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

2012 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.


Summary (Year 3)

Our field experiment testing four different cover crop termination treatments was planted for a second field season in fall 2011 at the Center for Environmental Farming Systems (CEFS) on certified organic land. Three cover crop species (hairy vetch, crimson clover and Austrian winter pea) were grown and in 2012 were spring-terminated using four approaches (organic herbicide, flail mowing, rolling, and incorporation), then their decomposition rate and nitrogen delivery compared to plots using no cover crops and no fertilizer. Cover crop biomass production, N-mineralization and mycorrhiza colonization were evaluated. Results of the survey to evaluate use and handling of rhizobia inoculants were analyzed and summarized, resulting in an additional presentation at the Soil Science Society of America national meeting. Two workshops were delivered at Southern SAWG reaching almost 200 individuals. Project results were also delivered at the national Ecology Society of America meetings and at the Soil Science Society of America national meetings. Three soil science lesson plans were developed and taught to underrepresented groups through our community partner’s gardening education program. Nodulation capacity and rhizobia diversity on our demonstration farms is underway in our laboratory.

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 three 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: In 2010 a survey containing questions related to legume inoculant use was administered to over 500 farmers, resulting in 82 responses. The survey focused on cover crop choice, inoculation approaches, inoculant care and maintenance, information sources, and farm demographics. The growers’ responses were compared with the most recommended practices by extension, manufacturers and the literature. Final results were analyzed in 2012 and suggested that 41% of growers choose inoculants based on recommendations from the seed vendor, but obtain information about what practices to follow nearly equally from the seed vendor (25%), inoculant vendor (22%), organic online information (18%), and the seed package (23%). Furthermore, it was found that only 55% of growers surveyed follow recommendations of storage temperature practices of 40 F and 26% follow the most highly recommended application practices, of using a sticky agent other than water with solid inoculant. Unexpectedly, new growers (less than three years of experience) who likely have the greatest need for inoculation, regularly inoculate the least. From these observations it was concluded that in a majority of cases growers do not follow the practice that is most highly recommended for inoculation success. Proper practices could be increased and implemented with greater outreach to growers through workshops and publications. Results were presented at one national (Soil Science Society of America annual meetings) and one regional conference (UNC undergraduate research symposium) in 2012.

Objective 2: Fall of 2010 we 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. Legume biomass, harvested in the spring of 2011 and was analyzed throughout 2012. Cover crop root nodules from all farms and cover crop varieties were harvested to analyze nodule occupancy. Five-hundred-fifty nodules were used total – half from inoculated treatments, and half from nodules only exposed to native soil populations. When the DNA of nodules of inoculated and un-inoculated plants were analyzed, about 3% or less of the inoculated plants were shown to contain DNA similar to the commercial inoculant strain, suggesting that native rhizobia may be out-competing the inoculant for nodule occupancy. In general, the intra-species diversity was high. In one species of rhizobia, 54 different DNA fingerprint profiles were identified. Many fingerprints were found on all 3 farms. The Kinston site had very low initial rhizobia populations (less than 10 cells per gram of soil), and was also shown to have the majority of the rhizobia population to contain one or two of the DNA fingerprints. Future work will include isolating representative rhizobia strains from the identified DNA fingerprints, and sequencing their functional regions of DNA, possibly seeing differences in the rhizobia between soil types, host plants, or inoculants.

Objective 3: In October 2011, experimental plots were established at the Center for Environmental Farming Systems (CEFS) in Goldsboro, NC, grown throughout the winter, and terminated in Spring 2012 to investigate impact of termination method on N mineralization. In this study, four leguminous winter cover crop species, Austrian winter pea, hairy vetch, and balansa and crimson clovers, were terminated using a roller-crimper, flail mower, disk, or an herbicide. Bi-weekly inorganic soil tests and Plant Root Simulator ion resin probes were used to measure plant available NO3- and NH4+. Cover crop biomass, total carbon, and total nitrogen were measured for each species prior to termination. Second year field plots (2012) were also assessed for the following arbuscular mycorrhizal fungi (AMF) responses: 1) AMF species diversity in spring, prior to cover crop termination; 2) AMF root colonization of each cover crop prior to cover crop termination; and 3) AMF root colonization of corn, 3 weeks after cover crop termination. Extra radical mycelium (ERM) quantification of soil collected pre- and post cover crop termination from 2011 and 2012 field plots is in process. Fifteen AMF species were found in the second year field plots: Acaulospora laevis, A. scrobiculata, Glomus clarum, Gl. etunicatum, Gl. intraradices, Gl. mosseae, Gl.sinuosum, Gigaspora gigantea, Gi. roseae, Gi. margarita, Scutellospora fulgida, Sc. heterogama, Sc. gregaria, Sc. pellucida and Sc. reticulata. Austrian winter pea, hairy cetch and crimson clover all supported higher AMF colonization than balansa clover. 3) The same trend was evident in 3 week corn colonization, with enhanced AMF colonization following hairy vetch and suppressed AMF colonization following balansa clover. Lastly, AMF colonization of 3 week corn was significantly lower in the Disk/ Till treatment than in the other cover crop termination treatments.

Objective 4: Funded in-part by this award, we organized two workshops at the Southern Sustainable Agriculture Working Group meetings in Little Rock, AK describing advantages of cover crops, how to estimate fertility impact on cash crops, and presenting data from our two-years of cover crop termination method trials. Students in Dr. Julie Grossman’s Soil Agroecology course designed three 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 sites where the units were taught included 1) a manufactured home community in Raleigh (i.e. trailer park) with predominantly African American and Hispanic/Latino residents and 2) Young Farmer Training Program, supported by our community partner, designed for lower income participants 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: Results from this objective indicated that awareness of effective inoculant use and handling procedures among Southeast organic growers may need to be increased. Although our study represents only those who participated in the survey, results indicated a lack of proper handling procedures and care for rhizobia inoculants that contained live cultures. This could possibly result in decreased nitrogen fixation resulting from active symbiosis in crops were improperly handled inoculants are applied to legume seeds at planting.

Objective 2: Cover crop production on our three demonstration organic farms ranged from 2,300 – 5,800 lbs of biomass per acre, with lbs of N ranging from 72-183 lbs N per acre. There was no correlation between soil characteristics, including organic matter, soil pH, or naturalized soil rhizobia population size, to effect of inoculation on plant biomass. Despite no observation of an inoculation effect on cover crop production, inoculation significantly increased biomass and plant nitrogen contribution at one farm site (Clayton), for both Austrian winter pea and hairy vetch, by 21-23% above un-inoculated plants. Crimson clover, however, at Clayton had significantly more cover crop biomass production when plants were exposed only to native soil rhizobia, than when they were inoculated, suggesting that the strains of native Rhizobium leguminosarum bv trifolii native to North Carolina, could be better suited to growing conditions than commercially available strains and should continue to be investigated. Native strains of soil rhizobia were shown to, in many cases, outcompete the inoculants added at cover crop planting. This indicates that in some cases less effective or efficient N-fixing strains may be responsible for the N fixed in many of North Carolina’s popular cover crop species. Such a phenomenon demands further analysis in order to determine relative N-fixation capacity of native strains occupying cover crop nodules.

Objective 3: 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. Over the two years of our study, mineralized nitrogen was most available from Austrian winter pea and hairy vetch across all termination treatments at six to ten weeks after kill. Disked hairy vetch contributed the greatest plant available nitrogen amongst all 16 combinations. Biomass contributions of cover crops ranged from 2.4 Mg ha-1 to 9.7 Mg ha-1 in balansa clover and crimson clover, respectively. Hairy vetch had the greatest overall average biomass nitrogen of 226.4 mg kg-1, while Austrian winter pea grew 188.71 kg ha-1 and crimson clover averaged 181.1 kg ha-1. We have learned that species differ in their N mineralization release rates, with hairy vetch contributing the most N at six-ten weeks after termination.

Results of this objective have resulted in one Masters thesis (Matt Brown), currently being prepared for publication in a peer-reviewed journal, as well as three abstracts presented at national or regional conferences:

1. Brown, M. 2012. Evaluating termination methods of four winter annual leguminous cover crops for optimizing nitrogen synchrony. Masters Thesis at North Carolina State University.
2. Brown, M., Grossman, J. Horton, S.C. and Shi, W. The influence of termination techniques on plant available nitrogen mineralized from winter annual leguminous cover crops. Soil Science Society of America Annual Meeting. October 22-24th 2012, Cincinnati, OH.
3. Brown, M., Grossman, J. Shi, W., Reberg-Horton, S.C. Evaluating termination methods of leguminous cover crops for optimizing nitrogen synchrony, Ecological Society of America Conference, 97th Annual, August 5th-10th, Portland, OR.
4. Brown, M., Grossman, J.M., Reberg-Horton, C. and Shi, W. Evaluating Termination Methods on Leguminous Winter Cover Crops for Optimizing Nitrogen Synchrony in Corn. Annual Meeting of Soil Science Society of North Carolina. January 17-18, 2012. Raleigh, NC.

Objective 4: The SARE-funded workshops held this past year reached over 195 individuals, many of them small organic farmers in the Southeast. Over 80% of attendees said they learned something useful, and almost 67% said they would absolutely use the information they learned in the next year. Specific comments included appreciation for calculations on how to determine N contribution from cover crops, as well as for the scientific results from SARE-funded field trials.

A total of 16 local youth participated in the lessons developed through this grant in Y2 and three lesson plans were developed and returned to the IFFS for use in their gardening education program in low-income neighborhoods. In Fall 2012, evaluation efforts were expanded to collect data on community youth’s learning gains, as well to learn how the lessons conducted by NCSU students are helping to increase community youth interest in agriculture and soil science. Preliminary results show that 75-100% of the participating community youth now know more about the important functions of soils in our daily lives; and 75% of participant youth feel they have greater knowledge about career paths related to agricultural fields. All but one of the participating youth described an interest in teaching others what they had learned through the lessons. One youth stated that he is excited about taking further courses in agriculture because of what he learned from NC State students regarding environmental problems, pushing him to “become more aware of the environment.” Former NCSU students also chose career paths related to teaching within under-served and low-income communities. At least two former students are now teaching science in rural areas with the Teach for America program, and several more have indicated interest in working with low-income youth and agricultural education in their future. In a letter to Dr. Grossman, one student stated “I have been labeled as the token ‘Ag Kid’ in my Teach for America cohort among my humanity-urbanite colleagues. I have you to thank for that and so much more. My college career and life trajectory changed so much because of the relationship I had with your [community engagement program].”

Three scholarly manuscripts are either under review or published based in-part on results of this SARE grant:

1. Smith, S. Grossman, J.M., Bradley, L., Hesterberg, D. Preparing students for a diverse future: Using service-learning to train students for careers in agricultural community outreach, In Review, HortTech.
2. Niewolny, K.L., Grossman, J.M., Byker, C., Helms, J. Clark, S.F., Cotton, J., Jacobson, K.L. 2012. Sustainable Agriculture Education and Civic Engagement: The Significance of Community-University Partnerships in the New Agricultural Paradigm. Journal of Agriculture, Food Systems and Community Development, May, 2012, pp. 27–42.
3. Grossman, J. Sherard, M., Prohn, S., Bradley, L., Goodell, S., Andrew, K. 2012. An Exploratory Analysis of Student-Community Interactions in Urban Agriculture, Journal of Higher Education Outreach and Engagement, 16(2):179-196.

Four presentations were delivered on this aspect of the project, with one including two of the PI’s (Grossman and Schroeder-Moreno):
1. Grossman, J. Service-learning in agriculture. Invited talk at Pennsylvania State University, January 18, 2013, Oral.
2. Grossman, J., Smith, S., Schroeder-Moreno, M. Laying the groundwork for soil science education through urban agriculture service-learning (Invited). Ecological Society of America Conference, 97th Annual, August 5th-10th, Oral.
3. Smith, S, Prohn, S, Grossman, J. Preparing Students for a diverse future: designing and evaluating a cultural competency training program for community engagement in agriculture. Graduate Student Research Symposium, NCSU. March 20, 2012
4. Grossman, J., Smith, S., Bradley, L. and Driscoll, L. 2012. Teaching soil science through community gardens (Invited). Annual Meeting of Soil Science Society of North Carolina. January 17-18, 2012. Raleigh, NC, Oral.


Dr. Michelle Schroeder-Moreno
Assistant Professor
North Carolina State University
2406 Williams Hall Campus Box 7620
Raleigh, NC 27695
Office Phone: 9195130085
Dr. Wei Shi
Associate Professor
North Carolina State University
4302 Williams Hall, Campus Box 7619
Raleigh, NC 27695
Office Phone: 9195134641
Dr. Sarah Bowen
Assistant Professor
North Carolina State University
1911 Bldg 362
Raleigh, NC 27695
Office Phone: 9195150452
Dr. Amy Germuth
150 Solterra Way
Durham, NC 27705
Office Phone: 9194015403