To fulfil the demand of a growing population for rice food, a better understanding of soil biological systems is urgently required to improve the efficient use of chemical fertilisers, especially phosphate (P)-based fertilisers. Microorganisms play an important role in the decomposition of organic compounds and solubilisation of inorganic P. Presently, whether P availability affects the incorporation of rice root-derived carbon and the activity of P-solubilising bacteria remains uncertain. Herein, stable isotope probing of phospholipid fatty acids and DNA and high-throughput sequencing methods were used to investigate active P-solubilising bacteria and the microbial community in a paddy soil treated at 0 (controls), 50, and 200 mg P₂O₅ kg⁻¹. Compared with controls (23.4–62.2‰), the δ¹³C values of 16:1ω7c, 18:1ω7c, and 18:1ω9c (Gram-negative bacteria) increased to 69.2–167.4‰ with low P input, but decreased to 6.6–58.6‰ under high P input. Similarly, higher ¹³C enrichment (1.04–1.47 ng g⁻¹ dry weight soil) was observed in phospholipid fatty acids (16:1ω7c, 18:1ω7c and 18:1ω9c) with low P input. High-throughput sequencing indicated that low rather than high P input promoted the growth of P-solubilising Bacillus, Alicyclobacillus, and Clostridium. The relative abundance of these organisms increased significantly in light DNA fractions following both ¹²C and ¹³C treatments, suggesting they seldom use rice root-derived carbon as carbon sources. By contrast, P-solubilising Rhizobiales, Rhodospirillales, and Myxococcales preferred root-derived carbon under P input conditions. Community and phylogenetic analysis of genes related to P solubilisation (alkaline phosphatase phoD and pyrroloquinoline quinone pqqC) indicated a dominant role for Rhizobiales and Actinomycetales in paddy soil P solubilisation.