Cover crop mixtures enhance belowground carbon input and suppression of spontaneous flora under Danish conditions

Cover crop (CC) mixtures offer a unique set of advantages that can enhance soil health and agricultural productivity when compared to pure stand CC. However, a quantitative understanding of the varying contributions of different carbon (C) input pathways in CC mixtures is lacking. To address these gaps, a field experiment with multiple-pulse labelling with ¹³CO₂ was used to quantify C-derived from CC mixtures via plant biomass, as well as via phyllo- and rhizodeposition. We assessed the impact of preceding main crops (barley, barley-pea, pea, and faba bean) on soil C input to 75 cm depth by two CC treatments (pure stand ryegrass versus a mixture of chicory, plantain, and ryegrass) and their effect on spontaneous flora (SF) biomass and diversity. In topsoil layers (0–25 cm), net C lost to soil via phyllo- and rhizodeposition was higher with mixed CC (30 g C m⁻²) than pure stand ryegrass (25 g C m⁻²). Between 25 and 75 cm, mixed CC and pure stand CC had similar C inputs via rhizodeposition despite larger root biomass in mixed CC. Cover crops reduced SF biomass and diversity, with mixed CC exerting the strongest suppressive effect, reducing biomass (individuals counted) by 57 % compared to the control. The improved efficiency of mixed CC was attributed to species complementarity in leaf and root patterns, resource utilization, and nutrient uptake. In conclusion, well-designed mixed CC had a greater positive impact on soil C inputs and suppression of SF compared to CC pure stand with ryegrass, resulting from complementary above and belowground traits.