Hypothetically, there is a terrestrial sink for carbon dioxide (CO2). The real issue lies deeper than understanding the processes of photosynthesis and respiration. The real unknown factor evolving around the terrestrial ecosystem is in the feedback mechanisms and not the sequestering of carbon by crop yield. So, rather than debate whether biofuels are displacing the carbon budget, I'd like to bring to light how changes to world climate can influence how carbon is stored in a terrestrial environment.
The negative feedbacks or possible sinks would be CO2 fertilization of soils and warming- enhanced mineralization of nitrogen. Both play large rolls in the carbon-to-nitrogen (C:N) ratio of soils, which is less than that observed in woody tissue and more than that observed in soil organic matter. Theoretically, the CO2 fertilization allows for an increase in the growth of woody tissue and the storage of carbon in biomass. Nitrogen mineralization has the same affect in that it also increases plant growth.
Other processes suggest that global warming is a positive feedback or a loss of terrestrial carbon storage. Respiration rates are positively correlated with global warming. So, warmer climates allow for biomasses to give more CO2 back into the atmosphere
The issue at hand is that these processes are very poorly understood, and concrete evidence for large-scale sources and sinks is not a direct measurement. Is a CO2-enhanced warming trend going to feedback positively or negatively with the terrestrial ecosystem?
After asking ourselves if un-accounted for feedback mechanisms are the reason we have an unbalanced model of the global carbon budget, we can then debate the carbon mass existing in the terrestrial environment. Do changes in land use outweigh the terrestrial sink? Do we respire, decay and burn more organic carbon than we store?
Editor's Note: This is Part 2 in a multi-part series. Part 1 is already on-line.
References:
1. Chester, Roy. Marine Geochemistry Second Edition. Malden, MA: Blackwell Science Ltd. 2000, 2003
2. Richard Houghton, Senior Scientist, Carbon Research. Understanding the Global Carbon Cycle. Woods Hole Research Center, http://www.whrc.org/carbon/
3. Hesshaimer, Vago, Heimann, Martin, Levin, Ingeborg. Radiocarbon evidence for a smaller oceanic carbon dioxide sink than previously believed. Nature (London). 370 (6486), p. 201-203, 1994.
4. Frankignoulle, Michel , Borges, Alberto V. European Continental shelf as a significant sink for atmospheric carbon dioxide. Global Biochemical Cycles. Pages 569-576, September 1, 2001
5. Houghton, R. A. , Davidson, E. A. , Woodwell, G.M. Missing sinks, feedbacks, and understanding the role of terrestrial ecosystems in the global carbon balance, Global Biochemical Cycles. Vol. 12, No. 1, Pages 25-34, March 1998
6. Tsunogia, Shizuo , Ono, Tsuneo , Watanabe, Shuichi. Increase in total carbonate in the western North Pacific water and a hypothesis on the missing sink of anthropogenic carbon, Journal of Oceanography, Vol. 49, Pages 305-315, 1993
7. Burdige, David J. , Alperin, Marc J., Homstead, Juliana, Martens, Christopher S., The role of benthic fluxes of dissolved organic carbon in oceanic and sedimentary carbon cycling. Geophysical Research Letters, Vol.19, Pages 1851-1854, 1992
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Re: <B>The Missing Carbon Problem (Part 2)</B>
