Potentially harmful effects of seed treatment and pre-inoculation on soybean biological nitrogen fixation and yield

Biological nitrogen fixation (BNF) is a crucial process for the successful development of soybean plants. Nevertheless, many management decisions can affect the symbiosis between the bacteria and the plant, impacting soybean growth and yield. The use of pesticides in soybean seed treatment and the application of Bradyrhizobium sp. inoculant to the seeds a few days or even weeks before sowing are two management practices that can have a detrimental effect on the BNF process and always raise concerns. To shed light on the potential impact of these two management practices, the objective of this study was to evaluate under laboratory, greenhouse, and field conditions the effects of pre-inoculating soybean seeds with Bradyrhizobium sp. for up to 30 days prior to sowing with and without common pesticides used for seed treatment. One laboratory, one greenhouse, and six field experiments were conducted from 2016 to 2019. Pre-inoculation time (3 h and 30 days before sowing) and pesticides seed treatment (control - without pesticide application; pyraclostrobin + thiophanate-methyl + fipronil; thiabendazole + fludioxonil + metalaxyl-M; and carbendazim + thiram) were the two fixed effects controlled by the experimental design. Results from laboratory and greenhouse trials showed that pre-inoculation and pesticide seed treatment can negatively affect the recovery of colony-forming units of Bradyrhizobium elkanii inoculated to the seed, ureides concentration in plant shoots, BNF efficiency, and plant growth. Pooled analysis of the six field experiments demonstrated that although none of the BNF variables assessed were affected by pre-inoculation or pesticide seed treatment compared to the control, thiabendazole + fludioxonil + metalaxyl-M caused significant yield loss, whereas the weight of thousand grains for inoculation 3 h before sowing was significantly higher than inoculation 30 days before sowing.