Background/Question/Methods While the negative effect of invader weeds on crop yields have been widely regarded in agroecosystems, few studies, to our knowledge, have devoted to the role of farming system on invasion process, although invasion by alien and native weeds could also dramatically affect biodiversity as occurs in a wide range of ecosystems. The aim of this paper is to analyze if invasibility could be affected by farming system. To this end, the growth, reproduction, and the predation by slugs and insects of the invader weed Amaranthus retroflexus in relation to farming system were evaluated. The study was carried out in maize crops under organic and conventional farming practices within a long-term experiment [(DOK (biodynamic, bioorganic, Konventionell) trial, Therwil, Switzerland]. Two organic farming systems (biodynamic and bioorganic) and two conventional systems (using mineral fertilizer plus farmyard manure and using mineral fertilizer exclusively) were emulated in a replicated field plot experiment. Furthermore, the above and belowground biodiversity and the biological activity was higher in organic than conventional systems. The experiment was based on simulated invasion by A. retroflexus through seedling transplant introductions into farming systems.

Results/Conclusions The growth of A. retroflexus, assessed as aboveground biomass, plant height and number of leaves, was significantly higher in conventional systems. The higher mineral fertilization and its fast release of nutrients lead a greater availability and a more efficient uptake of limiting resources (i.e. N and P). The survey of predation impact on transplanted seedlings has shown that pre-reproductive mortality was mainly related to slug predation on soft plant tissues at above and below-ground. The higher pre-reproductive mortality in conventional systems could be explained by the higher abundance of Arion hortensis in those systems and, in turn, to lower abundance of insects-eating slugs. The strong demands of N and its high accumulation in its tissues of A. retroflexus coupled with the higher availability of N_min in conventional systems can lead that palatability could be higher in plants thriving in conventional than organic systems because slugs preferred high N content tissues. Furthermore, the greater abundance of resident weed vegetation could provide more shelter and more food availability. The outcome of the balance between the negative effect of predation by slugs and the positive effect of resources availability on growth of A. retroflexus through the different systems reflects that invasibility is enhanced in conventional systems.