Soil aggregates differently modulated bacterial community and function composition with fertilized regimes

Soil aggregates influence bacterial spatial distribution by providing varying substrate-laden micro-habitats. Understanding the nuances of bacterial variation within aggregates is essential for comprehending the involvement of soil microbes in biogeochemical processes. However, as a common practice for improving crop yields, it remains unclear how aggregates influence the redistribution of bacterial communities and functional composition across different fertilizer regimes. In this study, we used high-throughput sequencing of the 16S rRNA to examine the variation in soil bacterial communities across three different aggregate sizes (macroaggregate: >250 μm; mesoaggragate: 250-63 μm, and microaggregate: <63 μm) under three fertilizer regimes (CK, unfertilized soil; NP, nitrogen & phosphorus; NPM, nitrogen, phosphorus & manure) in calcareous soil in a 38-year experiment. The higher richness (Chao1) and diversity (Shannon) were found in microaggregate in the NP treatment, whereas the higher values were found in the macroaggregate after farmyard manure application (NPM) compared with meso and microaggregates. Bacterial community compositions had significantly difference among aggregate sizes from non-metric multidimensional scaling (NMDS). Further, we observed more complex co-occurrence networks (more links and a higher average degree) in the mesoaggregates compared with both micro and macroaggregates. The relative abundance of metabolic pathways related to C5-Branched dibasic acid metabolism, Biosynthesis of vancomycin group antibiotics and Fatty acid biosynthesis were higher in macroaggregates. However, the relative abundance of Biosynthesis of ansamycins was higher in mesoaggregates. Bacterial richness was positively correlated with biological index, but negatively with quantity and quality of soil carbon (dissolved organic carbon, fluorescence index and humification index). Our study highlights the distinct distribution characteristics of bacterial communities in different-sized aggregates with fertilization regimes. These findings contribute to a better understanding of the complex interactions between soil aggregates, bacterial communities, and agricultural practices.