To what extent replacing N fertilization by a companion service cover crop maintains crop productivity and N use efficiency?

Replacing mineral nitrogen fertilizers with organic sources or service crops is often proposed to enhance resource nutrient use efficiency and reduce environmental impacts. However, in semi-arid irrigated systems, evidence of their agronomic performance is limited. A three-year field experiment in a wheat-wheat-rapeseed rotation compared five strategies: unfertilized control (0 N), full mineral fertilization (FM), pre-sowing slurry + mineral top-dressing (BAU), full slurry fertilization (FS), and a legume companion cover crop (CCC). In both wheat cropping seasons, CCC yielded less than FM, BAU and FS, and showed lower water use efficiency, with no benefits from biological N fixation. A non-significant trend of lower yields in CCC compared to 0 N was also observed in 2021 and 2022. FM and FS achieved equivalent yields at comparable N rates, indicating that slurry can fully replace mineral fertilizer for both pre-sowing and top-dressing applications without yield or nitrogen use efficiency penalties, while also increasing soil organic carbon in surface layers. These results highlight the need for critical evaluation of CCC in semi-arid irrigated systems, as they may not improve -and can even reduce- productivity and resource use efficiency. • A three-year field experiment compared fertilization and management strategies. • The companion cover crop reduced wheat yields compared to fertilized treatments. • Pig slurry achieved yields and NUE equivalent to mineral fertilizer. • Slurry fertilization increased surface SOC compared with mineral fertilization. • Companion crops compromised productivity without improving NUE, WUE or SOC stock.