Simulated Glyphosate Drift Influences Nitrate Assimilation and Nitrogen Fixation in Non-glyphosate-Resistant Soybean

Nontarget injury from glyphosate drift is a concern among growers using non-glyphosate-resistant (non-GR) cultivars. The effects of glyphosate drift on nitrate assimilation and nitrogen fixation potential, nodule mass, and yield of non-GR soybean were assessed in a field trial at Stoneville, MS. A non-GR soybean cultivar ‘Delta Pine 4748S' was treated with glyphosate at 12.5% of use rate of 0.84 kg of active ingredient/ha at 3 (V2), 6 (V7), and 8 (R2, full bloom) weeks after planting (WAP) soybean to simulate glyphosate drift. Untreated soybean was used as a control. Soybeans were sampled weekly for 2 weeks after each glyphosate treatment to assess nitrate assimilation and N₂ fixation potential. Nitrate assimilation was assessed using in vivo nitrate reductase assay in leaves, stems, roots, and nodules. Nitrogen fixation potential was assessed by measuring nitrogenase activity using the acetylene reduction assay (ARA). Nitrogen content of leaves, shoots, and seed and soybean yield were also determined. In the first sampling date (4 WAP), glyphosate drift caused a significant decrease in NRA in leaves (60%), stems (77%), and nodules (50%), with no decrease in roots. At later growth stages, NRA in leaves was more sensitive to glyphosate drift than stems and roots. Nitrogenase activity was reduced 36−58% by glyphosate treatment at 3 or 6 WAP. However, glyphosate treatment at 8 WAP had no effect on nitrogenase activity. Nitrogen content was affected by glyphosate application only in shoots after the first application. No yield, seed nitrogen, protein, or oil concentration differences were detected. These results suggest that nitrate assimilation and nitrogen fixation potential were significantly reduced by glyphosate drift, with the greatest sensitivity early in vegetative growth. Soybean has the ability to recover from the physiological stress caused by glyphosate drift.

Keywords: Acetylene reduction assay; glyphosate; nitrate assimilation; nitrate reductase activity; nitrogen fixation; nitrogenase activity