How can fertilization regimes and durations shape earthworm gut microbiota in a long-term field experiment?

The positive roles of earthworms on soil functionality has been extensively documented. The capacity of the earthworm gut microbiota on decomposition and nutrient cycling under long-term fertilization in field conditions has rarely been studied. Here, we report the structural, taxonomic, and functional responses of Eisenia foetida and Pheretima guillelmi gut microbiota to different fertilization regimes and durations using 16S rRNA gene-based Illumina sequencing and high-throughput quantitative PCR techniques. Our results revealed that the core gut microbiota, especially the fermentative bacteria were mainly sourced from the soil, but strongly stimulated with species-specificity, potential benefits for the host and soil health. The functional compositions of gut microbiota were altered by fertilization with fertilization duration being more influential than fertilization regimes. Moreover, the combination of organic and inorganic fertilization with the longer duration resulted in a higher richness and connectivity in the gut microbiota, and also their functional potential related to carbon (C), nitrogen, and phosphorus cycling, particularly the labile C decomposition, denitrification, and phosphate mobilization. We also found that long-term inorganic fertilization increased the abundance of pathogenic bacteria in the P. guillelmi gut. This study demonstrates that understanding earthworm gut microbiota can provide insights into how agricultural practices can potentially alter soil ecosystem functions through the interactions between soil and earthworm gut microbiotas.