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 the need for nitrogen fertilizer. This strategy depends on a symbiotic relationship between the legume and nitrogen-fixing Rhizobium so biological nitrogen fixation can occur. Under certain conditions, this symbiosis fails to form despite legume inoculation with Rhizobia. This was evident in a large winter cover crop trial on an organic farm in the Rio Grande Valley of deep south Texas, where three legume species were planted with little evidence of nodulation and nitrogen fixation.

This study seeks to explain this failure in nodulation and to assess the viability of interventions that could encourage nodulation and nitrogen fixation. Research suggest that BNF is highly sensitive to water stress and requires high levels of bioavailable phosphorus for successful symbiosis. Since the Rio Grande Valley is a semi-arid region with calcareous soils, moisture, phosphorus, and micronutrients are suspects in the nodulation failure and will be investigated in a controlled greenhouse study. In addition, legumes can form a tripartite symbiosis with both Rhizobia and arbuscular mycorrhizal fungi. Evidence suggests that AMF symbiosis can enhance water and nutrient uptake and improve rates of nitrogen fixation. AMF inoculation will be tested in the greenhouse study as an intervention, and, if promising, replicated under field conditions. Lessons learned in this study will help improve the quality of information on nitrogen fixation by cover crops available to farmers in the region, so they can make more informed decisions about their cover cropping practices.

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.