DU Jun-bo, HAN Tian-fu, GAI Jun-yi, YONG Tai-wen, SUN Xin, WANG Xiao-chun, YANG Feng, LIU Jiang, SHU Kai, LIU Wei-guo, YANG Wen-yu

1 Sichuan Engineering Research Center for Crop Strip Intercropping System/Key Laboratory of Crop Eco-physiology and Farming System in Southwest China, Ministry of Agriculture/College of Agronomy, Sichuan Agricultural University, Chengdu 611130,P.R.China

2 Institute of Crop Sciences, Chinese Academy of Agricultural Sciences, Beijing 100081, P.R.China

3 National Center for Soybean Improvement, Nanjing Agricultural University, Nanjing 210095, P.R.China

1. Introduction

As population expansion, urbanization, environmental pollution, and climate change, the global food crisis is currently aggravated in the world (Godfrayet al. 2010; Foleyet al. 2011). To fix the food crisis, the overarching task of the developing countries is to boost grain production on the finite cropland especially for those countries with large population and limited arable land. China has the largest population with limited arable land in the world. Drastic burst of human population and natural disasters have led to food crisis in modern Chinese history. Even nowadays,food supply and food safety are a major public health issue in China (Lamet al. 2013). Although grain production has been improved after establishment of the new China, the acreage of arable land decreased drastically and people’s demands for higher living quality are ever-increasing in recent years. Traditional cropping systems are not able to afford the food demand any more. The limited farmland in China impels people to max the crop yield without expanding the farmland size. Chemical fertilizer use has made a big contribution to increase crop productivity and alleviate the food crisis in the last decades. Increase of food production and crop yield were mainlyviahigh input of fertilizer and water irrigation, which finally aggravates environmental deterioration such as serious water and air pollution, soil acidification, and erosion in a unsustainable way (Tilmanet al. 2002; Feikeet al. 2012; Liuet al. 2013; Zhanget al.2013). These serious problems bring new challenges in food security and sustainability.

Environmental problems caused by overfertilization lead to the rediscovery of intercropping systems in China in recent years. Intercropping is a practice involving growing two or more crops simultaneously in the same field during a growing season to produce more yields (Liet al. 2014;Brookeret al. 2015). Intercropping has shown potential as a land-use efficient and sustainable agricultural practice (Liet al. 2014; Brookeret al. 2015), which is as well the main model to increase the land use efficiency in both traditional and modern Chinese agriculture. Intercropping principally contains four subcategories: mixed intercropping, row intercropping, strip intercropping, and relay intercropping,which have been employed about 1 000 years in Chinese agricultural history (Ofori and Stern 1987; Kn?rzeret al.2009). It is estimated that intercropping has been widely employed between a range of more than 2.8×107ha (Liet al. 2007) and 3.4×107ha (Liet al. 2001a) of annually sown areas in China. In the long Chinese agricultural history, farmers often grow maize, soybean, peanut, potato,wheat, millet, faba bean, tobacco, cotton, sorghum, sesame,garlic, vegetables, cassava, etc., in intercropping systems(Kn?rzeret al. 2009). Previous studies demonstrated that intercropping systems increased biodiversity, soil quality, soil carbon sequestration, and land-use efficiency, enhanced nutrient-use efficiency and lowered pathogen infection to a greater extent compared with continuous monoculture systems (Zhuet al. 2000; Liet al. 2007; Wanget al. 2014;Conget al. 2015).

2. Chinese traditional soybean intercropping problems

China has a long history for intercropping practice, which can track back to Dong Zhou and Qin dynasties (770–206 BC) initiated from crop rotation. Crop rotation and intercropping were first found to be recorded in an famous ancient encyclopaedia “Important Means of Subsistence for Common People” in China (Kn?rzeret al. 2009).Intercropping practice was initiated from forests with grains or cereals, and then evolved into green plants with soybean,hemp, mung bean, rice, and cotton, etc., in Chinese history.Farmers perform intercropping practice in the history due to their desire to use all available land for more production owing to the rare arable land. In rural areas, farmers always manage intercropping crops in the field with manpower in a inconstant way in Chinese traditional intercropping history, so called unconscious cropping system. However,as a matter of fact, these intercropping systems are not optimal for strong increase of crop yield in a long history. At present, China has about 137.1 million hectares of arable land occupying 16% of total national territorial area, in which 20–25% of arable land were covered by intercropped species, such as maize, soybean, wheat, potato, millet,peanut, cotton, sorghum, sesame, cassava, tobacco, faba bean, garlic and vegetables, etc. (Liet al. 2001b, 2007;Kn?rzeret al. 2009). Previous studies demonstrated that traditional intercropping system has its advantage specially when input is low, while this advantage decreased as the inputs increase (Wubset al. 2005). In recent years,chemical fertilizer has been broadly overcommitted in China.Total food yield in China has been raised by more than 5.4 folds (616.24 million tons) in 2016 than that in 1949(113.18 million tons) largely rely on chemical fertilizer.However, overfertilization and extensive irrigation lead to decrease of crop yield and environmental deterioration in traditional Chinese intercropping practice.

An excellent study revealed that soybean-based cropping can reduce soil carbon and nitrogen losses and thus improve soil fertility and yields, which is important for sustainable agricultural development on a regional and global scale(Drinkwateret al. 1998), suggesting that soybean-based intercropping systems are ideal for increasing land-use efficiency and sustainability in modern agriculture. As one of the most important crops worldwide, soybean is capable of providing plant proteins and oil for humans and concentrated feeds for animals. Soybean was first domesticated in China(Leeet al. 2011), which is currently the largest soy consumer and one of the major soy producing countries worldwide. In China, farmers traditionally intercrop soybean with maize,wheat, sugarcane, cassava, sweet potato, potato, tobacco,and fruit crops, etc. (Kn?rzeret al. 2009; Liet al. 2013; Yanget al. 2014). However, there’s no special high yield varieties suitable to the soybean-based intercropping systems.What’s more, in recent years, most young and middle-aged farmers chose to enter the nearby cities to make money rather than farming, which results in labor famine in rural areas of China. Traditional intercropping practice is not able to adapt to modern agriculture due to several major disadvantages and needs to be innovation. For instance,the use of a conventional intercropping field layout often results in low light-use efficiency, low comparative profits of soybeans, and the inability to use agricultural mechanization systems. These disadvantages lead directly to the low efficiency of traditional crop production in China, which can not balance the high yield and agricultural sustainability.Furthermore, due to low yield and profitability, farmers have opted to grow more profitable crops such as maize, which has led to a national slump in the soybean planting area since 2009 (Fig. 1-A) (Ministry of Agriculture of People’s Republic of China, http://www.moa.gov.cn/). The soybean intercropping systems were suffering a slow death in last two decades (Kn?rzeret al. 2009; Feikeet al. 2012).

To meet the increasing domestic demand, Chinese soybean-processing enterprises have increased importation of soybeans. According to reports from China Customs,soybean imports increased rapidly from 26.59 million tons in 2005 to 83.91 million tons in 2016, while domestic production was only 13 million tons in 2016 (Fig. 1-B) (Ministry of Agriculture of People’s Republic of China, http://www.moa.gov.cn/). Agricultural self-sufficiency is of high priority to the Chinese government, and avenues need to be sought to improve soybean production by improving its profitability.

3. Innovation in maize-soybean strip intercropping systems

Soybean-based intercropping was rediscovered by agronomists in the modern society due to its feature of nitrogen fixation (Kn?rzeret al. 2009). Comparative analyses of various crops intercropped with soybean have indicated that maize is the best partner in a soybean intercropping system. This is because these species possess complementary characteristics and both are thermophilic seed crops with similar sowing seasons. Maize is a nitrogen-consuming C4crop that occupies a relatively higher ecological niche, while soybean is a nitrogen-fixing C3crop that occupies a relatively lower ecological niche;these characteristics enable them to coexist harmoniously.Therefore, a maize-soybean intercropping system can be implemented in any area worldwide where maize and soybean are grown. On the basis of the traditional soybeanintercropping technologies, we developed a modern maizesoybean strip-intercropping model one decade ago. This model was developed through the application of three critical strategies: row spacing expansion, in-row plant spacing reduction, and optimal cultivar screening (Yanget al. 2014).The strip-intercropping model contains two systems, i.e.,regular strip intercropping, which is used in most regions in China (Fig. 1-C), and relay-strip intercropping, which is popular and principally applied in southwestern China(Fig. 1-D). Wide- and narrow-strip were used in the two strip intercropping systems. The wide strips are wide enough to permit happy growth of two or three rows of soybean, while the narrow strips are narrow enough for happy growth of two rows of maize. Maize and soybean are sown simultaneously in different strips in a regular strip-intercropping system, while soybean is sown in the wide strips at a later stage of maize life span in a relay-strip-intercropping system. In the present strip-intercropping model, a soybean strip contains two rows with a row spacing of 40 cm, or three rows with a row spacing of 30 cm. A two-row strip with row spacing of 30–40 cm is used for maize. The maize and soybean strips are alternated and are adjacent to each other (Fig. 1-C and D) (Yanget al.2014). A minimum distance of 60–70 cm between the outer rows of maize and soybeans is maintained instead of the traditional row distance of 40–50 cm between the maize and soybean. Previous studies show that the yield of maize growing in the border rows was enhanced compared with that in the inner rows in the strip intercropped maize system,which indicates that the advantages of the border row effects were apparent in this system (Iragavarapu and Randall 1996;Liet al. 2001b). Current maize-soybean strip intercropping is a major innovation compared with the single-row layout used in traditional soybean intercropping systems (Lvet al. 2014;Yanget al. 2014). This innovation by row-spacing expansion not only allows maize to use the border row effects efficiently(Yanget al. 2014) but also allows a high light transmittance(as high as 50–60%) for soybean (Cuiet al. 2014; Yanget al.2017). Furthermore, the strip intercropping accommodates the use of mini-tractors in the field (Fig. 1-E–H).

Fig. 1 A new maize-soybean strip intercropping model was developed in China. A, soybean-cropping acreage declined drastically in China after 2009. B, due to the implementation of maize-soybean strip intercropping, the drastic decline in the soybean-growing acreage since 2009 has apparently not affected the annual soybean production in China. The use of this technology needs to be rapidly expanded to diminish the gap between domestic soybean production and import. C, the regular maize-soybean stripintercropping system is used in most regions in China. D, the maize-soybean relay-strip-intercropping system is used mainly throughout Southwest China. E, the field operation of a combined seed and fertilizer drill for a regular strip maize-soybean intercropping system. The seed boxes located at the two terminals are for maize sowing, whereas the two sowing boxes in the middle are for soybean. F, a mini-tractor sowing soybean and applying fertilizer in a wide soybean-planting strip. G, a maize harvester working in a narrow maize-planting strip. H, a soybean harvester working in a soybean-planting strip.

Similar to the traditional intercropping systems, maize would shade the shorter crop, which is soybean in our case. Most soybean cultivars grow under shade exhibit stem elongation, lodging, and a reduction in leaf size,which ultimately results in yield decline (Liuet al. 2016b).Variety analyses have indicated that among the shadeavoidance phenotypes, stem elongation and lodging,which directly influence soybean yield, are two of the most typical traits for the majority of shade-sensitive soybean varieties in a maize-soybean strip intercropping system(Liuet al. 2015). Recently, we have identified an increasing number of soybean landraces that exhibit shade-insensitive phenotypes according to our assessment criteria. These landraces have been used to breed cultivars for this intercropping model for different regions in China. The deployment of numerous semi-dwarfed but high-yielding maize cultivars can also reduce the influence of shade on intercropped soybean (Yanet al. 2010; Gonget al. 2014).The semi-dwarfed varieties facilitate close planting, which minimizes the differences in biomass and leaf area index between a maize-soybean strip intercropping system and a monocropped maize or soybean system (Yanget al. 2014,2015). Combined with compact-planting strategies, the crop density of strip-intercropping is increased. As a result, the yield of maize is similar to that in sole cropping while extra soybean production can be obtained, providing additional benefits compared with maize-sole cropping. Another important innovation is that several types of agricultural machines used for sowing, fertilizing, and harvesting have been developed for the strip-intercropping model. These machines can even be used in the hilly areas in southwestern China (Fig. 1-E–H). These breakthrough innovations in our improved intercropping model have the potential to make an important contribution to develop soybean production in China and the world.

4. Maize-soybean strip intercropping balances both high yield and sustainability

The land equivalent ratio (LER) and yield are critical indices to assess land output. A previous study on Chinese traditional single-row maize-soybean intercropping model indicated that the highest LER did not exceed 1.2, and the average grain yield reached 7 274 kg ha–1for maize and 1 004 kg ha–1for soybean (Lvet al. 2014). By contrast, the LER typically ranges between 1.64 and 2.27; the corresponding average grain yield is approximately 6 790–11 475 kg ha–1for maize and 1 510–2 364 kg ha–1for soybean in the maize-soybean relay-strip-intercropping system in Southwest China (Denget al. 2013; Cuiet al. 2014; Liuet al. 2014; Yanget al.2015; Rahmanet al. 2016), which indicates much higher values than those obtained using the previous traditional models (Table 1) (Raji 2007; Lvet al. 2014). The grain yield obtained in a recent two-year experiment in Shandong Province using the regular strip intercropping system was 9 765–11 710 kg ha–1for maize and 1 527–1 538 kg ha–1for soybean, with an LER of 1.4. In addition, the grain yield reached as high as 12 750 kg ha–1for maize and 1 650 kg ha–1for soybean in the regular strip-intercropping system in our field demonstration in Ningxia Hui Autonomous Region,China in 2014. These results indicate that the output value of the new intercropping model is much higher than that of the previous maize-soybean intercropping models (Verdelliet al. 2012; Lvet al. 2014).

Previous studies have demonstrated that soybean cropping increases the retention of soil carbon and nitrogen through the maize-soybean rotation system (Drinkwateret al. 1998). Recent studies also showed that maize yields in a two- and three-year maize-soybean alternative rotation were 9.4 and 12.6% higher, respectively, compared with a maize-sweet potato rotation system. The results suggest that the soybean-based maize strip intercropping increases the phosphorus availability and APase activity in the soil thus improving soil fertility and production of the following crop (Wanget al. 2012, 2017). Actually, in the maize-soybean strip intercropping systems, annually alternative rotation between maize and soybean strips has been used to prevent soil continuous cropping obstacles and improve the sustainable production of the farmland.Comparing to successive maize-soybean strip intercropping systems, maize grain yield, absorption amount of nitrogen,phosphorus, and potasium in maize were increased by 7.5, 18.5, 9.1, and 14.2%, respectively, without significant changes in soybean yield, by annual maize- and soybeanstrip alternative rotation (Yonget al. 2015). Alternatively,extra soybean production is obtained without affecting maize yield in current strip intercropping systems, which balances the high crop productivity and sustainability. Because strip-intercropping is efficient and environmentally friendly,this model has been well accepted by Chinese farmers and the government. Thus far, it has been demonstratedand rapidly extended in 21 locations in China (Anhui,Chongqing, Gansu, Guangxi, Guizhou, Heilongjiang, Hebei,Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning,Ningxia, Qinghai, Shandong, Shaanxi, Sichuan, Yunnan,Zhejiang). The average yields of individual crop species in maize-soybean strip intercropping system in some of the demonstration plots in China are shown in Table 2. It is worth noting that while Sichuan Province was not a major soybean production area formerly, it now ranks the 6th in area and the 5th in soybean production in China due to the popularization of the strip-intercropping practice (http://www.moa.gov.cn/). In the past 13 years, maize-soybean strip intercropping has been extended on 3 million hectares of farmland in Southwest China, and produced 4.8 million tons of soybean (Liuet al. 2016). Studies of maize-soybean strip-intercropping have also contributed to the application of other strip-intercropping systems, such as sugarcanesoybean, cassava-soybean, and potato-soybean strip intercropping. Recently, soybean-strip intercropping has been recommended by the Ministry of Agriculture of China to farmers in maize and soybean planting regions.

Table 1 Comparison analysis of crop production in different maize-soybean intercropping systems

It has been demonstrated that the maize-soybean stripintercropping practice has improved soybean production and sustainable agriculture in China. This achievement also proposes a novel strategy to improve crop production and sustainable agricultural development, especially for developing countries that are still using traditional agricultural methods. First, this practice increases land-use efficiency,which is most important for developing countries with large populations but limited arable land. Given the land limitation,the application of maize/soybean rotation systems is almost impossible in many developing countries. It is therefore worth noting that the maize-soybean strip intercropping system would optimize sustainable land production. Second,considerably more food can be produced for humans and animals using this highly productive intercropping system.Third, the utilization of agricultural machines that are appropriate for this model would greatly increase labor productivity. Fourth, the nitrogen-fixing ability of soybean would improve soil fertility and offer more natural fertilization for other crops, which would effectively reduce pollution. The FAO statistical data (2009–2013) show that on an annual basis, croplands are covered by approximately 56.53 million hectares of maize and 19.12 million hectares of soybean in developing Asian countries, and approximately 33.13 million hectares of maize and 1.52 million hectares of soybean in Africa (http://faostat3.fao.org/browse/Q/QC/E). If this maize-soybean strip-intercropping practice is applied in all of these regions, more output would be obtained annually,indicating that it has great potential for the maize-soybean strip intercropping systems to prevent food crises and keep agricultural sustainability in developing countries.

However, to realize the ultimate goals - total mechanization, the maximization of yield potential of both maize and soybean, and nationwide and international application- more work still needs to be conducted to improve the present systems for application in all maize or soybean cropping regions in China and other developing countries. For example, optimal cultivars still need to be obtained or developed, such as semi-compact maize, shade-insensitive soybeans, and varieties that are suitable for the local climate and soil. The yield potential of both maize and soybean needs further exploration.Recent studies demonstrated that in the maize-soybean intercropping systems, the use of narrow strips grown with compact or semi-compact maize varieties facilitated the edge growth effects of maize in any rows of the narrow strips. In the meanwhile, the light transmittance at the top of soybean canopy was higher than that in the traditional single-row intercropping systems due to the border row effects of maize in current intercropping practice (Yanget al. 2014, 2015, 2017; Liuet al. 2017). By using the shade-tolerant varieties, soybean yield is almost hold in this model comparing to that of the soybean monoculture system(Liuet al. 2014, 2015, 2017; Yanget al. 2015). In addition,annual rotation of the crops in narrow and wide strips, as well as nitrogen-fixation of soybean and increase of phosphorus availability provide a sustaining high yield of both crops inthe maize-soybean strip intercropping systems (Wanget al.2017). However, more work needs to be done to elucidate more detailed mechanisms of sustainable production in the near future. Furthermore, additional more agricultural machines for different planting eco-regions have yet to be developed. Hence, a multi-disciplinary investigation should be performed to shed light on the detailed mechanisms of this model. This requires collaboration between interested research groups and scientists, as well as additional and continued financial support from the government.

Table 2 Individual crop yield of the maize-soybean strip intercropping system in different demonstration plots of China

5. Conclusion

A novel maize-soybean strip intercropping model with a wide-narrow-strip planting style was developed in China in recent years. This model refers to two systems, regular strip intercropping system, which is used in most regions in China,and relay-strip intercropping system, which is popular and principally applied in southwestern China. These systems achieved a balance between high yield and agricultural sustainability.

Acknowledgements

The authors are grateful to Dr. Hon-ming Lam at Chinese University of Hong Kong, Dr. Scott A. Jackson at University of Georgia, Dr. Sjoerd Willem Duiker at the Pennsylvania State University, Dr. Brett J. Ferguson at University of Queensland for their critical comments on the manuscript, and Ms. Siu Kit Wah Lydia for polishing the English. These studies are supported by the National Natural Science Foundation of China (31401308, 31371555 and 31671445).

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