Intercropping, i.e. the mixed cultivation of crop species, is recognized as a pathway towards more sustainable agriculture (Li et al., 2020; Lithourgidis et al., 2011; Maitra et al., 2021; Malézieux et al., 2009; Martin-Guay et al., 2018), because it has the potential to outperform sole cropping in several aspects. In particular, intercropping has been found to improve resource use efficiency (Hauggaard-Nielsen et al., 2008, 2011), increase above ground biomasses (Cardinale et al., 2007; Rauber et al., 2001; Trydeman Knudsen et al., 2004), provide favorable habitat for beneficial organisms (Potts et al., 2003), reduce weeds, diseases and insect pests (Aziz et al., 2015; Bedoussac et al., 2015; Lithourgidis et al., 2011; Lopes et al., 2016), and assist farmers to deal with adverse and unpredictable weather condition due to climate change (van Zonneveld et al., 2020; Waha et al., 2018). Moreover, cereal legume mixtures are known for complimentary use of nitrogen (N) sources due to symbiotic N2 fixation (SNF) from the atmosphere by legumes (Peoples et al., 2009). Most studies on mixtures have concentrated on final (grain) yield which integrates growth dynamics between species over the whole growing period. In contrast, only a few studies have investigated the degree of interaction between intercrops that may already be detected at a very early growing stage (Bellostas et al., 2003; Benincasa et al., 2012; Elsalahy et al., 2021), such as at crop emergence and the carry-on effects of mixing from early to later growth stages.
Crop emergence, i.e. the emergence of the shoot from a germinated seed through soil, is the first essential stage in the life cycle of plants because this process often affects components of plant fitness (Verdú & Traveset, 2005) and determines the future crop performance both at the individual (plant) and the population (crop stand) level. Presumably the final yield benefit of cereal legume crop mixture compared to their monocrops could not appear without earlier below- and aboveground competition and/or facilitation processes. In barley and pea intercropping, for example, the reduction in yield of pea has been found likely to be induced in early growth stages (Tofinga et al., 1993). However, the exact proportion and direction of the mixture effects on that early growing stage are currently largely unknown.
Seed germination and seedling emergence are influenced by the combination of different biotic and abiotic factors. Different studies show that germination and early crop establishment are affected by a range of factors, including temperature and soil water availability (Luo et al., 2018; Tribouillois et al., 2016), soil characteristics such as soil bulk density and the presence of soil crusts (Briggs & Morgan, 2011; Soureshjani et al., 2019), as well as the identity and density of neighboring seeds and seedlings (Fenesi et al., 2020; Leverett et al., 2016, 2018; Tielbörger & Prasse, 2009). Some studies demonstrate that the acceleration of germination is most likely influenced by the presence of a competitive neighborhood (Dyer et al., 2008; Orrock & Christopher, 2010). The entire soil chemistry related to early root and shoot growth could change due to volatile organic compound emission from the germinating seed coat (Fincheira et al., 2017; Tielbörger & Prasse, 2009) and thus affect their neighbors. However, the growth dynamics in a crop mixture depend on the balance between competitive (Renne et al., 2014; Tielbörger & Prasse, 2009) and facilitative interactions (Orrock & Christopher, 2010; Schiffers & Tielborger, 2006) between seedlings. Indeed, facilitation was observed to enhance seedling survival and growth, especially in extreme weather events such as harsh winters (Batllori et al., 2009). By accelerating or reducing the emergence rate, crop mixtures might have a decisive role on plant biomass in the early growth stage, and thereby also on the balance of competition among partners at later growth stages.
Crop mixtures of cereals and grain legumes have a high potential of increasing the system productivity compared to the respective sole crops (Xiao et al., 2018). In general, in such crop mixtures, cereals have been shown to gain a higher relative yield than grain legumes (Xiao et al., 2018; Yu et al., 2016). However, estimating the final yield as an overall growth dynamic is obviously insufficient for drawing conclusions about lifetime dominancy over the whole growing stage of the cereal over the legume. It is currently unknown when the domination of the cereal over the legume starts, in particular, whether this domination is evident already at an early growing stage. Further, it is not known if there is a legacy of early dominance (if any) on later performance of the crops. It is important to understand the competitive balance between the two intercropping partners at an early growing stage because this may play a major role in determining productivity in mixtures. To understand the complexity of crop mixtures and predict their productivity and the relative performance of the partners, it is vital to study the entire life cycle of the interacting plants from crop emergence.
We therefore conducted an experiment to test the mixture effect of spring wheat/ faba bean crop mixtures of emerging seedlings in three different environments. In particular, we tested the following four hypotheses: (H1) There is a positive mixture effect of spring wheat/faba bean crop mixture at the early growing stage (emergence); (H2) The early mixture effect depends on factors such as SW cultivar, FB cultivar, environment, and sowing density; (H3) Domination of SW in the mixture can be observed already at crop emergence; here, by domination we mean that the relative proportion of dominating partner in the mixture is higher compared to the other partner; (H4) There is a legacy of early mixture effects on later growth stages as measured by crop biomass, i.e. the strength of domination of one partner over the other at crop emergence is carried over to later stages.