Elizah Z. Stephens

Postdoctoral Researcher, Homyak Lab

Environmental Science

University of California Riverside

Wildfires are a growing concern as climate change causes hotter and drier conditions across the globe. What will happen to our local ecosystems as fire intervals shrink? Can even fire-adapted ecosystems like California chaparral handle such frequent and severe fires? To tackle one angle of this long-term multidisciplinary question, I ask how the cycling of essential nutrients like nitrogen (N) and carbon (C) are impacted by fires and how the chemistry and biology of soils are changed by the transformative event of fire.

Research Interests

Investigating the impacts of wildfire on soil nitrogen cycling and emissions of the potent greenhouse gas nitrous oxide

Linking microbial community dynamics and post-fire soil nitrogen and carbon cycles

Understanding the functions of "fire-loving" or pyrophilous fungi and thier unique roles in post-fire nutrient cycling

Current Projects

Increasing wildfires in chaparral ecosystems can increase soil fluxes of NOx and N2O

In chaparral ecosystems that undergo seasonal N fluxes, fires may allow the buildup of large amounts of N by leaving behind ash rich in NH4+ and organic N, leading to high levels of N loss with the onset of rain. However, only about half of the post-fire N flux in these systems has been accounted for following fire. One important mechanism behind this may be significant gaseous N loss from soils in the form of NOx and N2O via abiotic and microbial process. Better understanding the impacts of fire on NOx and N2O fluxes from soils in chaparral ecosystems will lead to a clearer understanding of N-limitation in chaparral ecosystems, and also to understanding trace N gas inputs to the atmosphere.

This work is funded by CAL FIRE.

eos.org/articles/wildfires-may-alter-the-nitrogen-cycle-and-air-pollution

For a review on this topic: link.springer.com/article/10.1007/s10533-023-01072-5

2. Investigating how wildfires alter communities of nitrifiers and denitrifiers using isotopic and molecular tools

The rate-limiting step of nitrification is mediated by ammonia-oxidizing bacteria (AOB) and archaea (AOA) which oxidize NH4+ to nitrate (NO3-) and release NO and N2O as byproducts. Since AOB are associated with higher NO and N2O emissions and can be favored by higher pH and NH4+ soil conditions (such as after a fire), I expect that NO and N2O emissions will increase as AOB communities become dominant after fire. I also expect that as nitrifiers generate NO3-, denitrification processes will be stimulated to increase N gas emissions. To understand these processes, I measured trace N gas emissions from soils collected pre- and post the KNP complex fire in the Sequoia National Park, CA and used selective inhibitors of AOB and AOA communities to measure their contributions to NO and N2O emissions, assesed thier abundaces using qPCR, and measured the isotopic composition of N2O to better understand the processes responsible for post-fire N2O production.

As more frequent, high-intensity wildfires affect larger areas each year in tandem with the progression of climate change, quantifying post-fire soil emissions of NO and N2O becomes increasingly important to understand ecosystem N loss and the potential implications for climate feedbacks, particularly in fire-prone shrubland ecosystems.

3. Understanding the biogeochemical functions of pyrophilous fungi

The specialized roles that post-fire microbes may play in cycling nutrients is relatively unknown for many species--I aim to understand thier ability to cycle N and produce nitrous oxide

Using cutting-edge isotopic tools to capture the fingerprint of N2O released by fungi allows us to better understand their contributions to soil N cycling

Fungi play important roles in global nitrogen (N) cycling and are suspected to contribute a substantial portion of the nitrous oxide (N2O) typically measured from soils. An emerging method for partitioning fungal N2O sources from in-situ soils is measuring the natural abundances of isotope ratios of N2O such as δ15N2Obulk, δN218Obulk, and site preference δ15N2OSP . By comparing isotopic signatures from soil to isotopic values derived from pure culture experiments, we can draw conclusions about the fungal derived proportion of N2O, which can aid in predicting responses to disturbances that cause microbial communities to shift.

Publications

Dissertation: https://escholarship.org/uc/item/2p06j96j

Stephens, E.Z, and Homyak, P.M. 2023. Post‑fire soil emissions of nitric oxide (NO) and nitrous oxide (N2O) across global ecosystems: a review. Biogeochemistry 165: 291–309. link.springer.com/article/10.1007/s10533-023-01072-5

Stephens, E.Z., C. Murar, D. Tinker, and P.E. Parry. 2018. Environmental determinants of recruitment success of subalpine fir (Abies lasiocarpa) in a mixed-conifer forest. Western North American Naturalist 79(4): 481–495. www.jstor.org/stable/26909527

Krichels, A.H., Aral C. Greene, Elizah Z. Stephens, Sharon Zhao, Joshua P. Schimel, Emma L. Aronson, Erin J. Hanan, and Peter M. Homyak. 2024. Nitrifier controls on soil NO and N2O emissions in three chaparral ecosystems under contrasting atmospheric N inputs. Soil Biology and Biochemistry 196 (109482). https://doi.org/10.1016/j.soilbio.2024.109482.

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