Functional diversity promotes multifunctional cover cropping systems
Increasing ecosystem service provision is an emerging goal of contemporary agriculture. Exploiting biodiversity to meet this goal is a promising approach, though relationships between diversity and ecosystem services remain largely unexplored for innovative practices such as cover crop polycultures. In this study, we sought to answer two questions: (1) Does increasing species richness lead to greater ecosystem service provision by cover crops? (2) Beyond richness, are there other components of cover crop diversity that influence the simultaneous provision of multiple services (multifunctionality)? To answer these questions, we carried out a two year field study of seventeen cover crop systems ranging in diversity from one to eight species. In each system we measured five ecosystem services: weed suppression, nitrogen (N) retention, N provision, N supply, and subsequent corn crop yield. Using linear models, we examined the relationship between species richness and individual services. We also assessed the effect of increasing species richness on multifunctionality using both an averaging approach and a new multiple threshold approach. To identify additional components of diversity that may influence ecosystem service provision, we examined the relationship of average multifunctionality to taxonomic evenness and to several functional diversity indices.
Increasing cover crop species richness had a positive impact on weed suppression (p<0.01, R2=0.16), N retention (p<0.01, R2=0.15), and N provision (p=0.01, R2=0.04), but negatively affected N supply (p=0.01, R2=0.05) and did not predict subsequent crop yield. We identified a positive relationship between species richness and average multifunctionality (p=0.01, R2=0.05). The multiple threshold approach revealed variability in multifunctionality between years. Increasing functional diversity and taxonomic evenness also led to increased multifunctionality and explained a greater proportion of variation in multifunctionality than species richness. A quadratic relationship between multifunctionality and functional specialization explained the greatest proportion of variability (p<0.01, R2=0.41) and indicated that cover crop stand evenness and inclusion of species with complementary N acquisition strategies maximized multifunctionality. We have shown that although increasing species richness can increase ecosystem service provision, using cover crop diversity to create multifunctional agroecosystems requires polycultures that incorporate functional diversity and exhibit evenness. These analyses are the first of their kind to examine the relationship between cover crop diversity and ecosystem services derived from agriculture.