Annual, Annual Self-Seeding and Perennial Legume Living Mulches (LLMs) Shape Weed Community Composition and Diversity During the Wheat–Sorghum Fallow Period

Among the agroecological practices that can be used for sustainable weed management, legume living mulches (LLMs) are receiving increasing attention. This study provides a detailed report on how different LLMs established in durum wheat affected the weed community composition and diversity during the fallow period in a wheat–sorghum crop sequence. We hypothesised that differences in the life cycle of the seven legume species used as LLMs, namely perennial, annual self-seeding and annual species, would result in contrasting soil cover dynamics during the fallow period and, consequently, in distinct effects on weed community composition and diversity. To this end, two complete wheat–sorghum crop sequence cycles were performed. While LLMs did not significantly affect weed community composition during the intercropping with wheat (IC stage), their influence became evident in the post-intercropping fallow period (post-IC stage). In both experimental fields, perennial LLMs (Hedysarum coronarium (L.) B. H. Choi & H. Ohashi, Medicago sativa L. and Trifolium repens L.) and self-seeding legumes (Medicago polymorpha L. and Trifolium subterraneum L.) establish consistent ground cover during the post-IC stage of the wheat–sorghum crop sequence, shifting weed communities towards perennial species dominance. Perennial/biennial weed species such as Plantago lanceolata L., Cynodon dactylon (L.) Pers., Beta vulgaris L. and Silene latifolia subsp. alba Poir. thrived due to their underground storage organs, which conferred resilience against LLMs' competition. However, while LLMs hindered most annual weed species, Papaver rhoeas L. responded positively to LLMs, producing larger, more vigorous plants compared to control plots. Results showed that, except for the annual LLMs in the first year, all treatments reported a reduction in weed species richness compared to control conditions in both experimental fields. The lower level of weed species richness under LLMs reflects a high capacity for weed suppression that may have reduced the number of weed species germinating and therefore their richness. However, no effect was found for species' evenness, indicating that none of the weeds became dominant despite the lower species' richness. The role of legume species traits for LLMs are discussed, and it is concluded that the adoption of LLMs within diversified crop rotations, which imply alternating soil disturbance practices, can be recommended to maintain a balanced composition between annual and perennial weed species.