1. “Carbon Isotope Systematics in Soil” -or- “Plant Poo and Microbe Farts” Justin Yeakel, UCSC

2. Soil Pathway Summary Organic matter finds it’s way to the forest floor and decomposes SOM (Soil Organic Matter) is further decomposed by microbes which emit CO 2 as a biproduct Under certain conditions, water and this carbon dioxide can form carbonates

3. Each step in this sequence is met with a fractionation of the original carbon. Diffusion of carbon dioxide and mixing with the atmosphere is responsible for the drop in δ 13 C values of CO 2 and CaCO 3 as you descend through the soil layers. Cause of rise in δ 13 C values with depth in SOM is debated.

4. Since C3 and C4 plants are distributed based on specific environmental parameters… And the carbon that is cycled through soils is from these plants…  values in the carbon found in soils will be representative of these respective photosynthetic pathways

6. What else could possibly contribute a significant amount of carbon to this process?

7. Parent material

8. Most carbon in soil carbonates is from soil gas (vegetation, respiration, atmosphere) and not from the parent material that soil is found on.  13 C Soil Carbonates Altitude (meters)

9. Phase 1: Enrichment of SOM δ 13 C values -Trees respire -Roots respire -Litter on the forest floor gets eaten by bacteria This CO 2 is representative of the particular photosynthetic pathway (C3 or C4), and is therefore about -27‰ or -13‰ respectively.

10. CO 2 Initially,  13 C values of the SOM look exactly like the plant. But with time and/or depth, SOM  13 C values rise.

11. Depth (cm)  13 C -300 -100 0 -20

12. Torn et al. 2001

13. Why the SOM  13 C values rise?  13 C Depth 1)Suess effect - recent drop in δ 13 C values of plants due to fossil fuel burning, etc. 2)Differential loss of components (lipids, lignin, protein, etc.) with different δ 13 C values. 3)Progressive 13 C-enrichment of residual SOM due to respiratory loss of 12 C- enriched CO 2. 4)Carboxylation in soils in the presence of soil gas that is 13 C-enriched relative to typical plant matter!!

14. Although there is a rise in the  13 C values of SOM material at greater depths, most carbon is digested by microbes and released into the soil as CO 2, leaving little remaining carbon in deeper SOM. This 13 C-enrichment in deeper SOM does not have a great effect on the  13 C values of soil CO 2 because is occurs at such low concentration.

15. Phase 2: Soil CO 2 Via decay of SOM and root respiration, soils are a huge source of CO 2, which diffuses into the atmosphere.

16. Phase 2: Soil CO 2 Via decay of SOM and root respiration, soils are a huge source of CO 2, which diffuses into the atmosphere. The diffusion of CO 2 has a direct impact on the  13 C values, and is a affected by properties of the soil. The pattern produced by diffusion can be modeled to estimate the  13 C values of the CO 2 at differing depths.

17. There is an initial 4.4‰ enrichment due to diffusion of CO 2 out of the soils Based on soil properties, atmospheric CO 2 plays a role at differing depths in a soil  13 C values at depth reflect the respiration, whereas values nearer the surface are more affected by the atmosphere Cerling et al. 1991

18. *Per mil value of CO 2 in the soil as a function of depth 13 C/ 12 C ratio of isotopic standard PDB Constructed to put number into delta format Soil Properties: Production rate of respired CO 2 Diffusion coefficient for CO 2 in soil Depth Soil Tortuosity Ratio of 13 C in diffused CO 2 Per mil value for soil- respired CO 2 Per mil value for atmospheric CO 2 Concentration of CO 2 in the atmosphere

19. This equation is set by parameters that limit the function. -The uppermost boundary condition is that soil CO 2 concentration equals that of the atmosphere (z=0) -CO 2 production is modeled with depth and an impermeable lower boundary at some finite z.

20. In more productive soils (red), the effects of the atmosphere are only seen in the top few cm. Soil gas has a  13 C value = respired CO 2 + offset due to diffusion. In less productive soils, atmospheric affects gas isotope values deep in the soil (blue).

21. Phase 3: Carbonate Formation In the presence of water, the soil CO 2 goes through a number of chemical reactions that can result in the formation of soil carbonates.

22. CO 2 + water  H 2 CO 3  H + + HCO 3 -  Ca 2+ + 2HCO 3 -  CaCO 3 + CO 2 + H 2 O

23. Acid formed by hydration of soil CO 2 dissolves minerals near the soil surface and is slowly buffered by this process. At deeper levels in the soil, either because of high Ca 2+ concentration (due to mineral dissolution or seasonal evaporation) or simply due to the degassing of CO 2 from soils, the reaction above is driven to the right, leading to soil carbonate precipitation.

24. CO 2 + water  H 2 CO 3  H + + HCO 3 -  Ca 2+ + 2HCO 3 -  CaCO 3 + CO 2 + H 2 O Fractionations that occur along these reactions are temperature-sensitive. Between the 4.4‰ impact of diffusion and these equilibrium reactions, soil carbonate is 13 C-enriched by 13.5 to 16.5‰ relative to soil organic matter.

25. CO 2 + water  H 2 CO 3  H + + HCO 3 -  Ca 2+ + 2HCO 3 -  CaCO 3 + CO 2 + H 2 O In addition, the  13 C is strongly correlated to that of the SOM and overlaying flora Cerling & Quade 93

26. CO 2 + water  H 2 CO 3  H + + HCO 3 -  Ca 2+ + 2HCO 3 -  CaCO 3 + CO 2 + H 2 O The fractionations that occur during carbonate formation enrich the  values from 13.5 and 16.5 per mil between 0ºC and 25ºC respectively.

27. How deep a carbonate is when it forms also has an impact on it’s carbon isotope value. This is directly correlated to the  13 C value of soil CO 2, which differs according to depth.

28. The model described previously is used in the graph above, and shows a desertine C4 environment. Cerling & Quade 93

29. Progression of  13 C values in soil CO 2 to soil carbonate ~14‰ enrichment in 13 C

30. Overall Process: SOM Soil CO 2 Soil Carbonates 13 C-enrichment with depth Cause unclear, but no large effect overall on δ 13 C of soil CO 2 Microbial digestion of SOM and root respiration releases CO 2 into soil. Diffusion leads to 13 C enrichment of ~4.4‰ relative to respired CO 2. With increasing depth, atmosphere affects  13 C of soil CO 2 less and less, and soil CO 2 is more representative of vegetation As soil CO 2 reacts to form soil carbonate, C isotopes are fractionated differently according to temperature. Overall fractionations amount to 13.5 to 16.5 ‰ enrichment relative to plants or soil CO 2.

31. All Summed up

32. T H E E N D