border rows and 9% higher in center rows than in SW, respectively (Fig. 6A–H).

3.5. Chlorophyll relative content (SPAD) of wheat and maize The SPAD value of wheat leaves gradually increased in the early stage, reaching a peak in the anthesis stage, and then decreased dramatically in both years. During the whole growing stages, the SPAD value of wheat in center rows and the first border rows in intercropping was higher (p < 0.05) than that of SW, especially in the anthesis stage, grain filling stage and maturity stage (Fig. 7A and C). For maize, the SPAD value in all treatments showed the trend of increasing at first, reaching a peak at V12 and then declining as the growth developed. Before wheat harvest (V3 and V6), the SPAD value of maize in both center rows and border rows of intercropped maize were significantly (p < 0.05) lower than that in SM. After wheat harvest, the SPAD value of intercropped maize increased markedly, and the SPAD in center rows was significantly higher than that of SM (V12 and VT). However, the SPAD value in the border rows was close to that of SM at V12 and VT (Fig. 7B and D).

3.4.2. Maize Before wheat harvest, the Pn of the border rows and the center rows were 31% and 13% lower than that of SM during the co-growth period (V3 and V6 periods). After wheat harvest, the Pn in border rows and center rows increased markedly, and exceeded SM at some periods (V12 and R1 periods for border rows, V12, R1 and R2 periods for center rows). The pattern of Gs was consistent with the Pn. In two years, the Tr of the border rows was the lowest in all treatments. The Tr of the center rows was lower than that of SM except for V12 and R1 stages. The average Tr over the whole growth period of the border rows and the center rows were 22% and 7% lower than that of SM, respectively. Before wheat harvest, the WUEleaf in intercropped maize was similar to SM at V6 stage, but the WUEleaf both in border rows and the center rows were 23% and 12% lower (p < 0.05) than that of SM at V6 stage, respectively. After wheat harvest, the WUEleaf both in border rows and center rows were higher than that of SM at V12, R1 and R2 stages (Fig. 6I–P).

3.6. Dynamics of soil water storage The change of soil water storage at different locations showed similar patterns, which decreased with the development of growth period except for the heading stage in 2015 (Fig. 8). In 2015, the soil water storage at wheat heading stage at three locations increased, which was due to the heavy rainfall before. During the whole wheat growing period, the soil water storage at BWM was significantly (p < 0.05) 7

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Fig. 7. Dynamics of the chlorophyll relative content (SPAD) of wheat and maize at different growth stages in two years.

Fig. 8. Soil water storage in the 0–160 cm profile of intercropping plots and monocropping plots in two years. SWR, IWR, SMR, IMR, and BWM represents sole wheat row, intercropped wheat row, sole maize row, intercropped maize row and the row between wheat and maize strips in intercropping. Different lowercase above bars indicates significant difference (p < 0.05) among different positions during the same growth stages.

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higher than that in intercropping wheat row and sole wheat row. The soil water storage in sole wheat row was slightly higher than that in intercropped wheat row, but the difference was not significant (Fig. 8A and C). In contrast, soil water storage at BWM was significantly (p < 0.05) lower than that in intercropped maize row and sole maize row. Soil water storage in sole maize row was slightly higher than that in intercropped maize row, and significant difference was only observed in VT period of two years and R2 period in2016 (Fig. 8B and D).

4.2. Border row effect for intercropped wheat To further understand the reasons for the high yield of wheat in intercropping, it is necessary to analyze the yield and yield components at different row positions in this study. In this study, the first border rows and the second border rows had the yield advantage in both years (Figs. 4G, H and 6). This phenomenon was also observed in the study of Li et al. (2001a). Two reasons could explain the yield advantage in the second border rows. The first one was that the plant height of wheat in first border rows was lower than that of the other intercropped rows (Appendix 1 Table S1) and the intercropped maize did not shade on wheat (Appendix 1 Table S2), which benefited the light transmission, increased the light interception. The second is that the wheat border row accessed more water and nutrient from the adjacent maize strip, thus alleviating the competitive pressure of the second border rows (Ma et al., 2018; Li et al., 2001b). Thus, the wheat grain production in the second border rows were facilitated by the more favorable aboveground and below-ground environments, and enhance the grain yield. However, the other rows in intercropped wheat did not increase yield relative to sole wheat (Fig. 4G and H). Therefore, if the wheat strip is widened and the number of wheat rows is increased, the contribution ratio of the border row to the wheat yield is reduced. On the contrary, if the number of rows is reduced, the contribution of border row yield will become larger, and the increase in intercropped wheat production will be obvious. Compared to SW, the yield in the first border rows was significantly greater in two years, mainly due to the increase in EN, KN and TKW (Fig. 4A–F; Table 4). The soil water storage was higher at BWM than in intercropped wheat and sole wheat row (Fig. 8A and C), so the first border rows in intercropped wheat had better water availability, which was favorable to the formation of tillers. So the EN in the first border rows was significantly higher than in SW and this was consistent with the study of Zhu et al. (2016). At the anthesis stage, the Pn in the first border rows was significantly higher than in SW (Fig. 6A and B), and this contributes to the formation of KN (Appendix 1 Table S3). Furthermore, light availability enhances the differentiation rate of inflorescences during anthesis stage resulting in a higher number of spikelets and florets per inflorescence (Geisler, 1983), due to the higher light interception in border rows (Wang et al., 2017a,b; Zhang et al., 2018). The increase in TKW contributed to the increase of grain yield in the first border rows, this was the actual situation we observed. However, the reduction of TKW was observed in the study of Gou et al. (2016), different from our study. This was because during the cogrowth period, intercropped wheat was usually more competitive than intercropped maize in our study, the Pn was large, so the grain filling was fully supplied (Appendix 1 Table S3). In the study of Gou et al. (2016), during the late co-growth period, the intercropped wheat was suppressed by intercropped maize, grain filling was limited, leading to a lower TKW. Previous studies demonstrated that photosynthesis is the basis for yield formation, and 90% of dry matter comes from photosynthesis (Gaju et al., 2016). Higher Pn was observed for border row in intercropped wheat compared to sole wheat during the whole growth period (Fig. 6A and B). There were two reasons to explain this situation. First, the physiological characteristics was changed, and the photosynthetic ability enhanced. Good light and nitrogen environment in border rows is conducive to the synthesis, formation and stability of chlorophyll (Wang et al., 2017a,b). The Gs and SPAD in wheat border rows were significantly increased (Figs. 6C, D and 7). The SPAD and Gs are significantly positive correlation with the photosynthetic rate (Gaju et al., 2016; Swiader and Ame, 2002). Second, the improvement of light as a power and the improvement of moisture as a raw material further promote the improvement of photosynthesis. Intercropping not only increased the photosynthetic rate of wheat, but also extended the green area after flowering, preventing premature senescence of leaves (Fig. 7), thereby the intercropped wheat can get a sufficient supply of

4. Discussion 4.1. Yield performance and land use efficiency The net primary productivity of terrestrial ecosystems generally increases with increasing biodiversity (Hector et al., 1999). The increase in the biodiversity of intercropping improves productivity through efficient of light, temperature and water resources due to temporal and spatial niche differentiation (Yu et al., 2015). In the present study, the wheat yield was increased and the maize yield was reduced in intercropping, but the increase in wheat yield was greater than the reduction in maize yield. Therefore, the total yield of wheat/ maize intercropping is still greater than that of weighted means of monoculture (Table 1). It should be noted that different row arrangements can affect light transmission, light interception and root distribution, in turn influence grain yield of intercropping. So, it requires further research to clarify the yield advantage of intercropping with varied row arrangements. LER was higher than 1 under rainfed conditions in both years (Table 1), indicating that this relay strip intercropping improved land use efficiency, that is to say, intercropping improved the grain yield without increasing the area of arable land. This was consistent with the situation under fully irrigated conditions, in which intercropping showed yield advantage and land use advantage, such as the combination of wheat/maize, maize/peanut, and soybean/maize (Liu et al., 2018; Gou et al., 2017a; Li et al., 2001a; Yang et al., 2017). This indicated that the wheat/maize intercropping under rain-fed conditions was advantageous in semi-humid areas with rainfall of about 585 mm. Under rainfed conditions, the yield of intercropped wheat was significantly higher than that of SW (Table 3), it was because intercropped wheat had higher EN, KN and TKW compared to SW (Table 2). Two reasons could explain this, one reason was that before maize sowing, more resources in the adjacent vacant area were available to intercropped wheat. Another reason was that wheat was more competitive than maize in intercropping during the co-growth period, so intercropped wheat could capture more soil water, nutrient and light, contributing to the wheat growth and yield formation. However, the yield of intercropped maize was lower than that of SM (Table 1). This was because that the yield components such as KN and TKW in intercropped maize was significantly lower than SM (Table 2). Previous studies showed that after wheat harvest, maize has growth recovery, which is the basis for high yields of intercropped maize (Li et al., 2001b). The recovery growth of maize is subject to a variety of factors. The recovery of maize after wheat harvest is facilitated by better water conditions, the yield of intercropped maize was improved compared to SM with sufficient water supply (Li et al., 2001b; Wang et al., 2015a). Our previous studies (Ma et al., 2018) showed that the roots of the intercropped wheat extended laterally to the maize row, resulting in reduced soil moisture in the maize strip (Fig. 8B and D), especially under rainfed conditions, resulting in the inability of maize to fully recover, leading to lower yield of intercropped maize (Table 1). It confirmed that water deficit is an important limiting factor for the recovery growth of intercropped maize. Furthermore, the study by Gou et al. (2016) showed that the temperature and radiation in Netherlands are low, so that the recovery and growth of the intercropped maize was limited. Similarly, under water limited conditions, it is not favorable to maize growth recovery. 9

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photosynthetic products and obtain higher yield compared to SW.

4.5. Implications Recently water scarcity has been critical issue worldwide (Moiwo et al., 2010; Sun et al., 2015). Intercropping has emerged as a suitable approach for sustainable agriculture, especially under water limited conditions (Martin-Guay et al., 2018). Our research has found that in semi-humid areas with annual precipitation of 585 mm approximately, intercropping without irrigation increased the production and the land use efficiency. As the labor costs are increasing, intercropping as a labor-intensive system, the development of are limited. The planting pattern of 8:4 wide relay strip intercropping system is more conducive to mechanization, suitable for large-scale promotion and development. Intercropping significantly increased wheat yield in wheat/maize intercropping, similar results have been found in other studies (Li et al., 2001a; Gao et al., 2014). This was because that more radiation aboveground (Gou et al., 2017a; Wang et al., 2015b) due to wheat is the dominate crop, and the wheat roots extend laterally the adjacent crops, and then intercropped could capture more resources such as water (Ma et al., 2018) and nutrient (Li et al., 2001a). At the same time, the number of ears in the border rows is significantly higher than in SW, and the number of ears was significantly positively correlated to the yield (Table 4). Therefore, the planting density can be appropriately increased to better exert the advantage of the border rows. Furthermore, the yield advantage in wheat/maize intercropping was limited, because the yield of intercropped maize was lower compared to sole maize. Therefore, it is necessary to take appropriate measures to increase the yield of intercropped maize. For example, selecting a compact wheat cultivar and excellent shade-tolerant maize cultivar to reduce the adverse impacts of wheat shading on maize, would be an important and efficient agricultural practice. Improved water management for intercropped maize after wheat harvest is important for promoting the complete growth recovery of maize, and increase the productivity of intercropped maize. The use of straw or film mulching has been proved to improve soil hydrothermal conditions (Yin et al., 2015), and it could be an important practice to enhance the productivity of intercropped maize.

4.3. Border row effect for intercropped maize Different from the border row advantage of intercropped wheat, the intercropped maize showed border row disadvantage. The KN and TKW was significantly less in border rows than in SM, leading to a lower grain yield in border rows. However, no significant difference was observed between center rows and SM (Fig. 4I–L). Therefore, if the number of maize rows is increased, the contribution of border row yield will become smaller, and the yield reduction in intercropped maize will be further smaller. Our previous study showed that the soil water was depleted by intercropped wheat during the co-growth period, leading to a reduction of soil water in maize border rows (Ma et al., 2018). This was confirmed in our study that the soil water storage at BWM and intercropped maize was lower than sole maize row (Fig. 8B and D). Moreover, the Pn in the border rows was lower than that of SM during the co-growth period. Therefore, the KN in border rows was decreased due to water limitation and lower source of assimilates (Andrade et al., 1999). During the co-growth period, maize was shorter than wheat (Appendix 1 Table S1 and S2), so maize was shaded by wheat in intercropping. From the perspective of photosynthesis, this long-term shading seriously affected the synthesis of chlorophyll, resulting in a lower SPAD value in intercropped maize (Fig. 7B and D). Furthermore, shading reduced the thickness of the palisade tissue, but the chloroplasts are mainly present in the palisade tissue, so that the entry of CO2 into the chloroplast through the intercellular space is limited (Poorter et al., 2006; Terashima et al., 2006, 2011). Correspondingly, the Pn in border rows and the center rows was significantly lower than in SM, and the border row was reduced much more. Interestingly, after wheat harvest, the recovery of Pn in border rows and center rows were observed (Fig. 6I and J). This was because that wheat shading disappeared and maize light interception increased, which in turn improved the SPAD and stomatal conductance. However, the Pn recovery of border rows did not occur at R2 stage, which is the critical stage for grain filling (Appendix 1 Table S4). So, the supply capacity of the source in border rows was reduced, leading to a lower TKW than in SM, thus the yield of the border rows in intercropped maize was lower than that of SM. The Pn in center rows was higher than in SM in R2 period, so there was no significant difference in grain yield between center rows and SM. As such, the yield reduction of the intercropped maize was small (5% in 2015 and 7% in 2016).

5. Conclusions In the present study, the wheat/maize (planting ratio of 8:4) intercropping significantly increased the land use efficiency and the total yield compared to the weight means of sole crops in this region, and the yield advantage was mainly attributed to the wheat yield improvement. It indicates that wheat/maize intercropping has the yield and land use advantage under rainfed conditions with mean annual precipitation of 585 mm. Different from the previous studies, the yield increase of intercropped wheat derives not only from the first border rows but also from the second border rows. Ear number, kernel number per ear and thousand kernel weight were all responsible for the yield improvement in the first border rows, but only the ear number was responsible for the yield improvement in the second border rows. The yield of intercropped maize was slightly decreased, mainly attributed to the border rows. This was because that the SPAD and photosynthetic rate of maize in intercropping was suppressed during the co-growth period. Although the two had a partly growth recovery after wheat harvest, the growth recovery was not complete. Future studies on the productivity of intercropping are needed to focus on the cultivar selection and water management of late maturing crop after the early maturing crop is harvested.

4.4. Leaf water use efficiency (WUEleaf) Under rainfed conditions, intercropping significantly increased the WUEleaf of wheat (12%), and slightly increase the WUEleaf of maize (2%) (Fig. 6G, H, O and P). This indicating that, the total WUEleaf of intercropping was enhanced. This reflects the physiological reasons for the improvement of water use efficiency at the field level (Morris and Garrity, 1993; Yin et al., 2015). The increase of WUEleaf in intercropped wheat was mainly attributed to the border rows, because the wheat border rows had better radiation and soil water (Ma et al., 2018; Wang et al., 2017a,b). There was no significant difference in WUEleaf between intercropped maize and SW during the whole growth period, for that the increase in center rows was higher than the decrease in border rows (Fig. 6O and P). It is worth noting that, the WUEleaf both in border rows and center rows was significantly higher than that of maize after wheat harvest, especially at V12 stage. This stage was also the key stage for the growth recovery of intercropped maize and kernel number formation. Therefore, under rainfed conditions, combined with the characteristics of high WUEleaf during this period, water conservation measures should be adopted to promote the recovery of maize and improve productivity.

Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work. 10

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Acknowledgements

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