Biogeochemistry wrap-up

1. Biogeochemistry wrap-up K. Limburg lecture notes, 12 February 2002

2. Outline: • Biogeochemistry of • carbon cycle • phosphorus cycle • Relevance at the watershed scale

3. Carbon is, by definition, the basic element of life on Earth The major pools: After Schlesinger 1997

4. After Schlesinger 1997

5. Schlesinger 1997

6. World Soil Resources (USDA)

7. Clearly, one of the major factors driving carbon cycling is primary production – reflected in the annual patterns of atmospheric CO2 After Schlesinger 1997

9. Global terrestrial NPP U. Montana EOS Center

10. Ocean Primary Productivity Group, Rutgers University

11. Some important biomolecules: Engines of photosynthesis Lehninger (1977) Bioenergetics

13. Decomposition of plant matter yields complex molecules – humic and fulvic acids Dr. R. Town, School of Chemistry, Queens Univ. Belfast Oak Ridge National Laboratory

14. U.S. Forest Service

15. Skinner et al. 1999. Blue Planet. Wiley.

16. World Soil Resources (USDA)

17. World Soil Resources (USDA)

18. C-fluxes to the atmosphere Production/respiration + fossil fuel burning  increase in “greenhouse effect”

19. Limited by mixing rate of deep and surface waters After Schlesinger 1997

20. Methane (CH4) is another greenhouse gas Less abundant than CO2, but potentially 25X as effective at trapping heat in atmosphere Increasing 1%/yr After Schlesinger 1997

21. The phosphorus cycle. Phosphorus (P) is one of the more abundant of elements on earth, but by no means the most biologically available. P Figure source: “Global Change” course notes, University of Michigan

22. 19

23. The phosphorus cycle differs from other major elemental cycles in one important way: there is no atmospheric pathway (except for dust transport). www.toms.nasa.gov

24. Where does P come from?  The origin of phosphorus in the biosphere comes from volcanic eruptions. Although P is part of many minerals, the most common form is apatite:  Ca5(PO4)3 (F, Cl, OH) Ca5(PO4)3F - fluorapatite Ca5(PO4)3Cl – chlorapatite Ca5(PO4)3OH – hydroxylapatite The mineral apatite is an essential component of bones and teeth

25. Phosphorus becomes available in the lithoshere via two main pathways:  Rock weathering  Rock mining Mechanical weathering is important in extreme environments where rock is exposed to seasonal extremes of temperature, moisture, wind, etc. Chemical weathering occurs when rocks and soils react with acids and oxidizing agents. Typically, minerals are dissolved and ions exist in solution that can be taken up by organisms or, more often, bound in soils. Rates of weathering depend on the mineral types, moisture, temperature, and pH.

26. One very important chemical reaction that promotes weathering is the carbonation reaction:  H2O + CO2 H+ + HCO3- H2CO3 . Microbial activity (decomposition of organic matter) can increase the CO2 concentration in soil waters far above its atmospheric concentration (360 ppm or 0.036%). For example, [CO2] in soils beneath wheat fields in Missouri were reported to reach > 7% (= 70,000 ppm) This sets up a strong gradient that drives the reaction to the right:  H2O + CO2 H+ + HCO3- H2CO3 (carbonic acid)

27. Apatite can undergo weathering via a congruent reaction (co-occurs with carbonation) that releases P:  H2O + CO2 H+ + HCO3- H2CO3 Ca5(PO4)3OH + 4H2CO3 5Ca2+ + 3HPO42- + 4HCO3- + H2O The HPO42- is called orthophosphate and is a form readily taken up by plants.

28. P is most biologically available at pH values near 7. That’s why farmers have to lime their fields, if they are acidic. The availability of orthophosphate is strongly governed by pH:

29. Most P is precipitated into unavailable forms, particularly if oxides of Fe or Al are present. (Because such oxides are widespread in tropical soils, P is relatively unavailable there.) • P bound by FeOH or AlOH is termed occluded because it is held in the interior of the oxide crystals and is thus biologically unavailable. • Nonoccluded P forms can be bound onto the surfaces of soil minerals.

30. Over a long period of time, the weathering of apatite goes from occluded and nonoccluded forms being most abundant, to occluded and organic-P forms (i.e., biologically fixed P). Very old weathered soils are called laterites (clay-like soils) and contain essentially no available P. Photo: J.R. Smyth, U. Colo.

31. Other important features of soil chemistry that determine rates of chemical weathering include • cation exchange capacity (important in temperate soils) affecting soil buffering • anion adsorption capacity (mostly important in tropical soils) Phosphate anion (PO43-) is one of the most strongly adsorbed onto tropical soil particles, which explains its low bioavailability. P in many tropical ecosystems is thus almost exclusively recycled organic P.

32. Phosphate rock mining – the other source of P Phosphate ore deposits are fairly widespread throughout the continents, and so are available for mining. Global production (mining) of phosphate rock from 1995-1999 averaged 138.8 x 106 metric tons, equivalent to around 19 x 106 metric tons of P.

33. The US is the single largest producer of mined phosphate (27.3% of world production, 1995-99), and these come from 18 mines. However, 86% of this production comes from 12 mines in Florida and 1 mine (the world’s largest phosphate mine) in Beaufort, North Carolina. Photos: Aurora Potash Corp of Saskatchewan

34. Most (93%) is used to produce chemical fertilizers and animal feed supplements. So-called superphosphate is produced by crushing the parent rock, mixing it into a slurry with H2SO4, and extracting the phosphate.

35. 1. Phospholipids – key component of cell membranes source: http://ampere.scale.uiuc.edu/~ecoscoll/fsi/pictures/phospholipids.gif Phosphorus use in organisms.  P is a key component in a number of biomolecules and biochemical reactions:

36. 2. DNA, RNA Phospho-diester bridges link nucleotides 3. ATP, ADP, AMP– the energy molecules of organisms

37. Because P is involved in important biochemical processes, and because it can be unavailable due to soil chemical characteristics, it is often a limiting nutrient.

38. P required for primary production: Redfield’s (more complete) stoichiometric equation of photosynthesis in the ocean plankton: 106CO2 + 16NO3- + HPO42- + 122H2O + 18H+  (CH2O)106(NH3)16(H3PO4) + 138O2 . Redfield ratio: 106 atoms C per 16 atoms N per 1 atom P. This ratio is a good indicator of nutrient limitation, at least in aquatic ecosystems.

39. What’s important about C, N, P, and other elements in a watershed context? • patterns of production • nutrient transformation (what chemical species, and where are they? • nutrient transfers (fluxes) • nutrient ratios • structural influences on fluxes • biotic influences on fluxes

40. What’s important about C, N, P, and other elements in a watershed context? • knowledge of the effects of: • land use • land use change • position in the landscape (?) • location of “hot spots” (?)