Intercropping maize and soybean increases efficiency of land and fertilizer nitrogen use; A meta-analysis
Introduction
In the 21th century, the major challenge of agriculture is to reconcile these two needs: increasing food production while lowering environmental pollution (Matson et al., 1997; Wittwer et al., 2017). Intensive farming contributes much to crop production, but it meanwhile leads to negative consequences because of high chemical inputs and low biodiversity (Giller et al., 1997; Tilman et al., 2002; Guo et al., 2010; Liu et al., 2013). Intercropping has recently been called the “new green revolution” since it has the potential to raise land productivity by exploiting species complementarities, and provide an opportunity to achieve sustainable intensification of agriculture (Martin-Guay et al., 2018). However, adoption of intercropping in the developed world is slow. On the other hand, intercropping is a main way of production for smallholders in comparatively underdeveloped areas (Snapp et al., 2010; Ngwira et al., 2012; Rusinamhodzi et al., 2012; Huang et al., 2015).
Maize is one of the most important food crops in the world. It was grown on a global acreage of 228 million ha in 2016 (FAO, 2018 http://www.fao.org/faostat/en/#data/QC). Soybean is an important source of plant-derived protein in animal feed and accounts for more than half of the world’s oil production (Ainsworth et al., 2012).
Land and water resources are lacking to expand soybean production area in major producing countries such as the United States and Brazil. Soybean cultivation is regarded as one of the underlying direct causes of tropical deforestation (Barona et al., 2010). Here, we argue that maize/soybean intercropping has the potential to solve the conundrum of maintaining or even increasing maize and soybean production, while using less land and reducing environmental impact.
Intercropping is an effective way to increase system productivity (Yu et al., 2015). It enhances farmland biodiversity and increases utilization efficiencies of resources, i.e. land, nutrients, light and water (Clark and Francis, 1985; Li et al., 2001a; Good et al., 2004; Zhang et al., 2007, 2008a, b; Li et al., 2009). However, trade-offs exist. In many maize/soybean intercrop systems, soybean is shaded and therefore yields less than sole crops (Wolff and Coltman, 1989; Wu et al., 2016). Studies on maize/soybean strip intercropping in the USA found that maize plants in intercrops had approximately 25% more yield than maize plants in sole crops, while soybean yield in rows that were immediately adjacent to maize was reduced by 10% to 27% (West and Griffith, 1992; Ghaffarzadeh et al., 1994). However, yield in maize/soybean intercropping varies from study to study and a synthesis of existing data is required to assess the benefits of intercropping maize and soybean using all the available information across the world.
Previous meta-analyses of land equivalent ratio in intercropping indicate that the average LER is around 1.22 (Yu et al., 2015) to 1.30 (Martin-Guay et al., 2018). These previous studies are global meta-analyses, but they consider a wide range of species combinations, with only a small sample of studies on maize/soybean intercropping, e.g. seven out of 100 publications in the study of Yu et al. (2015) and 66 out of 939 data records (126 publications) in Martin-Guay et al. (2018). No meta-analysis has been made of the potential yield advantages that might be achieved by intercropping maize and soybean. Because of the global importance of these crop species, we conducted a specific meta-analysis on the land equivalent ratio in maize/soybean intercropping. Land equivalent ratio is used in this analysis because it expresses the potential land sparing of intercropping as compared to sole crops (Mead and Willey, 1980). Furthermore, we use here a related and new metric, fertilizer nitrogen equivalent ratio (FNER), to express how intercropping can spare fertilizer resources. We address two main questions: (1) What is globally the land equivalent ratio and fertilizer N equivalent ratio of maize/soybean intercropping? (2) How are LER and FNER affected by management of the intercropping systems, particularly, the use of simultaneous or relay-intercropping, input of fertilizer N, and soil organic matter?
Section snippets
Data collection and extraction from the publication
We searched for relevant publications using the terms (“Maize” or “Corn”) and “Soybean” and “Intercrop*”, in the topic field in three databases: CNKI (http://www.cnki.net/), WanFang DATABASE (http://g.wanfangdata.com.cn/index.html) and Web of Science (http://apps.webofknowledge.com/). The set of publications was refined by selecting publications mentioning “Nitrogen”, “Grain yield” and “Field” in the topic field. In Web of Science, we searched literature published from 1980 to 2018, and
Distribution of LER and FNER
First of all we checked whether there were differences in LER and FNER between studies originating from different databases. We found that the frequency distribution of LER values originating from “CNKI” and “WanFang” and those originating from “Web of Science” were similar (Appendix: Fig. S1a,b). The estimated cumulative probability distributions were not significantly different according the Kolmogorov-Smirnov test (D = 0.1665, P = 0.091554; Appendix: Fig. S1d). We therefore conclude that
Discussion
Results of this global meta-analysis show that maize/soybean intercropping has a high LER, from 1.32 with simultaneous intercropping (two species sown and harvested simultaneously) to values in the range 1.25–1.46 with relay intercropping. The high LER means that intercropping uses less land for the same yields, or – alternatively – total production may be increased by using intercropping on the same land area. The FNER was 1.31 for simultaneous intercropping, while a higher value of 1.71 was
Conclusions
In conclusion, maize/soybean intercropping is a promising practice to meet the challenge of sustainable development and food security. It is important not only for smallholder agriculture in developing countries, e.g. in Africa, to meet demands for calories and protein (Snapp et al., 2010), but also for organic farming and land sparing in developing countries (Fischer et al., 2008; Phalan et al., 2011). Maize/soybean intercropping can result in major increases in the efficiency of land and N
Declaration of Competing Interest
The authors have declared that no conflict interests exist.
Acknowledgements
We acknowledge funding from the Chinese National Key R & D Program (grant number 2017YFD0200200), the Chinese National Basic Research Program (NO. 2015CB150400) and the European Union’s Horizon 2020 Programme for Research & Innovation under grant agreement n°727217 (ReMIX: Redesigning European cropping systems based on species MIXtures).
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