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The main goal of our project is to discover how the unique changes in the soil environment brought about by organic management impact N-fixation and apply this knowledge to improve management of leguminous green manure. We aim to investigate the complex interactions between soil environment, legume species and soil N-fixing microorganisms and develop management tools that can be used by organic farmers. <P>In the course of this research we also develop a model system for using on-farm experiential learning as a technique for training undergraduates. Specific objectives for this proposal:<OL> <LI>We will test hypotheses about how management-induced changes in the soil environment accruing over the long-term interact with microorganisms and plants that govern biological nitrogen fixation and assess the economic consequences of these interactions on working organic farms.<LI>We will assess the nitrogen-fixing potential for a variety of novel legume species and legume/non-legume species mixtures that could be used in managing biological nitrogen fixation. <LI>We will develop a management tool based on a quick, visual method for estimating total nitrogen content for key legume species.<LI>We will develop and test methods for training undergraduates and graduate students in organic agriculture, on-farm research and small farm management through implementation of a structured experiential learning initiative, the Sustainable Agriculture Scholars Program.
Non-Technical Summary: Biological nitrogen fixation is the major source of new nitrogen in organic agriculture, yet there has been almost no research devoted to understanding how organic management practices impact this process. Furthermore, few widely used, temperate green manures have been characterized in terms of their nitrogen fixing traits. Our research aims to understand how organic management strategies and their resulting long-term soil legacies interact with plant and microbial species to regulate nitrogen fixation and to assess the economic consequences of these interactions. <P> Approach: The research will be conducted on organic farms that we have been studying for the past five years. The field sites have been under organic management for 2-24 years and provide a management induced fertility gradient that reflects the accrual of SOM that typically occurs under long-term organic management. We believe these fields on working organic farms provide an ideal backdrop for studies aimed at understanding the dynamics of biological N fixation (BNF) in organic production systems. A complex suite of factors govern BNF making it necessary to use a holistic approach to study this process in order to understand the role of management practices, environment, and plant and microbial species. We will use mass balances as an indicator of relative N abundance and as a key defining characteristic in our studies of organic farming systems. We have used this approach combined with a number of soil measurements that characterize soil organic matter and microbial traits as the basis for characterization of the N fertility status of our study sites. We will use 15N natural abundance to measure N fixation rates in a number of legume species across a variety of organically managed farms. We will also conduct exploratory work on free-living N fixation on a limited number of organically managed farms. We will characterize the rhizobium communities in soils and in root nodules. A combination of culture-independent (i.e. molecular) and culture-based techniques, as we propose here, has been shown to be the preferred method to describe the ecology of complex microbial communities. The use of such a combined approach allows us to access both the cultural and un-culturable soil microbes that are likely responsible for much of the nutrient cycling that is so critical for organic farm functioning. Because much of this work will be conducted on working farms, we will also be able to conduct an economic assessment of these green manure crops that takes environmental variation into account. Economic assessments of cover crops have focused on comparisons with inorganic fertilizer costs and few studies have compared costs and benefits of organic N amendments with leguminous green manures. Economic assessments rarely incorporate environmental variation such as soil N fertility in comparing the value and costs of particular management practices. Finally, the integrated approach to our research goes beyond scientific inquiry. By linking our on-farm research to training of undergraduates, we will test a formalized experiential teaching approach. We also impact students who are likely to become key future players in nutrient cycling research, the cornerstone of organic management. Students that are chosen to participate in our program will not only become skilled in research techniques that answer critical questions asked by organic farmers, but will also learn to link this research to current issues in organic farm management.