Lighting up the black box: Improving legume performance on organic farms by optimizing microbially-mediated plant and soil nitrogen cycling processes.
A field experiment was established to determine how four different cover crop termination techniques impact microbial activity and nitrogen (N) delivery of cover crops, including hairy vetch, crimson clover, balansa clover, and Austrian winter pea. Additionally, three demonstration plots were established across the state to teach farmers about nitrogen fixation, and the role of bacteria in improving N-delivery, through a SARE-funded workshop. An inoculation survey was designed and 80 farmer participants gave us 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.
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.
Funding for this project was received July, 2010. In Fall 2010, an undergraduate researcher (Erika Larsen) designed a 5-page paper and internet-based survey in collaboration with Dr. Sarah Bowen in NCSU’s Rural Sociology department. The survey contained questions related to legume inoculant use and handling procedures, sources of information on legume inoculation, legume nodulation, and perceived benefits of leguminous cover crops. It was promoted to over 400 farmers, resulting in 82 responses. We surveyed growers at two regional conferences: the Carolina Farm Stewardship Sustainable Agriculture Conference in Winston-Salem, NC and at the Organic Growers School in Asheville, NC, and through the Internet. The project was explained orally to participants in both conferences and Southeast SARE acknowledged. Following the conferences, we emailed approximately 200 growers in the southeast and directed them to our internet-based survey. 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. Statistical analysis of the results is currently underway. We plan to present our findings to at least one national and one regional agriculture conference in 2012. Upon presentation of the results, we expect that organic farmers throughout the Southeast will gain a better understanding of patterns of effective inoculant use and leguminous cover crop management practices that allow for increased nitrogen availability.
We have established three demonstration plots on organic land in North Carolina. The inoculation trials are located on 3 organic farms (2 private, and one state research) in Johnston, Lenoir, and Rowan counties of North Carolina. The organic farms were chosen based on differences in soil type (Lower Coastal Plain, Upper Coastal Plain, and Piedmont respectively), limited field history of legume cover crops, and general farmer interest in cover cropping. Sites were disked in the fall and planted in October 2010 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. Fields were planted using 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. Plots are 15 ft by 120 ft to accommodate a grain drill that is split in half, creating one inoculated subplot 7.5ft x 120ft and another subplot uninoculated. Additionally, the inoculated subplot is divided again, and one half will be fertilized with Microstart ® organic pellets, as a positive control. Commercial rhizobia inoculants are N-Dure, by INTX microbials – for vetches and winter pea and R/ WR, by Nitragin – for crimson clover. Plants were inoculated according to manufacturers’ recommendations. Plots were replicated 4 times at each site. Cover crops were allowed to overwinter and data will be collected in the spring of 2011.
Since it has been shown that the legume response to inoculation is inversely correlated with the amount of native soil rhizobia, we selected fields that had little inoculation history. We will carry out a technique called Most Probable Number (MPN) on uninoculated soils from the 3 field sites to quantify native rhizobia that may be competing with inoculant. Soil samples were taken across the entire field at time before cover crop planting, estimating native rhizobia present in the soil from each legume variety. The same soil samples from the 3 farms were sent to the NCDA for laboratory soil tests and a KCl extraction was done for inorganic nitrogen.
In October 2010, 144 experimental plots were established at the Caswell Research Station in Kinston, N.C on a sandy loam soil. Baseline soil tests were taken at the time and analyzed by the North Carolina Department of Agriculture (NCDA). Based on the recommendations by the NCDA, 1000 lbs/acre of lime was applied to correct for a slightly low pH for the upcoming corn crop to be planted this May. We are investigating growth, productivity, and N contribution of four cover crops, one positive control of 150 lbs/acre of nitrogen fertilizer, and one negative control on bare ground where nothing is planted or fertilized. The four cover crops are crimson clover, hairy vetch, Austrian winter pea, and balansa clover. These four cover crop treatments and the positive and negative control will be subjected to four different kill methods at the end of April, 2011. These treatments include 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. By the end of 2010 we observed typical winter growth of the cover crop, however we expect to see the bulk of the biomass accumulate as the climate warms up and day length increases. The same should hold true for biological nitrogen fixation, with the maximum accruing at the time of the kill in the last week in April.
At the time of kill, we intend to take biomass measurements of each cover crop; commencing a detailed monitoring program that will quantify not only the rates of decomposition of the unique biomass of each legume, but also measure the rate of mineralization of nitrogen. This will be accomplished through four different methods identified by the literature to be the most effective in doing so. These include: Ion resin exchange probes that mimic the nitrate and ammonia absorption characteristics of plant roots; inorganic soil nitrogen tests of both ammonia and nitrate; litter decomposition bags, where a known quantity of biomass is placed in a small mesh sack and subjected to in-situ decomposition variables. The litterbags are removed over time and reweighed to quantify decomposition rates and nitrogen mineralized. The final method includes assessing microbial biomass as a function of time. This is to identify potential nitrogen immobilization and remobilization with precision. These measurements start on the day of the kill and continue for the following three months. About two weeks after the kill date, we intend to plant corn (Doeblers N631).
We have successfully recruited and hired a graduate student, Mr. Matthew Brown, to design and carry out this work. Matthew comes to us from New York City, where he worked as a soil scientist in Central Park carrying out traditional soil analyses in the Park’s laboratory, and designing educational programming for city youth. We feel he will be a great asset to the research and educational aspects of this project.
Mycorrhizal analyses have also begun on the plots. Most of the work to-date has included optimization of the methodology for the mycorrhizal hyphal extraction and quantification. Trap cultures have been established using soil from each plot in order to extract mycorrhizal hyphae and learn the best and most efficient way to extract them. This work has proven to be challenging, primarily because extracting identifiable mycorrhizal hyphae and distinguishing them from other hyphae can be difficult. Another sampling will take place in coordination with cover crop sampling in April in order to determine mycorrhizal diversity at time of cover crop termination.
A project has been designed in conjunction with Dr. Julie Grossman’s Soil Agroecology course in which enrolled students will design and teach lessons on cover crop establishment, management, nitrogen fixation, and soil ecology to low-income youth in the Raleigh area over the duration of the SARE project. The project has been designed with our community partner, the Inter-Faith Food Shuttle (IFFS), who manages a series of community gardens in low-income neighborhoods in Raleigh with the goal of helping to increase food security.
Impacts and Contributions/Outcomes
The surveys will be analyzed in the coming months. Once we receive statistical data from the surveys, we will be able to analyze the results and present our findings. We plan on presenting a poster at various conferences in the southeast in the Summer and Fall of 2011. Participants at two regional conferences (Organic Growers School in Asheville, NC and Carolina Farm Stewardship Association meetings in Winston Salem, NC) were made aware of project goals and support from SE SARE.
So far this project has introduced 3 farmers to planting cover crops using grain drills that they readily have on their farm. By adjusting the calibration of the drill to accommodate a variety of seed sizes and seeding depths, we demonstrated that equipment farmers already use and have available can be used in cover cropping techniques, no matter the seed size or type.
A large part of this objective is to tackle whether or not inoculant bacteria get into the nodule, but possibly of equal or more interest to the farmer, is which cover crops will grow best on their farm. We planted 4 different 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. 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.
In the spring of 2011, we will be using one of the demonstration farms for a SE-SARE funded workshop to describe advantages of cover crops, how to estimate their fertility impact on cash crops, as well as have a visual example of what happens when you do or do not inoculate these legumes. It will be a hands-on workshop experience for farmers to come ask questions, identify possible varieties that would work for their specific farming application, and get a feel for another source of organic fertility at the time of year when spring cover crops are at their peak. Growers will be able to see 4 cover crop varieties at full maturity.
The outcome of this objective 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 while not leaching from the soil due to it not being taken up. Currently the only time-release fertilizers that can accomplish this task are extremely costly and not permissible for use by certified organic farms. Biologically fixed nitrogen has great potential to replace this with the only cost being seed and fuel.
Two soil science lessons were developed and taught by NCSU undergraduate students in 2010 and are now in the hands of our community partner for use in their community gardening educational programming with low-income Raleigh youth. The two lessons that were developed are: 1) Soil Erosion and 2) Roots. A total of 25 local youth participated in the lessons, 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. Further, two scholarly presentations were delivered on this aspect of the project including two of the PI’s (Grossman and Schroeder-Moreno):
Grossman, J., Schroeder-Moreno, M., and Prohn, S. Assessing Student Learning Gains in Community Gardening Service-Learning Initiatives. National Outreach Scholarship Conference, Raleigh, NC. Oct 4-6, 2010.
Sherard, M. Prohn, S. Grossman, J. Into the Garden: Assessing a garden-based service-learning project in inner-city Raleigh, NC. National Outreach Scholarship Conference, Raleigh, NC. Oct 4-6, 2010.
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