pea strip intercropping

European Journal of Agronomy 113 (2020) 125986

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Expanding row ratio with lowered nitrogen fertilization improves system productivity of maize/pea strip intercropping

T

Yan Tana,b, Falong Hua,c, Qiang Chaia,c,*, Guang Lib,**, Jeffrey A. Coulterd, Cai Zhaoa,c, Aizhong Yua,c, Zhilong Fana,c, Wen Yina,c a

Gansu Provincial Key Laboratory of Arid Land Crop Science, Lanzhou, 730070, China College of Forestry, Gansu Agricultural University, Lanzhou, 730070, China c College of Agronomy, Gansu Agricultural University, Lanzhou, 730070, China d Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN, 55108, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Intercropping Interspecific interactions Crop productivity Strip ratio N fertilizer

Intercropping is increasingly applied to confront food security issues in a sustainable manner. However, whether changing of row ratio and lowering N fertilization will affect interspecific interactions in maize (Zea mays L.)/pea (Pisum sativum L.) strip intercropping is unknown. Here we determined interspecific competition and yield response of pea and maize in a substitutive strip intercropping field experiment with irrigation in arid northwestern China from 2009–2011. Expanding the maize-to-pea row ratio from 2 : 4 to 3 : 4 intensified interspecific competition by 39, 96 and 154 % at 45, 60 and 75 d after maize sowing, and increased crop growth dynamics index of maize by 102, 89 and 16 % during 75–105, 105–135, and 135–165 d after maize sowing. Accordingly, grain yield of pea and maize with row ratio 3 : 4 was improved by 12 and 8 %, respectively, compared to row ratio 2 : 4. Meanwhile, the overyielding effect of pea and maize with row ratio 3 : 4 at 28 and 35 %, respectively, was significantly greater than row ratio 2 : 4. Lowering N fertilizer rate from 450 to 300 kg N ha−1 intensified interspecific competition without affecting grain yield of pea and maize in the intercropping system. Grain yield of pea was positively correlated with interspecific competition, while that of maize was positively correlated with crop growth dynamics index of maize. Consequently, expanding row ratio with reduced N fertilizer rate could optimize interspecific interactions and improve system productivity of maize/pea strip intercropping.

1. Introduction Global food demand is projected to double by 2050 due to an ever growing human population (Chen et al., 2014). Meeting this demand will require a 70 % increase in yield of present crops (Johnson et al., 2014). Unfortunately, potential increases in yield are predicted to be much less, only 0.86, 0.63, and 0.83 % for wheat, rice, and maize, respectively (Alexandratos and Bruinsma, 2012), causing agriculture to face unparalleled pressure and challenges. The situation is expected to become worse as urbanization constricts available cropland (Tilman et al., 2001). In addition, long-term extreme high input of fertilizers, especially for nitrogen (N), have led to serious biodiversity loss and environmental harm (Foley et al., 2011; Chen et al., 2014). It is imperative that agriculture must simultaneously address global food security and environmental sustainability (Tilman et al., 2002). Intercropping, with two or more species growing in the same field,

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improves the efficiency of the utilization of land, light, water, and nutrients (Willey, 1990). In this system, intercrops, often with differing growth period, canopy structure, and root system (Brooker et al., 2016), help promote biodiversity, reduce pest and disease incidence, improve soil fertility, and boost system productivity (Lithourgidis et al., 2011; Hu et al., 2016). Among the numerous advantages, increasing yield and reducing environmental impacts are of significant importance (Li et al., 2009). Intercropping is increasingly adopted as a sustainable pattern for crop production (Martin-Guay et al., 2018), particularly when it combines a cereal with a legume (Dhima et al., 2007; Liu et al., 2018). Generally, legumes can facilitate cereal growth though nutrient sharing (Li et al., 1999). In situations with insufficient soil available N, symbiotic N fixation by the legume intercrop is the dominant source (Li et al., 2009; Hu et al., 2017). The fixed N can also be assimilated by the component cereal through root expansion (Hauggaard-Nielsen et al., 2001). Thus, N fertilizer rate can be lowered (Liu et al., 2011) and

Corresponding author at: Gansu Provincial Key Laboratory of Arid Land Crop Science, Lanzhou, 730070, China. Corresponding author. E-mail addresses: [email protected] (Q. Chai), [email protected] (G. Li).

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https://doi.org/10.1016/j.eja.2019.125986 Received 16 February 2019; Received in revised form 12 November 2019; Accepted 15 November 2019 1161-0301/ © 2019 Elsevier B.V. All rights reserved.

European Journal of Agronomy 113 (2020) 125986

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however, it is particularly suitable for intercropping. Long-term average annual precipitation is 156 mm, occurring mainly in June to September; and annual evaporation is > 2400 mm. The soil at the experimental site was classified as an Aridisol (FAO/UNESCO, 1988), with 8.0 pH (1:2.5 soil:water), 14.3 g kg−1 organic carbon, 0.78 g kg−1 total N, 1.41 mg kg−1 available phosphorous (Olsen phosphorus), and 121.4 mg kg−1 available potassium (ammonium acetate-extractable potassium) prior to initiation of this experiment.

environmental feedbacks could be reduced. Inter- and intraspecific competition as well as facilitation are important processes in intercropping (Vandermeer, 1989; Zhang and Li, 2003). Interspecific competition often leads to one crop growing superiorly, while the growth of the other partner is depressed (Li et al., 2011). In contrast, facilitation can promote the growth of both intercropped components (Fan et al., 2006; Mei et al., 2012). In the majority of intercropping studies, a cereal is the dominant crop (Zhang and Li, 2003; Dhima et al., 2007); nevertheless, in maize (Zea mays L.) /faba bean (Vicia faba L.) and maize/pea (Pisum sativum L.) intercropping, the legume has been observed to be the dominant species (Li et al., 2009; Hu et al., 2016). In maize-legume strip intercropping, maize contributes greatly to the total system yield, mainly due to its compensatory growth, a growth leading to maize repair form the depression by the component species, i.e. recovery (Li et al., 2001a). Fixed N in legume strips that is utilized by maize is vital for a maize recovery and improves potential yield of maize and the overall cropping system (Senaratne et al., 1995; Zhang and Li, 2003). In order to promote yield of the cereal crop, strategies that can increase biological N2 fixation have raised a great interest (Van Kessel and Roskoski, 1988; Karpenstein-Machan and Stuelpnagel, 2000; Abi-Ghanem et al., 2011). The direct way is to reduce the N fertilizer rate (Li et al., 2009; Hu et al., 2017). This is also known to affect interspecific competition, and the subsequent recovery (Hu et al., 2016). Expanding the maize-to-legume row ratio is another effective approach, which significantly reduced the nitrate-N accumulation in the soil and increased the total N accumulation of maize (Zhang et al., 2015). However, whether the depletion of soil nitrate-N will increase N2 fixation was not addressed. Maize/pea strip intercropping is a popular cropping system adopted largely in northwestern China (Chen et al., 2015; Zhao et al., 2016). It is also a promising model for sustainable production (Chai et al., 2014). However, in practice, pea and maize are fertilized with high amounts of urea, which had counterproductive influence on yield as well as negative environmental impacts. Therefore, reducing the N fertilizer amount is a potential strategy for improving maize/pea intercropping systems. In addition, the ability to increase contribution of maize yield to the system has been restricted due to inadequate recovery growth (Hu et al., 2016). In this study, the N input was reduced and maize-topea row ratio was expanded so that maize can sustainably contribute more yield to the system. The primary objective of this study was to investigate how expanded row ratio in combination with lowered N fertilizer rate will affect interspecific interactions and the productivity of intercropped pea and maize. We hypothesized that (i) lowered N fertilizer rate could intensify the interspecific competition of intercropped pea to maize, (ii) expanding the maize-to-pea row ratio could improve recovery growth of maize, and (iii) optimized competition and compensation would promote productivity of both pea and maize. In testing the hypothesis, we determined (i) interspecific competition, (ii) compensation of intercropped maize, and (iii) yield response of pea and maize.

2.2. Experimental design The experimental design was a split-plot in randomized complete block with three replicates. The main plot factor was cropping system, consisting of sole pea (P), sole maize (M), and two maize/pea strip intercropping systems, i.e.; I1, with a maize-to-pea row ratio of 2 : 4, and I2, with a maize-to-pea row ratio of 3 : 4. The subplot factor was N fertilizer rate, consisting of 0 kg N ha−1 (N0), 90 kg N ha−1 (N1) and 135 kg N ha−1 (N2) for pea, and 0 (N0), 300 (N1) and 450 kg N ha−1 (N2) for maize, respectively. Each of the sole and intercropped species received the same area-based N fertilizer rate. For pea, all N fertilizer was broadcast and incorporated into the soil prior to seeding as base fertilizer. For maize, 30 % of the total N fertilizer was broadcasted and incorporated into the soil prior to seeding as base fertilizer, 60 % was applied at pre-tasseling stage (12-leaf phenological development), and the remaining 10 % was applied at grain filling stage (kernel blister stages phenological development). In conjunction with N fertilizer application prior to seeding, all plots received a base application of phosphate fertilizer at 150 kg P ha−1. In the N0 treatments, calcium superphosphate (0-16-0 of N-P-K) was used, while in the N1 and N2 treatments, diammonium phosphate (18-46-0 of N-P-K) was used. For N fertilization at the pre-tasseling stage and grain filling stage of maize, a fertilizer placement drill was used to make a 3-cm diameter hole (10-cm deep) which 4−5 cm away to the side of maize stem, and urea (46-0-0 of N-P-K) was applied in the hole. Once the drill was pulled out, the hole was sealed with soil. Irrigation water was applied on the day after each N application to dissolve fertilizer into the soil. In each year, field pea (cv. Long-wan No.1) was seeded in early April and harvested in early July, and maize (cv. Wu-ke No. 2) was seeded in late April and harvested in late September. The plot size was 38.4 m2 (4.8 m × 8 m) for I1 treatment, 48 m2 (6 m × 8 m) for I2 treatment, and 48 m2 (6 m × 8 m) for sole cropping treatment. The system layout of I1 was 160-cm-wide strips consisting of two rows (40 cm interrow) of maize with a strip with of 80 cm, and four rows (20 cm interrow) of pea with strip width of 80 cm. While the layout of I2 was 200-cm-wide strips consisting of three rows (40 cm interrow) of maize with strip with of 120 cm, and four rows (20 cm interrow) of pea with strip width of 80 cm (Fig. 1; Table 1). There were three pairs of maizepea strips in each intercropping plot. Therefore, in I1 intercropping, 1/2 of the plot occupied by maize and another 1/2 by pea. In I2 intercropping, 3/5 of the plot occupied by maize and 2/5 by pea. Maize strips were all mulched with clear plastic film at seeding to boost maize productivity (Gan et al., 2013). The density of pea was 1,800,000 plants ha−1 and of maize was 90,000 plants ha−1. For each crop, the same area-based density was employed in intercropping and sole cropping (Table 1). Due to low precipitation at the testing areas (< 155 mm annually), supplemental irrigation was applied. All plots received 120 mm of irrigation the previous fall just before soil freezing, and then various irrigation quotas during the growing season were applied to the crops by drip irrigation to satisfy the treatment requirements. The same area-based irrigation quota was implemented for both pea and maize in the sole cropping and intercropping at each irrigation event.

2. Materials and methods 2.1. Experimental site The experiment was conducted in 2009–2011 at the Oasis Agricultural Experimental Station (37°30′ N, 103°5′ E; 1776 m a.s.l.) of Gansu Agricultural University near Wuwei, China. This station is located in the eastern part of the Hexi Corridor of northwestern China in the temperate arid zone of the Eurasian Continent. Long-term (1960–2009) average annual mean air temperature is 7.2 °C with accumulated air temperature above 0 °C of 3513 °C and above 10 °C of 2985 °C, solar radiation is 5.67 KJ m−2 with 2945 h of sunshine duration, and frost-free period is 156 d. The experimental site represents a typical agroecosystem in this region, with abundant light and heat for growing single crop per year but insufficient for two per year;

2.3. Plant sampling and analysis Both pea and maize in the sole and intercropping systems were sampled at 15-d intervals before pea harvest and at 30-d intervals after 2

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Fig. 1. System layout of maize-pea intercropping with the row ratio of (a) 2 : 4, having maize strip of 80 cm and pea strip of 80 cm, and (b) 3 : 4, having maize strip of 120 cm and pea strip of 80 cm.

2.4.2. Crop growth dynamics index of intercropped maize After pea harvest, intercropped maize often received a recovery growth (Hu et al., 2016). It can be quantified as crop growth dynamics index (CGDI) according to Yin et al. (2017). The CGDI means that CGR (kg ha−1 d−1) of intercropped maize exceeds than that of sole maize, and was determined as:

Table 1 Agronomic practices and structural layout of sole and intercropping systems at the Wuwei Oasis Agricultural Experimental Station in northwestern China in 2009–2011. Item

Cropping pattern

Pea

Maize

Sowing date Harvest date Row width (cm) Strip width (cm)

Sole or intercrops

1–5 April 5–10 July 20 80 80 4 4 1,800,000 900,000 720,000 165 82.5 66

20–25 April 25–30 September 40 80 120 2 3 90,000 45,000 54,000 405 202.5 243

Plant rows per strip Plant density (plants ha−1) Irrigation quota (mm)

Intercropping Intercropping Intercropping Intercropping Sole Intercropping Intercropping Sole Intercropping Intercropping

1 2 1 2

(I1) (I2) (I1) (I2)

1 (I1) 2 (I2) 1 (I1) 2 (I2)

(Wim2-Wim1) ⎤ ⎡ (Wsm2-Wsm1) ⎤ CGDI = CGRim − CGRsm = ⎡ − ⎢ ⎥ ⎥ (T2-T1) (T2-T1) ⎣ ⎦ ⎦ ⎢ ⎣ (2) where CGRim and CGRsm are the crop growth rate of intercropped and sole maize, respectively, Wim1 and Wim2 are DM weight of maize in intercropping sampled at date T1 and T2, and Wsm1 and Wsm2 are DM weight in sole cropping sampled at date T1 and T2. For CGR of maize in I1 and I2 intercropping, they were calculated on a same area base as sole cropping. 2.4.3. Grain yield (GY) Plots were harvested when the crops reached full maturity, and the grain was air-dried, cleaned, and weighed to determine GY, meanwhile, the dry matter content was also measured.

pea harvest. The first sampling was conducted at 15 d after maize sowing (or pea emergence). At each sampling date in each plot, four adjacent rows of pea with a row length of 30 cm were sampled to assess pea aboveground dry matter (DM, kg ha−1), and 10 individual maize plants were randomly selected to determine maize aboveground DM. For each sampling date, all aboveground samples were oven-dried at 105 °C for desiccation and then oven-dried at 80 °C until constant mass. Crop growth rate (CGR, kg ha−1 d−1) was determined as the difference of DM between previous and current sampling divided by the sampled time interval.

2.4.4. Overyielding Overyielding of intercropped crops relative to sole crops was assessed by an increase or decrease in the intercropped crops over the corresponding mono-cropped crops according to Li et al. (2011), which was calculated as:

Overyielding = 2.4. Measurement and calculation

(3)

where Yintercrop and Ysole crop are the GY of a given crop in intercropping and sole cropping, respectively, and P is the proportion of a given crop in the intercropping system. A positive overyielding value indicates a yield advantage for intercropping and a negative value denotes yield disadvantage.

2.4.1. Interspecific competition in maize/pea intercropping The competition of pea relative to maize during the co-growth period was determined using aggressivity (Apm) according to Hu et al. (2016), an index commonly applied to assess competition between different species, and was calculated as:

DMip ⎞ DMim ⎞ ⎟ − ⎛ Apm = ⎜⎛ DMsp Fp DMsm × × Fm ⎠ ⎝ ⎝ ⎠

Yintercrop -(P× Ysole crop) × 100% (P× Ysole crop)

2.4.5. Land use efficiency Land equivalent ratio was applied in this study to evaluate the use efficiency of cropland. It was defined as the total land area of sole crops required to achieve the same yields as intercrops (Willey, 1979). In an intercropping system, the partial land equivalent ratio (PLER) of each component comprise the total land equivalent ratio (TLER), they were calculated as:

(1)

where DMip and DMsp are DM accumulation of intercropped and sole pea, respectively, DMim and DMsm are DM accumulation of intercropped and sole maize, respectively, and Fp and Fm are the proportion of land area occupied by pea and maize in the intercropping system, respectively. If Apm is greater than 0, then intercropped pea is the dominant species; if Apm is less than 0, then intercropped maize is dominate; and if Apm = 0, then both crops are equally competitive. 3

PLER =

Yip Yim or Ysp Ysm

(4)

TLER =

Yip Yim + Ysp Ysm

(5)

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However, no significant difference was found between N1 and N2 in each of the three studying years. Dry matter accumulation of maize (DMm) in co-growth stages (before 75 d) varied greatly from that in recovery stages (after 75 d, Fig. 3). At 75 d after maize sowing, DMm was significantly affected by the year × cropping system × N level interaction (P < 0.001); however, the effect of cropping system × N level interaction was not significant (P = 0.597). In 2009, I1 and I2 with DMm of 12,024 and 12,411 kg ha−1, was 10 and 7 % lower than sole cropping. N2 with DMm of 14,442 kg ha−1, was 15.2 % greater than N1. In 2010, I1 and I2 with DMm of 9465 and 8718 kg ha−1, was 6 and 15 % lower than sole cropping. N2 with DMm of 10,745 kg ha−1, was 15 % greater than N1. Similarly, in 2011, I1 and I2 had 6 and 10 % lower DMm than sole cropping. N2 had 12 % greater DMm than N1. At date 165 d after maize sowing, the year × cropping system × N level interaction (P < 0.001) and cropping system × N level interaction (P = 0.033) all significantly affected DMm. In each testing years, N1 and N2 greatly improved DMm compared to N0, but had no significant difference between each other. Compared to I1, DMm in I2 was improved by 16, 13 and 10 % with N0, N1 and N2, respectively.

where Yip and Yim are GY of intercropped pea and maize, respectively, and Ysp and Ysm are GY of sole pea and maize, respectively. A TLER > 1.0 indicates a yield advantage for intercropping relative to sole cropping, and a TLER < 1.0 indicates a yield disadvantage for intercropping. 2.5. Statistical analysis Data were analyzed at P ≤ 0.05 using Statistical Analysis Software (SPSS software, 17.0, SPSS Institute Ltd, Chicago, USA). Analysis of variance was conducted using the standard split-plot design analysis method to test for the significance of year and treatment effects and their interactions. Means were compared using Fisher’s protected LSD test at P ≤ 0.05. Pearson’s correlation coefficient was used to assess the linear association between parameters. 3. Results 3.1. Dry matter accumulation Dry matter accumulation of pea (DMp) had a similar trend in the three study years, and the values increased as growth progressed (Fig. 2). Generally, DMp in sole cropping was less than that in intercropping. For total DMp (i.e. at 75 d after maize sowing), a significant (P < 0.001) year × cropping system × N level interaction affected it; however, the effect of cropping system × N level interaction was not significant (P = 0.123). In 2009, total DMp in I1 was 7602 kg ha−1, and in I2 was 8300 kg ha−1; they were increased by 9 % and 19 % compared to sole pea. In 2010, total DMp in I1 and I2 were 7890 and 8574 kg ha−1, which were increased by 8 and 17 % compared to sole pea. Similarly, in 2011, it in I1 and I2 were 7808 and 8765 kg ha−1, and were increased by 13 and 26 %. Compared to I1, DMp in I2 was increased by 9.2, 8.7 and 12 % in 2009, 2010 and 2011, respectively.

3.2. Interspecific competition of pea relative to maize Since middle co-growth (i.e. at 45 d after maize sowing) of maizepea intercropping, a significant (P ≤ 0.002) effect of year × cropping system × N level interaction affected Apm (Table 2). At 45 d after maize sowing, I2 had 44, 55 and 18 % greater Apm than I1 in 2009, 2010 and 2011, respectively. At 60 d after maize sowing, I2 had 81, 93 and 115 % greater Apm than I1 in the three studying years. Meanwhile, N1 had 9, 21 and 32 % greater Apm than N2 in 2009, 2010 and 2011, respectively. At 75 d after maize sowing, N1 had 25, 42 and 21 % greater Apm than N2 in the three studying years in I1; and had 61, 30 and 39 % greater Apm in the three studying years in I2, respectively. In Fig. 2. Dry matter accumulation of pea in sole and intercropping systems in a) 2009, b) 2010, and c) 2011, with different N levels. S, sole cropping; I1, intercropping with a maize-to-pea row ratio of 2:4; I2, intercropping with a maize-to-pea row ratio of 3:4. N0 is the control at N = 0 kg N ha−1. N1 and N2 represent an N fertilizer rate of 90 and 135 kg N ha−1 for pea, respectively. The smaller bars are LSD of dry matter at 75 day after maize sowing.

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Fig. 3. Dry matter accumulation of maize in sole and intercropping systems in a) 2009, b) 2010, and c) 2011, with different N levels. S, sole cropping; I1, intercropping with a maizeto-pea row ratio of 2:4. I2, intercropping with a maize-to-pea row ratio of 3:4. N0 is the control at N=0 kg N ha−1. N1 and N2 represent an N fertilizer rate of 300 and 450 kg N ha−1 for maize, respectively. The smaller bars are LSD of dry matter at 75 and 165 day after maize sowing.

(P < 0.001, Fig. 4). In sole cropping, CGR of maize with N1 was lowered by 8, 10 and 7 % compared to N2 in 2009, 2010 and 2011, respectively. In I1 treatment, the CGR of maize with N1 was lowered by 11, 13 and 11 % compared to N2 in 2009, 2010 and 2011, respectively. Similarly, in I2 treatment, it was lowered by 20, 17 and 15 % in the three studying years, respectively. After pea harvest (i.e. after 75 d), the effect of year × cropping system × N level interaction (P < 0.001) and cropping system × N level interaction (P < 0.001) also significantly affected the average

addition, I2 had 63, 127 and 270 % greater Apm than I1 in 2009, 2010 and 2011. 3.3. Recovery effect of intercropped maize 3.3.1. Crop growth rate Before pea harvest (i.e. before 75 d), the average CGR of maize was significantly affected by the year × cropping system × N level interaction (P < 0.001) and cropping system × N level interaction

Table 2 Interspecific competition of pea relative to maize (Apm, Eq. 1) at different co-growth stages as affected by cropping system and N level in 2009–2011. Cropping Pattern a

I1

I2

N rate

N0 N1 N2 N0 N1 N2

LSD (0.05) P > F Cropping system (C) N rate (N) C×N

b

2009

c

45 d

d

2010

2011

60 d

75 d

45 d

60 d

75 d

45 d

60 d

75 d

0.49 0.47 0.55 0.72 0.72 0.70 0.08

0.18 0.30 0.28 0.35 0.53 0.48 0.06

0.08 0.22 0.18 0.29 0.30 0.19 0.05

0.38 0.45 0.58 0.76 0.71 0.63 0.08

0.17 0.34 0.28 0.45 0.54 0.44 0.03

0.09 0.18 0.12 0.24 0.37 0.28 0.07

0.41 0.43 0.44 0.49 0.51 0.50 0.04

0.16 0.25 0.18 0.39 0.49 0.38 0.04

0.08 0.12 0.10 0.41 0.39 0.28 0.02

< 0.001 n.s. n.s.

< 0.001 < 0.001 n.s.

< 0.001 < 0.001 < 0.001

< 0.001 n.s. n.s.

< 0.001 < 0.001 0.001

< 0.001 < 0.001 0.020

< 0.001 n.s. n.s.

< 0.001 < 0.001 n.s.

< 0.001 0.002 < 0.001

a

I1 and I2, intercropping with a maize-to-pea row ratio of 2 : 4 and 3 : 4, respectively. N0 is the control at N =0 kg ha−1. N1 and N2 represent an N fertilizer rate of 90 and 135 kg N ha−1 for pea, respectively, and 300 and 450 kg N ha−1 for maize, respectively. c Data were presented by years due to significant year × cropping system × N level treatment interaction at date 45 (P < 0.001), 60 (P = 0.002) and 75 d (P = 0.001) after maize sowing. d 45, 60 and 75 d after maize sowing, respectively. b

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Fig. 4. Crop growth rate of maize in sole and intercropping systems in a) 2009, b) 2010, and c) 2011, with different N levels. S, sole cropping; I1, intercropping with a maize-to-pea row ratio of 2:4; I2, intercropping with a maize-topea row ratio of 3:4. N0 is the control at N=0 kg N ha−1. N1 and N2 represent an N fertilizer rate of 300 and 450 kg N ha−1 for maize, respectively. The smaller bars are LSD of crop growth rate at each sampling period.

CGR of maize (Fig. 4). In sole cropping, CGR of maize with N1 was lowered by 28 and 12 % compared to N2 in 2009 and 2010, respectively. While had no significant difference in 2011. In contrast, in I1 treatment, it with N1 was enhanced by 6 and 7 % compared to N2 in 2009 and 2011. In I2 treatment, it with N1 was enhanced by 15, 10.9 and 11.2 % compared to N2 in 2009, 2010 and 2011, respectively.

system × N level interaction affected it, however, the year × cropping system × N level interaction was not significant (P = 0.331, Fig. 5). Averaged across years, in sole maize, N1 with GY of 11,254 kg ha-1 was 7 % less than N2. While in I1 and I2, there was no significant difference between N1 and N2. In addition, I2 with GY of 13,689 kg ha−1 was 34 % greater than sole maize, and 8 % greater than I1.

3.3.2. Crop growth dynamics index A significantly effect of year × cropping system × N level interaction affected CGDI during the early (P < 0.001) and middle (P < 0.001) recovery stages, but not in the late (P = 0.231) recovery stages (Table 3). During early recovery stages, in I1, N1 had 11 and 66 % greater CGDI than N2 in 2009 and 2010, but had 8 % less CGDI in 2011. In I2, N1 had 27, 59 and 23 % greater CGDI than N2 in 2009, 2010 and 2011, respectively. During middle recovery stages, in I1, N1 had 10, 193 and 28 % greater CGDI than N2 in 2009, 2010 and 2011, respectively. Similarly, in I2, N1 had 42, 108 and 26 % greater CGDI than N2 in the three testing years, respectively. During late recovery stages, N1 with averaged CGDI of 30.5 kg ha−1 d−1, was 44 % greater than N2. Besides, I2 had 102, 89 and 16 % greater CGDI than I1 in early, middle and late recovery stages.

3.4.2. Overyielding effect Average across years, I2 achieved overyielding effect of 28 % for pea and 35 % for maize, they were significantly greater than that in I1 (Table 4). Furthermore, lowering N fertilizer rate significantly improved overyielding effect of maize. Compared to N2, the overyielding effect of maize with N1 was improved by 64 %. 3.4.3. Land use efficiency On average, PLER of pea in I2 was lowered by 11 % compared to I1 (Table 4). There was no significant effect of N level affecting PLER of pea. For PLER of maize, I2 increased it by 30 % compared to I1. And N1 increased it by 8 % compared to N2. In terms of LER of intercropping system (i.e., TLER), I2 increased it by 11 % compared to I1, and N1 increased it by 6 % compared to N2.

3.4. Yield performance 3.5. Correlation of Apm, CGDI, overyielding, and LER with DM and GY 3.4.1. Grain yield There was no significant (P = 0.084) effect of year × cropping system × N level interaction affecting GY of pea, but the cropping system × N level interaction significantly (P = 0.002) affected it (Fig. 5). On average, I2 yielded 3336 kg ha−1 with N0, which was improved by 13 % compared to I1. Similarly, I2 yielded 4627 kg ha−1 with N1, and 4582 kg ha-1 with N2, were improved by 12 and 11 %, respectively. For GY of maize, a significant (P = 0.002) cropping

There were highly significant positive correlations between Apm and DM, and between Apm and GY of pea (Table 5). This implied that intensifying the competition of pea relative to maize greatly improve DM and GY. For maize, highly significant positive correlations between CGDI and DM, and between CGDI and GY were found, implying that maize yield was dependent on the level of recovery growth after pea harvest. 6

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Table 3 Crop growth dynamics index of intercropped maize (CGDI, kg ha−1 d−1) at different recovery stages as affected by cropping system and N level in 2009–2011. Cropping Pattern

a

N rate

b

2009

c

2010

2011

d

I1

I2

LSD (0.05) P > F Cropping system (C) N rate (N) C×N

N0 N1 N2 N0 N1 N2

Middle Late Early Middle Late Early Early ———————————————————————————————————————————— kg ha−1 d−1 ————————————————————————————————————————————

Middle

Late

52.0 53.5 48.0 89.3 147.2 115.9 18.9

62.3 86.5 79.0 108.1 127.7 89.9 10.7

13.4 23.7 12.6 18.9 29.3 11.6 6.5

28.7 83.1 50.2 140.8 117.0 73.8 25.4

49.8 78.1 26.7 72.9 190.5 91.8 33.4

23.8 30.9 25.5 19.1 36.9 26.2 9.6

22.2 78.4 85.4 127.4 104.8 85.2 18.1

70.9 116.6 90.8 115.2 238.3 189.4 23.3

18.0 29.2 21.5 22.7 33.0 30.2 10.0

< 0.001 0.001 0.002

< 0.001 < 0.001 0.001

n.s. < 0.001 n.s.

< 0.001 0.002 0.000

< 0.001 < 0.001 0.005

n.s. 0.006 n.s.

< 0.001 0.042 < 0.001

< 0.001 < 0.001 0.001

n.s. 0.021 n.s.

a

I1 and I2, intercropping with a maize-to-pea row ratio of 2 : 4 and 3 : 4, respectively. N0 is the control at N =0 kg ha−1. N1 and N2 represent an N fertilizer rate of 90 and 135 kg N ha−1 for pea, respectively, and 300 and 450 kg N ha−1 for maize, respectively. c Data were presented by years due to significant year × cropping system × N level treatment interaction during early (P < 0.001) and middle (P < 0.001) recovery stages. d Early, middle, and late recovery stages are 75–105, 105–135, and 135–165 d after maize sowing, respectively. b

4. Discussion

In cereal/legume intercropping, N is a vital element either competitively or complementarily used by the component crops (Ofori and Stern, 1987; Hauggaard-Nielsen and Jensen, 2001). For cereals in the intercropping, they mainly rely on competition to acquire soil inorganic N (Jensen, 1996; Corre-Hellou and Crozat, 2005). In contrast, legumes mainly depend on symbiotic N2 fixation (Crozat et al., 1994). However, high level of fertilizer N may inhibit the function of N2 fixation (Li et al., 2009). In this study, both pea and maize in intercropping received 1/3 reduction of fertilizer N, but there was no significant difference of grain yield between N1 and N2. It means that traditional N regimes with 135 kg N ha−1 for pea and 450 kg N ha−1 for maize was overused and lead to luxury consumption of N fertilizer. Furthermore, over-application counterproductively depressed N2 fixation (Hu et al., 2016), leading to low N efficiency and high negative environmental impacts (Chen et al., 2014).

4.1. Intercropping effects on grain yield Numerous studies have reported that a legume crop in intercropping is effective at promoting yield of the component cereal (Lithourgidis et al., 2011; Betencourt et al., 2012). In maize/cowpea (Vigna unguiculata L.) intercropping, maize yield was increased by 25 % (Latati et al., 2014). In maize/soybean [Glycine max L. (Merr.)] intercropping, maize yield was increased by 22 % (Ghaffarzadeh et al., 1994). Similarly, in wheat (Triticum aestivum L.)/soybean intercropping, wheat yield was increased by 29 % (Li et al., 2001b). In the present study, maize was strip intercropped with pea, which improved maize yield by 29.9 and 45.6 % in the I1 and I2 intercropping system, compared to sole maize. In addition, this kind of intercropping also improved grain yield of pea. Compared to sole pea, grain yield of pea in I1 and I2 was improved by 19.0–23.1%. This may attributable to the coordinated interspecific interactions (Hu et al., 2016). As capacity of light interception, soil nutrient utilization, and supplementary effects were all altered by interspecific interactions (Zhang and Li, 2003).

4.2. Interspecific interactions in intercropping Interspecific competition, the ability of a crop to depress the growth of another, is fundamental in overall yield performance (Zhang and Li,

Fig. 5. Grain yield of a) pea and b) maize in the sole and intercropping systems with different N levels. S, sole cropping; I1, intercropping with a maize-to-pea row ratio of 2:4; I2, intercropping with a maize-to-pea row ratio of 3:4. N0 is the control at N=0 kg N ha−1. N1 and N2 represent an N fertilizer rate of 90 and 135 kg N ha−1 for pea, respectively, and 300 and 450 kg N ha−1 for maize, respectively. Different letters indicate significant differences (P < 0.05) among treatments and the smaller bars are standard error of means (n = 3). 7

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greater than that in sole cropping, implying that compensation was occurred (Li et al., 2001a). Compared to I1, expanding the maize-to-pea row ratio improved crop growth dynamics index by 102, 89 and 16 % during the early, middle and late recovery stages. Generally, more efficient recovery growth requires more N fertilizer input (Hu et al., 2016). However, in this study, lowing N rate significantly improved the crop growth dynamics index. It is because lowered fertilizer N could intensify the competition, a stimulater for recovery (Hu et al., 2017). In addition, intensify the competition could improve the biological N2 fixation, which benefits the maize in lateral root expansion. This can be shown by crop growth rate of intercropped maize in recovery period was 6.5 % greater in I1 treatment, and 12 % greater in I1 treatment, with N1 compared to N2.

Table 4 Overyielding and land use efficiency of pea and maize as affected by cropping system and N level across 2009–2011. Treatmenta

Pea

Maize

Overyielding (%) Cropping system effect (C) I1 14.0 I2 27.6 P > F < 0.001 LSD (0.05) 3.6 N level effect (N) e N0 20.0 N1 22.9 N2 19.5 P > F n.s. f – LSD (0.05) C×N n.s.

PLER

b

TLER

Overyielding (%)

PLER

0.57 0.51 0.001 0.02

24.5 34.8 0.008 5.5

0.62 0.81 0.001 0.05

1.19 1.32 0.011 0.08

0.54 0.55 0.53 n.s – n.s.

36.0 31.5 21.6 0.001 8.6 n.s.

0.75 0.73 0.67 0.002 0.03 n.s.

1.29 1.27 1.20 0.001 0.03 n.s.

c

d

4.3. Mechanisms of yield improvement in maize-pea intercropping In intercropping systems, productivity of dominant species increased with intensified competition (Li et al., 2001b; Corre-Hellou et al., 2006). The present study also shows further evidence for this with the significant positive correlation between Apm and dry matter, and between Apm and grain yield of intercropped pea. Furthermore, expanding the maize-to-pea row and lowering N fertilizer rate both intensified Apm, resulting in growth advantages of intercropped pea. Generally, dry matter of pea with I2 was increased by an average of 21 % compared to sole pea, and by 10 % compared to I1. However, there was no significant difference between N1 and N2. As a result, grain yield of pea with I2 was increased by an average of 28 % compared to sole pea, and by 12 % compared to I1. Whereas, grain yield of pea with N1 consistently showed no significant difference from that with N2, indicating lower N fertilization did not depress production of pea. The possible reason may attributable to competition induced soil available N (Crozat et al., 1994; Hu et al., 2017). Due to pea dominate the intercropping system, dry matter of maize was suppressed during the co-growth period, while after pea harvest, it was greatly improved. Therefore, dry matter and grain yield of maize all positively correlated with crop growth dynamics index. Furthermore, expanding the maize-to-pea row ratio from 2:4 to 3:4 greatly improved crop growth dynamics index. This may be attributable to the optimized light interception and coordinated N utilization (Bedoussac and Justes, 2010). Consequently, grain yield of maize in I2 was 34 and 8 % greater than that in sole maize and I1. For the N fertilizer effect on crop growth dynamics index and grain yield, we assumed the following possibilities: (i) lowered N fertilizer rate intensified competition of pea, (ii) greater competition increased biological N2 fixation, and (iii) increased N2 fixation supported maize recovery growth. However, how N balanced in the strip intercropping was not addressed in the present research, which is fundamentally needed in clarifying of the increased yield and assessing of the system sustainability. The monitoring of soil mineral N during cropping season, determination of N concertation at each crop growth stages and estimating of N2 fixation are therefore imperative in further researches.

Data were averaged across years due to no significant (P ≥ 0.250) year × cropping system × N level treatment interaction. b PLER, partial land equivalent ratio. c TLER, total land equivalent ratio. d I1 and I2 are the intercropping with a maize-to-pea row ratio of 2 : 4 and 3 : 4, respectively. e N0 is the control at N =0 kg ha−1. N1 and N2 represent an N fertilizer rate of 90 and 135 kg N ha−1 for pea, respectively, and 300 and 450 kg N ha−1 for maize, respectively. f LSD not provided when the corresponding P > F from analysis of variance is not significant at P ≤ 0.05. a

Table 5 Pearson’s correlation coefficients (df = 11) of dry matter and grain yield relative to interspecific competition (Apm), crop growth dynamics index (CGDI), overyielding, and partial land equivalent ratio (PLER) of pea and maize. Item

Pea

Maize

Apm Dry matter Grain yield a

0.649** 0.563**

a

Overyielding

PLER

CGDI

Overyielding

PLER

0.464** 0.459**

−0.207 −0.077

0.570** 0.484**

−0.186 −0.198

0.075 0.006

**, correlation coefficients that are significant at P ≤ 0.01.

2003; Hu et al., 2016). It may occur when intercropped components use solar radiation, water and nutrients in the same time (Lv et al., 2014; Yang et al., 2017). In this study, Apm all greater than 0, indicating pea was the dominant crop. In addition, expanding the maize-to-pea row ratio intensified competition, as Apm in I2 was 39, 96 and 154 % greater than I1 at 45, 60 and 75 d after maize sowing. For cereal and legume intercrops, the quantity of competition is highly affected by N fertilization (Hu et al., 2016). In this study, N level effect on interspecific competition was revealed until 60 d after maize sowing. Compared to N2, lowing fertilizer N rate intensified competition by 9, 21 and 32 % in 2009, 2010 and 2011 at 60 d after maize sowing; and by 43, 36 and 21 % in 2009, 2010 and 2011 at 75 d after maize sowing. This indicates that lower rate of N fertilizer intensified Apm during late co-growth of pea and maize. It can be attributable to the following possibilities: (i) pea mainly relied on fertilizer N in early establishment, but shifted to biological fixed N in subsequent stages (Crozat et al., 1994); (ii) greater N fertilizer rate inhibited N2 fixation in late cogrowth stages (Li et al., 2009); and (iii) constant insufficient soil available N aggravated N competition between pea and maize (Hu et al., 2017). Recovery, is another type of interspecific interactions (Li et al., 2001a). In maize/pea strip intercropping, pea was early harvested before maize reproductive growth, allowing maize to vigorously recover the depressed growth (Hu et al., 2016). Since after 75 d after maize sowing (i.e., after pea harvest), crop growth rate in intercropping was

5. Conclusions In maize/pea strip intercropping, interspecific interactions greatly affected growth status of the component crops, and thereby influenced yield response. Generally, dry matter, grain yield, and overyielding of intercropped pea were increased with intensified competition. Expanding row ratio enhanced crop growth dynamics index of intercropped maize, leading to a 13, 8, and 35 % increase in dry matter, grain yield, and overyielding, respectively. As a result, total LER was improved by 11 %. N reduction from 450 to 300 kg N ha−1 for maize consistently had no significant effect on grain yield of intercropped pea and maize. The significant positive correlation between grain yield and competition of pea and crop growth dynamics index of maize indicates that the coordination of interspecific interactions is vital for system 8

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productivity improvement in strip intercropping.

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