Although it is well established that soils are the dominating source for atmospheric nitrous oxide (N ₂ O), we are still struggling to fully understand the complexity of the underlying microbial production and consumption processes and the links to biotic (e.g. inter- and intraspecies competition, food webs, plant–microbe interaction) and abiotic (e.g. soil climate, physics and chemistry) factors. Recent work shows that a better understanding of the composition and diversity of the microbial community across a variety of soils in different climates and under different land use, as well as plant–microbe interactions in the rhizosphere, may provide a key to better understand the variability of N ₂ O fluxes at the soil–atmosphere interface. Moreover, recent insights into the regulation of the reduction of N ₂ O to dinitrogen (N ₂ ) have increased our understanding of N ₂ O exchange. This improved process understanding, building on the increased use of isotope tracing techniques and metagenomics, needs to go along with improvements in measurement techniques for N ₂ O (and N ₂ ) emission in order to obtain robust field and laboratory datasets for different ecosystem types. Advances in both fields are currently used to improve process descriptions in biogeochemical models, which may eventually be used not only to test our current process understanding from the microsite to the field level, but also used as tools for up-scaling emissions to landscapes and regions and to explore feedbacks of soil N ₂ O emissions to changes in environmental conditions, land management and land use.

Global nitrogen fixation contributes 413 Tg of reactive nitrogen (N _r ) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic N _r are on land (240 Tg N yr ⁻¹ ) within soils and vegetation where reduced N _r contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer N _r contribute to nitrate (NO ₃ ⁻ ) in drainage waters from agricultural land and emissions of trace N _r compounds to the atmosphere. Emissions, mainly of ammonia (NH ₃ ) from land together with combustion related emissions of nitrogen oxides (NO _x ), contribute 100 Tg N yr ⁻¹ to the atmosphere, which are transported between countries and processed within the atmosphere, generating secondary pollutants, including ozone and other photochemical oxidants and aerosols, especially ammonium nitrate (NH ₄ NO ₃ ) and ammonium sulfate (NH ₄ ) ₂ SO ₄ . Leaching and riverine transport of NO ₃ contribute 40–70 Tg N yr ⁻¹ to coastal waters and the open ocean, which together with the 30 Tg input to oceans from atmospheric deposition combine with marine biological nitrogen fixation (140 Tg N yr ⁻¹ ) to double the ocean processing of N _r . Some of the marine N _r is buried in sediments, the remainder being denitrified back to the atmosphere as N ₂ or N ₂ O. The marine processing is of a similar magnitude to that in terrestrial soils and vegetation, but has a larger fraction of natural origin. The lifetime of N _r in the atmosphere, with the exception of N ₂ O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 10 ² –10 ³ years), the lifetime is a few decades. In the ocean, the lifetime of N _r is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N ₂ O that will respond very slowly to control measures on the sources of N _r from which it is produced.