Investigating Controls Over Nodulation and Nitrogen Fixation in Leguminous Cover Crops in Subtropical South Texas

View the project final report

Commodities

Practices

• Animal Production: inoculants
• Crop Production: cover crops, fertilizers, nutrient management

Many farms use leguminous cover crops as a nutrient management strategy to reduce their need for nitrogen fertilizer. When they are effective, leguminous cover crops are a valuable tool for sustainable nutrient management. However, the symbiotic partnership between legumes and nitrogen fixing rhizobia is vulnerable to several abiotic and biotic stressors that reduce nitrogen fixation efficiency in real world contexts. Sometimes, despite inoculation with rhizobial strains, this symbiosis fails to form. Such failure was observed in a 14-acre winter cover crop trial in the Rio Grande Valley (RGV) of Texas when three legume species produced no signs of nodulation or nitrogen fixation. This study examined the role of nitrogen, phosphorus, moisture, micronutrients and native microbial communities in the nodulation of Vigna unguiculata and assessed arbuscular mycorrhizal fungi as an intervention to improve nodulation. Results from two controlled studies confirm moisture and native microbial communities as major factors in nodulation success. Micronutrients showed mixed impacts on nodulation depending on plant stress conditions. Nitrogen and phosphorus deficiencies, however, were not likely causes, nor was mycorrhizal inoculation an effective intervention to improve nodulation. Inoculation method also had a major impact on nodulation rates. Continued research on improved inoculation practices and other ways to maximize nitrogen fixation efficiency will be required to increase successful on-farm implementation. The lessons learned in this project were shared widely with the south Texas agricultural community through 3 field days, 4 conference presentations, an educational video and online nodulation troubleshooting tool, and one peer-reviewed publication. 

Objective 1: Determine the likely cause(s) of legume nodulation failure in the cover crop trials at Hilltop Gardens in Lyford, TX.

Objective 2: Assess the potential of inoculation with arbuscular mycorrhizal fungi to enhance nodulation and N fixation by buffering the legume-Rhizobium symbiosis from these three stressors: water stress, phosphorus availability, and micronutrient deficiencies.

Objective 3: If preliminary greenhouse data shows increased nodulation and N fixation, incorporate inoculation with mycorrhizae into winter 2018-2019 cover crop trials at Hilltop Gardens to determine the utility of this intervention under field conditions.

Objective 4: Disseminate project findings so that producers in this area and elsewhere can make better informed decisions about cover cropping as part of their farm management strategies.