Importance of soil fungi in the nitrogen cycle of arid ecosystems

Background/Question/Methods

Nitrification and denitrification are key processes in the nitrogen (N) cycle of terrestrial and aquatic systems. Both are performed by microorganisms, leading to N losses from ecosystems as nitrate or as trace gases such as nitrous oxide (N₂O). Nitrification in soils is thought to be performed primarily by chemoautotrophic prokaryotes such as bacteria and archaea, and denitrification by bacterial heterotrophs. Arid and semi-arid ecosystems experience high temperatures and low moisture conditions, both of which may favor growth and survival of fungi. Recent evidence suggests that fungi are responsible for nitrification and denitrification in some dryland soils, but the extent of these processes have not been explored. We investigated the role of heterotrophic fungi in nitrification and N₂O production in a range of soils from arid ecosystems in the US Southwest. Soils were collected from urban and Sonoran Desert sites across the Phoenix metropolitan area, and from higher elevation grasslands in Arizona and New Mexico. N₂O production was measured from soils incubated with biocide treatments (fungal and bacterial inhibitors) under two water conditions. Nitrification potentials of soils were measured with and without biocides and acetylene, an inhibitor of autotrophic nitrification. 

Results/Conclusions

Potential nitrification in all soils decreased with additions of acetylene, indicating that autotrophic microorganisms are responsible for this process rather than heterotrophic microbes (e.g. fungi). Fungi also played a minor role in N₂O production from urban lawn soils that were highly managed with irrigation and fertilizers. In contrast, fungi played a major role in N₂O production from desert and native grassland soils. These data suggest that fungi are responsible for denitrification in a wide range of aridland soils, and that this function changes with land cover and land use associated with urbanization.