Soybean yield response toBradyrhizobiumstrains in fields with inoculation history in Southern Brazil

Soybean [Glycine max (L.) Merr.] responses to seed inoculation with rhizobium are inconsistent in sites with inoculation history. Field trials were conducted in South-central region of Paraná State (Southern Brazil) aiming to evaluate yield response to inoculation in areas with history of inoculation at a regional-scale (21 trials, 1999/2000 to 2014/2015), and identify nodulation, plant dry weight and nitrogen (N) components underpinning yield formation (10 trials, 2015/2016 and 2016/2017). Seed yield varied from 1853 to 5352 kg ha−1 (first dataset), characterizing a wide range of environmental conditions across seasons. Response to inoculation was inconsistent at regional-scale, with a similar seed yield of inoculated (3286 kg ha−1) and non-inoculated (3298 kg ha−1) soybean. Lack of differences were also observed in all the variables analyzed of second dataset. Research efforts should be applied aiming to identify rhizobia persistence in the soil and its efficacy at N fixation after continuous cropping without inoculation.