Anthropogenic nitrogen plays a double role in climate change

Increased nitrous oxide emissions from fertilised soils offset climatic benefits from carbon sequestration

August 03, 2011

Human nitrogen additions to the soil may reinforce the greenhouse effect.  Nitrogen additions tend to boost plant growth, so that terrestrial ecosystems absorb more carbon dioxide from the atmosphere. But recent studies have shown that they also stimulate nitrous oxide release from the fertisilied soils – a much stronger greenhouse gas than carbon dioxide. According to a new paper by researchers around the team of Sönke Zaehl from the Max Planck Institute for Biogeochemistry, nitrogen's detrimental effects on the climate roughly correspond to its climactic benefits. In fact, the scientists' findings suggest that the negative impacts of nitrogen may even slighty prevail.

Vor allem durch Düngung hat sich die Stickstoff-Menge in Land-Ökoystemen wie etwa in landwirtschaftlich genutzten Gebieten, aber auch in Wäldern und Feuchtgebieten seit Mitte des 19. Jahrhunderts verdoppelt – mit zwei gegensätzlichen Effekten für das Klima: Die Ökosysteme speichern dadurch mehr Kohlendioxid, was den Treibhauseffekt abschwächt. Allerdings entweicht aus stickstoffreichem Boden mehr Lachgas, das deutlich klimaschädlicher ist als Kohlendioxid und die positive Wirkung des Stickstoff-Eintrags zunichte macht.

Human activities have more than doubled nitrogen inputs to the terrestrial biosphere since the 1860s through increased atmospheric nitrogen deposition 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. But  anthropogenic perturbations of the nitrogen cycle through the additions of fertilisers are known to affect the terrestrial sources and sinks of greenhouse gases such as carbon dioxide (CO2) and nitrous oxide (N2O), better known as "laughing gas".  An international research team lead by  Soenke Zaehle,  researcher at the Max Planck Institute for Biogeochemistry, has now determined the magnitude of such effects on the two greenhouse gases.

“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.

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