Changes in rhizosphere phosphorus fractions and phosphate-mineralizing microbial populations in acid soil as influenced by organic acid exudation

Interactions were explored between organic acids, phosphate-mineralizing microorganisms, and phosphorus fractions in rhizosphere soil. Bulk soil, rhizosphere soil, and root samples were randomly collected from a mature lemon tree plantation. Composition of acid (phoC) and alkaline (phoD) phosphatase gene harboring bacterial populations in rhizosphere soil were different. Whereas only soil properties affected the phoC-harboring bacterial population, soil properties and organic acids shaped the phoD-harboring bacterial population. Organic acids, especially oxalic acid, rather than phosphatase significantly directly and indirectly affected phosphorus fractions. When oxalic acid concentration increased, abundances of phoD-harboring Pseudomonas and Bacillus also increased. Those genera then released alkaline phosphatase, which reduced the contents of calcium-bound organophosphorus, insoluble organophosphorus, and residual phosphorus. By contrast, when oxalic acid concentration decreased, abundance of phoC-harboring Xanthomonas increased. The genus released acid phosphatase, which reduced calcium-bound organophosphorus content. In addition, alkaline phosphatase released by phoD-harboring bacteria was more important than acid phosphatase in acid soil. In conclusion, oxalic acids have essential roles in directly and indirectly changing phosphorus fractions in lemon tree rhizosphere soil.