Author：Yu-Syuan Hsu¹, Cheng-Hua Huang^2,*

Abstrct：

   Phosphate fertilization is an important factor to influence crop yields. It has been suggested that inoculation of crops with phosphate-solubilizing bacteria (PSB) can enhance phosphate uptake, and thus commercialized products of PSB are increasingly available for growers. The purposes of this study were to evaluate the effects of culture media on phosphate-solubilizing activity and organic acids produced and to determine the efficiency of four commercial phosphate-solubilizing (PS) biofertilziers. Twelve PSB strains and four culture media including AFS3183-1 (glucose or sucrose as sole carbon source), NBRIP, and Pikovskaya’s medium (PVK) were used to evaluate their effects on PS activity, medium pH, PSB colonies, and organic acids secreted by PSB. In order to evaluate the effect of four commercial PS biofertilizers on the growth and P uptake of amaranth plants (Amaranthus mangostanus L.), commercial PSB2 and PSB3 were applied into an alkaline soil and the other two commercial PSB1 and PSB1 were used in an acid soil. Culture media significantly affected phosphate-solubilizing activities of PSB, and Pseudomonas grimontii CHB 1107 showed a significantly high level of PS activity up to 7946 g/mL/day. In general, PSB cultured in PVK resulted in a higher level of PS activity partly because of this medium containing yeast extract. There was a significantly negative correlation between medium pH and PS activity (r = -0.75, P < 0.0001), suggesting acidic conditions helpful for solubilizing tricalcium phosphate. In addition, culture media significantly affected the concentration of organic acids secreted by PSB. Of these organic acids, the concentration of 2-ketogluconic acid, gluconic acid, and pyruvic acid were the highest. PS activity was significantly correlated with the concentration of these three organic acids, with the highest Pearson correlation coefficient of -0.66 for 2-ketogluconic acid. Compared with half the recommended rate of chemical fertilizers, the alkaline soil applied with half the recommended rate of chemical fertilizers with non-sterilized or sterilized PSB3 resulted in a significant increase in dry weight by 125-140%. However, the chemical fertilizer treatment equal to the nutrient content in PSB3 also significantly increased dry weight by 83.6%, suggesting that the nutrients rather than PSB in PSB3 might significantly improve amaranth growth. In contrast, half the recommended rate of chemical fertilizers with non-sterilized or sterilized PSB2 did not significantly improve amaranth growth. The alkaline soil applied with half the recommended rate of chemical fertilizers with non-sterilized or sterilized PSB3 significantly increased P uptake of amaranth by 54.8-73.9% in comparison with half the recommended rate alone. However, the chemical fertilizer treatment equal to the nutrient content in PSB3 also significantly increased P uptake of amaranth up to 41.0%, suggesting that phosphate fertilization rather than PSB in PSB3 might significantly increase P uptake of amaranth. Conversely, half the recommended rate of chemical fertilizers with non-sterilized PSB2 significantly increased P uptake of amaranth by 38.0% compared to half the recommended rate with sterilized PSB2, suggesting that PSB in PSB2 could improve P uptake by amaranth roots. Compared with half the recommended rate of chemical fertilizers alone, the acid soil applied with half the recommended rate of chemical fertilizers with non-sterilized PSB1 or PSB4 significantly reduced dry weight of amaranth and did not increase P uptake of amaranth, suggesting a low efficiency of PSB in the acid soil. In conclusion, media used for culturing PSB affect PS activity, and commercial PSB vary in improving P uptake and plant growth. Moreover, the nutrients in these commercial biofertilizers may considerably affect nutrient uptake and growth of plants.

Keywords：Phosphate fertilizer, Phosphate-solubilizing bacteria, Biofertilizer.

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* Corresponding author, e-mail: chhuang@nchu.edu.tw
¹ Graduate Student, Department of Soil and Environmental Sciences, National Chung Hsing University, Taichung, Taiwan, ROC.
² Associate Professor, Department of Soil and Environmental Sciences, National Chung Hsing University, Taichung, Taiwan, ROC.

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