A comprehensive quantification of global nitrous oxide sources and sinks.

Nitrous oxide (N2O) is a long-lived stratospheric ozone-depleting substance and greenhouse gas with an atmospheric lifetime of about 110-120 years. The concentration of atmospheric N2O has increased by more than 20% from 270 parts per billion (ppb) in 1750 to 331 ppb in 2018, with the fastest growth being in the last fifty years. Two key biochemical processes—nitrification and de-nitrification—control N2O production in both land and water ecosystems. These processes are regulated by multiple environmental and biological factors including temperature, water and oxygen levels, acidity, substrate availability (which is linked to nitrogen fertilizer use and livestock manure management) and recycling. In the coming years, N2O emissions are expected to continue to increase as a result of the growing demand for food, feed, fibre and energy, and an increase in sources from waste generation and industrial processes. In an effort to provide the global N2O picture across all sectors, Dr H. Tian and colleagues did a comprehensive analysis of N2O emissions which will reflect also in accumulation in the atmosphere. Their results were published in Nature, Volume 586 of 2020, page 248 to 256.  A comprehensive quantification of global nitrous oxide sources and sinks.

The results presented by the authors provide a global N2O inventory that incorporates both natural and anthropogenic sources and accounts for the interaction between nitrogen additions and the biochemical processes that control N2O emissions. They used both a bottom-up (inventory, statistical extrapolation of flux measurements, process-based land and ocean modelling) and a top-down (atmospheric inversion) approach to provide a comprehensive quantification of global N2O sources and sinks resulting from 21 natural and human-induced sectors between 1980 and 2016.

The estimates showed that global N2O emissions were 17.0 (minimum–maximum estimates: 12.2–23.5) teragrams (Tg) of nitrogen (N) per year for the bottom-up approach and similarly 16.9 (15.9–17.7) Tg of nitrogen per year for the top-down approach, between 2007 and 2016. The global human-induced emissions, which are dominated by N additions to croplands, increased by 30% over the past forty years to 7.3 (4.2–11.4) Tg N per year, which implies that agriculture is currently responsible for more than 40% of N2O emissions. The increase was mainly responsible for the growth in the atmospheric burden. Most of the increase in N2O emissions came from emerging economies—particularly Brazil, China and India. Analysis of the process-based model estimates revealed an emerging N2O–climate feedback, resulting from interactions between N additions and climate change. The recent growth in N2O emissions exceeds some of the highest projected emission estimates, emphasizing the urgency of mitigation.

This assessment by the authors (1) enhances understanding of the global nitrogen cycle, and (2) will inform policy development for N2O mitigation, which could help to curb warming to levels consistent with the long-term goal of the Paris Agreement. What does it mean for the dairy sector: Nitrous oxide emissions result from N fertilization and to some extent herbicide and pesticide use in crop and pasture management. It is crucial that these applications are kept to the minimum or even almost eliminated as in regenerative agriculture methodology.