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Abstract Lowland tropical forest soils are relatively N rich and are the largest global source of N₂O (a powerful greenhouse gas) to the atmosphere. Despite the importance of tropical N cycling, there have been few direct measurements of N₂(an inert gas that can serve as an alternate fate for N₂O) in tropical soils, limiting our ability to characterize N budgets, manage soils to reduce N₂O production, or predict the future role that N limitation to primary productivity will play in buffering against climate change. We collected soils from across macro‐ and micro‐topographic gradients that have previously been shown to differ in O₂availability and trace gas emissions. We then incubated these soils under oxic and anoxic headspaces to explore the relative effect of soil location versus transient redox conditions. No matter where the soils came from, or what headspace O₂was used in the incubation, N₂emissions dominated the flux of N gas losses. In the macrotopography plots, production of N₂and N₂O were higher in low O₂valleys than on more aerated ridges and slopes. In the microtopography plots, N₂emissions from plots with lower mean soil O₂(5%–10%) were greater than in plots with higher mean soil O₂(10%–20%). We estimate an N gas flux of ∼37 kg N/ha/yr from this forest, 99% as N₂. These results suggest that N₂fluxes may have been systematically underestimated in these landscapes, and that the measurements we present call for a reevaluation of the N budgets in lowland tropical forest ecosystems. 

Abstract Denitrification plays a critical role in regulating ecosystem nutrient availability and anthropogenic reactive nitrogen (N) production. Its importance has inspired an increasing number of studies, yet it remains the most poorly constrained term in terrestrial ecosystem N budgets. We censused the peer‐reviewed soil denitrification literature (1975–2015) to identify opportunities for future studies to advance our understanding despite the inherent challenges in studying the process. We found that only one‐third of studies reported estimates of both nitrous oxide (N₂O) and dinitrogen (N₂) production fluxes, often the dominant end products of denitrification, while the majority of studies reported only net N₂O fluxes or denitrification potential. Of the 236 studies that measured complete denitrification to N₂, 49% used the acetylene inhibition method, 84% were conducted in the laboratory, 81% were performed on surface soils (0–20 cm depth), 75% were located in North America and Europe, and 78% performed treatment manipulations, mostly of N, carbon, or water. To improve understanding of soil denitrification, we recommend broadening access to technologies for new methodologies to measure soil N₂production rates, conducting more studies in the tropics and on subsoils, performing standardized experiments on unmanipulated soils, and using more precise terminology to refer to measured process rates (e.g., net N₂O flux or denitrification potential). To overcome the greater challenges in studying soil denitrification, we envision coordinated research efforts based on standard reporting of metadata for all soil denitrification studies, standard protocols for studies contributing to a Global Denitrification Research Network, and a global consortium of denitrification researchers to facilitate sharing ideas, resources, and to provide mentorship for researchers new to the field. 

Abstract Nitrogen is a critical component of the economy, food security, and planetary health. Many of the world's sustainability targets hinge on global nitrogen solutions, which, in turn, contribute lasting benefits for (i) world hunger; (ii) soil, air, and water quality; (iii) climate change mitigation; and (iv) biodiversity conservation. Balancing the projected rise in agricultural nitrogen demands while achieving these 21st century ideals will require policies to coordinate solutions among technologies, consumer choice, and socioeconomic transformation.