Cover crop mixtures result in a positive net biodiversity effect irrespective of seeding configuration

Abstract

Mixtures of plant species are often higher yielding due to species complementarities. Such effects may be beneficial to agriculture but are under-used in practice. It is therefore important to demonstrate beneficial effects of mixtures that are easy to implement for farmers. Here, we study yield in mixtures of agricultural cover crops. We hypothesized that the degree to which species express complementarity might be affected by the spatial configuration of the mixture. To test this hypothesis, we seeded mixtures using two configurations: (1) seed mixtures in each row (intimate mixing), and (2) different species in separate (alternate) rows (less intimate mixing).Three years field experiments were conducted to compare growth and N uptake by sole stands and the two pairwise mixture configurations of three cover crop species: vetch (Vicia sativa L.), bristle oat (Avena strigosa) and oilseed radish (Raphanus sativus L.). Shoot biomass production and N uptake were determined. Results were analysed using additive partitioning to determine the net biodiversity effect and its components.Mixtures, overall, accumulated significantly more shoot biomass and shoot N than the corresponding pure stands did on average; hence, the net biodiversity effect was positive. The yield benefit of mixtures (observed minus expected) was 0.66 t biomass ha−1 and 10.7 kg N ha-1 compared to an average sole cover crop of 2.58 t biomass ha−1 and 53.1 kg N ha−1. Contribution to the net biodiversity effect for biomass was, on average, equally divided between the complementarity effect and the selection effect. For N uptake, the net biodiversity effect was mostly realized through complementarity. Experimental year and species combination had a significant effect on biomass and N yield of mixtures. Mixtures containing vetch had the greatest gain in biomass compared to expectation. Whether within or between row mixing gave the highest yield and N uptake depended on species combination and year, but in most cases, the effect of configuration was not significant. Thus, overall, using cover crop mixtures resulted in a positive net biodiversity effect irrespective of seeding configuration.