Microorganisms play a key role in cycling nutrients in terrestrial and aquatic habitats. Nitrification is the conversion of ammonium (NH4+) to the more mobile nitrate (NO3-), which may pollute groundwater or be converted to a greenhouse gas through denitrifying processes. In contrast, some microbial activity is beneficial for agriculture by transforming nitrogen (N) compounds into usable forms for plants. Overall, microbes are vital in controlling the availability of N, directly and indirectly affecting ecosystem services. The scientific community is only beginning to uncover the identity of different types of microbes relevant for N cycling. However, this knowledge is lacking in both deserts and cities, despite urban growth and the commonness of arid and semi-arid ecosystems in the world. Additionally, 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. The aim of this research is to 1) investigate the role of heterotrophic fungi in nitrification and nitrous oxide (N2O) production in a range of soils under and between plants from arid ecosystems in the US Southwest and 2) characterize microbial contribution and describe the temporal dynamics between community composition and N cycling in response to environmental change.