Objective   Effects of straw-returning on phosphorus morphology and microbial phosphorus-cycling genes in paddy soil at rice tillering and maturing stages were investigated.

  Method   In consecutive 7 years on a rice field in southern China under a positioning experiment, spent straws were returned to the acidic soil. The implemented treatments included: (1) chemical fertilizer without straw-returning (CK), (2) chemical fertilizer + 100% straw-returning in same season (CKS), (3) CKS+ straws to replace 10% potassium fertilizer (S10), (4) CKS + straws to replace 20% potassium fertilizer (S20) or (5) CKS + straws to replace 30% potassium fertilizer (S30). At end of the treatments, Guppy soil phosphorus continuous extraction method and metagenomic technology were applied to determine the composition of phosphorus of different forms and microbial phosphorus-cycling genes.

  Result   Straw-returning significantly increased the available NaHCO₃-Pi in soil (P＜0.05)—the S10 and S20 treatments resulted in an increase by 5.88%-8.73% over CK. NaOH-Pi was the main form of phosphorus in the acid paddy soil in southern China with a content ranging from 154.03 mg·kg⁻¹ to 202.11 mg·kg⁻¹. By turning the spent straws into the field, the abundance of phosphorus-cycling genes, especially the inorganic phosphorus dissolution gene pqqC under CKS, was significantly affected. The genes, such as phnW, phnO, pqqB, and pqqC, activated the conversion of hydrochloric acid phosphorus and residual phosphorus into available form; those like appA, phnX, and ppx, participated in the formation of stable phosphorus; and NaOH-Pi played a key role in the long-term transformation of the mineral. And the main factors that governed the abundance of the functional genes appeared to be the organic carbon and pH of the soil.

  Conclusion   Through altering the soil physiochemical properties, returning spent straws to the ground significantly enriched the microbial phosphorus-cycling genes that promoted the mineral transformation of the acidic paddy soil in southern China.