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Lecture 4: Terrestrial Carbon Process I. Carbon Stocks and Fluxes in Terrestrial Ecosystems II. Terrestrial Ecosystems

Lecture 4: Terrestrial Carbon Process I. Carbon Stocks and Fluxes in Terrestrial Ecosystems II. Terrestrial Ecosystems A. Ecosystem Concept B. Ecosystem Carbon Balance (GPP, NPP, NEP, NBP) III. Missing Carbon sinks. Fluxes and Pools of Carbon on the Earth. Terminology.

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Lecture 4: Terrestrial Carbon Process I. Carbon Stocks and Fluxes in Terrestrial Ecosystems II. Terrestrial Ecosystems

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  1. Lecture 4: Terrestrial Carbon Process I. Carbon Stocks and Fluxes in Terrestrial Ecosystems II. Terrestrial Ecosystems A. Ecosystem Concept B. Ecosystem Carbon Balance (GPP, NPP, NEP, NBP) III. Missing Carbon sinks

  2. Fluxes and Pools of Carbon on the Earth Terminology Carbon pool: The reservoir containing carbon (carbon stock) Carbon flux: The rate of exchange of carbon between pools (i.e. reservoirs) Carbon sinks: Carbon reservoirs and conditions that take-in and store more carbon (e.g. carbon sequestration) than they release. Carbon sources: Carbon reservoirs and conditions that release more carbon than take-in and store Carbon sequestration: The uptake and storage of carbon

  3. 2). The Units of Carbon Pg C :petagrams of carbon (1 Pg C= 1015g C = 1 billion tons C) Gt C : gitatonnes of carbon (1 Gt C = 1 Pg C) Mt C : megatonnes of carbon (1 Mt C = 1012 g C) Tg C : teragrams of carbon (1 Tg C = 1 Mt C) (1 Pg C = 3.7 Pg carbon dioxide) 1 C = 3.7 CO2

  4. 1. Terrestrial ecosystems are important components in the global carbon cycle that create many of the sources and sinks of CO2, (methane, and nitrous oxide); • The Ecosystem Concept • The ecosystem including “ not only the organism-complex, but the whole • complex of physical factors forming what we call the environment” • (Tansley, 1935) • Whittaker (1975) suggested that “ an ecosystem is a functional system that includes assemblage of interacting organisms (plants, animals and microbes) and their environment, which acts on them and on which they act.” Ecosystem: Biotic community:Plant, animal, microbial community Abiotic environment:atmosphere, soil and geological substrate

  5. 2. The dynamics of terrestrial ecosystem depend on interactions between a variety of biogeochemical cycles, particularly the carbon cycle, the cycles of nutrient and water. For example: the interconnectedness of the various living and nonliving components of ecosystem that a change in any one will results in a subsequent change in almost all others.

  6. 3.  Carbon Balance of Terrestrial Ecosystem (4 important concepts) • GPP (Gross Primary Production): • The total amount of carbon fixed in the process of photosynthesis • by plants in an ecosystem, such a stand of trees. • Global total GPP: 120 Gt C yr-1 (b)NPP (Net Primary Production): The net production of organic matter by plants in an ecosystem – that is, GPP reduced by losses resulting from the respiration of plants (autotrophic respiration, Ra). NPP = GPP- Ra Global total NPP: 60 Gt C yr-1

  7. (c) NEP (Net Ecosystem Production): The net accumulation of organic matter or carbon by an ecosystem. NEP = NPP-Rh (heterotrophicrespiration) Rh includes losses by herbivory and the decomposition of organic debris by soil biota (= 50 Gt C yr-1) Global total NEP: 10 Gt C yr-1 (NEP is also called NEE- net ecosystem exchange)

  8. (d) NBP (Net Biome Production): The net production of organic matter in a region containing a range of ecosystem (a biome). NBP = NEP – non-respiratory C losses through ecosystem disturbances (harvest, forest fire, clearance, insect etc.) (by Schulze, E-D and M. Heimann, 1998) Global NBP: is small (about 1 Pg C for 1989-1998) NEP and NBP are key indicators used to describe the annual net C balance of forest ecosystems

  9. CO2 Disturbance (9 Gt C yr-1) Plant Respiration (60Gt C yr-1) Decomposition (50Gt C yr-1) GPP = 120 Gt C yr-1 Med.-term Carbon Storage Long-term Carbon Storage Short-term Carbon Uptake CO2 assimilation NPP 60 Gt C yr-1 NEP 10 Gt C yr-1 NBP ±1 Gt C yr-1 0.5% <5% 50%

  10. 4.Carbon Stocks and Flows in Major Biomes of Terrestrial Ecosystems ·-Total ecosystem area: 151.2  106 km2 ·       -Vegetation carbon: 500 Gt C ·       -Soil carbon: 2000 Gt C ·       -Total terrestrial biomes:  2500 Gt C ·Forests contain a large part of carbon stored on land, in the form of biomass (trucks, branches, foliage, roots etc.) and in the form of soil organic carbon.       ·Grassland ecosystems store most of their carbon in soils, where turnover is relatively slow.

  11. Wetlands are important carbon reservoirs. • - Undrained peatlands in high latitude - C sinks • (0.2-0.5 t C ha-1 yr-1), but, sources of methane • (0.03-0.3 t CH4 ha-1 yr-1); • - Peatlands that are drained for agriculture • or for afforestation release carbon as CO2, • No significant methane release. In croplands, carbon stocks are primarily in the form of below-ground plant organic matter and soil

  12. Atmospheric evidence of large carbon exchanges by the biosphere

  13. 3). Major carbon pools on the Earth • The Earth contains about 1023 g of carbon • Active surface Pools • Atmosphere: 7.5 x 1017 g ( = 750 Pg ) • Land plants: 6.1 x 1017 g (=610 Pg) • Soils: 1.6 x 1018 g (=1, 600 Pg) • Ocean Dissolved Inorganic carbon: 3.9 x 1019 g • (surface and deep oceans) (=39,000 Pg) • Fossil Fuels and cement production: 4 x 1018 (=4,000 Pg) • - Sedimentary rocks • organic compounds: 1.56 x 1022 g • carbonate: 6.5 x 1022 g

  14. 4). Major carbon fluxes on the Earth NPP (net primary production) on land: ~60 Pg yr.-1 - Photosynthetic Uptake – 120 Pg yr.-1 - Plant Respiratory Loss - 60 Pg yr.-1 Soil Respiration - 60 Pg yr.-1 Fossil Fuels ~5.5 Pg yr.-1 Land Use Changes ~1.6 Pg yr.-1 Ocean Uptake (physiochemical diffusion) ~92 Pg yr.-1 Ocean Release (physiochemical diffusion) ~90 Pg yr.-1

  15. Two important factors contributing global carbon budgets Contemporary carbon cycle is controlled primarily by the rate at that carbon moves in and out of these pools, not by their size; For example: Calcium carbonate in desert soil: 930 Pg C (930 x1015 g C) Flux from this pool to atmosphere: 0.0023 Pg C yr –1 (85,000 years turnover time!) Small change in the rate of large carbon pools can have a dramatic impact on the atmospheric CO2 For example: A 0.1% increase in the rate of decomposition on land, release about 0.6 Pg C /yr to the atmosphere

  16. 2. The Missing Carbon Sinks Atmospheric Imbalance - the Missing Carbon Sink? Net sources of CO2 to the atmosphere (in units of 1015g) Fossil Fuels - 5.5 Pg yr.-1 Land Use Change - 1.6 Pg yr.-1 Net sinks for CO2 from the atmosphere Atmospheric Increase ~3.2 Pg yr.-1 Oceanic uptake (increase) ~2.0 Pg yr.-1 Net carbon emissions = Net carbon sinks Imbalance ~1.9 Pg yr.-1(unknown carbon sinks) (ref. 1980 to 1989)

  17. Missing Carbon Sinks: The imbalance between carbon emissions and sinks (of about 1.9 Pg C yr-1 for 1980’s) is often refereed to as the “missing carbon sinks”

  18. Finding the Missing Carbon Sinks Hypothesis 1: There is a large terrestrial sinks for anthropogenic CO2 in the Northern Hemisphere Hypothesis 2: The ocean carbon sinks will continue to increase in response to rising atmospheric concentrations, But, the rate of increase will be modulated by changes in ocean circulation, biology and chemistry

  19. Take home message:Two Questions 1. What are the evidences in support of the terrestrial carbon sinks? 2. What are the major factors contributing the terrestrial carbon sinks?

  20. 1. What are the evidences in support of the terrestrial carbon sinks? A. Expected CO2 North-South Gradient: The smaller than expected north-south gradient of atmospheric CO2, combine With data on the partial pressure of CO2 in ocean surface waters, suggests that There is a large terrestrial carbon sinks at temperate latitudes in the Northern Hemisphere (Tans et al., 1990) Tans, P., I.Y. Fung, and T. Takahashi. 1990. Observational constraints on the global atmospheric CO2 budget. Science, 247: 4131-1438.

  21. B. Isotopic Fractionation:The existence of a large terrestrial sink at northern latitude is supported by 13C/12C ratio measurement in atmospheric CO2 (Ciais et al., 1995) and by measurements of the O2/N2 ratio (Keeling et al., 1996). • Ciais, P., P.P. Tans, M. Trolier, J.W. C. White, and R.J. Francey. 1995. A large north hemisphere • terrestrial CO2 sinks indicated by the 13C/12C ration of atmospheric CO2. Science: 269: 1098-1102. • Keeling, R.F., S.C. Piper, and M. Heimann. 1996. Global and hemisphere CO2 sinks deduced from • changes in atmospheric O2 concentration. Nature, 381: 218-221.

  22. C. New Technique of Eddy Covariance:Flux measurements obtained by this technique in different ecosystems have demonstrated the ability of some forest to act as significant net sinks for atmospheric CO2 (Wofsy et al., 1993) • Wofsy, S.C et al. 1993. Net exchange of CO2 in a mid-latitude forest. Science, 260: 1314-1317. D. Recent Forest Inventory:suggested that there has been a substantial increase in the carbon stock in northern forest biomass, of the order of 06-0.8 Pg C y-1 (Caspersen et al., 2000; Fang et al. 2001; Goodale et al. 2002) Caspersen, J. P., et al. 2000: Contributions of land-use history to carbon accumulation in U.S. forests. Science290, 1148-1151.7 Fang, J. A. Chen, C. Peng, X. Zhao, and L. Ci, 2001: Changes in forest biomass carbon storage in China between 1949 and 1998. Science292, 2320-2322. Goodale, C.L. et al. 2002. Forest carbon sinks in the Northern Hemisphere. Ecological. Application, 12: 891-899

  23. E. Remote Sensing Estimates:Alarge carbon sink, of order 0.68 Pg C, Was found in the woody biomass of Northern forests based on 19 years of data from remote-sensing spacecraft and forest inventory. Myneni, R.B. et al. 2001. A large carbon sink in the woody biomass of Northern forests. PNAS: 98: 14784-14789

  24. 2. What are the major factors contributing the terrestrial carbon sinks? • Several factors contribute to carbon sinks including: • Direct human impact (e.g. reforestation and tree regrowth) • Indirect human impacts (e.g. CO2 and nitrogen fertilization) • Natural factors (e.g. climate variability, Fire )

  25. Schimel et al. (1995) attributed the missing carbon sinks to: • Enhance forest growth due to CO2 fertilization (Beta factor) • e.g. CO2 stimulation of plant growth (32-41%) • (2) Forest regrowth due to land-use and land cover change • (3) Increase N deposition • (4) Positive response to climate anomalies (climate variability)

  26. Land Cover Change in USA Ref. G. C. Hurtt et al., Proc. Natl. Acad. Sci. USA 99, 1389 (2002).

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