Maximizing the efficiency of layered double hydroxides as a slow-release phosphate fertilizer: A study on the impact of plant growth-promoting rhizobacteria

Layered double hydroxides (LDH) have gained considerable attention for their potential application in agriculture, serving as a slow-release source of essential nutrients for plants. This study aimed to investigate the effects of Mg-Al-LDH materials (with M²⁺/M³⁺ ratios of 2:1 and 3:1) intercalated with phosphate, both with and without the presence of two plant growth-promoting rhizobacteria (PGPR) strains, on maize growth and the uptake of phosphorus (P), magnesium (Mg), and manganese (Mn). The LDH materials were synthesized, and their properties were assessed by X-ray diffraction (XRD) patterns, Fourier-transform infrared (FT-IR) spectra, and elemental analysis. In an in vitro experiment, the P solubilization capacity of five PGPR strains was evaluated, and the two most effective strains (Bacillus anthracis: B1 & Pseudomonas sp.: B2) were selected for the subsequent pot trials (greenhouse). In the pot experiment, the effectiveness of these two bacterial strains was evaluated in relation to the availability of P for maize plants from specific quantities of Mg-Al-LDH-P (2:1) and Mg-Al-LDH-P (3:1) materials amended in 500 g of soil, providing P concentrations of 0, 50, and 100 mg kg⁻¹. A treatment of triple superphosphate (TSP) was also included as a fast-release P source (50 mg P kg⁻¹). Irrespective of the type of LDH used, the application of LDHs significantly increased the shoot and root biomass of maize plants; the LDH effect was similar or even higher than that of the TSP, driven by the LDH type and P content. The application of LDHs and TSP resulted in higher shoot P concentration in maize plants compared to the untreated control. Inoculation with both bacterial strains increased the shoot P concentration of maize plants in the absence of any P treatments. Moreover, both bacterial strains increased the shoot P concentration of plants in LDH 2:1 (100 mg P kg⁻¹) and TSP treatments, indicating synergistic interactions between PGPR and LDH 2:1 and TSP (while no synergism was found between PGPR and other LDH treatments). Without any LDH/TSP application, only the B1 strain increased the shoot and root biomass of plants compared to the non-inoculated control. In terms of shoot biomass, no synergistic effects were observed in any PGPR-LDH combinations compared to the individual application of PGPR and LDH, which could possibly be due to the short growth period (40 days) of plants. Overall, this study suggests that LDH (2,1) and LDH (3,1) can effectively serve as a P source for enhancing plant growth under P deficiency conditions. Furthermore, the results underscore the potential of PGPR strains to release P from P-containing LDH compounds, which could vary based on the molar ratios of metal cations in LDHs.