Agronomic evaluation of Herbaspirillum seropedicae strain ZAE94 as an inoculant to improve maize yield in Brazil

Diazotrophic bacteria applied as a seed inoculant can improve the grain yield of several crops including maize. The current study aimed to test the agronomic efficiency and contribution of biological nitrogen fixation (BNF) of the endophytic diazotroph Herbaspirillum seropedicae strain ZAE94 to maize under field conditions. Eighteen field assays were conducted in four different locations during consecutive years on two hybrids and two varieties of maize in a random block design with four replicates using a peat-based inoculant. The inoculant containing the ZAE94 strain was applied without nitrogen (N) fertilization or with 40 kg N ha^–1 and was compared to the application of 40 and 80 kg N ha^–1 without inoculation. Crop productivity and N accumulation in the grain were evaluated in addition to ¹⁵ N natural abundance (°¹⁵ N) to evaluate BNF in the treatments without N fertilization. Fertilization at 40 kg N ha^–1 plus bacterial inoculation produced crop yields similar to the treatment with 80 kg N ha^–1 and increased grain N content, especially in the off-season with 40 kg N ha^–1. The inoculation treatments showed lower °¹⁵N values than the non-inoculated treatments, which was most evident in the off-season. The BNF contributed about 30% of N accumulated in plants inoculated with ZAE94. On average, 64% of the N fertilized plots showed an increase of the parameters evaluated in the inoculated treatments, compared with the control. Inoculation also increased root length, root volume, and leaf area, and these parameters were positively correlated with plant weight using a hydroponic assay. This study revealed that the application of H. seropedicae inoculant increased the amount of N in plants owing to BNF, and there is a better chance of yield response to inoculation under low N fertilizer application in the off-season.