The role of Nitrogen

This video highlights how nitrogen is a natural resource that we don’t worry enough about. Nitrogen is essential for plant growth and therefore a vital agricultural input. However we are wasting it through inefficient use of fertilizers (Eickhout, 2006).

In the early 20th century the Haber-Bosch process was discovered for synthesizing ammonium nitrate (converting Nitrogen into a reactive form). This increased availability of this limiting resource enabled huge increase in food production and therefore population growth (Steffen et al., 2007).

This has dramatically altered the natural nitrogen cycle, as worldwide, more nitrogen fertilizer is now used per year than can be supplied through natural sources as agricultural inputs currently exceed inputs from natural N fixation. More nitrogen is now converted from the atmosphere into reactive forms than by all the natural processes in terrestrial ecosystems put together (Steffen et al., 2007).

This diagram shows how the flow of nitrogen has been altered by human interruption.

Figure 1. Global terrestrial nitrogen buged for a) 1890 and b) 1990 in Tg N yr^-1. (Steffen et al., 2007)

The emissions from NOy reflect those from fossil fuel combustion. Those from the vegetation include agricultural and natural soil emissions and combustion of biofuel biomass and agricultural waste. The NH_x emissions from the cow and feediot reflect emissions from animal wastes. The transfers to the fish box represent the lateral flow of dissolved inorganic nitrogen from terrestrial systems to the coastal seas.

The enormous amount of N₂ converted to NH₃ in the 1990 panel compared to 1890 represents human fixation of nitrogen through the Haber-bosch process, made possible by the development of fossil-fuel based energy systems (Steffen et al., 2007).

Artificial inputs of nitrogen mean the nitrogen cycle is no longer a closed loop, which has led to huge losses of Nitrogen from agroecosystems (Eickhout et al., 2006). This is generally due to over-application of fertilizers and the inefficient use by crops. ‘The recovery of fertilizer N in global crop production is about 50%’ (Eickhout et al., 2006). The fertilizer that is not recovered by the crop ends up in our environment, mostly in surface water or in ground water. This can then contribute to eutrophication and pollution of aquifers and can also contribute to emission of the greenhouse gas nitrous oxide (Tilman et al., 2002).

As our population continues to grow, our agricultural yields will also have to, therefore even more nitrogen fertilizer will be required. However, Tilman et al. (2002) state that increased application of nitrogen is unlikely to be as effective at increasing yields as it previously was due to diminishing returns. Figure 2 shows how efficiency declines with higher levels of addition. Therefore as more is applied to the land in the hope of increasing yields, the greater the losses and pollution of the environment will be.

Figure 2. trends in nitrogen-fertilisation efficiency of crop production (annual global cereal production divided by annual global application of nitrogen fertiliser). (Tilman et al., 2002)

Total reactive nitrogen loss will increase dramatically with the worlds increasingly intensive agricultural systems (Eickhout et al., 2006). Therefore we need to improve nutrient use efficiency so that less nitrogen is lost to the environment.