Challenges of developing sustainable nitrogen sources in agriculture: Cover crops, nitrogen fixation and ecological principles
Substantial increases in nitrogen additions to agricultural ecosystems over the past century have increased crop yields, but have also led to ecological damage due to nitrogen losses to water and the atmosphere. Traditional inputs of nitrogen were in the form of organic material such as manure or legumes, which led to longer retention of nitrogen in the agroecosystem compared to synthetic fertilizers. These methods remain a viable option for contemporary farmers, but are rarely used instead of inorganic nitrogen fertilizers. I examined methods to improve integration of organic nitrogen sources, particularly legume winter cover crops, into cropping systems. Specifically, I tested the effect of planting date, temperature, varieties and mixtures on the ability of hairy vetch (Vicia villosa) to meet nitrogen demands of common cash crops (Chapters 1, 2). In addition, I examined the relative rates of nitrogen fixation by another legume cover crop, red clover (Trifolium pratense), across a gradient of management intensity (Chapter 3).
Variation existed among hairy vetch varieties in growth, morphology and phenology, and this variation translated into differences in relative success within different cropping systems. Yet, planting date and accumulated growing degree days after planting outweighed the variability among varieties in determining the amount of biomass produced by hairy vetch. In two long-term cropping experiments, the rate of nitrogen fixation by red clover differed across environments, with a trend towards higher rates of nitrogen fixation in systems managed with chemical sources of nitrogen fertilizer.
Together, these results indicate that legume winter cover crop performance is influenced by genetic information and environmental characteristics. Species and varieties offer different opportunities within different cropping systems, and management history can influence the rate of nitrogen fixation of legume cover crops. In addition, red clover may act as a self regulating fertilizer source over time by fixing lower rates of nitrogen from the atmosphere when growing in more fertile soils, thus reducing the total amount of nitrogen entering the system and reducing long term costs for farmers and to the environment.
The final chapter of this dissertation is a review of the impacts of agricultural management on greenhouse gas concentrations in the atmosphere, and is written in a format for use in educational settings, particularly in college classrooms.