Flooding and straw returning regulates the partitioning of soil phosphorus fractions and phoD-harboring bacterial community in paddy soils

Flooding and straw returning are effective agricultural practices in promoting phosphorus (P) availability in paddy soils. However, little is known about the effects of these practices and their interaction on the soil P pools and functional microbes responsible for soil P mobilization. Our 4-year paddy field experiment aimed to analyze the responses of soil P fractions and phoD-harboring bacterial communities in a double-rice cropping system to intermittent flooding (IF) and continuous flooding (CF), in plots with (+S) and without (-S) straw return. Compared to IF, CF significantly increased soil citrate-P and marginally decreased the HCl-P fractions, suggesting that the stable inorganic P pools are transferred to labile inorganic P at lower redox potentials. Compared to the -S treatments, +S treatments significantly increased the labile organic fractions (enzyme-P). Correspondingly, a decreased soil total organic P concentration was observed in +S treatment. Additionally, +S treatment significantly increased the activity of acid phosphomonoesterase and alkaline phosphomonoesterase and the abundance of phoD-harboring bacteria. These results indicated that straw promoted organic P minimization to release orthophosphate. The diversity of the phoD-harboring bacteria and complexity of the co-occurrence network decreased under the CF + S treatment; however, all keystone species of the phoD-harboring bacteria were retained in this oxygen-deficient environment. This study highlights that irrigation regimes mediate the processes of inorganic P mobilization, while straw returns regulate the processes of organic P mineralization. Additionally, flooding could be a more effective agricultural practice than straw returning to promote soil P availability in paddy soils.