Impact of Graphite Nano Amendments on Soil Enzyme Activities, Functional Genes and Microbiome Composition in a Soil-Plant System

Graphite nano additive (GNA) has shown potential for enhanced soil N retention and plant productivity. To obtain mechanistic understanding on such beneficial effects, lettuce (Lactuca sativa) was grown in a greenhouse with an exposure of GNA at an application range of 0-500 mg/kg soil for seven weeks. Changes in microbial enzyme activities, N cycling gene abundances, and bacterial community composition were investigated as responses to GNA addition. GNA doses less than 100 mg/kg soil resulted in elevated soil microbial biomass carbon at week 3 and contributed to enhanced plant yield during the final harvest at week 7. GNA significantly influenced rhizosphere soil enzyme activities, with notable increases observed across all assayed enzymes at week 5, corresponding to the peak in lettuce growth. GNA addition decreased bacterial amoA abundance, which indicated suppressed soil nitrification potentials in both the bulk and rhizosphere soils. This was coupled with an increase in nifH-harboring bacteria in the bulk, but not rhizosphere, soil. Although the gene that encodes for the terminal step in denitrification (nosZ) was not significantly impacted, nirS and nirK abundances indicated a potential for enhanced denitrification in the bulk soil and suppression of denitrification in the rhizosphere soil. 16S rRNA gene-based abundances indicated no significant decreases in the total bacterial community with GNA amendment in both the bulk and rhizosphere soils which indicates that GNA had limited microbiocidal effect on the broad community. However, GNA imposed selection for certain microbial functional clades and taxonomic lineages and demonstrated a significant impact on the overall composition of the microbial community. GNA demonstrated the potential to augment several beneficial bacterial groups while suppressing others. Overall, these data indicate that GNA significantly impacted the bacterial composition, potential N cycling, and enzyme-based activities of the soil community in a fashion that positively impacted the growth of lettuce in our study, principally within the plant rhizosphere.