Nitrogen in the biosphere - a cliffhanger

In a recent report by Johan Rockström and many other leading scientists, Planetary Boundaries: Exploring the Safe Operating Space for Humanity the nitrogen cycle is identified as one of three areas (together with climate regulation and biological diversity) where we have surpassed a threshold for stable development. Nitrogen is the most common compound in the air, which is composed of some 78 percent nitrogen. Most of the nitrogen is, however totally inert biologically. It can be converted into active forms, such as nitrate and ammonia through thunderstorms, fire and biological processes such as symbiotic nitrogen fixation (by Rhizobium bacteria in the roots of leguminous plants) or fixation by blue green algae, nowadays called cyanobacteria. Nitrogen is a main building block in proteins and amino acids as well as in nucleic acids like RNA and DNA. The changes in the nitrogen cycles are manifold more market than the changes in the carbon cycle. The quantity biologically active nitrogen in the biosphere has increased nine times in hundred years, most of the increase has occurred the last fifty years (see figure). Increased use of synthetic fertilizers is the dominating reason. The increase is also projected to continue from 165 million tones year 2000 to 270 millions year 2050 (Millennium Ecosystem Assessment 2005). Locally we can see great effects of the release of all this Nitrogen, while the global impact is less certain, what is certain is that there will be an impact.




As mentioned above, nitrogen plays a critical role in life processes, and the short term effect of improved availability of Nitrogen is often improved growth, e.g. that the forests grow better. Nitrogen hungry plants are favored over those that don’t need so much nitrogen, e.g. grasses a re favored over herbs and over leguminous plants. Surplus of Nitrogen and Phosphorus are according to Millennium Ecosystem Assessment one of the single most important factor for changes in ecosystems. The effluent of Nitrogen to the sea has increased with 80 percent between 1860 and 1990. However, in some areas the effluent has been more or less constant while the North Sea and coasts off China and the USA receive up to 15 times as much nitrogen now compared to hundred and fifty years ago. This run-off leads to eutrophication with tremendous effects on the composition of species, and in particular it stimulate the bloom of algae and the therewith associated dead zones, such as part of the Mexican gulf and the Baltic Sea. The use of synthetic fertilizers has lead to higher and higher levels of nitrates in drinking waters. Nitrogen also plays a role in the formation of tropospheric ozone, which leads to damage on crops and plants (MEA 2005). Synthetic fertilizers cause losses of other important nutrients such as calcium and phosphorus as well as acidification of soils and waterway (IAASTD 2009). Finally, the production of Nitrogen fertilizers is very energy consuming and in addition it incurs great emissions of laughing gas, one of the greenhouse gases. Mono-cropping of grain which is rather closely associated to the use of synthetic fertilizers leads to a reduction of carbon in soils and thereby increase the green house gases.

Agriculture represent two thirds of the emissions of nitrous oxide, the third greenhouse gas of importance. These emissions are directly related to the nitrogen cycle. The increase of livestock and even the use of chemical fertilizers are key divers. The use of nitrogen fertilizers is extremely inefficient and a lot more nitrogen is added to the soil than what is taken away with the harvest. The rest of this nitrogen “gets lost”, some of it as nitrate run off some of it as emissions of nitrous gases. In addition, nitrogen fertilizers also consume a lot of energy for their production. Studies of grain production in Great Britain and Sweden shows an almost linear correlation between use of nitrogen fertilizers and GHG emissions (KRAV 2008). Reduced use of nitrogen fertilizer should be a main strategy for reduction of GHG emissions from agriculture.

In summary, there are good, and frightening, reasons to follow closely the development of the nitrogen cycle. We should not be surprised if we find effects and costs associated with disturbed nitrogen cycles as dramatic as those of the carbon cycle. Considering how farmers and farm lands have become “addicted” to the use of synthetic fertilizers it could become a real thriller to reduce nitrogen effluents. (Extract from Garden Earth)