Investigating the Biological Mechanism of N₂O Emissions from Arid Southern Californian Drylands

Citation

Isella, Emma Xueqian (2025) Investigating the Biological Mechanism of N₂O Emissions from Arid Southern Californian Drylands. Senior thesis (Major), California Institute of Technology. doi:10.7907/9a4y-mm41. https://resolver.caltech.edu/CaltechTHESIS:06122025-192128192

Abstract

Nitrous oxide (N₂O) is a powerful greenhouse gas, each molecule capable of warming the atmosphere 273 times more effectively than CO₂. Arid soils that have been rewetted by rainfall events can produce some of the highest instantaneous N₂O emission rates recorded globally. Recent work has shown that the majority of these emissions are biologically produced. While these emissions have classically been attributed to bacterial and fungal denitrification catalyzed by catabolic nitric oxide (NO) reductases (e.g. NOR), measured N₂O isotopic fingerprinting (site preference, SP) more closely matches flavohemoglobin enzymes involved in nitric oxide detoxification (e.g. Fhp). Analysis of the microbial community of the site demonstrates that fhp is significantly more phylogenetically abundant than nor. We hypothesize that NO detoxification pathways are responsible for the initial pulse of N₂O production after rainfall, with denitrification only becoming dominant after a few hours. N₂O production is only triggered once some critical saturation with the water is reached, suggesting that the soil community has to receive enough water to become anaerobic. Using coupled measurements of oxygen and N₂O concentration in soils, we show that N₂O production begins only once the added water depletes the soil of oxygen. Initial measurements of N₂O production from Pseudomonas synxantha, a bacterium isolated from soil, demonstrate clear differences in the timing and quantity of gas production following rewetting via the detoxification and denitrification pathways. We thus suggest that previously overlooked detoxification pathways may play key roles in observed biogeochemical events, as appears to be the case with soil N₂O emissions.

Thesis Files