Impact of microbial consortia on organic maize in a temperate climate varies with environment but not with fertilization

Inoculating crops with specifically designed microbial consortia (MC) is increasingly discussed as an instrument for sustainably promoting arable crop growth. While some promising results were achieved in laboratory, climate chamber, and greenhouse experiments, field applications of microbial inoculants under temperate climate conditions have been much less investigated. Here, we assess the effect of three novel (MC_B, MC_C, MC_C_AMF) and one commercial (Micosat F) MC in a factorial combination with three fertilization levels (unfertilized, 110 kg nitrogen ha-1, and 200 kg nitrogen ha-1) on organic maize (Zea mays L.) productivity. Split-plot experiments were performed in two consecutive years (2020 and 2021) at different locations in the Rhineland, Germany, resulting in three trial environments. The novel MC consisted of well-studied single strains of Azotobacter, Bacillus, Bur-kholderia, Pseudomonas, Rahnella, and Glomus. At stem elongation in 2020, crop growth (+17%), shoot nitrogen (+34%), and phosphorus uptake (+25%) were significantly enhanced upon inoculation with MC_C, possibly due to atmospheric N2-fixation of A. chroococcum LS132 and phosphorus solubilization of P. fluorescens DR54. However, MC_C failed to impact maize productivity in 2021. This difference between years was likely due to distinct weather and pre-crop effects. Significant increases in grain yield were observed only for MC_C_AMF (+10%) in 2021. Interactions between MC inoculation and organic fertilization were not observed. Therefore, the assumption that MC are more effective in low-input systems cannot be confirmed by this study. However, the effect of fertilization on crop growth and nutrition was more common and consistent than the impact of MC inoculation in both years. As revealed by the varying performance of MC across trial environments, achieving significant, agronomically relevant, and reproducible benefits from MC inoculation still remains a considerable challenge in field-grown arable crops in temperate climate.