STUDENT INSTRUCTIONS

Write before discussion

Study Figure 1 from a paper called "Human Alteration of the Global Nitrogen Cycle" by Vitousek et al. Follow the step one- step two procedure you have practiced in class.* As you are examining the data write down questions you have, important points in the data, or other topics you would like to bring up in the discussion. The overall question you should address is: why did the scientists who wrote this paper make this particular figure? What central points were they trying to get across?

Note: Tg = teragram or 1,000,000,000,000 grams

Background

Vitousek et al. published "Human Alteration of the Global Nitrogen Cycle" in a journal called Ecological Applications and a second article with the same title as an Ecological Society of America "Issue in Ecology" publication. Both articles are thorough reviews of this very large topic. In discussing sources of human-made nitrogen (N) the authors 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 (e.g. rain) on species diversity. The aquatic section focuses on N in surface and groundwater, acidification of water bodies, and a phenomenon called 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 data associated with Figure 1 are: 80 Tg/yr (teragram or 10¹² g) for current industrial fixation of N for fertilizers, >20 Tg/yr released to the atmosphere from burning of fossil fuels, 40 Tg/yr for leguminous crops and forages in agriculture.

___________________________

* In step one you first figure out how the figure or table is set up (e.g. what the labels on the axes mean). You also need to have a pretty good idea of the experimental design - how the scientists set up the experiments - and the hypotheses the scientists address. In step two you can go on to interpreting the data. For both steps write down any questions you have.