Biostimulant and biofertilizer functions of a bacterial consortia in Lolium perenne

The use of bioinoculants was emerging as an effective strategy to increase soil productivity, particularly in degraded areas where nutrient scarcity limits the potential of livestock systems. Inoculation with plant growth-promoting bacteria (PGPB) provides stimulation functions through the synthesis of phytohormones and available nutrients. Among PGPBs, the genus Azospirillum is known for its biostimulant capacity, while the genus Herbaspirillum includes nitrogen-fixing bacteria. Additionally, microorganisms from the genus Trichoderma are recognized for their ability to solubilize phosphorus. This study evaluates the efficacy of a bacterial consortium combining Azospirillum brasilense (A), Herbaspirillum seropedicae (AH), and Trichoderma haziarum (AT) to determine their potential as biostimulants and biofertilizers in Lolium perenne, a forage species with high nutritional value. The methodological set up included a laboratory phase where the microorganisms' ability to synthesize phytohormones was measured (Indole-3-acetic acid (IAA), cytokinins: trans-zeatin (ZT), trans-zeatin riboside (ZTR), and abscisic acid (ABA)). Also the nitrogen-fixing potential of H. seropedicae was evaluated using the acetylene reduction assay (ARA), and the phosphate-solubilizing capacity of T. harziarum was assessed using a semi-quantitative technique to measure solubilization halos. In a second phase, L. perenne seeds were sown in commercial substrate inoculated with A, AH, AT, and a control treatment (C). The following parameters were recorded: weekly longitudinal growth (WLG), at 30 days, total chlorophyll content (TCh), percentage of coverage (PC), dry weight of aerial biomass (ABiom), and root biomass (RBiom). The results showed detectable levels of growth-regulating hormone synthezed for all the microorganisms evaluated. Additionally, H. seropedicae exhibited nitrogen-fixing activity with a value of (8.33 ± 0.9) nmol C₂H₄ plant⁻¹ h⁻¹, while T. harziarum displayed a pH indicator shift, indicating a positive result for phosphorus solubilization. The growth parameter data demonstrated early seed emergence in inoculated treatments, with greater grass height (WLG) observed in co-inoculated treatments (C: 5; A: 7; AT: 8.5; AH: 8) cm. The consortia also showed higher root biomass development (RBiom: C: 0.76; A: 0.86; AT: 1.10; AH: 0.95) g and percentage of coverage (PC), with the H. seropedicae treatment standing out (C: 45.5%; A: 62.8%; AT: 60%; AH: 71.3%). In aerial biomass (ABiom, C: 0.89; A: 1.15; AT: 1.21; AH: 1.3 g) and total chlorophyll content (TCh, C: 0.68; A: 0.84; AT: 0.73; AH: 0.75 mg/g). Co-inoculated treatments did not show significant differences. Inoculations improved all the growth parameters studied; however, co-inoculations optimized the benefits, likely due to the combined potential to provide regulatory hormones and nutrient availability functions. In this regard, the AH combination stood out in the PC parameter, possibly due to the nitrogen-fixing ability of H. seropedicae. We conclude that joint inoculations should be further studied to optimize strategies for crop management.