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Arbuscular mycorrhiza suppresses microbial abundance, and particularly that of ammonia oxidizing bacteria, in agricultural soils

Summary

This study examined how beneficial fungal partners of plants (arbuscular mycorrhizal fungi) affect soil bacteria that convert ammonia to nitrate. Using 50 different soils from Czech agricultural fields, researchers found that these fungi suppress ammonia-oxidizing bacteria, but surprisingly this happens even when ammonia levels in soil are high. The findings suggest the relationship between these microorganisms is more complex than simple competition for nutrients.

Background

Arbuscular mycorrhizal (AM) fungi form symbiotic relationships with most terrestrial plants and play crucial roles in nutrient cycling. Ammonia-oxidizing (AO) microorganisms are important for soil nitrogen cycling but their interactions with AM fungi are complex and variable across different soil types.

Objective

To investigate quantitative and compositional responses of indigenous microorganisms in 50 different agricultural soils to actively growing mycelium of the AM fungus Rhizophagus irregularis, with particular focus on ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), and comammox Nitrospira.

Results

AM fungal activity systematically suppressed AOB and comammox Nitrospira abundances across diverse soils, while AOA abundances remained unaffected. Unexpectedly, AM fungal presence significantly increased soil ammonium concentrations while decreasing nitrate levels. Multiple microbial groups including bacteria, protists, and fungi were suppressed by AM fungal activity.

Conclusion

AM fungi suppress ammonia-oxidizing bacteria and comammox Nitrospira through mechanisms beyond simple competition for ammonium, potentially involving other nitrogen cycle pathways or biological nitrification inhibitors. The interaction between AM fungi and AO microorganisms varies with soil properties and may involve changes in nitrogen mineralization and cycling dynamics.
  • Published in:Frontiers in Microbiology,
  • Study Type:Experimental Study,
  • Source: PMID: 41341498, PMC ID: PMC12669096, DOI: 10.3389/fmicb.2025.1671859
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