Effects of cold temperatures on symbiotic nitrogen fixation in winter annual legume cover crops

Authors: THURSTON, CHARLOTTE - University Of Minnesota; GROSSMAN, JULIE - University Of Minnesota; FUDGE, REBBECA - University Of Minnesota; Mirsky, Steven; WIERING, NICHOLAS - University Of Minnesota; Maul, Jude

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/19/2022
Publication Date: N/A
Citation: N/A

Interpretive Summary: There are economic and environmental benefits for growers that rely on symbiotic nitrogen fixation (SNF) from winter annual legume cover crops to reduce the need for spring fertilization and build soil health. However, cold fall temperatures in the northern regions of the country potentially limit root growth and symbiotic bacterial establishment, reducing beneficial nitrogen balance contributions. This study aimed to examine effects of cold temperatures on nodulation and SNF in legume cover crops. The results indicate the lower limits of nitrogen fixation in three common cover crops, Hairy vetch (Vicia villosa), Austrian winter pea (Pisum sativa) and Crimson clover (Trifolium incarnatum) were different among species and all species failed to establish symbiotic relationships with bacteria below an ambient temperature of 10°C. This report is a first step in developing cover crop functional information to be used by farming system nutrient management modelers to estimate total cover crop nitrogen contribution annually.

Technical Abstract: Upper Midwest growers rely on symbiotic nitrogen fixation (SNF) in winter annual legume cover crops to reduce the need for spring fertilization and build soils. However, cold transitional fall temperatures potentially limit SNF, reducing soil contributions. This study aimed to examine effects of cold temperatures on nodulation and SNF in legume cover crops. Hairy vetch (Vicia villosa), Austrian winter pea (Pisum sativa) and Crimson clover (Trifolium incarnatum) were sown in growth pouches, inoculated with rhizobia, and placed in growth chambers set at 20°C, 15°C, 10°C or 5°C. After six weeks, plants were harvested, nodules counted, and nodules, roots and shoots dried and weighed. Shoots were analyzed for total carbon and nitrogen, and SNF quantified using the N-difference approach. Temperatures lower than fall average negatively affected biomass and nodule number. Coldest temperatures reduced SNF, with mean nitrogen fixation estimates decreasing ten-fold from 20°C to 5°C. Estimated SNF was greatest at 15°C, and did not occur at 5°C. Winter annual legume cover crops may not experience reduced growth, nodulation, or SNF until temperatures fall below 10', lower than previously reported for cover crop legumes. Findings support hairy vetch as a hardy winter cover crop for cold regions. This report is a first step in developing cover crop functional information to be used by farming system nutrient management modlers to estimate total cover crop nitrogen contribution annually.