Drought, wildfires, and “fire-loving” fungi effects on ecosystem nitrogen cycling: Understanding global change effects on denitrification using N2O isotopologues

Global changes caused by anthropogenic activities are altering the cycling of nitrogen (N) in terrestrial ecosystems. For example, droughts of increasing frequency and severity can stimulate large emission pulses of nitrous oxide (N₂O; a powerful greenhouse gas) when dry soils wet up. Further, increased fire frequency can favor the colonization of novel pyrophilous or “fire-loving” fungi on soils with the capacity to produce N₂O, yet N₂O isotopic ranges have been characterized in few fungal species, making generalizations difficult. To better understand how global changes are altering the N cycle, we studied drylands in southern California that can experience >6 months without rain, burned experimental “pyrocosms” to assess impacts of fire severity on soil biogeochemistry, and used a culture collection of pyrophilous fungi isolated from wildfire-burned soils to characterize their δ¹⁵N₂O_bulk,δN₂¹⁸O_bulk, and δ¹⁵N₂O_SP values. Despite the hot and dry conditions known to hinder denitrification, isotope tracers and natural abundance isotopologues of N₂O indicated NO₃^- was reduced within 15 minutes of wetting dry desert soils and that N₂O reduction to N₂ occurred. In post-fire environments, we found that while N₂O isotope values for Neurospora discreta and Fusarium tricinctum closely matched literature values when grown with NO₂^-, Aspergillus fumigatus, Coniochaeta hoffmannii, Holtermaniella festucosa, and R. columbienses did not. Further, Fusarium sp. δ¹⁵N₂O_bulk and δN₂¹⁸O_bulk values fell outside literature-derived values when grown with NO₃^-. Overall, we find that despite the hot and dry conditions known to make denitrification thermodynamically unfavorable in many drylands, denitrifiers can endure through hot and dry summers and are key to producing the surprisingly large N₂O emissions when dry desert soils wet up. Further, we find that novel pyrophilous fungi present an opportunity to further characterize the isotopic composition of N₂O as well as the factors controlling fungal denitrification as ecosystems are impacted by global changes.