Legume Genomics and Genetics 2025, Vol.16, No.6, 270-278 http://cropscipublisher.com/index.php/lgg 271 2 Theoretical Foundations of Intercropping Systems 2.1 Principles of resource complementarity and niche differentiation The core idea of intercropping is "resource complementarity". Different crops grow in the same field, and they use light, water, and nutrients in different ways and at different times, so they do not compete for resources. This practice can reduce competition and increase overall yields. Some crops are planted early and harvested early, some have deep roots, and some have shallow roots, so that the resources in the land can be more fully utilized, which is better than planting a single crop (Dong et al., 2018). For example, when legumes and cereals are planted together, if their growth periods only partially overlap, the "time difference" can be used to increase yields and land use efficiency (Zhang et al., 2020). 2.2 Role of facilitative interactions and reduced competition in legume-based intercropping When legumes are planted with other crops, the helping effect between them is obvious. Legumes can ‘grab’ nitrogen from the air and convert it into nutrients in the soil, which also benefits surrounding crops (Figure 1). In this way, less nitrogen fertilizer can be applied (Li et al., 2020; Xu et al., 2021). Studies have found that a large part of the yield-increasing effect of legume intercropping comes from the "complementarity" between crops (Yasin et al., 2024), especially when fertilizer is used little or even no. As long as the right crop combination is selected and their planting spacing and location are arranged, this help will be more obvious and competition will be reduced. Figure 1 Partial land equivalent ratio (PLER) of maize, soybean and peanut in maize/soybean and maize/peanut intercropping, based on grain yield, in relation to N and P fertilization in 2017 and 2018. Note that ‘N0’ and ‘N1’ means no N and N fertilizer added, respectively. ‘P0’ and ‘P1’ means no P and P fertilizer added, respectively. Total LER is the sum of the two PLER for maize and legume (soybean or peanut). Data show means ± SEMof each crop (n = 3). *p < 0.05; **p < 0.01; ns, not significant (Adopted from Xu et al., 2021) 2.3 Ecological intensification: maximizing productivity through spatial and temporal crop diversity The so-called "ecological intensification" is to improve the yield and resource utilization efficiency by rationally arranging the planting time and spatial location of crops. Intercropping crops with different growth habits and
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