Protists regulate microbially-mediated organic carbon turnover in soil aggregates

Soil protists, the major predator of bacteria and fungi, shape the
taxonomic and functional structure of soil microbiome via trophic
regulation. However, how trophic interactions between protists and their
prey influence microbially mediated soil organic carbon turnover remains
largely unknown. Here, we investigated the protistan communities and
microbial trophic interactions across different aggregates-size fractions
in agricultural soil with long-term fertilization regimes. Our results
showed that aggregate sizes significantly influenced the protistan
community and microbial hierarchical interactions. Bacterivores were the
predominant protistan functional group and were more abundant in
macroaggregates and silt + clay than in microaggregates, while omnivores
showed an opposite distribution pattern. Furthermore, partial least square
path modeling revealed positive impacts of omnivores on the
C-decomposition genes and soil organic matter (SOM) contents, while
bacterivores displayed negative impacts. Microbial trophic interactions
were intensive in macroaggregates and silt + clay but were restricted in
microaggregates, as indicated by the intensity of protistan-bacterial
associations and network complexity and connectivity. Cercozoan taxa were
consistently identified as the keystone species in SOM degradation-related
ecological clusters in macroaggregates and silt + clay, indicating the
critical roles of protists in SOM degradation by regulating bacterial and
fungal taxa. Chemical fertilization had a positive effect on soil C
sequestration through suppressing SOM degradation-related ecological
clusters in macroaggregate and silt + clay. Conversely, the associations
between the trophic interactions and SOM contents were decoupled in
microaggregates, suggesting limited microbial contributions to SOM
turnovers. Our study demonstrates the importance of protists-driven
trophic interactions on soil C cycling in agricultural ecosystems.