An Earth-System Perspective

Gruber and Galloway (2008) wrote in the journal Nature, as part of a ‘Year of Planet Earth’ feature, about the interactions between the nitrogen cycle, carbon cycle and climate in the Earth system, and their increasing importance in the face of ever greater anthropogenic factors.  One such factor, of vital importance, is how the availability of nitrogen will affect the capacity of the Earth’s biosphere to absorb carbon from the atmosphere and continue to mitigate climate change.  In order to determine this, and to properly understand the nitrogen cycle as a whole, Gruber and Galloway state that it is necessary to adopt an earth-system perspective, examining changing nitrogen and carbon cycles, climate and anthropogenic influence together.

They state that fossil fuel combustion and nitrogen use in food production have contributed in excess of 160 terragrams of N per year to the environment, a figure greater than the global N total supplied by natural biological nitrogen fixation on land or in the oceans.  This is shown in the diagram below:

Figure. 1 'Depiction of the global nitrogen cycle on land and in the ocean.' (Gruber and Galloway, 2008: 294)

The expected trend, given predicted population growth and human resource use, is that humans will double the turnover rates of the entire Earth’s nitrogen cycle.  This addition of nitrogen has many negative effects (e.g. eutrophication and global acidification), the situation is worsened considerably by the cascade of effects that occur as nitrogen compounds chemically react. For example, anthropogenic nitrogen oxide causes photochemical smog, the compound might then be oxidised to nitric acid in the atmosphere and deposited on the surface, causing ecosystem acidification.  The same quantity of nitrogen added has had double the negative impact. 

Gruber and Galloway stress that changes to the nitrogen cycle do not occur in isolation, humans are also altering the phosphorus, sulphur and carbon cycles.  Anthropogenic acceleration of the carbon cycle is a well documented and very prominent issue, due to the impact of additional CO₂ on the climate, but these changes are linked to the perturbation of the nitrogen cycle.  Anthropogenically produced Nitrogen oxides and ammonia are spread by the atmosphere and deposited on the ground where it can be used by plants, this results in greater productivity and greater uptake of CO₂. This link and its impact on carbon sinks will be featured in this blog soon.

The unaltered ‘natural’ components of the carbon and nitrogen cycles are linked closer still, this is due to the presence of living organisms linking the cycles of carbon, nitrogen and other elements by using them to produce the molecules needed for life.  Gruber and Galloway state that the ratios of C/N present in autotrophic organisms (those producing their own organic compounds) is vital to understanding both the carbon and nitrogen cycles.  This is because the coupling of the two cycles depends on how much the C/N ratios of autotrophs is able to vary; a ‘systematic alteration’ of these C/N ratios could mean that the earth’s biosphere could rapidly change in productivity without a change in the quantity of reactive nitrogen, so the cycles are coupled less closely.  If the C/N ratios were only able to vary a little, the coupling would be far tighter.

There are also links mentioned between the marine nitrogen and phosphorus cycle. Evidence suggests that phosphorus is essential in keeping the amount of fixed nitrogen in the oceans stable over thousands of years.  In this case phosphorus is actually the ultimate limiter of productivity and therefore the ocean carbon cycle.

Gruber and Galloway cite evidence from records of past climate, atmospheric CO₂, atmospheric NO₂, and the ¹⁴N/¹⁵N ratio of organic nitrogen from marine sediments, in order to test current knowledge of the nitrogen cycle.  The data presented below demonstrate a tight coupling between these factors.

 Figure 2. 'Changes in the climate system and the global nitrogen and carbon cycles over the past 75,000 years' (Gruber and Galloway, 2008: 295)

The paper makes it clear that when studying the nitrogen cycle and the extent and impacts of anthropogenic influences, it is vital to also consider other biogeochemical cycles and climate. The tight links between various cycles are a key factor that shows the importance of the global nitrogen cycle and recent anthropogenic alterations to it.