B32B-01
Optimizing Land use Decisions Under Future Bioenergy Scenarios
The proposed increase in bioenergy usage and production will have interdependent environmental and socioeconomic impacts. Several technological pathways connect the various biomass sources to diverse forms of bioenergy (fuels, heat, and power). Currently, the complexity and scale dependency of such decisions and their impacts are not understood, defined, or described with adequate clarity to enable policy makers to develop strategies to ensure a sustainable bioenergy future with acceptable environmental and socioeconomic consequences, particularly under a changing climate regime. We have developed systems-based conceptual model of the key environmental implications of bioenergy choices and are demonstrating the utility of this approach in addressing questions of biofuel selection and deployment through the development of a spatial optimization model (SOM) that optimizes land-use decisions within a watershed. The SOM efficiently distributes where dedicated energy crops should be grown while maximizing profits (or minimizing costs) while maintaining water quality limits (nitrogen, phosphorous and sediment concentrations) and acceptable land-use displacement (e.g., area of forest, cropland, and pastured converted to energy crops). These metrics were selected from the conceptual model to represent sustainability issues and farmer choices. The SOM is parameterized using SWAT; this integration combines the decision-making power of an optimization model (i.e. SOM) with a non-linear watershed simulation tool (i.e. SWAT). This integration is a significant advance for both optimization and hydrologic/ecologic modeling allowing, for the first time, the ability to optimize spatial decisions within a watershed while maintaining water quality throughout the basin. The SOM was formulated as a mixed integer linear program (MILP), an ideal way to combine complex and competing multiple objectives with conflicting constraints. The MILP approach easily allows constraints and objectives to be interchanged, allowing the SOM to analyze watershed land-use decisions from multiple perspectives; for example, costs can be converted to a constraint (i.e., a fixed budget) and the SOM can alternatively minimize the environmental impact within the watershed while meeting a biomass target
B32B-02
Impacts of Current and Previous Land Use on Greenhouse Gas Fluxes for Biofuel Cropping Systems
Biofuel cropping systems are both a source and sink of greenhouse gases (GHG). Fertilizer and pesticide manufacture and transport, farm machinery operation, and processing of biomass into fuel all lead to carbon dioxide (CO2) emissions, but the largest GHG sources for biofuel systems are often soil nitrous oxide (N2O) emissions and loss of organic carbon as a result of land use change. However, improved land management can increase soil carbon levels and decrease N2O emissions, thus complementing the CO2 sink from displaced fossil fuel combustion. Previously cropped land, grazed land, and Conservation Reserve Program (CRP) land is being converted to biofuel cropping. We report results for the central US because most of the land used for biofuel cropping is in the central region of the country (corn/soy belt). The primary tool for this analysis is the DAYCENT ecosystem model. The ability of the model to simulate soil GHG fluxes and crop yields is demonstrated and results from simulations of different land management scenarios are presented. Our analyses suggest that conversion of CRP or grazed land to corn ethanol cropping under conventional management leads to a net source of GHG, but that converting these lands to perennial cellulosic biofuel cropping results in a GHG sink. Previously cropped land converted to corn ethanol under conventional management is a small GHG sink, but improved management and conversion to cellulosic based crops can greatly increase this sink strength.
B32B-03
Unintended Environmental Consequences of a Global Biofuels Program
Biofuels are being promoted as an important part of the global energy mix to meet the climate change challenge. The environmental costs of biofuels produced with current technologies at small scales have been studied, but little research has been done on the consequences of an aggressive global biofuels program with advanced technologies using cellulosic feedstocks. Using a simulation modeling approach, we explore two scenarios for cellulosic biofuels production and find that with either one, biofuels could make a substantial contribution to meeting global-scale energy needs in the future, but with significant unintended environmental consequences. If forests are cleared to grow cellulosic biofuels crops, we estimate that about 105 Pg C would be released to the atmosphere as carbon dioxide and would cancel any greenhouse-gas savings from the substitution of biofuels for fossil fuels during the first half of the 21st century. Alternatively, if most cellulosic biofuels are grown on previously cleared land or land cleared of low-stature natural vegetation, we estimate that up to 30 Pg C would still be released to the atmosphere before a net greenhouse gas benefit from a global biofuels program is realized about the middle of the 21st century. With either alternative, we expect most of the world's cellulosic biofuels crops (14 to 15 million km2) to be grown on the relatively inexpensive but productive lands of the sub-tropics and tropics, with negative impacts on the biodiversity of these regions. Cellulosic biofuels may yet serve as a crucial wedge in the solution to the climate change problem, but must be deployed with caution so as not to jeopardize biodiversity, compromise ecosystems services, or undermine climate policy.
B32B-04
Projections of Biofuel Growth Patterns Reveal the Potential Importance of Nitrogen Fixation for Miscanthus Productivity
Demand for liquid biofuels is increasing because of the disparity between fuel demand and supply. Relative to grain crops, the more intensive harvest required for second generation liquid biofuel production leads to the removal of significantly more carbon and nitrogen from the soil. These elements are conventionally litter products of crops that are returned to the soil and can accumulate over time. This loss of organic matter represents a management challenge because the energy cost associated with fertilizers or external sources of organic matter reduce the net energy value of the biofuel crops. Plants that have exceptional strategies for exploiting nutrients may be the most viable options for sustainable biofuel yields because of low management and energy cost. Miscanthus x giganteus has high N retranslocation rates, maintains high photosynthetic rates over a large temperature range, exploits a longer-than-average growing season, and yields at least twice the biomass of other candidate biofuel grass crops (i.e. switchgrass). We employed the DAYCENT model to project potential productivity of Miscanthus, corn, switchgrass, and mixed prairie communities based on our current knowledge of these species. Ecosystem process descriptions that have been validated for many crop species did not accurately predict Miscanthus yields and lead to new hypotheses about unknown N cycling mechanisms for this species. We tested the hypothesis that Miscanthus hosts N-fixing bacteria in several ways. First, we used enrichment culture and molecular methods to detect N-fixing bacteria in Miscanthus. Then, we demonstrated the plant-growth promoting effect of diazotrophs isolated from Miscanthus rhizomes on a model grass. And finally, we applied 15N2 to the soil and rooting zone of field grown Miscanthus plants to determine if atmospheric N2 was incorporated into plant tissue, a process that requires N-fixation. These experiments are the first tests of N-fixation in Miscanthus x giganteus, and the ecosystem model allowed us to project how much nitrogen may be obtained from N-fixation to support sustainable high biomass yields.
B32B-05 INVITED
Nitrogen Limitation is Reducing the Enhancement of NPP by Elevated CO2 in a Deciduous Forest
Accurate model representation of the long-term response of forested ecosystems to elevated atmospheric
CO2 concentrations (eCO2) is important for predictions of future concentrations of CO2. For
biogeochemical models that predict the response of net primary productivity (NPP) to eCO2, free-air
CO2 enrichment (FACE) experiments provide the only source of data for comparison. A synthesis of
forest FACE experiments reported a 23% increase in NPP in eCO2, and this result has been used as a
model benchmark. Here, we provide new evidence from a FACE experiment in a deciduous forest in
Tennessee that N limitation has significantly reduced the stimulation of NPP by eCO2, consistent with
predictions from ecosystem and global models that incorporate N feedbacks. The Liquidambar styraciflua
stand has been exposed to current ambient atmospheric CO2 or air enriched with CO2 to 550
ppm since 1998. Results from the first 6 years of the experiment indicated that NPP was significantly
enhanced by eCO2 and that this was a consistent and sustained response. Now, with 10 years of data,
our analysis must be revised. The response of NPP to eCO2 has declined from 24% in 2001-2003 to
9% in 2007. The diminishing response to eCO2 since 2004 coincides with declining NPP in ambient
CO2 plots. Productivity of this forest stand is limited by N availability, and the steady decline in forest
NPP is closely related to changes in the N economy, as evidenced by declining foliar N concentrations. There
is a strong linear relationship between foliar [N] and NPP, and the steeper slope in eCO2 indicates that
the NPP response to eCO2 should diminish as foliar N declines. Increased fine-root production and root
proliferation deeper in the soil have sustained N uptake, but not to an extent sufficient to benefit
aboveground production. The mechanistic basis of the N effect on NPP resides in the photosynthetic
machinery. The linear relationships between Jmax and Vcmax with foliar [N] did not change from
1998 to 2008 or in response to eCO2; hence, lower foliar [N] resulted in significant reductions in
Jmax, Vcmax, and photosynthesis over time and in eCO2. It is not yet clear whether foliar [N]
and NPP will continue to decline or have reached a new steady state indicative of long-term forest response
to eCO2. These results are consistent with ecosystem models, which suggest that the NPP response to
eCO2 will include a transient increase in NPP followed by a decline to a lower level when fast C and N
pools reach quasi-equilibrium at eCO2. Our results also are consistent with optimization models of
carbon-water-nitrogen economy, which suggest eCO2 leads to increased fine-root production, declining
foliar [N], and diminishing enhancement of aboveground production. At a larger scale, a model incorporating
N feedbacks (CLM3-CN) predicts a much smaller enhancement of NPP in eCO2 than the CLM3-
CASÁ model without such a feedback. When applied across the terrestrial biosphere, the smaller
CO2 fertilization effect has important implications for the pace of climate change.
http://face.ornl.gov
B32B-06 INVITED
Disturbance history and nitrogen cycle controls on ecosystem response to increased CO2 and climate change
Land ecosystem response to increasing atmospheric CO2 concentration and changing climate depends on interactions among carbon and nitrogen cycles and previous changes in ecosystem state due to natural and anthropogenic disturbances. The current study quantifies these interactions using a model of coupled carbon, nitrogen, and disturbance dynamics (the Community Land Model with Carbon and Nitrogen, CLM- CN). CLM-CN is parameterized and applied for several sites where free-air CO2 enrichment (FACE) experiments have been performed. Simulations are performed with and without nitrogen limitations, and with and without including historical disturbance patterns. N-limitation is shown to reduce the CO2 fertilization effect in all systems, while recent disturbance is shown to have a strong transient effect on fertilization. Significant interactions among recent disturbance, elevated CO2, and nitrogen availability result in transient ecosystem responses to step-changes in atmospheric CO2. These responses are in qualitative and quantitative agreement with the observed fertilization responses at several FACE sites. The analysis is extended to present model-derived hypothetical responses to increased CO2 and warming, in anticipation of future multi-factor experiments.
B32B-07
Exploring carbon-nitrogen-albedo linkages in temperate and boreal forests.
The availability of nitrogen represents a key constraint on carbon cycling in terrestrial ecosystems and it is in this capacity that the role of nitrogen in the Earth's climate system has been considered. Despite this, few studies have included continuous variation in plant N status as a driver of broad-scale carbon cycle analyses. This is partly due to uncertainties in how leaf-level physiological relationships scale to whole ecosystems and because methods for regional to continental detection of plant N concentrations have yet to be developed. In recent work, we have shown that that ecosystem CO2 uptake capacity in temperate and boreal forests scales directly with whole-canopy nitrogen concentrations, mirroring a leaf-level trend that has been observed for woody plants worldwide. We further show that both CO2 uptake capacity and canopy nitrogen concentration are strongly and positively correlated with shortwave surface albedo. These results suggest that nitrogen plays an additional, and previously overlooked, role in the climate system via its influence on vegetation reflectivity and shortwave surface energy exchange. Here, we expand on this work by examining potential underlying mechanisms for the observed carbon-nitrogen-albedo relationships and by exploring their generality over a wider range of ecosystems using new data from the U.S. and Canada.
B32B-08 INVITED
Rhizosphere priming effects on soil N availability in forests exposed to elevated atmospheric CO2
The progressive nitrogen (N) limitation hypothesis suggests that the uptake of N due to rapid tree growth under elevated CO2 depletes pools of available N resulting in short-term increases in productivity under elevated CO2. To date however, a down-regulation of forest productivity under elevated CO2 has not been observed among the four forest FACE experiments suggesting that our understanding of the mechanisms by which trees influence soil N cycling needs further refinement. We sought to test the hypothesis that trees exposed to elevated CO2 increase soil N availability by 'priming' rhizosphere microbes via the release of root exudates. At the Duke Forest FACTS-1 site, NC, we collected exudates bi-monthly from intact fine roots of 25 year-old loblolly pine Pinus taeda trees exposed to elevated CO2 and N fertilization. In addition, we collected rhizosphere and bulk soil from the same plots in order to develop a time-integrated estimate of the plant- microbial response to the CO2 and N treatments. In general, there were strong interactive effects between CO2 and N fertilization on exudation and rhizosphere microbial activity. In non-fertilized plots, mass-specific exudation rates were 15% greater with CO2 enrichment. In fertilized soils the opposite patterns were detected, as CO2 decreased mass specific rates by 40% (relative to the ambient rates). In the soil, treatment effects on rhizosphere microbial activity were similar: elevated CO2 increased microbial activity in non-fertilized plots (29%) but decreased it in fertilized plots (15%). However, we found no differences in net N mineralization rates in the rhizosphere in response to either CO2 or N fertilization. Collectively, these results suggest that although changes in exudation and microbial activity are likely mediated by soil N availability, the degree to which such processes are responsible for increased soil N cycling in forests exposed to elevated CO2 remains unclear.