Cover Crop Mixture Diversity and Biomass Production.

The diversity-productivity hypothesis proposes that increasing plant diversity increases average biomass productivity. The goal of this study was to test this hypothesis in the context of cover crop mixtures. Eighteen species of cover crops were used in this study representing six pre-defined functional groups—cool season grasses, cool season legumes, cool season brassicas, warm season grasses, warm season legumes, and warm season broadleaves. Twenty to forty treatments reflecting varying levels of species and functional richness were planted at eleven sites across southeastern Nebraska. Cover crop treatments ranged from containing one species to containing all eighteen species. Planting dates ranged from July 19 to September 20. Species specific biomass measurements were taken approximately two months after planting. Of the sites planted, four sites were not sampled due to issues of cover crop establishment. Of the seven sites sampled, there was little evidence that increasing species richness without increasing functional richness increased productivity. However, increasing functional richness, while holding species richness constant, had a marked positive effect on average biomass productivity. The implications of this, however, are more mathematical than practical. The low yielding leguminous functional group simply dragged down the average productivity of the low functional richness category as compared to the high functional richness category where the high yielding grasses and brassicas compensated for the low production of the legumes. In terms of practical cover crop management, there was no evidence of any mixture out-yielding the highest yielding monoculture at each site. While the diversity-productivity hypothesis was supported—average productivity did increase with increased plant mixture diversity, this study suggests a rather simple, mathematical mechanism by which increasing diversity can increase average productivity.