Impacts
of vegetation change on stabilization and microbial accessibility of soil
organic matter:
A microbiological, isotopic, and molecular study.
Woody Plant Encroachment in Grasslands:
Implications of Increased Primary Production for Soil Carbon Sequestration.
Fire Management in Temperate Mixed-Grass Savanna:
Effects on Soil Carbon Sequestration.
Regional NPP and
Carbon Stocks in Southwestern USA Rangelands:
Land-Use Impacts on the
Grassland-Woodland Balance.
Scaling Soil C and N Storage in a Changing Savanna Parkland Landscape:
Spatial Structure, Prediction, and Uncertainty Assessment.
Impacts of vegetation change on stabilization and microbial accessibility of soil organic matter: A microbiological, isotopic, and molecular study. 2005-2009. 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 key ecosystem 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.
Woody Plant Encroachment in Grasslands: Implications of Increased Primary Production for Soil Carbon Sequestration. 2002-2006. USDA/CASMGS Program (T.W. Boutton and S.R. Archer).
Livestock grazing and fire suppression have caused woody plant proliferation in many grassland and savanna regions throughout the world over the past century. Estimates suggest that this land cover change may be responsible for sequestering >0.1 Pg C yr –1 in the USA alone, and >0.8 Pg C yr –1 globally. In the North American Great Plains region, this phenomenon is most prevalent along the grassland-forest boundary to the east, and along the grassland-desert boundary to the southwest.
Our recent work showed that C3 subtropical thorn woodlands dominated by N2-fixing tree legumes have largely replaced C4 grasslands in the Rio Grande Plains of Texas over the past 100-200 yr. This vegetation change has increased rates of aboveground net primary productivity (NPP) from 1.9-3.4 Mg ha yr –1 in remnant grasslands to 5.1-6.0 Mg ha yr –1 in areas now dominated by woody vegetation. Although belowground productivity has not been quantified yet, root biomass to a depth of 1.5 m is approximately 2-5X greater in wooded areas than in grasslands, suggesting belowground production is also accelerated significantly in wooded areas. These shifts in NPP are accompanied by significant changes in plant tissue chemistry (e.g., high-quality herbaceous tissue vs. poor quality lignified woody tissue) that likely alter ecosystem carbon storage and dynamics. Because trees are C3 plants (d13C ~ -27 ‰) and grasslands are dominated by C4 species (d13C ~ -13 ‰), this grassland to woodland conversion affords unique opportunities for isotopically tracing rates of soil organic carbon accumulation and turnover, and for identifying the specific physical and chemical mechanisms by which soil carbon is sequestered.
The purpose of this study is to evaluate the impact of grassland-to-woodland conversion on the carbon cycle in the Rio Grande Plains of southern Texas. More specifically, we will: (1) Quantify rates of soil organic carbon sequestration following grassland-to-woodland conversion; (2) Elucidate specific physical mechanisms of soil carbon sequestration in this system by determining where organic carbon is stored relative to soil aggregate structure; and (3) Utilize the natural isotopic difference between C4 grasses and C3 woody plants to quantify turnover rates of soil organic carbon in specific soil physical fractions isolated in Objective 2.
Research will be conducted at the Texas Agricultural Experiment Station LaCopita Research Area in southern Texas. We will utilize a chronosequence approach and collect soil cores in remnant grasslands (representing T0), and in woody plant stands ranging in age from 10-120 yrs. Ages of woody plant stands will be determined by tree-ring techniques and sequential aerial photography. Recently developed soil physical fractionation procedures will be employed to evaluate mechanisms of carbon sequestration. The mass and isotopic composition (d13C) of bulk soil organic carbon and carbon in specific physical fractions will be determined, and turnover rates of carbon in bulk soils and in physical fractions will be evaluated by compartmental analysis of d13C values.
Results of this study will enhance our understanding of the effects of land uses (livestock production) and land cover changes (grassland to woodland conversion) on NPP and soil carbon storage and dynamics in the southern Great Plains region. Furthermore, because similar grassland-to-woodland conversions have been geographically extensive in grasslands and savannas worldwide, changes in soil carbon storage and dynamics documented here could have implications for the global carbon cycle and climate.
Fire Management in Temperate Mixed-Grass Savanna: Effects on Soil Carbon Sequestration. 2002-2006. USDA/CASMGS Program (T.W. Boutton and R.J. Ansley).
Woody plant encroachment into grasslands and savannas can significantly diminish the productivity and economic viability of livestock production systems, and prescribed burning is often employed to prevent or slow woody invasion in rangelands. In the the southern Great Plains, fire is used to suppress encroachment and growth of aggressive woody species, especially honey mesquite (Prosopis glandulosa), an N2-fixing tree legume. However, fires have strong potential to modify ecosystem carbon cycles by altering: (a) standing stocks of carbon in vegetation, litter, and soil, (b) rates of net primary productivity and decomposition, (c) carbon allocation patterns, and (d) plant tissue chemistry due to changes in species composition and functional diversity. Furthermore, the production of relatively inert charcoal (or black carbon) during vegetation fires can have a major impact on the composition, formation, and turnover of soil organic matter, and may represent a significant fraction of the “missing carbon” in the global carbon budget. Despite its widespread use and significance as a rangeland management tool, little is known regarding the potential for fire to alter ecosystem carbon storage and dynamics.
This study will quantify the effects of repeated prescribed fires applied during different seasons (summer vs. winter) on carbon storage and dynamics in mesquite savannas in the southern Great Plains. More specifically, we will: (1) Quantify the effects of fire frequency and season of occurrence on soil organic carbon and total nitrogen, and on carbon in aboveground biomass and litter; (2) Identify the forms in which soil carbon is sequestered by determining where carbon is physically located relative to aggregate structure, and by quantifying charcoal carbon; (3) Utilize the natural isotopic signatures of soil organic carbon (d13C) and soil total nitrogen (d15N) to identify potential mechanisms responsible for gains/losses of soil C and N; and (4) Quantify changes in pool sizes and turnover rates of labile, slow, and recalcitrant carbon pools using long-term soil incubations.
Research will be conducted at the Texas Agricultural Experiment Station at Vernon in north-central Texas. Vegetation is comprised of C3 and C4 grasses in the herbaceous layer, and Prosopis glandulosa dominates the tree layer. Unburned controls and 4 fire treatments (differing in frequency and season of occurrence) were established in 1991 (3 replicates/treatment), and soils were collected in 1994 and 1996. New fires will be applied in spring and summer 2002. Vegetation composition and the mass of carbon in aboveground biomass and litter will be determined before and after fire treatments. Soils will be resampled (to 1 m) prior to the spring 2002 fires, and again in summer 2003. Bulk density, texture, organic carbon, total nitrogen, d13C, and d15N will be determined on all samples. Charcoal carbon will be quantified by a combination of physical separation, high-energy photo-oxidation, and solid-state 13C CP/MAS NMR. Soil organic carbon in physical fractions related to aggregate structure will be quantified, and d13C used
to determine source of carbon in those physical fractions. Long-term soil incubations will be conducted on soils from each treatment to evaluate changes in pool sizes and turnover rates of labile, slow, and recalcitrant soil carbon pools.
Prosopis glandulosa is a dominant woody plant on >45 million hectares of rangelands in the southern Great Plains and southwestern USA, and is one of the most important species associated with the dramatic and ongoing increase of woody plants in grasslands and savannas in those regions. The results of this study will provide key information needed to comprehend the impact of woody invasion on regional carbon storage and dynamics, and will enhance our understanding of the carbon cycle in rangeland ecosystems affected by this vegetation change. Furthermore, this study will provide key information regarding the influence of a common management technique (prescribed burning) on carbon storage and dynamics in mesquite-dominated systems. Our preliminary results suggest that the control of woody invasion through fire management may also promote ecosystem carbon storage.
Regional NPP and Carbon Stocks in Southwestern USA Rangelands: Land-Use Impacts on the Grassland-Woodland Balance. 2001-2006. NASA Carbon Cycle Science Program, CARBON-0000-0174 (C.A. Wessman, G. Asner, S.R. Archer, T.W. Boutton)
Tree/grass ratios profoundly impact the biogeochemistry of grasslands and
savannas by affecting: (i) decomposition of above- and belowground biomass, (ii)
vertical distribution, mass, and size of roots in the soil, and (iii)
microclimatic influences on soil microbial biomass and rates of soil organic
matter turnover. Because dryland ecosystems comprise half the terrestrial
surface, changes in tree/grass ratios likely influence global biogeochemical
cycles and climate. We propose to extrapolate our high-resolution, validated
studies to assess land-use impacts on NPP and C-storage in rangeland ecosystems
throughout the southwest. We will integrate aircraft, Landsat, and MODIS data to
retrieve, with increasing spatial coarseness, biogeophysical information
relevant to biogeochemistry, vegetation dynamics, and land management. Sequenced
validation of land cover fractions from plot-to-Landsat-to-MODIS scales using
spectral mixture analysis will enable us to determine scaling properties of key
biophysical variables (e.g. live vs. dead vegetation) from landscapes to regions
in contrasting bioclimatic zones. These variables will constrain the ecosystem
process model, TerraFlux, and thereby estimate regional productivity and
C-storage in vegetation and soil. δ13C
of soil organic carbon (SOC), a biogeochemical tracer of woody-herbaceous
inputs, will be obtained for our temperate savanna site and used to test model
performance and, hence, the adequacy of remote sensing inputs. δ13C
of SOC will also enable us to document long-term vegetation history, estimate
SOC turnover, and the relative contribution of grasses vs. woody plants to
ecosystem productivity and C-storage. This δ13C
database will be comparable to that completed at our subtropical savanna site;
therefore, we will compare and contrast effects of woody plant encroachment on
ecosystem C-storage in contrasting bioclimatic regions.
We will also develop a spatially explicit land-use history within our Texas
study region to distinguish among land-use practices influencing tree/grass
ratios (e.g., grazing, fire, brush clearing, cropland abandonment). Because
human management plays a dominant role in this region, we will test scenarios
encompassing the range of impacts that might result from contrasting land-use
policies. We cannot predict what state/federal policies might be enacted to
affect range management practices. Nor can we predict what economic incentives
pertaining to "carbon credits" might arise. However, we can use our
linked remote sensing-modeling approach to predict regional C-budgets in
response to potential policies or incentives that may emerge. Land-use scenarios
that define different policy environments (e.g., government subsidies to support
woody plant control, or carbon credit incentives that promote woody plant
proliferation) will be developed and played out through a simple GIS approach.
Consequent prescribed changes in vegetation structure, when coupled with
TerraFlux, will enable us to estimate the influence that policy changes might
have on trajectories in C-sequestration and liberation at the scale of the
southwest region.
Scaling Soil C and N Storage in a Changing Savanna Parkland Landscape: Spatial Structure, Prediction, and Uncertainty Assessment. 2001-2005. NSF Ecosystem Studies Program, DEB- 9981723 (X.B. Wu, S.R. Archer, T.W. Boutton).
It is widely known that woody plants significantly alter
soils subsequent to their establishment. This leads to the formation of Afertile
islands@ in shrublands, woodlands and
savannas. Although the biophysical characteristics of Aislands
of fertility@ are well-known, we know
little of their rates of formation, the extent of within-island variability or
how to develop an integrated landscape-scale assessment of nutrient pools in
relation to the spatial distribution of plant lifeforms or growthforms and their
change with time.
The proposed study will be conducted at a savanna parkland landscape where woody plant encroachment over the past century has been well-documented. We will quantify spatial variation in soil organic carbon (SOC), total nitrogen (TN) and root mass (0-20 cm) in relation to herbaceous and woody patch age-states across a topographically heterogeneous landscape, assess within-patch spatial structure of these variables, and generate landscape-scale estimates of SOC, TN, and root mass pools. Our goal is to understand the spatial structure of landscape-scale SOC, TN, and root mass and quantify changes which have accompanied the shift from grass to woody plant domination. The overall objectives are to: (a) compare and contrast patterns of SOC, TN, and root mass distribution within different woody plant and herbaceous patch types; (b) quantify the spatial structure of SOC, TN, and root mass distribution at the landscape scale using geostatistical tools; (c) utilize information on spatial structure and uncertainty patterns to estimate the intensity and distribution of Apoint samples@ needed to quantify landscape-scale SOC, TN, and root mass; (d) determine the geostatistical relationships between aboveground patch features (which can be readily quantified on aerial photos or satellite imagery) and SOC, TN, and root mass; and (e) quantify changes in SOC and TN pools accompanying woody plant expansion using information derived from historical (1930-2000) aerial photography; and (f) compare these estimates to those generated from linked biogeochemistry-succession models.
Specifically, we will test hypotheses that (1) there
are significant spatial structures in near-surface SOC, TN, and root mass
distributions across savanna parkland landscapes; (2) patterns of SOC, TN, and
root mass vary within patches as influenced by individual plants, patch type
(e.g., discrete cluster, groves, closed-canopy woodlands) and patch age-state;
(3) SOC, TN, and root mass distributions in savannas have nested spatial
structures, with local structures induced by vegetation pattern and large-scale
structure determined by geomorphology and surface hydrology; (4) efficient point
sample distributions across landscapes can be designed based on knowledge of
spatial structure and uncertainty distribution generated from stochastic
simulation and univariate statistics; (5) accuracy of landscape-scale SOC, TN,
and root mass inventories based on remotely sensed vegetation data can be
substantially enhanced using co-kriging and stochastic simulation at different
scales; and (6) an enhanced deterministic extrapolation model, based on an
understanding of the nested spatial structure and uncertainty, will generate
acceptable estimates of SOC, TN, and root mass compared against the estimates
using kriging and co-kriging. This approach will then be used to estimate
historical changes in landscape SOC, TN, and root mass distribution associated
with woody plant expansion as quantified on historical aerial photos.
Our approach will develop an extensive, spatially-explicit,
landscape-scale database of SOC, TN, and root mass that is linked to remotely
sensed and field vegetation data. These will be subjected to GIS-based spatial
analyses and modeling using univariate and geostatistical approaches (variogram
and cross variogram analyses, (co-)kriging, and stochastic simulation). Results
will (a) improve our understanding of the spatial patterns of soil properties
and shed light on processes that operate in these different patch and landscape
elements; and (b) increase our ability to develop accurate estimates of
landscape-scale SOC, TN, and root mass from point samples and aboveground
spatial attributes. The latter would c) help realistically constrain and
evaluate ecosystem simulation models now being parameterized for large
(pixel)-scale applications; and (d) provide a basis for estimating the magnitude
of C and N accumulation that has accompanied woody plant encroachment into
grasslands over the past century.