Divergent responses of phoD- and pqqC-harbouring bacterial communities across soil aggregates to long fertilization practices

Microbial community composition differs between soil aggregate sizes, which markedly affects phosphorus cycling in agroecosystems. The genes phoD and pqqC regulate phosphorus mobilization and act as molecular markers of phosphate-mobilizing bacteria, but differences in phoD- and pqqC-harbouring bacterial communities between different-sized aggregates remain unclear. The abundance and community composition of phoD- and pqqC-harbouring bacteria were determined in three aggregate size classes (> 250, 63–250, and < 63 µm) in calcareous soil from northwest China in a 38-year experiment. The > 250 and < 63 µm aggregates showed the lowest and highest cumulative relative abundance, respectively, of operational taxonomic units in phoD- and pqqC-harbouring bacterial communities. Simpler co-occurrence networks (fewer nodes, links, and shorter network diameter) were found in > 250 and < 63 µm aggregates, compared to 63–250 µm aggregates. For phoD and pqqC gene networks, > 250 and < 63 µm aggregates (phoD: α-Proteobacteria and Actinomycetia; pqqC: Actinomycetia) contained keystone taxa distinct from 63 to 250 µm aggregates (phoD: Rubrobacteria and γ-Proteobacteria; pqqC: β-Proteobacteria). Total nitrogen (TN) and Olsen-P significantly affected the abundance and community richness of phoD- and pqqC-harbouring bacteria. phoD and pqqC gene abundance was determined by TN in > 250 µm aggregates, whereas they were indirectly determined by Olsen-P in < 63 µm aggregates. Our results emphasized the distinct distribution characteristics of phosphate-mobilizing bacterial communities in different-sized aggregates and identified the main factors regulating phoD and pqqC gene abundance.