Impacts
of vegetation change on stabilization and microbial accessibility of soil
organic matter:
A microbiological, isotopic, and molecular study.
U.S. Department of Energy, Climate Change Research Division, Terrestrial Carbon Processes Program
Impacts of vegetation change on stabilization and microbial accessibility of soil organic matter: A microbiological, isotopic, and molecular study. 2005-2010. NSF Biogeosciences Program, EAR-0525349 (T. Filley, T. Boutton, D. Stott).
Soil organic matter (SOM)
represents the largest actively cycling pools of organic carbon (C) and nitrogen
(N) in the terrestrial environment, but an incomplete understanding of
multi-process soil/plant/microbe interactions limits our ability to
quantitatively account for the storage and dynamics of these elements in global
budgets. Species-dependent controls on plant chemical and physical
composition, microbial community structure and activity, climate, and edaphic
factors, all play a role in determining SOM stabilization and decomposition.
Current concepts of the physical and biological controls over soil C and N
storage emphasize that the long-term stabilization of SOM results largely from
the interplay among three factors: (i) physical protection within soil
aggregates, (ii) inherent chemical recalcitrance of the organic matter, and
(iii) association with mineral surfaces. This proposal seeks to document
and quantify how these protective mechanisms interact following a major
vegetation change from C4 grassland to C3 woodland dominated by N-fixing tree
legumes. Specifically, a chronosequence (120 yrs) of C3 woody plant invasion
into a subtropical C4 grassland will be utilized as a model system to
investigate the storage and dynamics of SOM in specific soil physical fractions.
The primary goal of this
project is to determine the quantitative significance of microbe community
structure and enzymatic activity, soil microfabric, and the specific chemical
forms of organic C and N in the stabilization of soil organic matter along a
chronosequence from remnant grasslands to woodland. The natural C-13 and
N-15 isotopic variations induced by the vegetation change from C4 grassland to
C3 woodland will be a key tool for tracing sources and fluxes of SOM in all
phases of the project. We will evaluate fundamental components of SOM dynamics
as summarized in the following four questions: (1) How does soil physical
structure determine C accrual and dynamics over time following woody plant
invasion? (2) What is the chemical composition, source, and turnover rate of
the plant and microbial carbon that is stabilized? (3) What is the role
of shifting populations of soil microbes and enzyme activity in the respiration
of litter and SOM fractions and how do they impact aggregation dynamics? (4)
What is the relative accessibility of the new C3 SOM pools to microbial decay
and can we relate physically
identifiable SOM
fractions with calculated mean residence time (MRT)
to potential respiration in inoculation experiments? We will employ innovative molecular, isotopic, and microbiological methods to develop a more fundamental
understanding of the processes that control soil carbon storage and dynamics.
This work has significant potential to benefit the SOM modeling community which
is searching for biologically, chemically, and physically meaningful approaches
to modeling of SOM dynamics. The broader impact of this work includes an
enhanced
understanding of the role of soil
processes in biogeochemical cycles and the earth system, which will be of
immediate significance to both scientists and policy-makers as mankind considers
the potential for manipulating the carbon cycle in order to mitigate the
potential for global climate change.
Soil
microbial communities and carbon dynamics in the southern Great Plains:
Influence of woody plant invasion and prescribed fire.
2006-2008.
NSF Ecosystem Studies Program, DEB-0608465 (T.W. Boutton and E.B.
Hollister).
Woody plant encroachment into grasslands and savannas is among the most
geographically extensive vegetation changes occurring in the world today, and
fire is a common management tool used to control it. Both woody encroachment
and fire often modify keyecosystem properties and processes such as plant
species composition, net primary productivity, decomposition, and nutrient
cycling processes. We will investigate the impacts of woody encroachment and
fire history on the diversity and function of soil microbial communities in the
southern Great Plains. Ribosomal RNA genes extracted from soils will be used to
characterize the biodiversity of the bacterial and fungal communities in the
soil, and functional gene arrays will be used to document their functional
attributes. This study will enhance our fundamental
understanding of the interactions and feedbacks
between above- and belowground components of ecosystems, and improve our ability
to manage and conserve natural resources in grassland and savanna ecosystems.
Soil carbon responses to atmospheric CO2. 2006-2009. U.S. Department of Energy, Climate Change Research Division, Terrestrial Carbon Processes Program , (J.D. Jastrow, R. Matamala, T. Filley, T.W. Boutton, M.A. Gonzalez-Meler, J. Six).
Determining the potential carbon sink strength of terrestrial ecosystems requires better understanding and improved quantitation of processes involved in soil carbon storage and turnover. Some soil carbon is stabilized because of its biochemical recalcitrance. More labile carbon can be physically protected from decomposition by incorporation into soil aggregates or chemically protected by association with soil minerals. The processes involved in soil aggregate formation and turnover form the theoretical basis for isolating measurable carbon pools with functionally meaningful relationships to soil carbon dynamics. We use a physicochemical fractionation approach, stable carbon isotopes, long-term incubations, and compound-specific isotope analysis of biopolymer structures to evaluate the dynamics, sources, and stability of these functional soil carbon pools and their responses to atmospheric CO2 enrichment. Studies carried out at free air CO2 enrichment (FACE) facilities use repeated 
measurements over time and the isotopic tracers available at these sites to investigate fundamental questions regarding potential saturation of soil carbon protection mechanisms, the effects of changes in input availability and source, the stability and longevity of accrued carbon, and the influence of species-specific responses and edaphic properties on soil carbon dynamics in ecosystems exposed to atmospheric CO2 enrichment. An important goal of this research is to contribute data and process knowledge that can be used to help parameterize and validate soil organic matter simulation models, thereby allowing extrapolation of our results to the broader scales necessary to predict
the role of terrestrial ecosystems in continental and global carbon cycles.