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

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

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 http://www.whrc.org/resources/online_publications/warming_earth/scientific_evidence.htm

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

Current Understanding (http://csp.unl.edu/public/G_carbon.htm) 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

Environmental Factors Influencing Carbon Allocation http://www.tva.gov/environment/air/ontheair/nitrogen.htm http://aspenface.mtu.edu/ Elevated CO2 Nitrogen Limited Systems Tropospheric O3

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 http://www.ehponline.org/docs/1996/104-1/forum.html

Elevated CO2 Free-air CO2 enrichment (FACE) experiments Opportunity to make long-term observations of forests under elevated CO2 in realistic forest stand conditions http://www.dukemagazine.duke.edu/dukemag/issues/111205/depgaz17.html

Elevated CO2 http://cdiac.ornl.gov/programs/FACE/whereisface.html

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)

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 http://www.physicalgeography.net/fundamentals/8h.html

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 1.5-1.9 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 http://harvardforest.fas.harvard.edu/research/nitrogensat.html

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 http://biology.usgs.gov/luhna/harvardforest.html

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 = 130 +1.92 * 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?

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?

PnET-CN & FACE Duke FACE Forest, Durham, NC (FACTS-I) http://www.ornl.gov/info/ornlreview/ v37_3_04/images/a09_sweetgum_full.jpg http://www.nicholas.duke.edu/people/ faculty/katul/project4.html http://aspenface.mtu.edu/ 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

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

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)

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

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

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

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