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Nitrification and denitrification are key processes that control nitrogen (N) availability and loss in terrestrial systems. Nitrification in soils is thought to be performed primarily by chemoautotrophic prokaryotes such as bacteria and archaea, and denitrification by bacterial heterotrophs. However, recent work in drylands suggests that fungi may play a larger role in N transformations than previously recognized. Arid and semi-arid ecosystems experience extreme temperatures and low moisture conditions, both of which favor growth and survival of some fungi, but the extent of fungal nitrification and denitrification in these soils has not been explored. We investigated the role of fungi in nitrate (NO(3) over bar) and nitrous oxide (N2O) production in soils from regions across the southwestern US. Soils were collected from urban and Sonoran Desert sites within the Phoenix metropolitan area in Arizona, and from grasslands in Arizona and New Mexico. Rates of potential nitrification were measured in soils subjected to biocide treatments (fungal and bacterial inhibitors) and acetylene, an inhibitor of autotrophic ammonia oxidation. Nitrous oxide production was measured from soils incubated at two moisture levels in the presence of the biocide treatments. Rates of potential nitrification decreased significantly in all soils with additions of acetylene, indicating that autotrophic microorganisms may dominate ammonia oxidation and NOT production rather than heterotrophs (e.g., fungi). Similarly, fungi played a minor role in N2O production in urban turfgrass soils that were highly managed with irrigation and fertilizers. In contrast, fungi were major sources of N2O production in desert and semi-arid grassland soils across a range of soil water content. We conclude that fungi are likely responsible for denitrification in aridland soils, and that land-use changes associated with urbanization alter the biotic pathways responsible for N cycling. (c) 2013 Elsevier Ltd. All rights reserved.