BER Research Highlights

Search Date: December 13, 2017

15 Records match the search term(s):


December 17, 2013

Improving Methods for Measuring Mesophyll Conductance in Response to CO2

Current studies with the chlorophyll fluorescence-based (i.e., variable J) method have reported that mesophyll conductance rapidly decreases with increasing intercellular CO2 partial pressures or decreasing solar irradiance. However, the current method can produce an artifactual dependence of gas conductance in the leaf. A new study at Oak Ridge National Laboratory has identified deficiencies in the chlorophyll fluorescence-based (i.e., variable J) and carbon isotope-based (i.e., online carbon isotope discrimination) methods for measuring mesophyll conductance and has proposed effective solutions. They also derived a new photosynthesis carbon isotope discrimination equation that considers multiple CO2 sources for carboxylation. The significance of this work lies in our understanding of mesophyll conductance, since it is crucial for understanding and predicting responses of photosynthesis to increases in atmospheric CO2. As a result of this study, scientists will be able to improve key methods for measuring mesophyll conductance that will improve the representation of photosynthesis and carbon isotope discrimination in carbon cycle models. Additionally, since photosynthesis is the foundation for the terrestrial carbon isotope ecology, this study will facilitate the application of carbon isotopes in studying ecological processes.

Reference: Gu, L., and Y. Sun. 2013. “Artefactual Responses of Mesophyll Conductance to CO2 and Irradiance Estimated with the Variable J and Online Isotope Discrimination Methods,” Plant, Cell and Environment, DOI:10.1111/pce.12232. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


November 25, 2013

Spatial Distribution of U.S. Anthropogenic Methane Sources

This study, jointly funded by the Department of Energy’s Atmospheric Radiation Measurement (ARM) and Terrestrial Ecosystem Science (TES) programs along with support from the National Science Foundation and National Aeronautics and Space Administration, sought to quantitatively estimate the spatial distribution of anthropogenic methane sources in the United States by combining comprehensive atmospheric methane observations, extensive spatial datasets, and a high-resolution atmospheric transport model. Using eddy covariance tower and aircraft-based atmospheric observations of methane, along with a high-resolution atmospheric transport model (STILT), results were compared to inventories from the U.S. Environmental Protection Agency and Emissions Database for Global Atmospheric Research (EDGAR) database. Current inventories from the database underestimate methane emissions nationally by a factor of ~1.5 to ~1.7. This study indicates that emissions due to ruminant animals (livestock) and manure are up to twice the magnitude of existing inventories. The discrepancy in methane source estimates is particularly pronounced in the south-central United States, where total emissions are ~2.7 times greater than in most inventories and account for ~24% of national emissions. The spatial patterns of emission fluxes and observed methane-propane correlations indicate that fossil fuel extraction and refining are major contributors (~45%) in the south-central United States. This result suggests that regional methane emissions due to fossil fuel extraction and processing could be nearly five times larger than in EDGAR, the most comprehensive global methane inventory. These results cast doubt on a recent decision to downscale estimates of national natural gas emissions by 25-30%. Overall, the investigators conclude that methane emissions associated with both the animal husbandry and fossil fuel industries have larger greenhouse gas impacts than indicated by existing inventories.

Reference: Miller, S. M., S. C. Wofsy, A. M. Michalak, E. A. Kort, A. E. Andrews, S. C. Biraud, E. J. Dlugokencky, J. Eluszkiewiczf, M. L. Fischer, G. Janssens-Maenhout, B. R. Miller, J. B. Miller, S. A. Montzkad, T. Nehrkornf, and C. Sweeney. 2013. “Anthropogenic Emissions of Methane in the United States,” Proceedings of the National Academy of Sciences (USA) 110(50), 20018-22. DOI:10.1073/pnas.1314392110. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Rickey Petty, SC-23.1, (301) 903-5548, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


November 15, 2013

Stability of Soil Organic Carbon: Impacts of Particle Size

Studies comparing the mineralization rate of organic carbon (C) associated with different particle size fractions are extremely limited. Organic C associated with the mineral fraction, in particular, is thought to have long residence times. Studies of C decomposition as a function of particle size should improve the representation of long-term C stabilization processes in terrestrial carbon cycle models. A recent study at Oak Ridge National Laboratory sought to quantify decomposition of native soil organic C and a newly added C substrate from both particulate and mineral soil pools. Five different soils were fractionated into particulate (> 53 µm) and mineral (< 53 µm) fractions, radiolabeled with glucose, and incubated for 150 days. Results indicated that the mineralization of native soil organic C was higher from the particulate fraction than the mineral fraction, while mineralization of glucose was similar from both fractions. Furthermore, native organic C in the soil mineral fraction was observed to be resistant to decomposition, in contrast to added glucose which was readily decomposed. Glucose additions therefore appear to be an inadequate surrogate for quantifying long residence times of organic C associated with soil minerals. Although we currently lack adequate experimental data on mineral-associated fractions, this study represents a significant step toward improving our understanding of long-term C stability of soil organic matter and representing these mechanisms in ecosystem-scale models.

Reference: Jagadamma, S., J. M. Steinweg, M. A. Mayes, G. Wang, and W. M. Post. 2013. “Decomposition of Added and Native Organic Carbon from Physically Separated Fractions of Diverse Soils,” Biology and Fertility of Soils, DOI:10.1007/s00374-013-0879-2. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


November 08, 2013

Worldwide Dataset Demonstrates Importance of CO2 Diffusion in Leaves on Photosynthesis

Oak Ridge National Laboratory scientists used worldwide datasets gathered through an online virtual laboratory for photosynthesis research (leafweb.ornl.gov) to determine the effects of CO2 diffusion inside leaves (i.e., mesophyll conductance) on photosynthesis across all major plant functional types and climates.   Molded mesophyll conductance (gm) generally has been assumed to be infinitely large.   Synthesis of LeafWeb data from over 130 C3 photosynthetic species in different countries showed that mesophyll conductance of most species is as important as stomatal conductance in affecting photosynthesis.   The study found that standard assumptions of an infinite mesophyll conductance resulted in a major underestimation of CO2 assimilation capacities of the photosynthetic machinery and a distortion of relationships between key biochemical processes.   Based on the study’s findings, a new functional model is proposed to facilitate the representation of mesophyll conductance in global carbon cycle models.   This study will lead to better understanding of photosynthetic processes under natural conditions and development of better global carbon cycle models.   A virtual laboratory like LeafWeb is a cost effective, efficient tool for promoting international collaboration, collecting spatially distributed datasets of global importance, and conducting synthesis research that would otherwise be difficult to carry out.

Reference: Sun, Y., L. Gu, R. E. Dickinson, S. G. Pallardy, J. Baker, Y. Cao, F. M. DaMatta, X. Dong, D. Ellsworth, D. Van Goethem, A. M. Jensen, B. E. Law, R. Loos, S. C. Vitor Martins, R. J. Norby, J. Warren, D. Weston, and K. Winter. 2013. “Asymmetrical Effects of Mesophyll Conductance on Fundamental Photosynthetic Parameters and Their Relationships Estimated from Leaf Gas Exchange Measurements,” Plant, Cell and Environment, DOI:10.1111/pce.12213. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


November 04, 2013

Sagebrush Carrying Out Hydraulic Lift Enhances Surface Soil Nitrogen Cycling and Nitrogen Uptake into Inflorescences

Plant roots are conduits for water flow from soil to leaves and from wetter to drier soil. This hydraulic redistribution through root systems occurs in soils worldwide and can enhance stomatal opening, transpiration, and plant carbon gain. For decades, upward hydraulic lift (HL) of deep water through roots into dry, litter-rich, surface soil also has been hypothesized to enhance nutrient availability to plants by stimulating microbe controlled nutrient cycling, but this link has not been demonstrated in the field. Working in sagebrush-steppe, where water and nitrogen limit plant growth and reproduction and where HL occurs naturally during summer drought, Department of Energy scientists from the Marine Biological Laboratory slightly augmented deep soil water availability (HL+) to plants throughout the summer growing season. The treated sagebrush lifted greater amounts of water than control plants and had slightly less negative predawn and midday leaf water potentials. Soil respiration also was augmented under HL+ plants. At summer’s end, they observed increased rates of nitrogen cycling in surface soil layers around HL+ plants and increased nitrogen uptake into HL+ plants’ inflorescences as sagebrush set seed. These treatment effects persisted even though unexpected monsoon rainstorms arrived during assays and increased surface soil moisture around all plants. Simulation models from ecosystem to global scales have just begun to include effects of hydraulic redistribution on water and surface energy fluxes. Results from this field study indicate that plants carrying out HL also can substantially enhance decomposition and nitrogen cycling in surface soils.

Reference: Cardon, Z. G., J. M. Stark, P. M. Herron, and J. A. Rasmussen. 2013. “Sagebrush Carrying Out Hydraulic Lift Enhances Surface Soil Nitrogen Cycling and Nitrogen Uptake into Inflorescences,” Proceedings of the National Academy of Sciences (USA) 110, 18988-991. DOI:10.1073/pnas.1311314110. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


October 30, 2013

Faster Organic Matter Decomposition Predicted for Well-Drained Boreal Soils Following Permafrost Degradation

Roots and litterfall can release readily decomposable carbon sources into soil. This newly added carbon may increase or suppress the decomposition of older soil organic matter phenomena known as positive or negative “priming effects.” In temperate regions, recent research suggests priming effects can be a critical mechanism controlling soil carbon dynamics, yet virtually nothing is known about priming effects in boreal ecosystems. To investigate the importance of priming effects in boreal forest soils, researchers at Argonne National Laboratory developed a mechanistic model that can simulate simultaneously occurring soil physical, chemical, biological, and hydrological processes and their interactions. The model was then used to perform sensitivity analyses for two black spruce forest sites, with and without underlying permafrost. Overall, priming effects were strongly controlled by the intensity and frequency of dissolved organic carbon (DOC) inputs to soil. Greater priming effects were predicted for the site with favorable soil water flow than for the site where soil water flow was limited because water flow can carry DOC to deep soil layers, which are rich in organic carbon in boreal soils. Modeling results suggest that priming effects might be accelerated for sites where permafrost degradation leads to drier soil conditions and favorable water transport into deeper layers. In addition to DOC dynamics, priming effects were most sensitive to changes in the composition of solid soil organic carbon, followed by biomass changes in the soil microbial community. The findings from this model sensitivity analysis highlight the urgent need to better study these key parameters in future laboratory and field experiments in permafrost regions.

Reference: Fan, Z., J. D. Jastrow, C. Liang, R. Matamala, and R. M. Miller. 2013. “Priming Effects in Boreal Black Spruce Forest Soils: Quantitative Evaluation and Sensitivity Analysis,” PLoS ONE 8, e77880. DOI:10.1371/journal.pone.0077880. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


October 08, 2013

Earthworms Affect Forest Soil Carbon Stabilization

The role of soils in mitigating increases in atmospheric carbon dioxide (CO2) is uncertain, in part due to the complex biotic and abiotic interactions determining soil carbon change. Earthworms, in particular, interact with the physical and chemical protection mechanisms of organic matter, major determinants of carbon storage capacity of soils. Protection of enhanced plant litter inputs from rapid decomposition by soil aggregates was a key mechanism facilitating the carbon gain observed in surface soils of the sweetgum forest Free-Air CO2 Enrichment (FACE) experiment in Oak Ridge, TN. To evaluate whether two earthworm species with different feeding behaviors played a role in soil aggregate formation and the stabilization of leaf and/or root litter in these aggregates, Department of Energy researchers conducted a laboratory incubation experiment with earthworms plus isotopically labeled soil and plant materials from the sweetgum FACE site. Compared to the experimental treatments without worms, the presence of either earthworm species increased the formation of soil macroaggregates (greater than 250 µm in diameter). The invasive European earthworm species, which feeds on both plant residues and soil organic matter, incorporated significant amounts of leaf- and root-derived carbon, in addition to soil-derived carbon, into newly formed aggregates. In contrast, the native earthworm species, which feeds mostly on soil organic matter, produced almost twice as many aggregates, but hardly any of the carbon in these aggregates was derived from the added plant materials. Overall, these findings suggest that the presence or absence of earthworms—and specifically the type of earthworm—could be an important factor contributing to the fate of increased plant litter produced as a result of rising atmospheric CO2 concentrations.

Reference: Sánchez-de León, Y., J. Lugo-Pérez, D. H. Wise, J. D. Jastrow, and M. A. González-Meler. 2014. “Aggregate Formation and Carbon Sequestration by Earthworms in Soil from a Temperate Forest Exposed to Elevated Atmospheric CO2: A Microcosm Experiment,” Soil Biology and Biochemistry 68, 223–30. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


September 03, 2013

Combining Ground Station and Satellite Data for Better Estimates of CO2 Emissions

Scientists from Lawrence Berkeley National Laboratory present one of the first estimates of the global distribution of CO2 surface fluxes using total column CO2 measurements retrieved by the SRON-KIT RemoTeC algorithm from the Greenhouse gases Observing SATellite (GOSAT). The international research team used a series of calculations to combine data from the satellite over an 18-month period with nearly 17,000 surface-level observations from 132 locations during the same period. The team used this data to estimate the CO2 sources and sinks around the world. Their global scale results compared favorably to independent estimates made by government agencies, while at regional scales some differences raised questions for future exploration. The study shows that assimilating the bias corrected satellite data on top of surface CO2 data reduces the estimated global CO2 land sink and shifts the net terrestrial carbon uptake from the tropics to the extratropics. It is concluded that while GOSAT total column CO2 provides useful constraints for source-sink inversions, small spatiotemporal biases €"beyond what can be detected using current validation techniques €"have serious consequences for optimized fluxes, even aggregated over continental scales.

Reference: Basu, S., S. Guerlet, A. Butz, S. Houweling, O. Hasekamp, I. Aben, P. Krummel, P. Steele, R. Langenfeld, M. Torn, S. Biraud, B. Stephens, A. Andrews, and D. Worthy. 2013. “Global CO2 Fluxes Estimated from GOSAT Retrievals of Total Column CO2,” Atmospheric Chemistry and Physics 13, 8695-717. DOI:10.5194/acp-13-8695-2013. (Reference link)

Contact: Wanda Ferrell, SC-23.1, (301) 903-0043, Mike Kuperberg, SC-23.1, (301) 903-3281, Rickey Petty, SC-23.1, (301) 903-5548, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


August 19, 2013

Accounting for Root-Driven Priming of Soil Organic Matter Decomposition Can Improve Model Performance

The interactions between plants and soil play a central role in the functioning of terrestrial ecosystems and the global carbon cycle. Most of these interactions take place in the rhizosphere, the zone of soil that surrounds and is directly influenced by plant roots. The rhizosphere priming effect is a key interaction between living roots and associated rhizosphere organisms that has the potential to alter soil organic matter dynamics by stimulating or suppressing decomposition rates. In a review for New Phytologist, a series of modeling exercises explored how the rhizosphere priming effect might result from an evolutionarily stable mutualistic association between plants and rhizosphere microbes, and how the physiological responses of rhizosphere microbes to different types of plant-derived substrates might help to explain the existence of both positive and negative priming effects. Further, the ability of a commonly used ecosystem model to correctly simulate data from the U.S. Department of Energy-sponsored Duke Free-Air CO2 Enrichment (FACE) experiment was significantly improved by including a priming-induced acceleration of soil organic matter decomposition in response to atmospheric CO2 enrichment. A 40% increase over ambient decay rates for one of the model’s soil organic matter pools enabled better predictions of the increases in plant growth and nitrogen uptake as well as the lack of change in soil carbon observed in the elevated CO2 treatment. This model-data-comparison case study demonstrates the potential importance of the rhizosphere priming effect in terrestrial ecosystems and highlights the value of research efforts to enable its mechanistic incorporation into future ecosystem and Earth system models.

Reference: Cheng, W., W. J. Parton, M. A. Gonzalez-Meler, R. Phillips, S. Asao, G. G. McNickle, E. Brzostek, and J. D. Jastrow. 2013. “Synthesis and Modeling Perspectives of Rhizosphere Priming,” New Phytologist 201, 31-44. DOI: 10.1111/nph.12440. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.2 Biological Systems Science Division, BER


July 23, 2013

Isoprene Discovered to be an Antioxidant

A fraction of net carbon assimilation can be re-released as isoprene by many tropical plants; however, much uncertainty remains about its biological significance. A comprehensive analysis of the suite of isoprene oxidation products in plants has not been performed and production relationships with environmental stress have not been described. Traditionally, isoprene is assumed to only be oxidized in the atmosphere to methyl vinyl ketone, methacrolein, and 3-methly furan. Abiotic stress (e.g., high temperature, light, and freeze-thaw) is known to induce oxidative stress in plants. A study conducted at Lawrence Berkeley National Laboratory, in conjunction with the Department of Energy’s GOAmazon campaign in Brazil, aimed to identify and quantify emissions of potential isoprene oxidation products from mango tree leaves as a function of temperature. Isoprene oxidation products including methyl vinyl ketone, methacrolein, and 3-methyl furan were measured as direct emissions from mango trees grown in environmental chambers. Isoprene oxidation also was measured in a tropical mesocosm (Bisophere 2). These measurements were taken at the leaf, branch, mesocosm, and whole ecosystem scale using chamber and tower sampling systems. The study’s results indicate that emissions of isoprene oxidation products from plants increase with abiotic stress and may be associated with lipid peroxidation at high temperatures. The results suggest that isoprene is an important ecosystem antioxidant involved in signaling processes through the formation of reactive electrophile species. These observations highlight the need to investigate further the mechanisms of isoprene oxidation in plants under stress and its biological and atmospheric significance.

Reference: Jardine, K. J., K. Meyers, L. Abrell, E. G. Alves, A. M. Yanez Serrano, J.  Kesselmeier, T. Karl, A. Guenther, J. Q. Chambers, and C. Vickers. 2013. “Emissions of Putative Isoprene Oxidation Products from Mango Branches Under Abiotic Stress,” Journal of Experimental Botany 64(12), 3669-79. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


July 18, 2013

Improving Estimates of Soil Organic Carbon Stored in Permafrost

Recent research has revealed that the amount of soil organic carbon (SOC) stored in the northern circumpolar permafrost region is far larger than earlier estimates, calling attention to the potential vulnerability of this carbon for release to the atmosphere. Even so, these new estimates of the quantity, decomposability, and combustibility of permafrost-region SOC stocks are poorly constrained, contributing to large uncertainties in model predictions of carbon–climate feedbacks under future warming. Two workshops held at Argonne National Laboratory during 2011 and 2012 led to a synthesis of the current differences between empirical and model estimates of the size and distribution of permafrost-region SOC stocks, and research needs to reduce this discrepancy were identified. Five research challenges for improving empirical assessments of the distribution and potential mineralization of SOC stocks in the northern permafrost region were highlighted. These include (1) improving the number and robustness of observations, (2) predicting the spatial and vertical distributions of SOC stocks, (3) characterizing existing carbon forms to better predict their fate, (4) using improved observation-based SOC estimates to inform model development, and (5) quantifying uncertainties in observations and predictions. These challenges are interlinked and suggest opportunities to organize, prioritize, and coordinate future research efforts to better understand and predict the impacts of permafrost SOC.

Reference: Mishra, U., J. D. Jastrow, R. Matamala, G. Hugelius, C. D. Koven, J. W. Harden, C. L. Ping, G. J. Michaelson, Z. Fan, R. M. Miller, A. D. McGuire, C. Tarnocai, P. Kuhry, W. J. Riley, K. Schaefer, E. A. G. Schuur, M. T. Jorgenson, and L. D. Hinzman. 2013. “Empirical Estimates to Reduce Modeling Uncertainties of Soil Organic Carbon in Permafrost Regions: A Review of Recent Progress and Remaining Challenges,” Environmental Research Letters 8, 035020. DOI:10.1088/1748-9326/8/3/035020. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


July 01, 2013

Evaluating the CO2 Component of Climate Models Using ARM Southern Great Plains Site Data

Aircraft data collected over the Southern Great Plains (SGP) site were used to analyze changes in the three-dimensional (3D) distribution of atmospheric CO2 for several greenhouse gas concentration trajectories adopted by the Intergovernmental Panel on Climate Change for its fifth Assessment Report. Using the Community Earth System Model–Biogeochemistry (CESM1-BGC), scientists first compared CO2 concentrations simulated for 1850 to 2005 to surface, aircraft, and column observations. Second, the evolution of spatial and temporal gradients within the SGP’s 3D observational footprint during the twenty-first century was examined. By upscaling the results, the study showed that the mean annual cycle in atmospheric CO2 was underestimated for the historical period throughout the Northern Hemisphere, suggesting that the growing season net CO2 flux in the Community Land Model (the land component of CESM) was too weak. Over the last half century, the growth rate of atmospheric CO2 was higher in the model than in observations, suggesting that the overall sensitivity of land and ocean CO2 uptake to rising atmospheric CO2 (and other human global change perturbations) was too weak (i.e., model parameterization of the land and ocean to rising CO2 needs to be adjusted) . The diagnostics that were developed in this paper provide a means to test future generations of coupled carbon–climate models.

Reference: Keppel-Aleks, G., J. T. Randerson, K. Lindsay, B. B. Stephens, J. K. Moore, S. C. Doney, P. E. Thornton, N. M. Mahowald, F. M. Hoffman, C. Sweeney, P. P. Tans, P. O. Wennberg, and S. C. Wofsy. 2013. “Atmospheric Carbon Dioxide Variability in the Community Earth System Model: Evaluation and Transient Dynamics during the Twentieth and Twenty-First Centuries,” Journal of Climate 26, 4447–75. DOI: 10.1175/JCLI-D-12-00589.1. (Reference link)

Contact: Wanda Ferrell, SC-23.1, (301) 903-0043, Rickey Petty, SC-23.1, (301) 903-5548, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


April 15, 2013

Improving Carbon Fluxes in Earth System Models

The extreme complexity of earth system models (ESMs) is necessary to represent the many processes underlying terrestrial carbon cycle processes. However, simple models may be useful to qualitatively understand projected dynamic responses to warming and to identify processes missing in the models. A U.S. Department of Energy scientist at Lawrence Berkeley National Laboratory developed a simple model for vegetation carbon response by tracking the movement of the most statistically similar climate at every location in an ESM over past time and recalculating the carbon flux within the Fifth Climate Model Intercomparison Project (CMIP5) ESMs. The most important area of disagreement between this simple method and the full ESM calculations are in the southern boreal forest, where ESMs project carbon gains, while the simplified approach projects carbon losses. This finding suggests that potential carbon losses such as forest disturbance and mortality, known to be missing in the ESMs, need to be better represented to robustly predict the carbon response in this region.

Reference: Koven, C. 2013. “Boreal Carbon Loss Due to Poleward Shift in Low-Carbon Ecosystems,” Nature Geoscience, accepted.

Contact: Dorothy Koch, SC-23.1, (301) 903-0105
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


March 25, 2013

Forest Water Use and Water-Use Efficiency at Two FACE Sites

Predicted responses of transpiration to elevated atmospheric CO2 concentrations are highly variable among process-based models. To better understand and constrain this variability, forest carbon and water flux data from the free-air CO2 enrichment (FACE) experiments at Duke University and Oak Ridge National Laboratory were compared to simulations from 11 ecosystem models. A primary objective was to identify key underlying assumptions in model structure that cause differences in model predictions of transpiration and canopy water-use efficiency. Model-to-model and model-to-observations differences resulted from four key sets of assumptions: (1) the nature of the stomatal response to elevated CO2; (2) the roles of the leaf and atmospheric boundary layer; (3) the treatment of canopy interception; and (4) the impact of soil moisture stress. The degree of coupling between carbon and water fluxes, and how that coupling is calculated, is one of the key assumptions that determines how well the models compare with observations. This study yields a framework for analyzing and interpreting model predictions of transpiration responses to elevated CO2. This approach highlights key areas for immediate model improvement, hypotheses for experimental testing, and opportunities for data synthesis to significantly reduce discrepancies among models.

Reference: Kauwe, M. G., B. E. Medlyn, S. Zaehle, A. P. Walker, M. C. Dietze, T. Hickler, A. K. Jain, Y. Luo, W. J. Parton, C. Prentice, B. Smith, P. E. Thornton, S. Wang, Y. P. Wang, D. Wårlind, E. S. Weng, K. Y. Crous, D. S. Ellsworth, P. J. Hanson, H. Seok-Kim, J. M. Warren, R. Oren, and R. J. Norby. 2013. “Forest Water Use and Water Use Efficiency at Elevated CO2: A Model-Data Intercomparison at Two Contrasting Temperate Forest FACE Sites,” Global Change Biology, DOI: 10.1111/gcb.12164. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER


January 20, 2013

Soil Feedbacks to the Climate System

A key question in Earth system science is: Will warming lead to increased soil organic matter decay and an accelerated release of soil carbon as CO2? If yes, a self-reinforcing feedback would result with warming begetting warming. In 1991, a replicated, in situ soil-warming experiment was established at the Harvard Forest in central Massachusetts to address this question. Rates of CO2 production have been measured monthly for microbial and root respiration from April through November. Initially, warmed plots had higher respiration than controls, but after about a decade, the warming-accelerated CO2 production decreased and returned to background levels. However, during the last seven years of the study (years 16–22), soil respiration again increased in the heated plots relative to the control plots – a long-term response to soil warming never before documented. Based on measurements made over the first 15 years that showed the depletion of the soil’s labile carbon pool, the investigators hypothesized that much of the carbon respired over the last seven years has come from the recalcitrant soil carbon pool. Using 13C compound-specific soil incubation studies, they found that long-term soil warming increases the microbial carbon-use efficiency (CUE) associated with the degradation of complex (recalcitrant) carbon compounds such as phenol, but that the CUE of simple carbon compounds such as glucose was not temperature sensitive. Additional preliminary data shows a shift in microbial community structure in the heated plots that indicates an increase in taxa or pathways adapted to recalcitrant carbon decomposition. This long-term study suggests that the soil microbial community will adapt to long-term warming in a way that will lead to a depletion of the recalcitrant soil carbon stocks and a self-reinforcing feedback to the climate system.

Reference: Frey, S. D., J. Lee, J. M. Melillo, J. Six. 2013. “Soil Carbon Cycling: The Temperature Response of Soil Microbial Efficiency and Its Feedback to Climate,” Nature Climate Change 3, 395–98. DOI: 10.1038/nclimate1796. (Reference link)

Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
Topic Areas:

Division: SC-23.1 Climate and Environmental Sciences Division, BER