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

The purpose of this project is to improve legume cover crop management by examining key soil microbial processes that regulate nitrogen (N) cycling in organic agriculture. In parallel, we seek to improve stakeholder knowledge of soil microbial processes by developing an integrated educational and outreach program involving farmers and soil ecology students. Organic grain production is an emerging market in the Southern region of the U.S. and many questions exist about how to increase efficiency of N delivery to crops from organic residues. This project will strengthen organic production in the long term by providing growers with information about how to best manage legumes to encourage beneficial microbial interactions, and how to manage cover crop residue for optimal N release in Southern climates. Short-term project impacts include improved farmer decision making about legume management, and improved student understanding of nutrient management challenges facing organic growers.

All farming systems, organic and conventional, require nutrients to be made available to crop plants at critical points in the plants lifecycle. Soluble inorganic fertilizers used by conventional growers are made available immedi­ately to plant roots, so nutrient application is timed to coincide with crop uptake need. In contrast, organic farmers rely on cover crops and other organic nutrient sources that require microbial activity to make them available to crop plants. It is critical that we know how to design agroecosystems where nutrient availability from certified organic sources is maximized, yet we are only just beginning to understand how organic management practices influence soil nutrient cycling and availability. 

Organic growers cite nutrient management as one of their most serious and intractable problems. Nitrogen management in particular is a critical, and often a costly, practice for organic growers. Long term USDA organic cropping experiments have shown that supplying adequate N for corn is the biggest challenge to achieving equivalent yields between organic and conventional cropping systems (Cavagelli et al., 2008). Nitrogen is often the most limiting nutrient in organic systems (Gaskell and Smith, 2007), while at the same time inefficient N management can lead to water body contamination and widespread environmental degradation (Mitsch, et al., 2002). As legume based systems have reduced carbon and N losses (Drinkwater et. al, 1998), efforts to maximize the efficiency of legume N delivery in organic systems at the time of greatest crop need (Crews et al., 2005) is in desperate need of study.

Growers with whom we work would appreciate additional information about two key microbially-driven processes they utilize on their farms, biological nitrogen fixation (BNF) and mineralization of legume residues. One common management practice in organic farms, as well as on increasing numbers of conventional farms due to costly synthetic N fertilizers, includes the use of legume cover crops. Legume cover crops can increase plant-available nitrogen through their symbiotic association with specific nodule forming, di-nitrogen fixing bacteria, or rhizobia, housed in legume root nodules.  Through the process of BNF, legumes associated with the appropriate root nodule bacteria can improve the nitrogen economy of an agricultural system and in some cases eliminate or reduce the need for supplemental N fertilization from manures or other organic sources. Biological nitrogen fixation is a major source of new nitrogen entering organic farming systems. Growers are eager to understand the functioning of the belowground soil community responsible for making nitrogen available to crops through cover crop management, including rhizobia ecological interactions, mycorrhizal fungi associations, and microbial mineralization of legume residue. Understanding these processes will help farmers better manage their legume cover crops for improved N cycling and availability.

Highly efficient strains (types) of rhizobia are often added with legume seeds at planting, a process called inoculation. While most organic growers inoculate their cover crop legumes with a recommended rhizobia inoculant, little is known about the ecology and the efficiency of rhizobia in fields under organic management. In many places where legumes are cultivated, indigenous or native populations of rhizobia are also plentiful in the soil. In some regions these native rhizobia can be beneficial as they may allow for improved legume production without inoculation. In contrast, soils containing an abundance of native rhizobia can be problematic if they are competitive with the strain introduced through inoculation and prove to be less efficient in fixing nitrogen than the inoculant strain (Obaton et al., 2002; McDermott and Graham, 1990)

Recent research results indicate that rhizobia ecology of legume cover crops is quite complex. Previous work conducted by Dr. Grossman at Cornell University found greater diversity in nodule occupancy among organic farm-grown legumes than conventional, even across sites where legumes received the same inoculant strain formulation. This calls into question the origin of the rhizobia diversity observed in the nodules; were the bacteria from the inoculant or resident soil populations? Here at NC State we have made some headway in answering this question with regards to cover crop nodules. Our preliminary results are indicating nodules of hairy vetch are more likely to contain native rhizobia strains rather than the applied inoculants, leading us to question the competitive ability of background rhizobia already present in soils where legumes have been planted for many years. Often organic growers have been using various legume species for many growing seasons, possibly increasing the number of background rhizobia able to compete with the introduced inoculant. Nodule formation by competitive resident soil rhizobia may result in reduced overall legume N contribution to the organic system if that strain is shown to be less efficient in N-fixation. As part of this project we will investigate rhizobia ecology and assess diversity of rhizobia within and among organic farm soils. Since organic farmers often use a variety of legumes in rotation, we hypothesize that native populations of rhizobia present at planting will affect nodulation and nodule occupancy in desired legume crops.

 Along with the ecological interactions of rhizobia in farm soils, the efficacy of the inoculants themselves is also of critical importance. There appears to be great variation in how farmers are handling and using inoculants, including practices that damage rhizobia and possibly render them useless. In conversations with organic grain growers at workshops in North Carolina we were told that some growers inoculate at planting, others do not, and most have a wide range of inoculant application and storage techniques. As inoculant contains live rhizobia, its effectiveness is based on the number of viable organisms present in the product at legume planting. Recent research carried out in the Grossman laboratory demonstrated that numbers of viable rhizobia in a random sample of commercial cover crop inoculants ranged from 4-68% of the commercially guaranteed minimum levels (Ballard and Grossman, 2008). These low numbers will limit the numbers of viable rhizobia entering the soil and reduce overall nodulation and nitrogen fixation from the inoculant strain. We would like to understand how farmers are handling inoculants so that we can improve the viability and success of rhizobia inoculation.

Further, organic growers have a wide range of legume cover crop choices to include in their farming system. As researchers at NC State, we are contacted on a regular basis by growers questioning which legume species might be their best options to provide plenty of plant biomass and fixed N to their cropping systems. We suspect that farmer decision-making regarding cover crop choice is based on a combination of economic and production data, as well as experience and information-sharing between growers. Using a rigorous survey of organic growers in North and South Carolina, we will assess current inoculation use and handling techniques, as well as perceived challenges to and benefits of using different species of winter annual legume cover crops.

In order for organic growers to fully benefit from legumes in their organic systems, it is critical that we understand many distinct pieces of the ‘N-input puzzle’, including the nodule occupants and their efficiency and the timing of N availability under various cover crop management practices. Growers are ultimately concerned with the total N that is made available for crop growth, either from soil organic matter or cover crop decomposition, so that N availability is synchronous with crop demand. There is a wealth of previous research demonstrating the utility of winter legume cover crop in increasing N availability to crops in the Southeast (i.e. Wagger, 1989; Rannels and Wagger, 1997a; 1997b). However, recent advances in cover crop breeding (Devine, 2008) have raised important questions about the performance of early-flowering cover crop populations, especially in the context of organic agriculture.  

Legume termination (kill) methods vary by farmer objectives and implement availability, but widely used practices include various combinations of mowing the cover crop, soil incorporation, and rolling/crimping using a large water-filled drum. Rolling and mowing leave residues on the soil surface where they are in less contact with decomposing soil organisms, while incorporation increases the decomposer-residue contact. Mowing and tillage intensity have been shown to increase N mineralization rate in hairy vetch (Snapp and Borden, 2005; Drinkwater et al., 2000), however we have found no studies comparing the 3 common termination practices mentioned above, especially in the absence of glyphosate treatment. Further, we predict that the warmer climes in the Southeast will impact both cover crop production and mineralization rates as microbes are known to ‘speed up’ their metabolic activities in warmer conditions and likely decompose residues at a much faster rate.  We will investigate how non-chemical legume kill method (rolling-crimping, flail mowing, and incorporation) impact N availability from decomposing legume residues. Since methods of legume termination place the killed cover crop at different points of contact with soil microorganisms (surface vs. in soil), we hypothesize that mineralization rates will be influenced.