Soil N, growing degree days, and plant C:N ratios predict cover crop mixture services more than diversity across farms

At a local scale greater biodiversity can increase the number and degree of ecosystem services (“multifunctionality”) provisioned by an ecosystem. In agriculture this phenomenon has been documented in diverse mixtures of cover crops: non-commercial crops planted during the fallow phase between cash crops. At larger geographic scales, for example, across multiple farms in a region, biodiversity and ecosystem services could also be influenced by, or depend upon, geographic location and environmental conditions. In cover cropping systems, where initial species diversity is artificially selected, environmental and management factors that may alter realized biodiversity and ecosystem services include soil conditions, landscape heterogeneity, growing degree days (GDD), and precipitation. However, we do not know to what degree environment and management versus biodiversity per se affect ecosystem services across farms. To test the prediction that cover crop mixture multifunctionality is influenced by cover crop diversity, environment, and management, we grew 5-species mixtures at one research station and eight organic farms across Pennsylvania and New York States. Eight ecosystem services pertaining to soil, plant, and insect health were assessed across locations for two growing seasons. In partial contrast to our prediction, only one diversity metric (Simpson diversity index) of cover crop mixtures predicted multifunctionality across sites. Some individual ecosystem services were positively or negatively correlated with multiple diversity indices. Path analyses showed that initial soil nitrogen, fall GDD, and spring GDD predicted individual ecosystem services and multifunctionality across sites by altering the C:N ratio of cover crop biomass. Our data suggest that at a wider geographic range traditional diversity metrics are poor predictors of multifunctionality; rather, environmental and soil factors interact with cover crop C:N ratios to affect ecosystem services provisioned by cover crop mixtures. Understanding how cover crop mixture diversity versus management and environment contribute to multifunctionality is necessary for making sound recommendations to farmers on which cover crops to plant to gain the services they desire for a given agricultural field. Growers and agronomists should take these factors into account, as well as the services farmers most desire, when designing and recommending cover crop mixtures to maximize ecosystem services to cropping systems.