Denitrification from terrestrial ecosystems: accounting for underrepresented sources and identifying paths forward

Over the past century, humans have more than doubled the amount of fixed nitrogen (N) in the biosphere. In doing so, we have kept pace with the food demands of a growing population, but excess N in the environment has also contributed to greenhouse gas production, air pollution, drinking water contamination, soil acidification, eutrophication, and biodiversity loss. Denitrification is a process that removes reactive N from the environment, turning it into either nitric oxide (NO), nitrous oxide (N₂O) or dinitrogen (N₂) gas. While denitrification remains poorly understood and quantified, recent research and technology development have allowed us to vastly improve our understanding. In this talk, I will discuss new findings regarding denitrification's three end products and suggest improvements for advancing this field of study. Nitric oxide is a key component of air pollution - a major cause of health concern in California's central valley. While the state currently considers the soil contribution of NO to the atmosphere to be negligible, we found that NO emissions from agricultural soils are high (25-42% of the state's budget) thereby presenting a new mitigation opportunity. Nitrous oxide is a powerful greenhouse gas with large fluxes from tropical forest soils, however it is unclear how these emissions might behave in response to future global change effects. We found that projected drought and N deposition conditions in tropical forests will likely produce a positive feedback effect on N₂O emissions. Dinitrogen is an inert form of N, and therefore a preferential loss pathway, however, it is difficult to measure in light of its high atmospheric background. Novel measurement techniques have allowed us to identify differential controls on N₂ and N₂O, as well as important loss pathways for N₂ from agricultural and forested ecosystems. Denitrification plays a critical role in regulating ecosystem nutrient availability and anthropogenic reactive N production, yet it remains the most poorly constrained term in terrestrial ecosystem N budgets. Nonetheless, we suggest that advances can be made using existing methods by standardizing the ways in which data are reported and by creating research coordination networks that will facilitate the sharing of ideas and resources among denitrification researchers.