Productivity benefits of cereal-legume cover crop mixtures under variable soil nitrogen and termination times

Mixtures of cereal and legume cover crops can merge the benefits of these two functional groups and sometimes increase biomass production and ecosystem services compared to monocultures. However, the multifunctionality and productivity of cover crop mixtures depend on environmental and management factors that influence species composition. We hypothesized that (i) the species composition in the cereal/legume cover crop mixtures will change in response to soil N level and growth stage, (ii) cereal/legume mixtures will accumulate more aboveground biomass and N and produce greater leaf area index and canopy cover than monocultures, particularly at low soil N levels and with late termination, and (iii) cereal/legume mixtures will reduce the risk of high C:N ratios associated with late termination of a cereal cover crop. We evaluated cereal/legume cover crop mixtures and monocultures in a long-term maize (Zea mays L.) N rate study at the University of Kentucky. The study had four inorganic N rates applied to maize since 1970 (0, 84, 168, 336 kg N ha^-1) and three winter cover crop treatments (cereal rye (Secale cereale L.) monoculture, crimson clover (Trifolium incarnatum L.) monoculture, rye/clover mixture). We evaluated the cover crops from fall 2020 through spring 2022. We collected repeated measurements of cover crop aboveground biomass, N content, leaf area index, and canopy cover. At the final sampling in late April or early May, we found that the cover crop mixture produced more biomass than both monocultures at intermediate N input rates, while low and high N input rates resulted in similar biomass levels as monoculture clover and rye, respectively. The treatment without N fertilizer application produced proportionally more clover in the mixture than in higher fertilizer rates. Clover in monoculture did not show significant differences in biomass among the N fertilizer rates, while rye increased its growth proportionally to the applied N. Early termination represented more biomass for rye in mixture, while late termination favored clover growth. The cereal/legume mixtures had moderate C:N ratios relative to both monocultures across N rates and sampling dates. The results of this study suggest that cover crop mixtures may perform equally or better than monoculture across different N levels and termination timings.