Straw retention combined with phosphorus fertilizer promotes soil phosphorus availability by enhancing soil P-related enzymes and the abundance of phoC and phoD genes

Straw retention has the potential to improve soil phosphorus (P) availability, but the question on how straw retention under different P level influences the activity of soil P-related enzymes and thus the transformfiation of soil organic P (Po) remains poorly understood. A four-year field experiment (2016–2019) was established to investigate the impacts of straw retention under different P fertilizer levels (0, 44, 88 kg P ha–1) on soil P fractions, the activity of P-related enzymes, and the abundance of phoC and phoD genes. Results demonstrated that P application alone significantly increased all soil inorganic P (Pi) fractions, but had no significant impact on soil Po fractions. Straw retention combined with P fertilizer mainly improved the proportion of NaHCO3-Po (labile P) and NaOH-Pi (moderately labile P), while decreased the proportion of dilute HCl-P and residual-P (non-labile P). The application of straw plus P fertilizer of 44 kg P ha−1 increased the contents of resin-Pi and NaHCO3-Pi more than straw plus 88 kg P ha−1. Also, the former had higher activity of alkaline (ALP) and acid phosphatase (ACP) and abundance of phoD gene than the latter. The structural equation model indicated that soil ALP and phytase (PHY) had a significant and positive effects in P cycling. Soil organic carbon (C), C:P ratio, and water-organic C were positively correlated with the activity of ALP and PHY, and the abundance of phoD gene. These findings together demonstrate that crop straw combined with a moderate amount of P fertilizer (44 kg P ha−1) is more beneficial for soil P availability through enhancing the activity of PHY and ALP and the abundance of phoD and phoC genes.