Dixie L. Smith

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Changes in Soil Carbon Dynamics During Juniper
Expansion into Tallgrass Prairie
Dixie L. Smith
Land Use Land Change Grant
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Flow diagram showing the integration and
interdisciplinary nature of the research program.

1.A. Assessment of Land-cover Change Using
Remote Sensing
1. A. Arc / Info GIS Database
1. B. Identification of Sites Undergoing Rapid
Land-cover Change
2. Biogeochemical Consequences of Land-cover
Change (In situ, process-level measurements)
Tallgrass Prairie Baseline Conditions (Konza
Prairie)
Ecosystem-scale Exchange of CO2, H2O and Energy
Soil and Plant C, N Pools and Fluxes
3. General Ecosystem Model
4. Predicting Ecosystem Consequences of Present
and Future Regional Land-cover Change
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Global Carbon Cycle
Atmospheric Pool
Terrestrial Pool
Oceanic Pool
Fossil Fuel Pool
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Terrestrial Carbon Cycle
Atmospheric Pool
Photosynthesis
Respiration
Respiration
Aboveground Fluxes (plant and animal)
Belowground Fluxes (plant and heterotrophs)
Detrital Inputs
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Overall Project Objective To quantify changes in
belowground carbon cycling caused by a dramatic
vegetation shift from tallgrass prairie to
juniper trees

• Less diversity
• Lower light levels
• Higher productivity
• Higher aboveground biomass

Closed canopy forest
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C4 Prairie
C3 Juniper Forest
C3-Derived SOC
C4 SOC replaced by C3 SOC
C4- Derived SOC
Time
Ti
At initial time, most SOC is from C4 grasses.
(Adapted from Balesdent and Mariotti, 1996.)
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Chapter 1Changes in soil organic carbon
dynamicsObjectiveTo determine changes in the
quantity and distribution of new C3-derived
carbon resulting from juniper expansion a stable
isotope approach
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Atmospheric CO2 Pool
1.1 13C and 99.89 12C
More Discrimination Against 13C
Less Discrimination Against 13C
C3-Trees
C4-Grasses
Less 13C in C3 Tissues
More 13C in C4 Tissues
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d 13 C
Less 13C in C3 Plant Tissues
-28 o/oo
More 13C in C4 Plant Tissues
-13 o/oo
Note LESS 13C Yields a MORE Negative
Del Value.
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Hypotheses As junipers invade tallgrass
prairie,

• forest-derived carbon will be concentrated in
  shallow soil layers, therefore changing SOC
  distribution, and
• evident in larger soil size fractions.
• Juniper forest soils will store more carbon
• than grassland soils.

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Delta 13C of Prairie and Juniper Soil Profiles
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New forest carbon replaced 38.8 of the
historic prairie carbon at the mineral soil
surface
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Afforestation did not change the quantity of SOC
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Particle-Size Soil Fractions
200 mm Fine Gravel
50 mm Sand
20 mm Fine Sand or Coarse Silt
2 mm Silt

( USDA Handbook 18, 1963)
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All grassland soil fractions had similar d13C
isotope abundance ratios
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Delta 13C of large soil fractions resembles that
of juniper carbon
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SummaryAs junipers invaded tallgrass prairie,

• forest-derived carbon is concentrated in shallow
  soil layers, therefore changing its distribution,
  and
• is evident in larger soil particles near the
  mineral soil surface.
• Soil carbon stocks did not change.

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Chapter 2
Mechanisms driving low soil respiration rates in
juniper forests Objective To determine if
vegetation type influences soil respiration rates.
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Terrestrial Carbon Cycle
Atmospheric Pool
Photosynthesis
Respiration
Respiration
Aboveground Fluxes (plant and animal)
Belowground Fluxes (plant and heterotrophs)
Detrital Inputs
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Atmospheric Pool
Soil Respired CO2

• Soil Respiration
• Heterotrophic respiration of SOC,
• Heterotrophic respiration linked to
• roots, and
• Respiration of living roots.

Belowground Fluxes
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Soil Respiration varies with

• soil temperature,
• soil moisture,
• root biomass,
• and microbial activity.

Vegetation potentially alters any or all of
these factors.
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• Hypothesis
• Grassland soil respiration rates will exceed
  forest rates due to factors such as
• higher soil temperature
• better quality substrate for microbial
• respiration, and
• greater root biomass in grasslands.

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Prairie soil respiration was 38 higher than
forest
Prairie Mean 4.60 (0.15) Forest Mean 2.87
(0.07)
Growing Season
Growing Season
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Growing season mean prairie soil temperature was
25oC, while forest temperature was 20o
Growing Season
Growing Season
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Different temperature response of soil
respiration in forest and grassland
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Soil temperature strongly influenced soil
respiration

• Grassland rates were 38 higher in the growing
  season, primarily due to higher soil
  temperatures.
• Soil respiration responded differently to
  temperature in forests and grasslands.
• Soil temperature explained 67 of the variability
  in grasslands, 37 in forests.

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• Vegetation affected soil respiration rates both
  directly and indirectly
• Different in temperature response of soil
• respiration in forests and grasslands (direct
• effect).
• Higher soil temperatures in grasslands
• increased soil respiration (indirect
• microclimate effect).

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Respiration may vary with

• soil temperature,
• soil moisture,
• root biomass,
• and microbial activity.

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Forest respiration rates were more responsive to
soil moisture levels than native grasslands.
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Respiration may vary with

• soil temperature,
• soil moisture,
• root biomass,
• and microbial activity.

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No difference in fine root biomass between
forests and grasslands
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Respiration may vary with

• soil temperature,
• soil moisture,
• root biomass,
• and microbial activity.

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No difference in microbial respiration between
forest and grassland
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Annual soil respiration from juniper forests and
grasslands
g C / m2 / year
Juniper forests 533.59 ( 21.7) Grasslands 858.3
8 (14.5)
Forest carbon stocks will take 16.5 (0.8)
years to cycle compared to 9.0 (1.0) years for
grassland carbon.
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• Regional conversion of grasslands to
• closed canopy juniper forest from 1981 1994
• affects 1million hectares in six states in
• the Midwest,
• lowers annual soil respiration by 3.4 x
• 106 Mg C,
• slows C-turnover by 7.5 years
• compared to grasslands.

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Chapter 3
As junipers expand into prairie, d13C indicates
recent inputs of C3-SOC form the substrate for
heterotrophic respiration Objective To determine
how vegetation affects microbial utilization of
soil organic C
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Atmospheric Pool
Soil Respired CO2
Labile pools of C cycle in years
to decades Recalcitrant C pools may
remain in the soil for thousands of
years
Belowground Fluxes
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Hypotheses
1. In grasslands, d13CO2 respired from soils
will reflect the bulk soil d13C. 2. In forests,
d13CO2 will initially reflect the microbial
utilization of new juniper carbon. Over time, as
labile juniper carbon is depleted, d13CO2 will
resemble prairie bulk soil d13C.
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No difference in microbial respiration rates in
forest and grassland incubations
a
a
b
b
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In forests, d13CO2 initially reflected the
microbial utilization of new juniper carbon
(-27.5 per mil) in bulk soil
-12.7
-14.6
-14.1
-17.7
Bulk soil 0-10 cm Bulk soil 10-20 cm
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In forests, d13CO2 initially reflected the
microbial utilization of new juniper carbon
(-27.5 per mil) in bulk soil. Thus, soil
microbes utilized C3-derived SOC preferentially.
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• Summary
• Heterotrophs in forest soils are
• preferentially utilizing newer soil
• carbon substrates (C3-derived).
• Respiration rates decreased over
• time and with depth, but were not
• affected by vegetation type.

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In summary, changes in soil carbon dynamics have
accompanied juniper expansion into tallgrass
prairie.

• New forest C replaced 38 of historic prairie C
  in 40 60 years near the mineral soil surface.
  However, there was no net increase above the
  original C stocks.

• In forest soils, new SOC predominated in
• larger soil particles near the mineral soil
• surface. Thus the distribution of new SOC
• is evident.

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• Vegetation affected soil respiration rates
• both directly and indirectly
• Different temperature response of soil
• respiration in forests and grasslands (direct
• effect).
• Higher soil temperatures in grasslands
• increased soil respiration (indirect
• microclimate effect).

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• Lower forest soil respiration slowed soil C
  turnover by 7.5 years.
• Microbes in forest soils are preferentially
  utilizing newer soil carbon substrates
  (C3-derived).

• Conversion of grasslands to closed canopy
• juniper forest can affect soil carbon dynamics
• over 1 million hectares of the Midwest

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Special Thanks To

• Mrs. Clarence Dobson
• Mr. Jim Ryan
• Mr. and Mrs. Leonard Borg
• Mr. Norman Winter
• Mr. Fred Carlson
• Mr. and Mrs. Steven Hargrave

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Acknowledgements

• Thank you for the advice and support of faculty
  at KSU Loretta Johnson, Alan Knapp, John Blair,
  Chuck Rice, Mickey Ransom, the Wind Erosion Lab,
  Ton Damman, and Walter Dodds. I also thank Chris
  Neill and Kris Tholke at the Ecosystems Center.
• Special thanks to Roxane Fagan, Greg Hoch, Mark
  Norris, Wendy Loya, Denise Walker, and Jim Clark.
  Thanks to the following assistants Sara Glaser,
  Bryan Stork, Jesse Nippert, Melissa Pline, Sarah
  Allen, Melissa Hill, Sarah Behrens, Shelly
  Harper, Matt Graeve, Vicki Charbonneau, Kathy
  Gaitros, Susan Fricke, David Schoolar, Jason
  Russell, and the Knapp Lab.
• My family, special friends, and the Bushnell
  Annex group.

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