Long-Term Phosphate Addition Changes Soil P Accumulation via phoD-Harbouring Bacterial Community in Loess Plateau

The bacterial phoD gene encoding alkaline phosphatase (ALP) plays a crucial role in the mineralisation of organic phosphorus (Po) to inorganic phosphorus (Pi). The purpose of this study was to explore the association between soil P fractions and the phoD-harbouring bacterial community. Based on a long-term P fertilisation experiment in calcareous soil on the Loess Plateau (started in 2014), this study analysed the results of treatments including four P fertilisation rates: 0, 60, 120 and 180 kg P2O5 ha-1 (denoted as P0, P60, P120 and P180, respectively). The abundance and community structure of the ALP-encoding gene (phoD) were analysed by PCR amplification and high-throughput sequencing, and the soil P fractions were measured using Hedley sequential fractionation approach. The majority of soil P was present in the form of HCl-Pi, and P fertilisation significantly increased the contents of Resin-P, NaHCO3-Pi, NaHCO3-Po, NaOH-Pi, NaOH-Po and HCl-Pi in soil. The ALP activity and phoD gene abundance in the P120 and P180 treatments were significantly less than those in the P0 and P60 treatments: P180 decreased by 14.52% and 46.83% compared with P0, respectively. ALP activity was positively correlated with pH, but negatively correlated with the contents of Resin-P, NaHCO3-Pi, NaHCO3-Po and NaOH-Pi. P fertilisation decreased the relative abundance of the genera Streptomyces, Bradyrhizobium and Rhizobacter. Sinorhizobium had the highest abundance in low-P (P60) soil and played an important role in improving ALP activity and bacterial community network stability. P fertilisation significantly affected the community assembly processes of phoD-harbouring bacteria, with high-P input promoting stochastic processes in soil. Soil microbial biomass carbon (MBC) and microbial biomass phosphorus (MBP) contents significantly affected the abundance of phoD, while pH and MBP contents significantly affected the composition of the phoD bacterial community. ALP activity was significantly correlated with phoD gene abundance, which played a key role in promoting Po turnover and improving soil P availability. The decrease in soil pH and the increase of MBC and MBP contents caused by long-term P fertilisation influenced the activity of ALP by regulating phoD gene abundance and community composition, thereby inhibiting the mineralisation of Po.