Efficiency of potassium-solubilizing Paenibacillus mucilaginosus for the growth of apple seedling

Chemical potassium (K) fertilizer is commonly used in apple (Malus domestica L. Borkh) production but K is easily fixed by soil, resulting in reduced K fertilizer utilization and wasted resources. K-solubilizing bacteria (KSB) can cost-effectively increase the soluble K content in rhizosphere soil. Therefore, the objectives were to select high-efficiency KSB from apple orchards under various soil management models and evaluate their effects on apple seedling growth. Maize (Zea mays L.) straw mulching (MSM) increased the total carbon (TC), total nitrogen (TN) and available potassium (AK) in the rhizosphere and improved fruit quality. The number of KSB in the rhizosphere soil of MSM was 9.5x10(4) CFU g(-1) soil, which was considerably higher than that in the other mulching models. Fourteen KSB strains were isolated with relative K solubilizing ability ranging from 17 to 30%, and five strains increased the dry weight per apple seedling. The most efficient strain was identified as Paenibacillus mucilaginosus through morphological observation and sequence analysis of 16S rDNA, named JGK. After inoculation, the colonization of JGK in soil decreased from 4.0 to 1.5x 10(9) CFU g(-1) soil within 28 d. The growth of the apple seedlings and the K accumulation in apple plants were promoted by irrigation with 50 mL JGK bacterial solution (1 x10(9) CFU mL(-1)), but there was no significant increase in the AK content of rhizosphere soil. High-performance liquid phase analysis (HPLC) data showed that the JGK metabolites contained phytohormones and organic acids. Hence, the JGK strain promoted the growth of two-month-old apple seedlings by stimulating function of the produced phytohormones and enhanced K solubility by acidification for apple seedling uptake. This study enriches the understanding of KSB and provides an effective means to increase the K utilization efficiency of apple production.