Cover Crop Mixture Diversity and Function

The objective of this study was to evaluate the effect of cover crop mixture diversity on cover crop function. Specifically, this study evaluated the effect of cover crop species and functional richness on aboveground biomass productivity, weed suppression, soil nutrient retention, soil microbial community characteristics, and performance stability. Twenty to forty cover crop treatments were replicated three to four times at eleven sites across southeastern Nebraska using a pool of eighteen species representing three cover crop species each from six pre-defined functional groups: cool-season grasses, cool-season legumes, cool-season brassicas, warm-season grasses, warm-season legumes, and warm-season broadleaves. Each species was planted in monoculture and the most diverse treatment contained all eighteen species. Remaining treatments represented intermediate levels of cover crop species and functional richness. Cover crop planting dates ranged from late July to late September with both cover crop and weed aboveground biomass being sampled prior to winterkill. Soil samples were taken in the following spring and analyzed for soil extractable nitrate, phosphorus, potassium, sulfate, and chloride as well as extracted for fatty acid methyl esters to characterize soil microbial biomass and community structure. Performance stability was assessed by evaluating the variability in cover crop biomass for each treatment across plots within each site. While increasing cover crop mixture diversity increased average aboveground biomass productivity, I argue that this was simply the result of the average performance of the monocultures being drawn down by low yielding species rather than due to niche complementarity or increased resource use efficiency. Furthermore, while increases in cover crop mixture diversity were often correlated with increases in weed suppression, increases in soil nutrient retention, increases in soil microbial biomass, alterations in soil microbial community structure, and increases in performance stability, I argue that this was a result of diversity co-varying with aboveground biomass, and that differences in aboveground biomass rather than differences in diversity drove the differences observed in weed suppression, nutrient retention, soil microbial community characteristics, and stability. The results of this study contradict many popular hypotheses regarding the relationship between plant mixture diversity and function. v Dedication To my husband, Robert, with thanks. vi Acknowledgements I have been incredibly fortunate in the people I have been able to work with during my time at the University of Nebraska. For these people and to these people I would like to express my heartfelt thanks. First and foremost I want to thank my major advisor, Dr. …