Summer cover crops impact weed biomass and C and N acquisition

Abstract Despite the potential to improve nutrient cycling, weed suppression, and system resilience, mixed‐species cover crops remain underutilized in organic irrigated systems. This study evaluated the influence of cover crop diversity and associated weeds on biomass production (both cover crops and weeds), as well as carbon (C) and nitrogen (N) acquisition, and weed suppression. Three summer cover crop–carrot rotation cycles (2018–2019, 2019–2020, and 2021–2022) were established on a clay‐textured soil in a 50‐ha organic pivot‐irrigated field in Alberta. Cover crops included a polyculture (POLY) and monoculture of mustard (MUST; white [ Sinapsis alba L.] and brown [ Brassica juncea (L.) Czern.]), buckwheat (BWHT, Fagopyrum esculentum Moench), faba bean (FABA; Vicia faba L.), and a no‐cover control (CONT). Cover crop performance varied by species and year. POLY consistently produced the highest biomass (up to 2.79 Mg ha −1 ) and, along with BWHT, achieved the greatest weed suppression (62%–72%), while FABA and MUST were least effective. FABA had the highest N concentration (27–32 g kg −1 ) and lowest C:N ratios (13–16), whereas BWHT had the highest C:N ratios (21–36). C and N uptake were generally greater in POLY (up to 1.20 Mg C ha −1 ; 74 kg N ha −1 ) and lowest in FABA or MUST, with POLY and BWHT accounting for the largest share of total biomass nutrient uptake (53%–69%). Overall, these findings demonstrate that cover crop performance is highly context‐dependent, with POLY and BWHT offering more consistent benefits in biomass production, weed suppression, and nutrient acquisition, and highlight the complementary role of weeds in nutrient cycling.