The Consequences of Considering Carbon–Nitrogen Interactions

Following on from the last post on the link between nitrogen availability and carbon sequestration, this is an interesting paper by Sokolov et al. (2008) in the Journal of Climate. They discuss the effect of including carbon-nitrogen dynamics in part of a particular earth systems model, the MIT Integrated Global Systems Model (IGSM), and how feedbacks from increased atmospheric CO₂ can change when the role of nitrogen is considered. 

The Link between Nitrogen Availability and Sequestration of Carbon

As mentioned in my last post, I’m going to look in more detail at the link between the increase in nitrogen deposition and the uptake of carbon from the atmosphere. This blog looks at a few recent papers on the topic.  The atmospheric deposition of nitrogen is thought to enhance the uptake of carbon dioxide from the atmosphere by terrestrial vegetation (Reay et al., 2008), and therefore may help reduce the warming effect produced by greenhouse gas emission.  This is an interesting and potentially beneficial impact of anthropogenic influences on the nitrogen cycle. 

In support of this idea, evidence has been highlighted by Thomas et al. (2010) who have used forest inventory data to examine the impact of nitrogen deposition on tree growth, survival and carbon storage across the north-eastern and north-central USA during the 1980s and 1990s. When their results are extrapolated to cover the entire globe, they estimate that nitrogen deposition could increase tree carbon storage by 0.31 Pg carbon/yr.  

Norby et al. (2010) have investigated the negative feedback on anthropogenic CO₂ present in the atmosphere due to increased CO₂ concentration stimulating plant production.  This feedback is included in models of CO₂ concentration, but not with great certainty as the impact of available nitrogen is anticipated to be a limiting factor for plant productivity.   The experiments of Norby et al. show that increases in net primary productivity under conditions where CO₂ concentrations were elevated drop off over an 11yr timescale.  Their nitrogen budget analysis supports the premise that nitrogen availability was limiting to tree growth and declining over time, and therefore global CO₂ and climate models that assume the CO₂ fertilization effect to be sustained are not supported by this evidence.  Norby et al. say that N limitation and N feedback effects should be incorporated into ecosystem and global models used in climate change assessments.

The nitrogen cascade on YouTube!

Here is a youtube video produced by Erin Siegel and Professor Myrna Jacobson Meyer at the University of Southern California.  It explains how the input of reactive nitrogen to the environment produces a cascade of effects, and that the same atom of reactive nitrogen can be responsible for several different environmental impacts, this was mentioned in my previous post about Gruber and Galloway's 'Earth-System Perspective' but hopefully the video will elaborate on and clarify the issue.  It is my intention to post as many relevant videos as possible throughout the course of the blog, although they are proving tricky to find!

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.

 

Nitrogen's Dark Side

Reactive nitrogen inputs as a result of human activity are known to have a range of far reaching concequences for the environment.  I feel that the best way to quickly introduce all of these impacts is visually, a long list of bullet points or boring chunk of text isn't very helpful in getting the basics across (that stuff will come later!).

Thankfully, Townsend and Howarth (2010) have done the job for me in an article for The Scientific American by producing the excellent diagram below.

  Fig. 1. 'Nitrogen's Dark Side' Townsend and Howarth (2010: 66)

The Nitrogen Cycle: A brief introduction.

I thought I'd start with a basic overview of the functioning of the nitrogen cycle.  Hopefully this will make my blog a little more readable when things get more in-depth later on.  Anybody who is well acquainted with this topic should probably give this a miss and move on to the more interesting stuff to follow.

Nitrogen has a vital role in the biosphere, it is an essential nutrient and major component of nucleic acids and proteins. Almost 80% of the atmosphere is made up of N2 gas, however before nitrogen is available to be used to support growth it must first be used to form ammonium NH4 or nitrate NO3 ions.  The availability of nitrogen in this 'fixed' form is the key limiting factor for plant growth and controls the productivity of most ecosystems.  Nitrogen fixation is a process performed by nitrogen-fixing organisms and involves the conversion of atmospheric nitrogen to ammonia by the nitrogenase enzyme.  Leguminous plants and some nitrogen fixing bacteria have a mutualistic relationship, where the bacteria live in the root nodules of the legume and produce ammonia in exchange for carbohydrate.  

The quality of soils can be improved by planting legumes to increase the soil nitrogen content, this is a fairly simplistic way for humans to influence the nitrogen cycle.  Nitrogen fixation also occurs artificially, the Haber-Bosch process is used in industry to create ammonia by combining N2 and Hydrogen at high temperature and pressure.  The ammonia formed industrially accounts for 30% of all fixed nitrogen, this is a major anthropogenic influence on the natural cycle. The combustion of fossil fuels is another anthropogenic process where atmospheric nitrogen is converted to a reactive form as a result of human activity.

The next process in the cycle is nitrification, this is where nitrifying bacteria oxidise ammonium in order to gain energy and create nitrate in the process, they also use carbon dioxide to synthesise organic compounds.  This causes problems as nitrate is easily leached from the soil, resulting in a loss of soil fertility and a potential build up of nitrate in groundwater, which can later contaminate drinking water supplies. The nitrate present can then be reduced to nitrite, a highly reactive compound. The health risks associated with this include 'blue baby syndrome' (respiratory problems in infants) and  the formation of carcinogenic compounds in the gut.

Other processes in this cycle replace nitrogen to the atmosphere. This is denitrification, which involves the conversion of nitrate to gaseous nitrogen compounds (e.g. nitric oxide, nitrous oxide and N₂ ).  In the natural nitrogen cycle the nitrogen released to the atmosphere by denitrification is approximately equal to the amount of nitrogen fixation occurring.  

For some further information and an outstanding summary of the processes of the global nitrogen cycle, as well as disturbances and their impacts that will be the subject of subsequent posts....see the Bernhard (2010) summary in Nature Education Knowledge.