Lighting up the black box: Improving legume performance on organic farms by optimizing microbially-mediated plant and soil nitrogen cycling processes.

Project Overview

Project Type: Research and Education
Funds awarded in 2010: $192,000.00
Projected End Date: 12/31/2014
Region: Southern
State: North Carolina
Principal Investigator:
Dr. Julie Grossman
University of Minnesota

Annual Reports


  • Agronomic: corn


  • Crop Production: biological inoculants, cover crops, no-till, nutrient cycling
  • Education and Training: farmer to farmer, on-farm/ranch research
  • Production Systems: organic agriculture
  • Soil Management: green manures, organic matter, nutrient mineralization, soil microbiology, soil quality/health

    Proposal abstract:

    The purpose of this project is to improve legume cover crop management by examining key soil microbial processes that regulate nitrogen (N) cycling in legumes, and by delivering grower information through participatory workshops. The information we propose to gather is specifically tailored to answer farmer questions generated at recent workshops for organic farmers in North Carolina addressing legume cover crop management for improved N. Growers are eager to understand the functioning of the belowground soil community responsible for making nitrogen available to crops, including both symbiotic nitrogen-fixing rhizobia bacteria associated with legume cover crops, mycorrhizal fungi associations, and microbes that mineralize legume residue. Competitive indigenous populations of rhizobia present in farm soils have been shown to impact the success of rhizobia inoculants added at legume planting. Since organic farmers use a variety of legumes in rotation, we hypothesize that indigenous rhizobia populations will affect nodulation and nitrogen fixation in desired legume crops. We will also examine mycorrhizal fungal diversity, as little is known about how diversity of legume cropping history and inoculation impact the mycorrhizal fungal community and activity. Lastly, we will investigate how non-chemical legume kill method (rolling-crimping, flail mowing, incorporation) impact N availability from decomposing legume residues. Our activities include 1) surveying organic growers to determine current rhizobia inoculant handling procedures and legume perceptions; 2) establishing legume inoculant demonstration plots and using them to determine how inoculation practices impact legume productivity, 3) determining how non-chemical winter annual legume cover crop kill methods impact indicators of microbial activity and N supply to crop plants; and 4) disseminating results and jointly educating North Carolina organic growers and students about soil microbial N-cycling processes in sustainable agriculture.

    Project objectives from proposal:

    Objective 1: Randomly survey 200 organic growers from North and South Carolina in order to determine current rhizobia inoculant handling procedures, and production challenges and perceived benefits of legume cover crop use. Using the Internet-based Survey Builder software provided free-of-charge at North Carolina State University, we will survey organic growers in North Carolina and South Carolina to determine current inoculant use and handling procedures, as well as perceived benefits and challenges, for a variety of popular and novel winter annual legume cover crop species in the Southeast. We would like to learn the answers to questions such as: which legume cover crops are commonly planted, where farmers acquire inoculant, how they store their inoculant, how they inoculate, and if they use a ‘sticker’ to adhere the inoculant to the seed. We will also include questions in the survey related to production challenges and perceived benefits of legume cover crop use. Cover crop use is limited to those temporal or spatial niches in a farming system where there is no cash crop currently grown, such as the winter months or between-row intercropping. This presents a number of challenges to growers who are trying to maximize legume biomass and N contribution. A better understanding of these challenges will help researchers address the constraints associated with legume cover crop use and optimizing N contribution to organic farms.

    Objective 2: Establish 3 demonstration plots on working organic farms to learn about the impact of inoculation on nodulation and legume production. We will establish 3 demonstration plots on working organic farms to determine how inoculation practices and background farm-soil populations of nitrogen-fixing rhizobia bacteria and mycorrhizal fungi impact legume productivity, nitrogen fixation, and nodule occupancy. These demonstration plots will also be used to collect valuable information on ‘who’ is inside the nodule, or nodule occupancy, which will help us determine how inoculation practices and background farm-soil populations of nitrogen-fixing rhizobia bacteria and mycorrhizal fungi impact legume productivity and nitrogen fixation.

    Objective 3: Determine how various methods of legume cover crop termination common in organic farming systems (rolling, disking, and flail mowing) impact indicators of microbial activity and N supply to crop plants. While BNF is critical to bring ‘new’ N into organic systems, a second area of microbial functioning that we would like to better understand includes the activity of the microorganisms involved in decomposing the legume residue and making N available to crop plants. In this objective we will learn how different methods of winter annual cover crop termination being used on organic farms around the Southeast affect N release and microbial activity in organic systems.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.