Climatic benefits from carbon sequestration are largely offset by increased nitrous oxide emissions, study finds

Human activities have more than doubled nitrogen inputs to the terrestrial biosphere since the 1860s. The two main causes for this are increased atmospheric nitrogen deposition from, for instance, fossil fuel burning, and the application of fertilizers in agriculture. Nitrogen is an essential nutrient for plant and microbial growth, and one of the key limiting nutrients in many natural ecosystems. The anthropogenic perturbations of the nitrogen cycle are known to affect the terrestrial sources and sinks of greenhouse gases such as carbon dioxide (CO₂) and nitrous oxide (N₂O). These changes are potentially very important as they may significantly affect the climate system, but their magnitude is still unknown.

"When added to nitrogen-limited ecosystems, it [nitrogen] can stimulate plant growth and/or suppress soil respiration, thereby leading to increased ecosystem carbon storage" explains Sönke Zaehle. However, there are also potentially negative consequences for adding nitrogen to ecosystems, as increasing nitrogen availability may enhance nitrogen losses from ecosystems, and eventually even have damaging effects on plant health. Particularly relevant for climate are elevated emissions of nitrous oxide, a long-lived greenhouse gas that is emitted from fertilised fields, as well as nitrogen-rich forest and grassland ecosystems.

Drawing on reconstructions of past and present anthropogenic nitrogen deposition and fertiliser applications, Sönke Zaehle and colleagues used a global computer model of the coupled terrestrial carbon and nitrogen cycles to better understand the consequences of this anthropogenic nitrogen perturbation for the climate system. Their results confirm that the anthropogenic nitrogen perturbation has profoundly affected terrestrial carbon dioxide and nitrous oxide fluxes. Human nitrogen additions are the principle cause for the increase in terrestrial nitrous oxide emission since 1960, and contribute to about one fifth of the current global net carbon uptake (1996-2005).

Sönke Zaehle and colleagues then determined the effect of anthropogenic nitrogen on the atmospheric concentrations of the greenhouse gases CO₂ and N₂O, and assessed the resulting consequences for present-day climate. The key finding is that the climatic effects of the anthropogenic nitrogen perturbation from both gases are very substantial but of opposite signs. The cooling effect due to enhanced carbon uptake of the terrestrial biosphere is more than compensated for by the warming effects from enhanced terrestrial N₂O emissions.

However, "the fact that in our study the N₂O effect appears stronger than the CO₂ effect should not be over-interpreted" cautions Zaehle. Rather, the study highlights the relevance of anthropogenic nitrogen in the climate system and the need to consider the effects of carbon and nitrogen cycling jointly. "I hope that this study fosters further research to better understand the effects of human N on ecosystem dynamics through joint observational and modelling studies," Zaehle adds.