Weed suppression in cover crop mixtures under contrasted levels of resource availability
Résumé
Cover crop (CC) mixtures appear as a promising agroecological tool for weed management.
Although CC supress weed growth by competing for resources, their suppressive effect under
contrasting levels of soil resource availability remains poorly documented. A two field: year
experiment was conducted to investigate the weed suppressive effect of four CC mixtures.
They were composed of 2 or 8 species including or not legume species and compared to a
bare soil control. The experiment included two levels of irrigation and nitrogen fertilisation at
CC sowing. The objectives were to assess (i) weed and CC aboveground biomass response to
CC mixtures and resource availability, (ii) the weed suppressive effect of CC mixtures across a
gradient of CC biomass and (iii) weed community response to CC mixtures and resource
availability. CC and weed biomass were mainly influenced by interactions between CC
mixtures and fertilisation and between CC mixtures and irrigation, with contrasted effects
between field: years. Nitrogen fertilisation increased biomass of non-legume based CC
mixtures but this only resulted into a further reduction of weed biomass of little biological
relevance. Legume-based CC mixtures suppressed weeds to a greater extent without nitrogen
fertilisation in year 2 but not in year 1, possibly due to contrasted initial soil nitrogen
availability (lower in year 2). Weed biomass generally benefited more from irrigation than CC
mixtures. Among the 33 weed species recorded, weed communities in each plot were
dominated by wheat volunteers, Geranium dissectum, Veronica persica and Echinochloa crusgalli, whose biomass varied depending on the interaction between year, CC mixture and
resource availability. Our results revealed that competitive outcomes between CC mixtures
and weed species were driven by a complex interaction between resource availability and
species traits. Further experiments focusing on plant traits should improve our understanding
of weed: CC competitive outcomes under various levels of resource availability.