Glyphosate- and imazethapyr-induced effects on yield, nodule mass and biological nitrogen fixation in field-grown glyphosate-resistant soybean

Over half of the 21 Mha of soybean planted in Brazil is now transgenic glyphosate-resistant (GM_RR). A field experiment was carried out to investigate whether the application of glyphosate or imazethapyr to the GM_RR variety reduced the input of N₂ fixation (BNF). No effects on yield, total N accumulation, nodulation and BNF (δ¹⁵N) could be assigned to the genetic modification of the plant. Imazethapyr reduced soybean yield but had no significant effect on BNF. Even though yields were not affected by glyphosate, the significant reduction of nodule mass and BNF to the GM_RR suggests that the use of this herbicide could lead to an increased dependence on soil N and consequently an eventual decrease of SOM reserves.