Issues : Figure Sets

Figure Set 1: How have humans altered the global N cycle?

Purpose: Help students grasp the large-scale anthropogenic changes to global N cycling.
Teaching Approach: "write before discussion"
Cognitive Skills: (see Bloom's Taxonomy) — comprehension, interpretation, analysis
Student Assessment: Replot the data


Vitousek et al. (1997 a, b) focus on N in these overview articles because N is a key determinant of species composition and diversity and of the function of all ecosystems. In addition, humans have greatly changed N dynamics worldwide. The abstract lists 8 alterations of the global N cycle as a result of human activity:1) doubled rate of N input to terrestrial systems with rates still increasing, 2) increased concentrations of nitrous oxide, an important greenhouse gas, 3) losses of soil nutrients (e.g. Ca and K), 4) acidification of water bodies, 5) greatly increased transfer of N to estuaries, 6) increased organic C stored in terrestrial systems, 7) accelerated losses of diversity, and 8) changes in composition and functioning of estuarine and nearshore ecosystems, including contributing to long-term declines in coastal marine fisheries.

Vitousek et al. (1997a, b) published "Human Alteration of the Global Nitrogen Cycle" in Ecological Applications and a second article with the same title as an ESA Issues in Ecology publication. Both are very useful reviews for ecology teaching. For N sources the papers deal with industrial N production for fertilizer, fossil fuel use, addition of N from nitrogen-fixing crops, and biomass burning. Effects on terrestrial ecosystems discussed include nitrogen saturation in forests and effects of N deposition on species diversity, The aquatic section focuses on N in surface and groundwater, acidification of water bodies, and eutrophication. There is also an interesting section about uncertainties such as marine N fixation (uncertain at least by a factor of 10).

According to Vitousek et al. (1997)

"The most fundamental human-caused change to the global N cycle is doubling of the transfer from the vast and unreactive atmospheric pool to the biologically available forms on land (termed "N fixation)... It is fair to conclude that human activity has doubled (or more) the transfer of N from the atmospheric pool to biologically available pools on land. The added N is spread unevenly over Earth's surface - some areas (e.g. northern Europe) are profoundly altered... while others... receive little direct input... but no place on Earth is unaffected."

The increase in N fixed for fertilizer over the last 25 years is clearly shown in Figure 3 in ESA's Issue in Ecology titled "Human Alteration of the Global N Cycle: Causes and Consequences; see Resources below including power point show). Here, Vitousek et al. attempt to compare industrial N fertilizer application to deforestation and carbon dioxide releases. The rate of change for anthropogenically fixed N is staggering. The authors point out that use of N for agriculture in developed countries stabilized in the 1970's and that the rapid increases since then is from use in developing countries. They say that more than half of all industrially fixed N for agriculture up to 1990 has been used since 1980.

You can use publication of these articles to stimulate discussion about why leading ecosystem scientists were interested in working together to address this global question - are humans altering the global N cycle - plus the challenges entailed in developing such estimates. For example, each scientists had to look beyond their usual turf (habitats or process) to reach consensus on pathways to focus on. Additional challenges in global budgets include dealing with huge numbers with large uncertainties and incomplete data. The Notes to Faculty section includes ideas for helping students appreciate these kinds of considerations.


Literature Cited

Specific Resources