Forage legume living mulch and rhizobial inoculation in Mediterranean organic durum wheat

Abstract

Intercropping rhizobia‐inoculated forage legumes in organic wheat systems reduces weed pressure and supplies nitrogen (N) to the companion crop via biological N ₂ fixation (BNF). However, adoption of combined intercropping and rhizobial inoculation remains limited. This 2‐year field study compared durum wheat ( Triticum turgidum L. subsp. durum (Desf.) Husn.), as a sole crop, or intercropped with forage legume ( Trifolium subterraneum L., Trifolium michelianum Savi, or Vicia sativa L.) in southern Italy. Legumes were rhizobia‐inoculated or non‐inoculated. Treatments were evaluated under reduced N fertilization (60 kg N ha ⁻¹ ) or standard N fertilization (120 kg N ha ⁻¹ ). The warmer and drier 2023/2024 season reduced wheat yields but increased grain protein, whereas the wetter 2024/2025 season enhanced yields, BNF (estimated using published empirical relationships linking N ₂ fixation to legume biomass yield and shoot N content), and wheat N recovery efficiency. Regardless of environmental conditions or agronomic treatments, wheat–legume systems consistently reduced weed biomass compared with sole wheat. Inoculated V. sativa and T . subterraneum improved wheat grain yield versus sole wheat, particularly under favorable conditions at reduced N rate. Vicia sativa exhibited the highest BNF, especially when inoculated at the low N rate, whereas T. michelianum showed smaller benefits. Inoculated T . subterraneum showed the highest nodule number and weight, and higher soil nitrate nitrogen (NO ₃ ⁻ ‐N), particularly at the higher N rate. Although results were context‐dependent on environmental conditions, legume living mulches integrated with effective rhizobial inoculation and reduced N fertilization support the agronomic sustainability of organic wheat systems in Mediterranean environments.