CSiTE Research Themes

Beginning in FY07, CSiTE reorganized around seven scientific themes and coordinated research activities at field experiments in switchgrass ecosystems managed for biomass production at Milan, Tennessee, and the Fermilab site at Batavia, Illinois.The seven themes (Figure 1) are:

1) Soil carbon inputs

2) Soil structural controls

3) Microbial community function and dynamics,

4) Humification chemistry,

5) Intrasolum carbon transport,

6) Mechanistic modeling

7) Integrated evaluation

Figure 1. The seven research themes of CSiTE and their relationships to each other.

Each of the five experimental theme’s research contributes to the sixth theme, Mechanistic Modeling which allows us to explore the effects of different processes, carbon inputs, and environmental conditions on enhancing of carbon sequestration at a local scale. The seventh theme, Integrated Evaluation of Carbon Sequestration Technologies, draws upon the Mechanistic Modeling theme for estimates of potential soil C sequestration across the wide range of soils, climate, and crops and management regimes possible in the U.S. By combining those estimates with the greenhouse gas (GHG) emissions and the economic value of those crops and management regimes, the seventh theme explores the economic consequences and GHG benefits for various strategies to enhance soil C sequestration at the national scale.

For the five experimental themes we have selected:

• An Alfisol located in western Tennessee (Milan) as our primary test soil type and a Mollisol located in northeastern Illinois (Fermilab) as our secondary test soil type. As research evolves we will explore other soils.
• Switchgrass ecosystems managed for cellulosic biomass production as our testbed ecosystem. We intend that poplar ecosystems be addressed in later years.
• Manipulations of C input and soil conditions to affect C sequestration processes at the sites.

We use the Erosion Productivity Impact Calculator (EPIC) (Izaurralde et al. 2006) as the basis of our mechanistic modeling activities. We use the systems modeling language STELLA® as a tool to engage the experimental scientists in building the conceptual and quantitative links among the five experimental themes in a way that can then be incorporated into the more multidimensional EPIC model and as a means for conducting model-based experiments for testing hypotheses about predicted effects of genotypic and environmental factors on soil C accrual.. The Forest and Agriculture Sector Optimizing Model (FASOM) model (McCarl and Schneider 2001), which is already integrated with EPIC and depicts total U.S. agricultural and forestry activities over time incorporating GHG issues of permanence, leakage, and additionality, forms the basis for our regional- to national-scale analysis of developed and potential soil C sequestration enhancement opportunities.

Research across all seven themes coalesces around the experimental switchgrass ecosystems managed for biomass production and is designed to address five overarching scientific questions:

I. What is the nature of belowground C inputs by switchgrass, and are they compatible with sustained aboveground biomass production and soil C sequestration?

II. What are the fundamental physical, chemical, and microbial mechanisms controlling C accrual and storage in soil, and how do they interact in space and time?

III. What processes control the movement and distribution of C through the soil profile?

IV. How are the fundamental processes controlling C distribution and movement manifested across landscapes and time?

V. How can fundamental knowledge best be used to identify and implement methods and practices for sustained enhancement of soil C in the context of biomass production for energy in an environmentally acceptable and economically feasible fashion?

Figure 2. Approach for transforming experimentally derived understanding to forecasting capabilities

CSiTE seeks to ensure that its fundamental science findings are used to inform and improve carbon sequestration forecasting capabilities by taking a vertically integrated approach which links field experiments to conceptual models, and conceptual models to forecasting models and regional models. (Figure 2). In this manner CSiTE strives to contribute to the effective application of carbon sequestration technologies to mitigate CO2-induced climate change.

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Click on a Theme to Expand. Click again to Close.

Theme 1. Soil carbon inputs – lead by Chuck Garten and Stan Wullschleger (genetics)

Participants – Julie Jastrow (ANL), Mike Miller (ANL), Roser Matamala (ANL), Cesar Izaurralde (PNNL), Carla Gunderson (ORNL), Robin Graham (ORNL), Rattan Lal (Ohio State University), Don Tyler ( University of Tennessee), Dave Parrish ( VA Polytechnical Institute)

Purpose and Objectives

The purpose of the soil C inputs theme is to quantify belowground C inputs and root dynamics within the framework of the four CSiTE experiments. The theme plan is designed to characterize treatment (i.e., cultivar and fertilization) differences and intra-annual variation in 1) root production, 2) root mortality and decomposition, and 3) root and microbial respiration at Milan, Fermilab, and possibly ORNL and to evaluate, on that basis, proposed strategies for enhancing soil C sequestration beneath switchgrass. In addition, the soil samples taken under the auspices of this theme will be used in the other four experimental themes. This theme is focused on addressing the timing, quality, quantity, and distribution with depth of belowground C inputs beneath switchgrass. Thus it directly addresses the first overarching science question: “What is the nature of belowground C inputs by switchgrass, and are they compatible with sustained aboveground biomass production and soil C sequestration simultaneously?“ Theme 1 also contributes to a better understanding of the distribution of C through the soil profile and thus also relates to the fourth overarching question: “How are the fundamental processes controlling soil C distribution and movement manifested across landscapes and time?”

Background and Science Questions

Advancements in quantifying belowground C inputs and the contribution of root production and turnover to soil C dynamics in terrestrial ecosystems are some of the grand ecological challenges of the 21st century. Currently, there is a diverse set of direct and indirect methods for measuring plant root production and mortality with no overall consensus on which method is best suited for accurate estimation of root dynamics (Vogt et al. 1998). Despite a widespread lack of agreement on which methods are best, there is universal agreement that belowground studies are labor-intensive and often carry large uncertainties about estimates of root production and mortality. Advantages and disadvantages of different methods are widely recognized and are an important consideration when selecting an overall approach to studies of plant root dynamics (Vogt et al. 1998). Root biomass is more than two-thirds of the total biomass in switchgrass plantations (Ma et al. 2001), and studies of root dynamics as they determine soil C inputs are an essential part of understanding soil C sequestration in these systems.

Depth profiles of coarse root biomass (>2 mm) for the Alamo switchgrass cultivar have been previously examined at Milan (Garten and Wullschleger 1999). Both coarse root biomass and soil organic C inventories decline in a semi-logarithmic manner with soil depth. Summation of measured and predicted amounts of biomass to a depth of 3 m at Milan indicates that >75% of the coarse root biomass resides in the top 40 cm of soil. This finding is similar to those of other investigations on the vertical distribution of switchgrass root biomass (Ma et al. 2000; Frank et al. 2004). Based on 13C natural abundance measurements, Garten and Wullschleger (2000) estimated an input of 210 g C m-2 y-1 beneath switchgrass at Milan. The former estimate was preliminary but represents approximately one-third of the C captured aboveground by annual switchgrass production. Preliminary estimates for the turnover time for C in coarse switchgrass roots were on the order of 1 to 2 years (Garten and Wullschleger 2000). Other investigations of root dynamics beneath switchgrass indicate that coarse roots are <20% of total root biomass (Tufekcioglu et al. 1999); therefore, much remains to be learned about the distribution and dynamics of switchgrass fine roots that undoubtedly will comprise most of the belowground biomass at Milan, Fermilab, and ORNL.

While the principal source of detritus and soil organic matter under switchgrass is the root system, the rate of soil C turnover may ultimately determine the potential for soil C sequestration. Soil respiration measurements integrate the biological activity of roots and microbes that determine soil C turnover rates. It is important to separate root from microbial respiration when assessing the effects of different treatments on soil C dynamics. For example, Parkin et al. (2005) reported differences in microbial respiration between landscape positions that were correlated with organic matter and microbial biomass content; however, the effect of landscape position was masked by differences in root respiration between crops. Environmental factors, particularly temperature and water availability because they influence plant activity and organic matter decomposition, are important in controlling soil respiration. In addition, substrate quality and soil nutrients influence the rates of C turnover. In a comparison with cool-season grasses, Tufekcioglu et al. (2001) found that switchgrass had the highest live, fine-root biomass and the lowest soil respiration. Another study indicated that differences in physiology (small root turnover or low specific root respiration) possibly lead to low rates of C turnover beneath switchgrass and contribute to greater soil C accumulation (Marquez et al. 1999). Finally, management practices are known to affect soil C turnover. Mulching and adding straw have been shown to have positive effects on soil C sequestration in croplands (Rees and Chow 2005), and Ma et al. (2000) showed that soil respiration and soil C turnover were greater when switchgrass was harvested once instead of twice in a sandy loam soil. However, while there is evidence that management practices can affect soil C turnover, the effects of management practices such as nutrient amendments on soil organic matter have not been extensively studied.

This research is driven by the following science questions:

How does C allocation and the attributes of belowground biomass (like tissue chemistry and rooting depth) influence the proclivity of different switchgrass varieties for soil C sequestration?
How do different switchgrass management strategies, like N fertilization, impact the dynamics of belowground biomass and how are such effects translated to the accrual of soil organic C?
What are the implications of increased stocks of soil organic matter for soil N transformations beneath switchgrass and to what extent do increased stocks of soil organic matter disrupt soil N supplies required for long-term sustainability of switchgrass plantations?

This theme will produce quantitative estimates of soil C inputs beneath switchgrass, by depth, for each treatment (e.g., fertilizer amount and switchgrass variety) in each of the planned experiments at Milan and Fermilab. It will establish relationships between organic matter inputs (largely from root turnover) and soil N availability that potentially affect switchgrass growth, rates of soil organic matter decomposition, and the function and dynamics of the soil microbial community. The studies of soil respiration will help to identify the relative importance of microbial processes to soil C sequestration under different switchgrass varieties and management regimes. The respiration studies will also provide quantitative information on the effectiveness of physicochemical stabilization (Theme 2), microbial activity and function (Theme 3), and humification rates (Theme 4). Studies with stable N and C isotopes will help place additional constraints on the process of soil C sequestration beneath switchgrass. Tracer studies will permit us to track the movement of 13C and 15N into soil aggregates, microbial biomass, and drainage waters.

Genetic component of Carbon inputs

A critical uncertainty in implementing carbon management research lies in understanding the many interactions between leaf and fine-root litter quality and decomposition, and the extent to which genetic variation in these traits can be harnessed to enhance carbon sequestration in managed ecosystems. CSiTE takes a multi-disciplinary approach to investigate how plant genetics control leaf and fine-root carbon inputs to ecosystems, how these inputs alter rates of litter decomposition, and how decomposition processes impact rates and magnitudes of soil carbon sequestration. We hypothesize that intra-specific variation in litter chemistry and decomposition rates among switchgrass and hybrid poplar genotypes will have large and sustained impacts on soil carbon sequestration.

In conducting this research, we will use novel genotypes of switchgrass and hybrid poplar differing in lignin biosynthesis in greenhouse and field studies to understand how variation in plant genotype and lignin composition of leaf and root tissues alter characteristics that determine rates of litter decomposition and soil carbon sequestration. We predict that understanding genotypic variation in leaf and fine-root litter chemistry and how these traits are related to decomposition and nutrient cycling will enable us to better understand how conventional breeding or marker-aided selection can be used to enhance rates and magnitudes of soil carbon storage in managed switchgrass and poplar ecosystems. Specific objectives include:

Quantify leaf and fine-root chemistry for novel genotypes of switchgrass and hybrid poplar that differ in lignin biosynthesis,
Measure leaf and fine-root decomposition and nutrient release for these genotypes using litterbags deployed in the field and in controlled laboratory incubations,
Assess the contribution of litter input from switchgrass and poplar genotypes contrasting in leaf and fine-root chemistry to labile and organo-mineral soil carbon pools using stable carbon isotope techniques, and
Integrate insights derived from these efforts into a STELLA®-based coupled plant-soil simulation model and use that model to examine the role that plant genetics (i.e., breeding) could play in enhancing soil carbon storage at local, regional, and global scales.

All of these expected results will help us to answer the overarching science question: “What is the nature of belowground C inputs by switchgrass, and are they compatible with sustained aboveground biomass production and soil C sequestration simultaneously?”

Sampling aboveground biomass and litter at Milan Tennessee

Sampling root structure under different hybrid poplar genotypes

Theme 2. Soil structural controls – lead by Julie Jastrow

Theme 2. Soil structural controls – lead by Julie Jastrow (ANL)

Participants – Chuck Garten (ORNL), Phil Jardine (ORNL), Vanessa Bailey (PNNL), Jim Amonette (PNNL), Rattan Lal (Ohio State University)

Purpose and Objectives

The purpose of this integrative theme is to improve understanding of soil structural controls on the transformation and stabilization of organic C inputs as soil organic matter (SOM). As the habitat for plant roots and soil organisms, soil structure affects plant growth and plays an important role in the regulation of decomposer activity. In addition, the physicochemical environment created by soil structure can influence both biotic and abiotic humification processes and can affect the transport of dissolved organic C (DOC) and other solutes through the soil profile. Thus, investigations in this theme will integrate with Theme 1 Soil Carbon Inputs, Theme 3 Microbial Community Function and Dynamics, and Theme 4 Humification Chemistry to address CSiTE’s second overarching science question: “What are the fundamental physical, chemical, and microbial mechanisms controlling C accrual and storage in soil, and how do they interact in space and time?” This theme will also contribute to understanding near-surface processes affecting the movement and distribution of C through the soil profile (Question III) by integrating with Theme 5 Intrasolum Carbon Transport. In addition, an important objective of this theme will be to quantify the effects of switchgrass production under different edaphic and climatic conditions on the size and dynamics of SOC pools defined by the spatial organization of soil structure. This information will be used by Theme 6 Mechanistic Modeling to incorporate soil structural controls into mechanistic models of soil C dynamics and predict the soil C sequestration potential of switchgrass production systems.

Background and Science Questions

Biochemical attack of SOM is inhibited at multiple scales by the physicochemical protection afforded by soil structure. Stabilization of otherwise decomposable SOM can occur via sorption to soil surfaces, complexation with soil minerals, occlusion within aggregates, and deposition in pores inaccessible to decomposers and extracellular enzymes. The relative importance and potential saturation of these physicochemical stabilization mechanisms vary depending on soil type, the nature of C inputs, management practices, and environmental conditions.

Current conceptual models of soil C cycling consider the interactions of decomposing C inputs with soil minerals at molecular to millimeter scales and the relationship of these developing organomineral associations to the structural organization and dynamics of the soil (e.g., Oades 1984; Golchin et al. 1994; Muneer and Oades 1989; Sollins et al. 1996; Six et al. 1999; Baldock and Skjemstad 2000; Christensen 2001). The physical location of SOM within the soil structural hierarchy and the dynamics of this structure together exert significant control on potential interactions between SOM, soil minerals, and decomposers (Elliott and Coleman 1988; Golchin et al. 1994; Christensen 2001; Plante and McGill 2002; Six et al. 2004, 2006).

In our working concept of soil structural controls on C cycling (adapted from Golchin et al. 1994 and Six et al. 1999), fresh organic inputs (especially root material and associated mycorrhizal fungi) are fragmented by soil biota and colonized by microorganisms. The mucilages and other residues produced by the activities of these organisms contribute to the stabilization of soil macroaggregates (>250 μm). As these fragmented residues are further broken down into finer particles, they become encrusted with mineral particles and form the organic cores of stable microaggregates (53-250 μm). The chances of microaggregates being formed and stabilized in this manner is thought to be greater inside stable macroaggregates, where this particulate organic matter (POM) is somewhat protected from rapid decomposition. While these organic cores are still rich in carbohydrates and chemically attractive to microorganisms, microbially produced mucilages, metabolites, and residues permeate the encrusting mineral particles and create very stable microaggregates. Once the more labile portions of the microaggregate cores are consumed, decomposition of more resistant plant structural materials proceeds more slowly. Eventually, deposition of new stabilizing microbial residues is exceeded by their decomposition, and the aggregate becomes unstable to disruptive forces such as wetting and drying, freezing and thawing, and root growth. Mineral particles and silt-sized aggregates that coated the organic cores are then freed to become associated with more labile POM.

At the next hierarchical level, microbial residues and small fragments of humified plant residues produced within microaggregates serve as nucleating sites for the formation of silt-sized aggregates. Thus, humified materials and microbial residues produced within microaggregates have a better chance of being protected long enough to become stabilized in silt-sized aggregates or finer-scale organomineral complexes than similar residues produced outside microaggregates. Along this spatially explicit decay continuum, microaggregates might be considered as “bioreactors” for the formation of new humified materials and new C-enriched silt-sized aggregates or organomineral complexes.

In this conceptual framework, factors controlling the capacity of soils to develop aggregate hierarchy and the turnover of macro- and microaggregates are important contributors to the physicochemical stabilization of SOM and to the humification processes of biochemical alteration and polymerization/condensation (Jastrow et al. in press). This paradigm is generally applied to inputs of plant detritus brought into direct physical contact with the soil matrix (i.e., root turnover or surface litter incorporation via bioturbation, physical disturbance, or mass flow) and to soils where SOM is a major aggregate binding agent. Thus, it is particularly appropriate for perennial grasses with extensive root systems, such as switchgrass, and is generally applicable to most Alfisols, Mollisols, and Ultisols that occur extensively throughout areas where switchgrass is likely to be grown.

Although the focus of our organizing concept is on the formation, stabilization, and turnover of soil aggregates, soil structure also includes the arrangement of pores within the soil. Hence, the development of aggregate hierarchy creates a parallel hierarchy of pores (Elliott and Coleman 1988; Young and Ritz 2000; Young and Crawford 2004). The pore system is where the decomposer community resides, and it also provides the environment that constrains the interactions between decomposers and their substrates (Strong et al. 2004; Ekschmitt et al. 2005). Biological factors (plant growth and turnover as well as decomposer activities) and management practices that affect soil aggregate dynamics can therefore have profound effects on soil porosity, which can then feed back to affect aggregation and plant growth.

A better understanding of these complex interactions and the factors controlling them is needed to maximize the stabilization of SOM derived from switchgrass production for biofuels while minimizing the economic and environmental costs of production. In other words, if C inputs exceed the capacity of a soil to protect and stabilize those inputs, then higher fertilization rates or other concerted efforts to increase inputs may not result in the desired or predicted enhancement of soil C sequestration and could, in fact, result in undesirable environmental consequences such as increased emissions of N2O or other greenhouse gases.

Planned research under this theme is driven by the following science questions:

How do the interactions and feedbacks between soil structure and switchgrass production control SOC stabilization and sequestration? How are these relationships affected by edaphic properties and climate?
Do microaggregates function as biophysical reactors that control the humification of fresh C inputs and the stabilization of SOC?
How does the hierarchical organization of soil structure affect microbial community structure and function?

Research conducted under this theme will:

Evaluate the interrelationships between soil structure (both aggregates and pores) and C inputs, microbial communities and their activity, humification processes, and DOC transport.
Assess the potential for using microaggregate-protected C as an early indicator of C sequestration and as a monitoring tool for predicting the capacity of a soil to protect and stabilize additional C inputs.
Provide quantitative estimates of how functional SOC pools and their dynamics are affected by the quantity and quality of switchgrass inputs and the specific manipulations designed to enhance humification processes (developed by Theme 4) under different edaphic and climatic conditions.
Evaluate the influence of aggregate hierarchy on microbial community structure, growth, activity, and spatial distributions.
Provide data to Theme 6 modeling efforts on the size and fluxes of C pools functionally related to soil C cycling.

These results, along with the findings of the other experimental themes, will contribute to an integrated understanding the fundamental physical, chemical, and microbial mechanisms controlling C accrual and storage in soil and the transport of C through the soil profile. The findings of this theme will also help to improve the capability of mechanistic models to predict soil C dynamics and sequestration.

Soil core collected in July 2007 at Switchgrass trial in Milan, Tennessee

Taking soil cores at Milan Tennessee in April 2007.

Theme 3. Microbial community function and dynamics – lead by Mike Miller

Theme 3. Microbial community function and dynamics – Lead by Mike Miller (ANL)

Participants – Chuck Garten (ORNL), Chris Schadt (ORNL), Hector Gonzalez (ORNL), Vanessa Bailey (PNNL), Jim Amonette (PNNL), Jeff Smith (USDA)

Purpose and Objectives

The purpose of this theme is to understand the influences of switchgrass varieties and crop management practices on soil microbial community structure and function. Identifying effects on microbial communities and soil C sequestration, together with knowledge of community function will improve understanding of fundamental mechanisms underlying maximization of the C sequestration potential. The contributions of this theme have ramifications to the other themes being presented, either directly through the quantification of microbial inputs, or indirectly through maintenance of soil structure or influence on humification. Hence, close coordination between this theme with Themes 1, 2, 4, and 6 will be necessary.

This theme will address the overarching science question of “What are the fundamental physical, chemical, and microbial mechanisms controlling C accrual and storage in soil, and how do they interact in space and time?” The theme will further contribute to the overarching science question of “How can fundamental knowledge best be used to identify and implement methods and practices for sustained enhancement of soil C in an environmentally acceptable and economically feasible fashion?”

Background and Research Questions

Organic matter decomposition and associated nutrient cycling are regulated by the soil microbial community. Therefore, developing methods and approaches to achieve a fundamental mechanistic understanding of microbial community composition and activity is important if we are to manage C stocks in soils. One of the major issues for dramatically increasing future biofuels production is the need to identify those characteristics and functions of soil microbial communities that are necessary for maintaining soil-ecosystem function and productivity under such scenarios (U.S. DOE 2006). Genotypic versus environmental variation in switchgrass root biomass production, architecture and morphology (e.g., root hair number and length; root branching patterns), degree of dependence on mycorrhizae, root longevity and tissue chemistry, and the reciprocal effects of potential management practices have not been investigated in depth, nor has the influence of these traits on soil microbial community structure and function.

Past CSiTE research has contributed to development and application of tools and experimental methods for quantifying the relative effects of fungal and bacterial presence and activity and the role of mycorrhizal fungi in coupled C/N cycling and storage in forests, agroecosystems, and prairie restoration. These approaches will be further refined and applied to switchgrass with a continuing focus on improved fundamental understanding of the coupled physical, biological and chemical processes controlling soil C sequestration. By understanding relationships among the characteristics of switchgrass varieties and their effects on the microbial communities and soil C sequestration, we will be better able to predict and inform resource managers on the effects of current biomass production processes on soil C sequestration.

Scientific questions to be addressed by research in this theme include:

How will variation in amounts and quality of belowground C inputs influence microbial community structure and dynamics?
Will the influence of microbial communities on soil C and N cycling under switch grass be similar or different from what was observed in earlier CSiTE research in restored prairie, agricultural systems, and forests?
Do specific microorganisms or groups of microorganisms exist that are predictive of soil C sequestration potential and whether a particular system is accruing or losing C?

The research addressed in the Microbial Community Function and Dynamics Theme will better quantify switchgrass influences on soil C sequestration by:

Determining the proportion of microbial C inputs derived from mycorrhizal fungi vs. saprophytic microbial processes.
Determining the relationship between root inputs, root morphology, root lignin content, and microbial structure and function (Theme 1).
Evaluating the potential for specific microbial groups or subgroups to act as sentinels of aggrading or degrading systems (Theme 6).
Determining the relationships of changes in microbial community structure and function to physical protection of soil C and soil structure (Themes 2 and 6).
Integrating expression of soil proteins with changes in humification processes (Theme 4).
Establishing a system of voucher samples for future Genomics: GTL studies in C sequestration.

Mike Miller holding switchgrass roots and rhizomes from Milan, Tennessee

Theme 4. Humification chemistry – lead by Jim Amonette,

Theme 4. Humification chemistry – lead by Jim Amonette (PNNL)

Participants – Chuck Garten (ORNL), Phil Jardine (ORNL) Julie Jastrow (ANL), Vanessa Bailey (PNNL), Carl Trettin (USFS)

Purpose and Objectives

The purpose of this theme is to develop a fundamental understanding of humification chemistry to guide the selection of potential manipulations that will enhance storage of organic C in soils under switchgrass cultivation. Specific objectives include 1) identifying the key chemical factors, such as pH, Eh, black C, Ca content, and N content of inputs, that can be manipulated economically to enhance C sequestration; 2) determining the optimal levels of these chemical factors for soils under switchgrass cultivation; and 3) developing measurement protocols that can be used to rapidly assess the current status of humification; that is, whether humic fractions are aggrading or degrading in a soil. Work under this theme primarily addresses the second overarching science question—“What are the fundamental physical, chemical, and microbial mechanisms controlling C accrual and storage in soil, and how do they interact in space and time?” By providing information about these mechanisms this theme also contributes to questions III, IV, and V, which focus on movement and distribution of C in soils as well as the identification and implementation of sustainable and economical practices to enhance soil C.

Background and Science Questions Our previous work suggests that co-catalysis of humification occurs by three mechanisms involving physical stabilization of tyrosinase, direct oxidation of the monomers, and promotion of the oxidation and condensation steps by alkaline pH (Amonette et al. 2003, 2004; Palumbo et al. 2004). Although tyrosinase activity is greatest at neutral pH, the large pH dependence of the condensation step drives the overall reaction to maximum rates under alkaline conditions. Following this hypothesis, liming of soils to slightly alkaline pH should enhance net C sequestration. Raising soil pH, however, is also likely to affect the activity of enzymes other than tyrosinase, such as various hydrolases. The hydrolase enzymes promote the breakdown of organic matter, and so the relevant question becomes one of whether the balance between humification and decomposition changes as the pH is altered. Preliminary evidence from the intermediate-scale experiment at the Santee Experimental Forest in South Carolina suggests that the balance does change and that decomposition increases relative to humification as a result of raising the pH. As a consequence, we broadened our enzyme analysis capabilities to allow monitoring of a suite of enzymes including tyrosinase, peroxidase, phosphatase, sulfatase, and other hydrolases by adapting methods of Marx et al. (2001) and Sinsabaugh et al. (1992) for microplate analysis. In addition, raising pH tends to decrease sorption of DOC to soil surfaces and thereby promotes leaching of DOC into deeper portions of the soil profile where adsorption can occur under acid conditions (Theme 5). Some evidence for this effect was also observed in the Santee experiment, confirming that two possible “desequestration” mechanisms (hydrolysis and leaching) could occur as a result of raising soil pH by alkaline fly ash amendments.

In contrast to the uncertain impact of alkaline pH, aspects of our previous work (Amonette et al., 2003) suggest that the presence of incompletely burned coal in fly ash can have a significant positive impact on the humification reaction, presumably by providing an organic surface where humic monomers preferentially accumulate and consequently react. In parallel with this observation is the renewed interest in the use of wood charcoal as an amendment to increase soil fertility while at the same time sequestering C (Glaser et al. 2003; Marris 2006). Pairing these two sets of observations, we think it likely that the physical stabilization of enzymes and humic monomers by charcoal-like materials (i.e., black C, whether from wood or coal), promotes humification. Accordingly, a strong focus of our research in this theme area will be on further understanding the role of black C on humification in a switchgrass cropping system.

The key scientific questions that will drive our research in the humification chemistry theme are:

How do macroscopic solution-phase soil-chemical properties such as pH and redox status influence the net rate of humification?
How do different types of soil surfaces (black C, minerals) influence humification?
How do different qualities of soil-C inputs (e.g., form and amounts of N resulting from different switchgrass fertilization regimes) influence humification?
How can we readily and rapidly determine whether humification is progressing or regressing in a soil at a particular point in time?

We expect work in this theme area to advance sequestration science in the following ways:

Improved fundamental understanding of the impact of soil chemical properties on humification rates.
Selection of potential chemical manipulations to enhance C sequestration under switchgrass.
Improved ability to simulate impact of changes in soil chemical properties on humification rates using EPIC (Theme 6).
Development of quick, reliable, inexpensive method(s) to determine humification status of soils (i.e., aggrading, degrading).
Quantitative assessment of aggregate development (Theme 2) and microbial activity (Theme 3) on humification processes and enzyme activities, and adsorption/desorption potential (Theme 5).

Moreover, the results obtained will help us answer several of the overarching scientific questions listed in the introduction. Most important, we will help provide a much better understanding of the fundamental physical, chemical, and microbial mechanisms controlling C accrual and storage in soil (Question II). Other overarching questions to which this work will contribute include identification of the processes that control C movement and distribution (Questions III and IV) and of the methods and practices that can be used to sustainably and economically enhance C sequestration in soils (Question V).

Sampling gases from Milan field site in Spring 2007

Jim Amonette sampling soil gases from Milan field site in April 2007. Note the dead and live switchgrass. Shortly after the switchgrass emerged there was a deep freeze that killed the new growth. Within a week new shoots emerged.

Theme 5. Intrasolum carbon transport – lead by Phil Jardine

Participants – Chuck Garten (ORNL), Melanie Mayes (ORNL), Vanessa Bailey (PNNL), Jim Amonette (PNNL), Jeff Smith (USDA)

Purpose and Objectives

The purpose of this theme is to test the hypothesis that deep subsurface soils can accumulate organic C and that accumulation will be affected by soil type, C inputs, and chemical effects induced by the addition of fertilizers and elements to enhance humification at the surface. The effort involves the immediate use of the Milan fertilizer experiment in years 1-3. In years 3-5 we will also use the Fermi manipulation experiment once switchgrass has been established. Our investigations are highly interactive with the other themes of this proposal by quantifying the impact of coupled hydrologic and geochemical processes on subsoil C and N dynamics and the processes that control enhanced organic C sequestration. This experimental information will then be used in Theme 6 to develop predictive models to assess subsurface C and N processes as a function of above ground manipulations and at larger scales. The specific objectives of this theme are to:

Quantify the magnitude of enhanced solid- and solution-phase C accumulation through soil profiles as a function of different fertilization rates, C inputs, humification additions, and soil types,
Quantify the impact of coupled hydrologic, geochemical, and microbial (Theme 4) processes on the fate and transport of solubilized organic C and N through the soil profile.
Quantify the chemical nature of the sequestered C and the mechanisms responsible for immobilization by the solid phase.

This theme is focused on quantifying the belowground movement and sequestration of organic C that is derived from switchgrass field manipulations and thus directly addresses CSiTE’s overarching questions II through IV, which seek to understand a) the fundamental physical, chemical, and microbial processes controlling C accrual and storage in soil, b) the processes that control C movement and distribution through the soil profile, and c) how these fundamental processes control soil C dissemination across the landscape as a function of time.

Background and Science Questions

As noted previously, widespread, highly developed mature soils such as Alfisols and Mollisols, which will be used in the proposed research, have deep soil profiles that have a tremendous capacity to sequester organic C. The physical and chemical properties of the lower horizons (B horizons) within these soils are ideal for maximizing organic C sorption to the solid phase (Sibanda and Young 1986; Jardine et al. 1989a,b, 1990b; McCarthy et al. 1993; Benke et al. 1999). This C pool (passive C pool) is significantly less dynamic than the C in upper soil horizons because it is strongly stabilized on mineral surfaces with estimated turnover times of millennia and longer (Trumbore 1997). Therefore, methods for enriching subsoil organic C can be a favorable technique to sequester appreciable quantities of C.

Subsurface conditions that create a favorable environment for enhanced carbon sequestration are: 1) a combination of high to moderate temperatures and large amounts of precipitation that enhance organic C decomposition rates and transport through the soil profile; 2) subsoil B horizons with suitable mineralogical components that strongly immobilize organic carbon; 3) subsoils with acidic pH and geochemical features for maximizing C sorption; 4) soils that are highly structured and have abundant microporosity that enhances solute attenuation; and 5) soils that are mature with deep profiles, thus enhancing C residence time prior to groundwater interception. These conditions are met very well in regions dominated by Alfisols, such as those at Milan, and Mollisols, such as those at Fermi.

Our goal is to test and resolve the hypothesis that deep subsurface soils can accumulate organic C as a result of near-surface manipulations such as land-use change and variations in fertilization amount. Our investigations are highly interactive with the other themes of this proposal by quantifying the impact of coupled hydrologic and geochemical processes on subsoil C and N dynamics and the processes that control enhanced organic C sequestration.

The research is driven by the following science questions:

How do different switchgrass management strategies (e.g., fertilization levels) influence the fate, transport, and sequestration of dissolved and solid-phase organic C through the soil profile?
In what capacity do the different soil horizons act as sources or sinks for DOC as it moves through the profile?
How does preferential flow and diffusion into the soil microporosity influence the transport and sequestration of DOC through the soil profile?
How do the chemical nature and form of the DOC change as it moves through the soil profile, and how do these changes influence sorption and microbial decomposition processes and thus sequestration?
How bioavailable are the dissolved and surface- bound organic phases in the different soil horizons, and what are the rates and mechanisms associated with the degradation (Theme 4)?
How significant are C, N, and P losses through the soil profile as a result of fertilization for the purposes of switchgrass crop productivity and non-sustainability?
Does the addition of fertilizer (e.g., N and P) drive DOC deeper into a soil profile, and is this DOC sequestered by the solid phase as a passive C pool?

Theme 5 will provide a quantitative understanding of coupled hydrological and geochemical subsurface processes of preferential flow, matrix diffusion, mass transfer kinetics, sorption, and degradation - useful for improving physical and chemical parameters in EPIC (Theme 6). It will also integrate the results of organic matter input measurements (Theme 1) and microbial assimilation rates (Theme 3) to quantify relationships between inputs, microbial communities, and DOC as affected by soil hydrology and chemical properties. The integrated theme strategy will not only provide an improved understanding and predictive capability of processes controlling C movement and storage in soil profiles, it will provide a fundamental understanding of how these processes control soil C dissemination across the landscape as a function of time.

Theme 6. Mechanistic modeling – lead by Mac Post

Theme 6. Mechanistic modeling – lead by Mac Post (ORNL)

Participants – Tris West ( ORNL), Chuck Garten (ORNL), Phil Jardine (ORNL), Cesar Izaurralde (PNNL), Allison Thompson (PNNL), Johan Six ( University of California, Davis), Steven DeGryze ( University of California, Davis), Bruce McCarl (TX A&M), Jimmy Williams (TX A&M)

Purpose and Objectives

The purpose of the mechanistic modeling theme is to improve our capability to mechanistically model/forecast soil organic C dynamics at local, regional, and national scales; facilitate the research of the five experimental themes; and enable evaluation of soil C sequestration technologies and the tradeoffs and complementarities of bioenergy and soil C sequestration. We will achieve this through improvements to and applications of the EPIC model. Our most significant improvement is that we will replace the current SOC decomposition submodel, which, like all widely used SOC decomposition models, is currently based on the use of conceptual SOC pools described by first-order kinetics and does not consider physicochemical protection. While this submodel is much improved by CSiTE (Izauralde 2006 a,b) it is still based on conceptual SOC pools. We will insert a totally new SOC decomposition submodel that combines measurable pools based on aggregate size and mineral association with differential equations capturing the relationships between these entities. This transition from conceptual pools to measurable pools characterized by physicochemical protection will enable a much greater degree of mechanistic process to be captured in the model and thereby enhance our ability to forecast soil C responses to novel land management and diverse crop types and reduce our reliance on empirical field trials for projecting C sequestration.

The work under this theme is broken into seven tasks. Each modeling task is directly linked to one of the other six themes. The work in this theme serves as a tool for analysis in themes 1-5 and is therefore connected to each of the five overarching questions in the Introduction. In particular, this theme is essential to addressing questions I, IV, and V by connecting the amount and nature of organic matter inputs to C distribution and dynamics in space and time.

Background and Science Questions

As a result of a need to integrate erosion impacts on soil C (Izaurralde et al. 2006b) and soil biogeochemical dynamics of trace gas exchange with the atmosphere (McGill et al. 2004), the development of EPIC (Williams 1995; Izaurralde et al. 2006a) and its landscape version APEX (Williams and Izaurralde 2005) has led to an advanced soil C dynamics model particularly suited for soil C sequestration analyses at site and regional scales (Izaurralde et al. 2006a,b; He et al. 2006; Thomson et al. 2006). Within CSiTE, we are well positioned to continue the development and application of EPIC by including the representation of additional processes and improving model parameters involved in soil C sequestration enhancement with an increased emphasis on perennial biomass energy crops. Improvements in model representations and suitable parameter estimates depend on experimental measurements to be investigated in themes 1-5.

The integration between CSiTE experimental and modeling activities will occur from both directions. From one direction, model development will benefit from knowledge emerging from CSiTE experimental sites. From the other, modeling activities will facilitate the design of field experiments by pre-testing hypotheses and helping design data collection. This theme will enhance the collaboration among CSiTE theme elements during study design, implementation of fieldwork, model development, and model evaluation by using modeling prototyping tools such as STELLA or MathLab to frame and evaluate model concepts in a way that encourages the participation of experimentalists.

The research is driven by the following science questions:

Would a model representation of a hierarchical organization of soil aggregates improve the correspondence between measured and simulated soil C pools? Further, would such a model representation improve the simulation of SOC sequestration over that of current, conceptual pool-based models?

Would a more refined treatment of microbial biomass dynamics in SOC models lead to improved simulations of SOC sequestration when field crops are replaced by perennial vegetation such as switchgrass? Currently, SOC models (including EPIC) contain a single microbial biomass pool that largely controls N mineralization-immobilization processes. Such models may not simulate a significant portion of the increase in SOC observed when perennial crops such as switchgrass replace field crops. Such replacements are known to induce shifts in bacteria to fungi ratios and changes in microbial conversion efficiency of POC to acid hydrolysis resistant material (lignin-like fungal residues, polymerized aliphatic materials, etc.). Nitrification and N2O evolution during nitrification is reduced or eliminated by a shift from bacterial-dominated decomposition to fungal-dominated decomposition.

Would incorporation into models of interactive effects of biochemical and physicochemical properties (e.g., soil pH, C content and quality, base saturation, cation and anion exchange capacities, mineralogy, texture, N content, oxidase/hydrolase activities, and fungal activity) on humification processes help explain results obtained in laboratory and field experiments?

Does modeling N cycling for deep-rooted perennial bioenergy crops require a detailed soil-profile level understanding of root distribution, root biochemistry, and C and N transformation dynamics?

Would an improved model representation of coupled hydrologic and geochemical processes controlling dissolved organic C and N transport and fate enhance EPIC’s predictive ability for determining C dynamics of deep soil layers? Furthermore, could such an improvement serve for assessing local and regional effects of manipulation strategies on C storage and N export?

This theme will develop and validate the mechanistic model EPIC using both information and data from previous CSiTE investigations and collaboration with the field and laboratory investigations of Themes 1 through 5.

This model will integrate and incorporate our process understanding of the roles of C inputs, soil structural controls, microbial community function and dynamics, humification chemistry, and intrasolum C transport on soil C sequestration.
It will enable testing of new technologies/approaches to enhance soil C sequestration. In particular, these modeling activities will improve our understanding of fundamental physical, chemical, and biological mechanisms controlling C accrual and storage in soil and how these mechanisms vary in time and space thereby addressing all five overarching research questions but especially questions IV, and V which address extrapolation of fundamental knowledge.
In producing results required for Theme 7, EPIC will facilitate the production of regional and national forecasts of SOC sequestration and provide information on how specific soil C sequestration practices could be implemented in an economically competitive, environmentally acceptable fashion.

Figure 1. Diagram of a SOC model based on integrating soil aggregated dynamics and SOC kinetics, which will provide the conceptual framework for modeling physicochemical processes in soil. Two classes of aggregates, macroaggregates (>250 m) and microaggregates (53-250 m), along with an unaggregated fraction consisting of silt and clay particles and their interactions are depicted. Each contains two organic matter classes—POC and MOC. Each of these organic fractions will be directly extracted in the soils from our four field experiments with a combination of sieving, density flotations, and combustion as described in themes 2 and 3.

Theme 7. Integrated evaluation – lead by Ron Sands

Theme 7. Integrated evaluation – lead by Ron Sands (PNNL)

Participants – Tris West ( ORNL), Cesar Izaurralde (PNNL), Allison Thompson (PNNL), Bruce McCarl (TX A&M)

Purpose and Objectives

This research theme uses fundamental mechanistic information garnered in themes 1-6 to provide broad-based evaluations of strategies to enhance soil C sequestration with a special focus on bioenergy. The primary objectives are to 1) build on CSiTE science and process modeling (EPIC) so that soil sequestration is integrated with strategies for realizing a national bioenergy economic sector and is recognized as significant in the broader C management community, and 2) facilitate the examination of competitiveness of dedicated bioenergy crops and soil C sequestration technologies in the context of the full suite of climate adaptation and GHG mitigation strategies. This theme directly addresses science question V: “How can fundamental knowledge best be used to identify and implement methods and practices for sustained enhancement of soil C in an environmentally acceptable and economically feasible fashion?”

Background and Research Questions

Soil C sequestration implications have not been fully considered in societal energy planning, including biofuel-related consideration and GHG management decision making. The potential for soil sequestration is represented either very simplistically or not at all in the analysis of alternative climate policies or large-scale expansion of biofuels. The process understanding and data requirements for extrapolation of CSiTE results to the regional and national levels require specialized knowledge and resources beyond those available to decision makers and modelers.

This theme is directed toward integrating and assembling CSiTE science and process understanding in a way that is broadly applicable and strengthens opportunities for evaluating potential prospects for sequestration and biofuel production. Therefore, we consider activities within Theme 7 as complementary to integrated assessment activities by other groups. A principal success measure will be met when IA modeling teams and policy bodies use CSiTE analyses to expand their consideration of GHG mitigation options to include terrestrial C sequestration on an equal footing with geologic sequestration, energy efficiency, nuclear power, and other energy and GHG mitigation options along with considering the soil sequestration consequences of biofuel possibilities.

Process and sectoral modeling, using EPIC and FASOM, are central to addressing the role of C sequestration. Jointly, these models allow economic and environmental simulation of alternate C sequestration strategies, their feasibility, and regional and national potential and how such potential changes with technological alternatives. EPIC simulates crop growth and environmental interaction processes, while FASOM uses results from EPIC to parameterize the biophysical and environmental tradeoffs across land uses and land management practices. Under this theme, additional research questions emerge including:

To what extent do soil C sequestration strategies improve the overall economics, GHG mitigation potential and environmental interactions of dedicated biofuel crops at a scale commensurate with competing energy and mitigation technologies?
What are the indirect environmental costs, with respect to carbon stocks and net carbon emissions, of changing from food crops to bioenergy crops?
How can we combine experimental science and mechanistic modeling to improve the effects of terrestrial sequestration and biofuel production strategies along with their adoption prospects?
How are the abilities to sequester carbon and mitigate emissions augmented or lessened by economic influences or incentives?

This involves a mixture of appraisals of the full environmental and economic implications of proposed strategies along with analyses of prospective practices to help guide research direction. We will examine a set of practices to see if some are more or less desirable and to identify aspects of practices that if changed would enhance environmental and economic attractiveness.

We expect to obtain an improved understanding of how soil C sequestration affects the overall economics and GHG mitigation potential of biofuel crops. We also expect to have an improved understanding of where C sequestration and biofuel activities might occur under various economic conditions. Specifically, results will be generated in terms of potential management manipulations arising from science findings under the first four overarching CSiTE research questions addressed by Themes 1-5. We expect that findings from these four science questions will be reflected in the EPIC parameters transferred to FASOM. Therefore, activities under this Theme are designed to directly address science question V: How can fundamental knowledge best be used to identify and implement methods and practices for sustained enhancement of soil C in an environmentally acceptable and economically feasible fashion?

Tris West discussing his modeling efforts at CSiTE modeling workshop in June 2007