Metam sodium influences the rhizosphere mycobiome more than the bulk soil mycobiome

Chemical soil fumigation is often assumed to have a detrimental effect on the soil microbiome. Metam sodium (MS) is a soil fumigant used preventively to control nematodes, weeds, and soilborne pathogens. This research aimed to determine whether MS affects the richness, diversity, and composition of the fungal community in both the soil and the rhizosphere, as well as whether these communities recover over time.
In an experimental field located in Cádiz, Spain, MS was applied in three replicates through drip-irrigation lines, under a Totally Impermeable Film (TIF). Untreated plots and untreated with TIF were used as controls. The TIF film was removed from all plots after six weeks, and all plots were then planted with tomatoes in January 2024. Soil samples from both bulk soil and the rhizosphere were collected from all replicates at two depths (0-15 cm and 15-30 cm) to analyze the composition of the fungal microbiome using ITS2 DNA metabarcoding. Bulk soil samples were taken before the application, as well as at 42 and 210 days after treatment (DAT). Rhizosphere samples were collected at 84 and 210 DAT. Fungal richness and diversity were assessed using the Chao1 and Inverse Simpson indices. Differences between treatments for each depth and timepoint were assessed for statistical significance. Functional guilds were assigned to the observed genera using FUNGuild, and ALDEx2 was used to determine statistically significant differences in community composition.
No significant differences between the three treatments were found in fungal richness and diversity for both the bulk soil and rhizosphere samples at each depth and timepoint. Limited significant compositional differences of the fungal community were observed in the bulk soil at 42 DAT. At 210 DAT, these limited compositional differences further reduced, indicating a recovery of the fungal microbiome over time . For the rhizosphere, compositional differences were observed after MS application compared to the control groups, and these effects were still present at 210 DAT. Although some changes in functional community compositions were noted, these changes were relatively small.
In this experiment, the effects of MS on the composition of the fungal community of the rhizosphere are still visible at 210 DAT. The fungal microbiome of the bulk soil shows near complete recovery. The application of MS does not affect the richness and diversity of the fungal community in bulk soil and the rhizosphere at any of the investigated timepoints.