1. Examining Total Belowground Carbon Allocation (TBCA) Using the PnET-CN Model
Kathryn Berger
UNH Department of Natural Resources
Research and Discover Fellow
Advisor: Scott Ollinger
Committee: Christy Goodale, Andrew Richardson,
& Mary Martin

2. Outline Significance of TBCA Current Understanding
Environmental Factors Influencing Carbon Allocation
Elevated CO2
Nitrogen Limited Systems & N Deposition
The PnET Model
Thesis Focus & Research
PnET-CN & FACE
Carbon and Nitrogen Availability Database
Summary

3. Significance
Anthropogenic sources have increased CO2 by 35% since the Industrial Revolution
Only 45% of all carbon released remains in atmosphere
Need for identification of missing carbon sink
Terrestrial ecosystems:
Large carbon pool in soils
Slow turnover rates
Implications for environmental policy

4. Current Understanding
Terrestrial ecosystems carbon neutral until 1990s
Crossover to carbon sink suspected result of land use changes in North America and Europe
Reforestation
Increased fire prevention
Changes in environment (longer growing seasons, fertilizing effects of air pollution)
Current net flux estimation: -1.4 (+/-) -0.7 Pg C y-1 (IPCC, 2001)
Uncertainty as to how forests will react to increased levels of atmospheric CO2:
Increased storage, or
More rapid processing of resources
Knowledge limited because of magnitude and duration of studies needed to draw conclusions

5. Current Understanding(
When carbon is allocated belowground it can:
Become immediately lost via soil respiration
Increase growth of root systems that undergo fast turnover rates & decompose quickly
Exude from root systems and used by microorganisms in the soil
Enter into woody portions of long-lived roots that promote carbon storage

6. Environmental Factors Influencing Carbon Allocation
Elevated CO2
Nitrogen Limited Systems
Tropospheric O3

7. Elevated CO2 Increased CO2 causes fertilization effect
Can alter chemistry of plant tissues; litter input into soil pool
Mediated by feedback related to microbial processes
C:N ratios, rates of litter decomposition, availability of N in soil
For C sequestration: nutrient availability must not deter plant growth
Organic C must be allocated to stable soil pools with low turnover periods

8. Elevated CO2 Free-air CO2 enrichment (FACE) experiments
Opportunity to make long-term observations of forests under elevated CO2 in realistic forest stand conditions

9. Elevated CO2

10. Elevated CO2
Oak Ridge National Laboratory (ORNL) FACE sweet gum plantation: CO2 enrichment increased fine-root production (Norby et al., 2004)
Highest increase in root production under elevated CO2 occurs in deeper layers of soil where sequestration suspected more likely (Norby et al., 2004)
Additional studies have reported that over half of carbon allocated belowground (in both elevated & ambient plots) is found in microaggregates protected from decay (Jastrow et al., 2005)
Demonstrated very little saturation in this protection mechanism after 5 years (Jastrow et al., 2005)

11. Nitrogen Limited Systems
Nitrogen most limiting nutrient in temperate forest ecosystems
Potential to down-regulate positive feedback loops caused by elevated atmospheric CO2
Progressive Nitrogen Limitation (PNL) caused by the rapid rate of N immobilization by plants and microorganisms
Scientists also hypothesize increased N could ameliorate the effects of rising CO2 by aiding N-limited systems
However, if growth is in nutrient rich (low C:N) with faster turnover, carbon sequestration may be minimal

12. Nitrogen Limited Systems
Additional studies have suggested alternative reasons for lack of growth:
15N-tracer studies in 9 forests suggest N deposition will play only minor role in C sequestration (Nadelhoffer et al., 1999)
Current N deposition accounts for less than 20% of annual Pg CO2 carbon uptake credited to forest growth (Nadelhoffer et al., 1999)
Körner et al., (2005) suggests soil microbial feedback mechanisms or ambient O3 to explain the lack of growth at a Swiss FACE experimental forest
Quantifying amount of carbon in terrestrial ecosystems as a result of anthropogenic N sources will have significant implications on the global carbon cycle and missing carbon sink

13. The PnET Model Developed at UNH (Aber and Federer, 1992)
A simple, daily-to-monthly time-step model of carbon and water fluxes
Uses a select number of parameters to portray essential interactions between nitrogen availability and leaf physiology as they influence photosynthesis and transpiration
Currently three grouped computer models that make up PnET:
PnET-DAY
PnET-II
PnET-CN

14. The PnET Model
The model estimates productivity in the plant pool by allocating biomass by tissue type: foliage, wood, and/or fine roots
PnET equation:
Fine-root carbon = * leaf carbon (Aber and Federer, 1992)
PnET model’s mechanism for TBCA is based on the following equations:
Raich & Nadelhoffer (1989)
Rs-Pa ≈ Pb + Rr
Pb + Rr is comparable to fine-root carbon
Davidson et al. (2002) using IRGA measurements:
(TBCA) = Rsoil – Litterfall-C
Both equations based on tentative assumption that carbon pools are at steady state
What about implications of global climate change? Environmental pollution? FACE experiments?

15. Thesis Focus & Research
PnET-CN & FACE
Research Question: Does the PnET-CN model’s TBCA mechanism correctly predict carbon allocation in soils under elevated atmospheric CO2 conditions?
Carbon and Nitrogen Availability Database
Research Question: Is there a trend between TBCA and nitrogen availability in terrestrial ecosystems?

16. PnET-CN & FACE Duke FACE Forest, Durham, NC (FACTS-I)
v37_3_04/images/a09_sweetgum_full.jpg
faculty/katul/project4.html
Duke FACE Forest
Aspen- FACE
Oak Ridge FACE
Duke FACE Forest, Durham, NC (FACTS-I)
Loblolly Pine Plantation w/ Sweetgum Understory
Oak Ridge FACE, Oak Ridge TN
Sweetgum Plantation
Aspen-FACE Aspen-FACE, Rhinelander, WI (FACTS-II)
Aspen Plantation

17. PnET-CN & FACE
PnET-CN has not yet been used to examine TBCA mechanisms in FACE experimental sites
Results from the PnET-CN model runs will be compared to published literature for each site to determine validity of the model’s mechanism for TBCA.
Each site needs individualized parameter values, climate and vegetation files
PnET-DAY will compare unknown values to published results from eddy flux towers at each site

18. Carbon and Nitrogen Availability Database
Database will include published values of foliar and soil metric measurements in a variety of temperate forests
Potential measurements to include:
Soil respiration, litterfall, N mineralization, foliar, N concentrations, carbon-to-nitrogen (C:N) ratios
Goal: Determine potential correlations between TBCA and nitrogen availability
Potential to contribute useful information on TBCA and nitrogen availability to PnET-CN model
Nutrient constraint mechanisms are often not developed or absent from most ecological models (Hungate et al., 2003)

19. Summary
Increase of CO2 in the atmosphere has created a “missing carbon sink”
Identifying where this missing carbon sink is of great importance to CO2 mitigation efforts
Terrestrial ecosystems, belowground soils suspected to be a substantial, long-term carbon sink
Knowledge of mechanisms related to belowground carbon allocation are still poorly understood
Implications of environmental pollution on soil carbon pools must also be taken into account in carbon models
More conclusive, long-term studies needed

20. Summary
Incorporating knowledge of N availability and TBCA will improve the PnET model
Including nutrient limitations and elevated levels of atmospheric CO2 into algorithms for TBCA will help scientists to understand the terrestrial ecosystem’s impact on climate change in the future
Understanding how models like PnET will incorporate rising CO2 into their TBCA mechanism will be of importance in modeling the effects of carbon sequestration under future climate change projections

21. Acknowledgments Special thanks to: My advisor: Scott Ollinger
My thesis committee: Christy Goodale, Andrew Richardson, & Mary Martin
Complex Systems Research Center: Rita Freuder, Sarah Silverberg
UNH-NASA Research and Discover Fellowship for allowing me to pursue this research for my Masters thesis

22. References
Canadell, J.G., L.F. Pitelka, J.S.I. Ingram The Effects of Elevated [CO2] on Plant-Soil Carbon Below-Ground: A Summary and Synthesis. Plant and Soil 187:
Davidson, E.A., et al Belowground Carbon Allocation in Forests Estimated from Litterfall and IRGA-Based Soil Respiration Measurements. Agricultural and Forest Meteorology 113:39-51.
Finzi et al Progressive Nitrogen Limitation of Ecosystem Processes under Elevated CO2 in a Warm-Temperate Forest. Ecology 87 (1):15-25.
Gill, R.A., et al Nonlinear Grassland Responses to Past and Future Atmospheric CO2 . Nature 417:
Hungate, B.A., et al Nitrogen and Climate. Science 302:
Intergovernmental Panel of Climate Change, Working Group Climate Change 2001: The Scientific Basis. Cambridge: Cambridge University Press.
Jastrow, J.D., et al Elevated Atmospheric Carbon Dioxide Increases Soil Carbon. Global Change Biology 11:
Johnson, D.W Progressive N Limitation in Forests: Review and Implications for Long-Term Responses to Elevated CO2. Ecology 87 (1)
Körner, C., et al Carbon Flux and Growth in Mature Deciduous Forest Trees Exposed to Elevated CO2. Science 309:
Luo, Y., D. Hui, and D. Zhang Elevated CO2 Stimulates Net Accumulations of Carbon and Nitrogen in Land Ecosystems: A Meta-Analysis. Ecology 87 (1):
Nadelhoffer, et al Nitrogen Deposition Makes a Minor Contribution to Carbon Sequestration in Temperate Forests. Nature 398:
Norby, R Inside the Black Box. Science 388:
Norby, R.J., et al Fine-Root Production Dominates Response of a Deciduous Forest to Atmospheric CO2 Enrichment. PNAS 101, 26:
Norby, R.J., et al Elevated CO2, Litter Chemistry, and Decomposition: A Synthesis. Oecologia 127:
Ollinger, S.V., et al Interactive Effects of Nitrogen Deposition, Tropospheric Ozone, Elevated CO2 and Land Use History on the Carbon Dynamics of Northern Hardwood Forests. Global Change Biology 8:
Raich, J.W., and K.J. Nadelhoffer Belowground Carbon Allocation in Forest Ecosystems: Global Trends. Ecology 70 (5):
Townsend, A.R., B.H. Braswell, E.A. Holland, J.E. Penner Spatial and Temporal Patterns in Terrestrial Carbon Storage Due to Deposition of Fossil Fuel Nitrogen. Ecological Application 6 (3)
Vitousek, P.M., et al Human Alteration of the Global Nitrogen Cycle: Sources and Consequences. Ecological Applications 7 (3):