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

2011 Annual Report for LS10-227

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

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


Our field experiment testing four different cover crop termination treatments (roll, spray, disk incorporation and flail mow) was assessed for biomass production, N-mineralization and mycorrhiza colonization. Through this experiment we have learned that cover crop N release rates vary by species, by termination method, and by time after termination. Three demonstration plots were established across the state to test recommended rhizobia inoculant effectiveness and to teach farmers about nitrogen fixation. A very successful SARE-funded workshop was held on one of the sites and information gathered about most useful information learned. Follow up calls were then made 6-months following the workshop to gain information regarding how new knowledge about cover crop N management was being put to use on their farms or in their jobs. An inoculation survey designed in year one and taken by 80 farmer participants is currently being analyzed and will provide information about how they manage their cover crop inoculants. Two soil science lesson plans were developed and delivered to low-income youth in Raleigh, NC.

Objectives/Performance Targets

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.

Objective 2

Establish 3 demonstration plots on working organic farms to learn about the impact of inoculation on nodulation and legume production.

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.

Objective 4

Disseminate results to agronomic organic growers and jointly educate organic growers and students nationwide about soil microbial N-cycling processes in sustainable agriculture.


Objective 1: Accomplishments

In 2010 a survey containing questions related to legume inoculant use and handling procedures, sources of information on legume inoculation, legume nodulation, and perceived benefits of leguminous cover crops was administered to over 500 farmers, resulting in 82 responses. Preliminary review of the data shows that a majority of growers do use inoculant. Most farmers, regardless of inoculant use, observed nodules on the legume cover crops. The original student assigned to the project left NCSU after data collection, and the project was transferred to a University of North Carolina (UNC) undergraduate honors student for data analysis in Fall 2011. Findings will be presented at least one national (likely the Soil Science Society of America annual meetings) and one regional agriculture conference (UNC undergraduate research symposium) in 2012.

Objective 2: Accomplishments

In fall of 2010 we had established three demonstration plots on organic land in North Carolina, located on 3 organic farms, each planted with 4 legume cover crop varieties: crimson clover (Trifolium incarnatum L) cultivar Dixie, hairy vetch (Vicia villosa Roth) cultivar Auburn Early, Austrian winter pea (Pisum sativum subsp. Arvense), and woolypod vetch (Vicia villosa Roth) cultivar lana. We used a randomized complete block split plot factorial design with the main factor of cover crop and the split plot factor being with or without inoculation, using four replications. Legume biomass produced on all plots ranged from 2000 to 3,500 lbs of biomass per acre, with lbs of N ranging from 80-120 lbs N per acre. Preliminary analysis of the data suggests that there are few differences in the amount of plant biomass or total plant N produced when legumes are inoculated compared to when they are not inoculated.

Objective 3: Accomplishments

In October 2010, 144 experimental plots were established at the Caswell Research Station in Kinston, N.C, grown throughout the winter, and terminated in Spring 2011 to investigate impact of termination method on N mineralization. We are investigating growth, productivity, mycorrhizal colonization, and N contribution of four cover crops, a positive control of 150 lbs/acre of nitrogen fertilizer, and a negative control on bare ground where nothing is planted or fertilized. The four cover crops (crimson clover, hairy vetch, Austrian winter pea, and balansa clover) were subjected to four termination treatments at the end of April, 2011, including flail mowing, tilling, an herbicide treatment, and rolling. This results in 24 total treatment combinations that are repeated in six research blocks over the entire field. Preliminary results from year one show that in both Austrian winter pea and hairy vetch, peak plant available N occurred at six weeks after termination (sampling time 2) across all termination treatments, with vetch biomass containing more N than pea (Figs 1a and b). At six weeks, rolled vetch produced significantly greater available soil N than any other termination treatment combination. Biomass contributions of crimson clover were higher (7,528 kg ha-1) than hairy vetch (6,555 kg ha-1) and Austrian winter pea (5,813 kg ha-1), which both contributed similar amounts of biomass to the system. Balansa clover was the lowest contributor of biomass at 4,880 kg ha-1. Austrian winter pea and hairy vetch had the highest total nitrogen content (216 and 191 kg ha-1, respectively), followed by crimson clover (177 kg ha-1), and balansa clover (108 kg ha-1). These preliminary results suggest that termination method is as important as cover crop species choice in order to maximize nitrogen contributions from annual winter leguminous cover crops. Treatment plots have been assessed for the following arbuscular mycorrhizal fungi (AMF) responses: 1) baseline AMF species diversity in fall 2010 prior to cover crop treatments being established), 2) AMF root colonization of each cover crop prior to kill treatments in 2011, and 3) AMF root colonization post kill treatments, taken 3-weeks after cover crop termination and corn planting in 2011. We are in the process of also analyzing samples from treatment plots of AMF extra-radical hyphae collected from soils pre- and post kill treatments. Thus far, we have found 6 AMF species collected from the baseline samples across the field treatments, including: Glomus clarum, Glomus intraradices, Acaulospora mellea, Gigaspora margarita, Scutellospora sp. (coraloidia-like), and Acaulospora sp. The latter AMF species will be submitted to the International Cultural Collection of Vesicular Arbuscular Mycorrhiza (INVAM) for species confirmation. We have also found that Austrian Winter Pea and Hairy Vetch supported higher AMF colonization that the Crimson Clover and two N fertilizer control treatments (Figure 2). Lastly, AMF root colonization of 3 week corn roots was significantly lower in the roll treatments than the other cover crop kill treatments (Fig 3.).

Objective 4: Accomplishments

On April 19, 2011 we held a SE SARE funded workshop to describe advantages of cover crops, how to estimate fertility impact on cash crops, as well as offer a visual example of legume growth immediately prior to spring termination. Forty farmers, extension agents, and agricultural professional attended the event. The workshop, entitled “Growing Your Own N: Improving Legume Cover Crop Management” was held at the certified organic Abundant Life Farms in Clayton, NC. It was a hands-on workshop experience in that farmers were able to sample biomass and determine N contribution through common and easy to use “rules of thumb” related to biomass N content of typical cover crop legumes. Growers were also able to see and compare cover crop varieties at full maturity.

Additionally, students in Dr. Julie Grossman’s Soil Agroecology course designed two lesson plans for low-income youth and taught them in conjunction with the Inter-Faith Food Shuttle’s Farm and Community Garden project over an 8-week period. The first lesson taught principles of vermicomposting and established a vermicomposting system at the two sites. The second lesson taught concepts of nutrient cycling as related to legume cover crops, including biological nitrogen fixation and rhizobia, and also established winter annual cover crop plantings at both sites. The sites where the units were taught included 1) Longview School in Raleigh, an alternative school for behaviorally challenged middle and high school students, and 2) a ‘manufactured home community’ (i.e. trailer park) with predominantly African American and Hispanic/Latino residents. Lessons were transferred to the Inter-Faith Food Shuttle at the end of the semester for use in their community outreach activities.

Impacts and Contributions/Outcomes

Objective 1: Impacts

We expect that by delivering information about improved inoculant management to organic farmers throughout the Southeast, we will improve awareness of effective inoculant use for increased nitrogen availability. Information has yet to be summarized and disseminated.

Objective 2: Impacts

This objective was designed to determine success of inoculation when faced with competition with resident rhizobia soil bacteria, but possibly of equal or more interest to the farmer, is which cover crops will grow best on their farm. In 2010 we planted 4 cover crop legume varieties: crimson clover (Trifolium incarnatum L) cultivar Dixie, hairy vetch (Vicia villosa Roth) cultivar Auburn Early, Austrian winter pea (Pisum sativum subsp. Arvense), and woolypod vetch (Vicia villosa Roth) cultivar lana on two organic demonstration farms and one organic research station unit. One of the hairy vetch varieties – Auburn Early – is specifically adapted for southeastern growers. Exposure to varieties that are adapted to North Carolina climate is an important step in promoting a positive grower view of cover crops that are more traditionally adapted to northern climates.

Using our external evaluator following the cover crop workshop, we gained information regarding specific knowledge workshop attendees gained through observation of our demonstration plots and workshop activities, including these verbatim comments from growers about what they learned:

• Crop choice – plant and kill time plant and kill methods
• Better understanding of legume crop Nitrogen cycle
• Nitrogen being tied in the plant material not leached in soil
• N yield per species
• Most N is in the above ground portion of the cover crop
• The availability of the nitrogen; how much; biomass
• Tons of Stuff!
• How to measure N in cover crop
• everything, new topic for me
• all this was new to me
• seed sources, prices
• Hairy vetch one to use for best bang for N

Regarding which practices participants stated they would use on their farm or their job, farmers mentioned:

• Wait until plants are mostly flowering before tilling under
• Inoculate my legumes
• Feel more confident about using cover crops.
• Modifying the types of cover crops I am using.
• Mowing the cc versus incorporating into soil
• I will be planting a cover crop this fall
• Planting timing for cover crops
• Restrictions on manure use due to high zinc, etc. levels

Six months after the workshop follow up phone calls were made to participants to determine how they were using knowledge gained in the workshops on their farms or in their jobs, gathering information from a total of eight farmers and seven extension and Natural Resources Conservation Service (NRCS) personnel. Out of the eight growers, seven were using information learned in the workshop to help them make decisions on the farm, including planting new species of cover crops, new mixes, and calculating N contribution from cover crops. Extension and NRCS personnel were asked which aspects of the workshop they found particularly useful. The most important concepts these participants learned included termination time, amount of N contributed through cover crop biomass, visualizing species differences, and the process of biomass mineralization after termination.

Objective 3: Impacts

The outcome of this research is to identify leguminous winter cover crops and methods for incorporation that achieve the optimal synchrony of plant-available nitrogen for a crop. With fertilizer prices increasing sharply and nitrogen from runoff being one of the biggest problems in streams and open waterways, one of the key goals of this study is to insure enough mineralized nitrogen is present when the plant needs it. Biologically fixed nitrogen has great potential to replace this with the only cost being seed and fuel. We have learned that species differ in their N mineralization release rates, with hairy vetch contributing the most N at six-weeks after termination as compared to two other common legume species. Further, we have evidence that termination method impacts how much N is released, with differences observed between cover crops that have been rolled, incorporated or flail mowed and left on the surface. Information gained through portions of this SARE funded project were summarized in this peer-reviewed publication, with one additional manuscript in preparation:

1. Reberg-Horton, C, Grossman, J., Kornecki, T.S., Meijer, A.D., Price, A.J., Place, G., Webster, T.M. Utilizing cover crop mulches to reduce tillage in organic systems in the Southeastern USA. Renewable Agriculture and Food Systems, In Press; available online: doi:10.1017/S1742170511000469.

Objective 4: Impacts

A total of 15 local youth participated in the lessons developed through this grant, and we assume learned valuable information that they can use to help grow food in their own neighborhoods through support of IFFS and their community gardening program. Two lesson plans were developed and returned to the IFFS for use in their gardening education program in low-income neighborhoods. Three scholarly presentations were delivered on this aspect of the project, with one including two of the PI’s (Grossman and Schroeder-Moreno):

1. Grossman, J., Schroeder-Moreno, M., Thraves, T. and Creamer, N. , Good food for all: Using service-learning to teach agriculture to diverse audiences. Invited symposium speaker in: Experiential Learning and Action Education, ASA-CSSA-SSSA International Annual Meetings, Oct 16-19, 2011, San Antonio, TX.

2. Smith, S., Grossman, J., Prohn, S. Preparing students for a diverse future: designing and evaluating a cultural competency training program for community engagement in agriculture, ASA-CSSA-SSSA International Annual Meetings, Oct 16-19, 2011, San Antonio, TX.

3. Sherard, M., Grossman, J.. Prohn, S. Smith, S. Building soils, building minds: Evaluating learning gains resulting from community-engaged coursework in soil science, ASA-CSSA-SSSA International Annual Meetings, Oct 16-19, 2011, San Antonio, TX.


Dr. Michelle Schroeder-Moreno

[email protected]
Assistant Professor
North Carolina State University
2406 Williams Hall Campus Box 7620
Raleigh, NC 27695
Office Phone: 9195130085
Dr. Wei Shi

[email protected]
Associate Professor
North Carolina State University
4302 Williams Hall, Campus Box 7619
Raleigh, NC 27695
Office Phone: 9195134641
Dr. Sarah Bowen

[email protected]
Assistant Professor
North Carolina State University
1911 Bldg 362
Raleigh, NC 27695
Office Phone: 9195150452
Dr. Amy Germuth

[email protected]
150 Solterra Way
Durham, NC 27705
Office Phone: 9194015403