Weeds are not always evil: crop-weed relationships are context-dependent and cannot fully explain the positive effects of intercropping on yield

Abstract

6 1. Implementing sustainable weed control strategies is a major challenge in agriculture. Intercropping offers a 7 potential solution to control weed pressure by reducing the niche space available for weeds; however, available 8 research on the relationship between crop diversity and weed pressure, and its consequences on crop yield is not 9 fully conclusive yet. 10 2. In this study, we performed an extensive intercropping experiment using eight crop species and 40 different 11 species mixtures to examine how crop diversity affects weed communities and how the subsequent changes in 12 weeds influence crop yield. Field experiments were carried out in Switzerland and in Spain, which differ drastically 13 in terms of climate and soil, and included monocultures, 2-and 4-species mixtures, and a control treatment without 14 crops. Weed communities were assessed in terms of biomass, species richness and evenness, and community 15 composition.16 3. Results indicate that intercropping per se does not reduce weed performance or diversity, but crop species 17 composition does. In particular, the presence of cereals in crop mixtures significantly reduced weed biomass and 18 diversity.19 4. Despite the lack of crop diversity effect on weeds, crop yield increased with crop species richness, particularly 20 in Switzerland. Moreover, in Switzerland, where soil resources were abundant, increasing crop yield correlated 21with increased weed suppression and reduced weed diversity. In Spain, where water and nutrients were limited, 22 crop yield was not related to weed biomass or diversity. 23 5. Synthesis and applications: We demonstrate that in our study, increased crop yield in mixtures was not due to 24 increased weed suppression in diverse crop communities, but must be the result of other ecological processes. We 25 also show that crop-weed relationships vary across environmental conditions, which emphasizes the need for a 26 better understanding of weed communities' assembly mechanisms, as well as locally adapted weed management 27 strategies. 28

Positive effects of crop diversity on productivity driven by changes in soil microbial composition

“…Results from our SEM in Switzerland suggest that the changes in soil microbial communities induced by crop diversity further affected ecosystem functioning and services, such as soil microbial activity and crop yield. We had previously shown that there was a positive effect of crop species richness on crop yield in Switzerland, demonstrating a positive diversityproductivity relationship (Chen et al, 2020;Engbersen et al, 2020). Here, we provide evidence that these positive effects of intercropping on crop yield can partially be explained by changes in microbial communities.…”

Facilitation and biodiversity–ecosystem function relationships in crop production systems and their role in sustainable farming

“…Instead, weed control in crop mixtures may require the use of highly weed‐competitive species or varieties (e.g. Stefan et al., Submitted), integration with ‘living mulches’ and modified agronomic operations such as the timing and density of seed sowing, mechanical weeding and cleaning to remove weed seeds from the harvested grain. Consequently, we are not suggesting that enhancing biodiversity in farming systems is a straightforward panacea.…”

Positive Effects of Crop Diversity on Productivity Driven by Changes in Soil Microbial Composition

“…In Switzerland, the soil consisted of 45% sand, 45% silt, and 10% clay, and contained 0.19% nitrogen, 3.39% carbon, and 332 mg total P/kg. Spanish and Swiss soils had a mean pH of 6.30 and 7.25, respectively (Stefan et al, 2021).…”

“…This demonstrates that other mechanisms must play a role in increasing crop productivity in diverse mixtures. These processes could include a complementary use of resources, such as nutrient or light partitioning (Jesch et al, 2018;Engbersen et al, 2020), trait differentiation (Cadotte, 2017), or changes in crop-weed interactions (Stefan et al, 2020).…”