Since the 1860′s nitrogen additions to the terrestrial biosphere have more than doubled, due to human activities. Since nitrogen is a key nutrient needed for plant growth and therefore used as a fertiliser, the additions have made a drastic increase in plant growth possible, which was accompanied by a rise in net uptake of CO2 and possibly methane by the terrestrial biosphere.
However, it also stimulated the release of nitrous oxide, another powerful greenhouse gas, from the soil, according to a study in Nature Geoscience.
It is hard to quantify the exact climatic impact of anthropogenic nitrogen inputs since the magnitude of uptake of CO2 and release of N2O are uncertain and the carbon and nitrogen cycles are tightly coupled. Human influence on the nitrogen cycle are however known to affect CO2 and NO2 sources and sinks and may therefore have a significant influence on the climate.
By using a model of the coupled terrestrial carbon and nitrogen cycles the researchers found that between 1996 and 2005 nitrogen introduced by humans accounted for about a fifth of the carbon uptake by terrestrial ecosystems. The same nitrogen, mostly introduced because of agricultural intensification, was also responsible for the increase in global N2O emissions in the past decades.
Purely looking at quantities, much more CO2 was sequestrated than N2O was released. But since N2O has a global warming potential about 300 times as high as CO2 the net results was an increase in radiative forcing. To put it in numbers the total reduction of CO2 radiative forcing due to nitrogen deposition was approximately 96 mW/m2 while the increase of NO2 radiative forcing amounts to about 125 mW/m2.
Not to be over-interpreted
So it appears that the warming effect of from N2O emissions far outweigh the cooling effects of carbon uptake. The scientists stress however that the apparent stronger N2O effect should not be over-interpreted. The key finding is that nitrogen introduced by humans has significant opposite effects on the climate and it is important to always consider carbon and nitrogen cycles jointly to get a clear picture of human influence on climate.
© Jorn van Dooren | www.bitsofscience.org