The effect of intra- and interspecific diversity of cover crops on ecosystem services in agricultural systems
Expanding on the foundation of biodiversity ecosystem function research (BEF), we are now looking beyond species richness and productivity, to different measures of diversity as well as other ecosystem functions. From an applied perspective, the ability to enhance the delivery of specific ecosystem services through increased diversity would be a valuable tool, especially in managed landscapes. In multifunctional agroecosystems our goals include yield, as well as other ecosystem services. Cover crops are a crucial tool in promoting a range of ecosystem services from carbon sequestration and nitrogen fixation and retention to weed suppression and erosion reduction. We are interested in how increasing diversity at different levels in cover crop mixtures can improve targeted delivery of ecosystem services. Here we implemented a field experiment (2 seasons) with a gradient of diversity in cover crop mixtures from a single cultivar of one species to multiple cultivars of six species. Additionally, we used three legumes and three grasses allowing us to examine the effect of functional diversity as well as genotypic and species richness. We measured three ecosystem services; productivity (aboveground biomass at flowering), weed suppression, and nitrogen fixation. Finally, as we are also interested in the impact of environmental conditions on the relationship between diversity and ecosystem services, we supplemented half the plots with additional nitrogen fertilizer. Also, in the second year, we added four on-farm sites to further examine the environment interaction.
Preliminary results show an intriguing relationship between cultivar diversity and the ecosystem service outcomes of cover crop biomass and weed suppression by the cover crop. For hairy vetch, we observed greater biomass in the five-cultivar mixture than any of the cultivars grown as monocultures. Additionally, the five-cultivar mixture suppressed more weeds than any of the cultivars grown as monocultures. The trends found in vetch hold with the other five species (crimson clover, winter pea, cereal rye, wheat, ryegrass) with regard to weed biomass; increased cultivar richness in a mixture of a single species increases weed suppression by the cover crop. For cover crop biomass however, there was not a consistent response across all species. This suggests there may be other mechanisms reducing weeds other than just aboveground competition from the cover crops. We expect to present additional results from the legume-grass mixtures, as well as the impact of nitrogen availability on the relationship between diversity and the three measured ecosystem services.