Biogeochemistry – the bio-mediated cycles of elements on Earth

1. Biogeochemistry – the bio-mediated cycles of elements on Earth K. Limburg lecture notes, 5 February 2002

2. Shahid Naeem, biodiversity expert, visiting SU last week – comments on the thickness of the biosphere relative to the Earth In an earlier lecture, we heard about the massive physical processes that drive the movements of rock cycles, continental uplift, erosion, and denudation, tectonic plate dynamics, volcanoes… Biogeochemistry is about how biotic and abiotic processes interact to drive our global ecosystems.

3. Thickness of the biosphere ……….? ~100 m 12,756 km Diameter of the Earth ……………? 12,756 km = 12.756 x 106 m The biosphere is over 120,000 times thinner than the Earth By analogy, if the biosphere is a piece of paper, the Earth is a book-case that is 33 feet across

4. Hard as it is to believe, this thin “skin” – of which we are a part – has profound effects on the climate, the hydrologic cycle, the cycles of major elements, and thus even the geological cycles Source: W.H. Schlesinger 1997. Biogeochemistry (2nd edition). Academic Press

5. A hypothesis is that the biosphere possesses a complex set of feedback mechanisms that regulate the climate and temperature – a “global homeostatic mechanism” James Lovelock (1979) – coined the term “Gaia” to refer to this set of mechanisms – published a book and a series of papers with others

6. An example of this biotic mediation is one we’re familiar with now…the increase in [CO2] in the atmosphere Source: Schlesinger 1997

7. Source: Vitousek 1994. Ecology 75: 1861-1876 Just how big a change is this, really? What kind of scale can we rank this on?

8. Source: Vitousek 1994

9. The distribution of elements related to their formation – biogeochemistry focuses on a few of the key lighter elements Source: Schlesinger 1997

10. The elements C, H, N, O, P, and S form the proteins, carbohydrates, lipids, and hard elements (e.g., bone) that form living organisms – other important elements include Ca, Na, K, Mg, Si…trace elements… Life resists thermodynamic equilibrium! The molecular bonds in living tissue are reduced (e- enriched) In the presence of a strong oxidizing agent (O2), these should tend to be broken down spontaneously to CO2, H2O, and NO3 – but they’re not. Why?

11. For the biosphere in general, and for watersheds in particular, we’re concerned with the distribution and cycling of carbon, nitrogen, and phosphorus primarily, and secondarily a slough of other elements In future lectures, we’ll look at how some of these elements move through different parts of watersheds

12. Atmosphere Biosphere Anthroposphere Hydrosphere Lithosphere

13. Much as we outlined a series of steps in doing an ecosystem analysis earlier, the study of elemental cycles requires a similar approach*: • 1st - Accounting: Accounting tells you "where things are", or the distribution of the element in different pools within the ecosystem.  • 2nd - Cycling: Cycling tells you "where things are going", and how fast they are moving from different pools in the ecosystem.  • 3rd - Controls: Determining the controls tells you "how does the system function, and what factors drive the cycling".  * taken from “Global Change” course notes, University of Michigan

14. We need to consider the major reservoirs of N, C, and P – where are they? • N – in the atmosphere (78% of the atmosphere; 3.87 x 1021 g) • C– in sedimentary rocks and fossil fuels (approx. 8 x 1022 g); secondarily in oceans • P – in rocks (1016 – 1017 g) What are the major forms of N, C, and P that are relevant to watersheds?

15. The nitrogen cycle • N is an essential nutrient • an essential part of proteins (16%) • until recent human intervention, not all that available to the biosphere – molecular nitrogen (N2) has trivalent bonds • NN  requires 950 kJ/mol to break

16. Global accounting of nitrogen – how much, where? Data sources: Schlesinger 1997, O’Neill 1998

17. Cycling Source: Schlesinger 1997

18. N2 fixation photosynthesis N2O Organic matter NH4+ NO2 NO3 nitrification mineralization denitrification N2O N2 Detrital O.M. NH4+ Atmospheric gases (N2, N2O) Controls (non-human) Aerobic conditions Anaerobic conditions

19. Inorganic N species Organic N Source: O’Neill 1998. Environmental Chemistry (3rd ed) Chapman & Hall

20. Acetobacter C. Kennedy, Dept Plant Pathology, Univ of AZ Examples of natural N fixers www.agroforester.com Rhizobium D.M. Krempels, U. Miami

21. N fixation by the Haber-Bosch process: artificial fertilizer www.nobel.se 3H2 + N2 gas + catalyst + heat + pressure  2NH3 (ammonia gas, can be cooled down & liquified)

22. Source of data: The Fertilizer Institute, Washington, DC

23. Source: Vitousek 1994

24. Source: Vitousek 1994

25. Atmospheric effects of anthropogenic N Burning fossil fuels (in power plants, in furnaces, in combustion engines) heats air enough so that N2 and O2 react to form oxides of nitrogen (Nox) Although the average global levels are fairly low (0.003 ppm), in cities the concentrations can reach as high as 2 ppm – a key ingredient of smog NO can react with ozone (O3), another air pollutant, to form nitrate, and when in contact with water this can form nitric acid (HNO3)  acid rain

26. Source: O’Neill 1998.

31. Anthropogenic N in water -- issues Nitrate is: • highly soluble, stable over a wide range of environmental conditions. Can readily enter streams, lakes, & groundwater • toxic to animals, including fish and small humans • methemoglobinemia – “blue baby syndrome” NO3 oxidizes Fe2+ to Fe3+ - won’t carry O2 in blood. Action limit is 10 mg/L in water.

32. Anthropogenic N in water – issues, cont’d. Ammonia: • occurs in two forms, NH3 and the ionized form, NH4+. • NH3 is far more toxic to aquatic life (values > 0.5 mg/L toxic to fish) • toxicity increases with temperature and pH • however, NH3 is also volatile, can readily enter the air

33. Next time: carbon, phosphorus cycles