Temporal crop diversification could reduce pesticide use by increasing the proportion of crops with low pesticide use (dilution effects) or enhancing the regulation of pests, weeds and diseases (regulation effects). Here, we use the French National DEPHY Network to compare pesticide use between 16 main crops (dilution effect) and to assess whether temporal crop taxonomic and functional diversification, as implemented in commercial farms specialized in arable field crops, could explain variability in total pesticide use within 16 main crops (regulation effect). The analyses are based on 14,556 crop observations belonging to 1334 contrasted cropping systems spanning the diversity of French climatic regions. We find that cropping systems with high temporal crop diversity generally include crops with low pesticide use. For several crops, total pesticide use is reduced under higher temporal crop functional diversity, temporal crop taxonomic diversity, or both. Higher cover crop frequency increases total pesticide use through an increase in herbicide use. Further studies are required to identify crop sequences that maximize regulation and dilution effects while achieving other facets of cropping system multiperformance.

Functional and phylogenetic diversity (FD and PD respectively) of the resident community are expected to exert a key role in community resistance to colonization by surrounding species, and their establishment success. However, few studies have explored this topic experimentally or evaluated the interactive effects of these diversity measures. We implemented a diversity experiment to disentangle the role of FD and PD by sowing mixtures of 6 species, drawn from a pool of 19 species naturally coexisting in central European mesic meadows. The mixtures were designed to cover four independent combinations of high and low FD and PD. Species covers were estimated in spring and late summer over two growing seasons. We then assessed the establishment success of colonizers as a function of their mean traits and phylogenetic distance to the resident (i.e. sown) communities, as well as the resistance of the resident communities to natural colonizers as a function of their functional and phylogenetic structure. Results generally indicated a temporal shift regarding which trait values made a colonizer successful, from an acquisitive strategy in early stages to a more conservative trait syndrome in later stages. FD decreased community resistance to natural colonization. However, PD tempered this effect: with high PD, FD was not significant, suggesting complementary information between these two components of biodiversity. On average, colonizing species were more functionally distant from the resident species in sown communities with high functional diversity, i.e. those that were more colonized. Synthesis. Our results confirm an interplay between FD and PD during community assembly processes, namely resistance to colonizers, suggesting that these two descriptors of biodiversity only partially overlap in their contribution to the overall ecological structure of a community. The hypothesis that higher FD increases resistance through a more complete use of resources was challenged. Results rather suggested that greater FD could provide an unsaturated functional trait space allowing functionally unique species to occupy it.

In Europe, conservation agriculture (CA) is currently challenged by higher weed pressure, potential glyphosate ban and reduced crop yield. Based on preliminary results and a critical analysis, we provide insights on how to assess the effect of introducing different levels of tillage intensity, after a long-term CA sequence, on weed communities and crop yield. The experiment compared three types of fallow management (ploughing (CT), reduced tillage (RT), no-till with glyphosate (NT)) on four fields after 17 years of no-plough, which ended with 7 years of CA. The introduction of tillage proved to be a major driver of weed communities before weeding (density, richness and composition) in winter wheat. Weed density and species richness before weeding was greatest in RT, intermediate in CT and lowest in NT. The number of grains per ear and crop yield increased with tillage intensity (+11% for RT, +31% for CT). We provide avenues for future research through detailed methods and key references. Differences in winter wheat productivity were possibly related to enhanced soil structure and increased mineralisation of soil organic matter. Potential benefits of occasional ploughing will depend on the density and composition of the newly upwelled weed seedbank, which will need to be assessed before implementing tillage. From a multicriteria perspective, the long-term benefits associated with CA could largely exceed short-term yield increases associated with occasional tillage. Future studies will need to characterize the impact of occasional tillage operations on the long-term multiperformance of CA systems.