Reducing outcome uncertainty in cereal-legume intercrop by leveraging insights on farming history and practices as drivers of soil microbial functions and soil fertility

Intercropping or mixed cropping, i.e., cultivation of different species together in one field, can benefit (i) ecosystem resilience and sustainability through increased biodiversity and (ii) farmers in terms of yield, harvest quality and profitability through complementarity and facilitation effects. An ancestral practice that is regaining interest is to mix cereals with legumes. But despite welldocumented benefits, this practice remains unpopular among farmers, partly for agronomical or socio-economic reasons (e.g., lack of know-how or fitting equipment, underdeveloped down-stream sector, research focused on monocultures). Yet, another issue is the uncertain outcome of this practice. Indeed, its efficiency varies due to (i) plant genotypes; (ii) environmental factors; and (iii) chosen metric to assess efficiency. While regional agricultural research centres screen crop combinations to advise farmers, little is known about the effect of the environment. The environment of crops is strongly shaped by soil, rhizosphere and hyphosphere microbiomes. These biomes drive biogeochemical cycles, influence plant nutrition, act as soil architects and participate in auxiliary or pathogenic interactions. They contribute to N-cycle and N-nutrition through (i) mineralization, (ii) (de)nitrification, (iii) biological N fixation, (iv) connection of Rhizobium to its legume partner and construction of hyphal highways for various molecular compounds and bacteria (e.g. hyphae of arbuscular mycorrhizal fungi). In turn, agricultural soil microbiomes are the result of farm history, farming practices, soil quality and pedo-climatic context. So, could it be that the efficiency of mixed cropping, however defined, depends on the influence of (industrial) farming on soil microbiome? We propose a review of the literature to identify trends associated with “success” in cereallegume intercropping. In a holistic view, we extend our focus from plant genotype to the “black box” that is the farming system as a whole, influencing the soil microbiomes that determine this “success”. We ask what soil conditions ensure a positive outcome for a mixed crop in terms of yield, harvest quality, profitability and system sustainability, with a focus on the N-cycle. Doing so, we hope to gather insights on the contribution of soil microbiomes to the success of the intercrop and relate it to environmental factors such as agricultural practices (including fertility management) and farm and plot history.