Metagenomics reveals the effects of potassium fertilizer gradients on soil phosphorus-cycling microbial communities and functional genes, and their relationship with yield.

The rational application of potassium (K) is essential for soil nutrient transformation in orchards. Microbial communities drive phosphorus (P) transformation and activation, yet their responses to potassium (K) gradients and the underlying regulation of functional potential remain unclear. Accordingly, a field trial was conducted on 7- to 8-year-old Korla pear trees to evaluate changes in microbial community structure and functional potential across topsoil (0-20 cm) and subsoil (40-60 cm) layers under different K gradients (0, 75, 150, and 225 kg/ha K2O). Results indicated that microbial communities under K treatments were distinctly separated from the control. Microbial responses in subsoil were more pronounced than in topsoil, with soil available phosphorus and pH associated with these shifts. At the functional gene level, under K150, microorganisms were characterized by a reduced relative abundance of genes annotated for phosphorus-acquisition-related functions (ugpE, phy) and genes associated with nucleotide biosynthesis (guaA). With increasing K application, the relative abundance of phosphorus-cycling genes in microbial communities exhibited notable differences across K gradients. Soil available K and microbial biomass nitrogen were closely linked to these functional gene profile variations. Furthermore, K fertilization correlated with fruit yield variations, peaking under K150. PLS-PM showed that the community composition of phosphorus-cycling microorganisms and related functional genes had positive relationships with fruit yield. Overall, these results suggest that the K150 treatment corresponded to a favorable balance between fruit yield and microbial genetic potential for nutrient cycling under the conditions of this study.