2/18/2016
Depth-Dependence of Biologically
Forced Mineral Weathering
Dan Richter, Duke Univ
Calhoun CZO
“Biological Weathering” -- a topic that
reminds us that Darwin could not have been
Darwin without the great geologist,
Sir Charlnes Lyell
Southside of the Missouri, George Catlin 1832
Belowgrd CZ system
In 20th c., we allowed disciplines to become entrenched:
in 21st c. we need to be more boldly interdisciplinary
in studies of soil, ecosystems, & Earth’s critical zone
>50 m
Our objective
Southside of the Missouri, George Catlin 1832
Biological processes affecting mineral weathering
Bio-agent
Nitric acid
Soil horizon
A&B
Sulfuric acid
Organic acids
A&B
A & deeper
horizon
rhizospheres
A, B, & C
Richter, Driscoll
Plant ion uptake
Root growth
A, B, & C
pressure
C-Fe redox cycling B & C
Carbonic acid
B&C
Contributors
Van Miegroet, Johnson,
Cole
Johnson
Boyle, van Breeman
April, this research
Fimmen, this research
DWJohnson, this research
First, mechanical effects of rooting:
Physically altering mineral structures
Three effects of biota on mineral weathering,
especially in the wider soil environment
• Mechanical
• Carbon-iron redox cycling
• Carbonic acid weathering
Three understudied biotic effects on minerals &
mineral weathering
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2/18/2016
Rich April’s 1992 SEM “of a silt-sized muscovite
that appears to have been bent by the growing root”
Generally plant roots & mycorrhizal hyphae preferentially
follow pores greater than their own diameters
20 um
20 um
Ectomycorrhizal fungi
at 1.5-m soil depth -Figure 3. ESEM images (8.0 kV, 20um on horizontal scale) of rhizospheres at 1.5 m dep
in Appling soil of
B horizon
at the
Calhoun Experimental Forest, SC. Rhizospheres are of
Rhizopogon genus associates
Pinus
taeda
basidiomycete hyphae of the species, Rhizopogon fuscorubens.
at Calhoun Experimental Forest, SC (Richter et al., 2007)
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8
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Preferential rooting at a mesoscale
Photo: Horst Kiechle
Over time, radial expansion of coarse roots exerts enormous
pressures, shattering rock and soil minerals
Figure 3. ESEM images (8.0 kV, 20um on horizontal scale) of rhizospheres at 1.5 m depth
in Appling soil B horizon at the Calhoun Experimental Forest, SC. Rhizospheres are of
basidiomycete hyphae of the species, Rhizopogon fuscorubens.
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20 um
20um
20um
20 um
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20
Vertical uplift (cm)
18
Tree diameter (DBH)
15
13
10
8
5
20 um
3
0
0 12 24 36 48 60 72 84 96
41cmTree
62cmTree
63cmTree
74cmTree
20um
20 um
Rhizosphere effects
exerted to 1-m in 70 yr
Richter et al. 2007
Richter et al.
2007
Radial distance from tree (cm)
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Figure 4. Soil microtopography surrounding four 70-year-old loblolly pine (Pinus teada)
trees in the Duke Forest, North Carolina. Diameters are given for each of the four trees.
In B horizons, tap root growth pressures, modeled at
>100 mPa, are relieved
1.9
by consolidation, 0
with soil in
1.8
50-cm radial
50
1.7
distances
1.6
100
from tap root
having BD
1.5
BD
150
up to
1.4 g/cc
3
2.0 g/cm
Depth, cm
In a 70-year old pine forest on the Duke Forest,
physical effects of tap roots quantified:
In A horizons, root pressures were relieved
by upward & outward displacement
10
200
20um
1.3
n = 180 samples
250
0
100
200
1.2
300
400
500
Horizontal distance, cm
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Figure 5. Bulk density of soil surrounding two 70-year old loblolly pine trees.
Bulk density in g/cm3. Depth and horizontal distance are in cm. Bulk
densities were obtained with conventional slide hammer for 180 samples on
the face of the excavation. Isolines of densities were obtained using Matlab’s
interpolation via a shading function (‘INTERP’).
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2/18/2016
Profiles often have two systems:
Second, we expand upon the
too-sleepy hypothesis of
Fimmen et al. (2008) BGC
Upper system: intensely rooted & physically mixed
Lower system: formed in place, far less mixing, microsite hot spots
In many upland subsoils, in
periodic saturated conditions,
oxygen-deficiency leads to
rhizogenic redox cycling of C-Fe
affecting rooting, microbiology,
acidification, & weathering
Photo: Allan Bacon
Depth to
mottling
= 70cm
1m
More sedentary,
heterogenous
lower system
with preferential,
even ancient,
root zones;
chemically
weathered
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Soil’s root zone historically
indexed by
“depth to mottling”
• In plant productivity studies,
DTM often better correlated
with productivity than most
fertility properties
• DTM indexes rooting volume
with consistently ample
supply of 20% O2
(Ralston
1978)
O2
Soil’s redox features indicate
periodic
oscillations in redox
potential -- C-Fe cycling
Periodic
C-Fe
cycling
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H2O O2
1. Hydraulic conductivity
Well mixed,
upper
“biomantle”*
of Don Johnson;
Physically &
chemically
weathered
Three controls
• Hydraulic & gas
conductivity
• Climate & weather
• Root & microbiology
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H2O O2
2. Climate and weather
• Most upland soils across a
range of climates at least
occasionally perch water
• Water in soil profile slows
Zone of
diffusion of gases (O2)
periodic
C-Fe
by orders of magnitude
Zone of
periodic
C-Fe
cycling
cycling
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Organic reductants
Tree
roots
3. Root system biology
In upland subsoils, rhizospheres are often
considered to be relic features
• Roots and rhizosphere microbes
drive C-Fe cycling with inputs of
organic reductants & respiratory
demands for O2
• At some depth-dependent
frequency, perched water
exhausts O2 & electrons transfer
from organic reductants to
redox-active metals
to ~4 m
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However, upper B horizon soil of a wind-blown tip up plate
of a <150-yr old post oak (Quercus stellata): Duke Forest
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Patterns
can be
spectacular
Under
longleaf pine,
Pinus palustris,
SE USA
~30 cm
Relic rhizospheres
Rhizogenic redox cycling under
loblolly pine (Pinus taeda)
in rooted fractures of underlying
granite
Fe made
New field sampling methods developed by
Bacon et al. to study microsites:
backhoe, a field table, care, & patience
visible by
oxidation
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2/18/2016
C budget
FACE forest
Third, a new look at carbonic
acid weathering, as ecologists
continue to emphasize that
ecosystem metabolism
is only about CO2 exchange
between plants, upper soil, &
the atmosphere
& geologists
continue to credit raincharged CO2 with mineral
weathering
Finzi et al. 2005
The new look at carbonic acid weathering requires a full
accounting of photosynthetic C & respiratory CO2 and O2,
a full accounting of ecosystem metabolism
CO2
Data from Markewitz & Richter 1998
While most soil CO2 returns to the atmosphere,
consumption of CO2 at the weathering fronts drives
CO2 diffusion downward, assuring continued weathering
Mean
CO
2
Max
(summer)
SOIL CO2 (%)
C
CO2 (%)
Where bio meets
geo:
Carbonic acid
system
CO2  H2CO3 
H+ + HCO3Richter & Markewitz
1995, 2001
Oh & Richter 2004,
2005
Richter & Billings 2015
Changes in water chemistry quantify weathering
reactions
H+ + feldspars  Ca2+ + Na+ + HCO3Water
quality
controlled
by
ecosystem
metabolism!
Soil depth (m)
Min
(winter)
SOIL HORIZONS
SOIL DEPTH (meters)
Ecosystem metabolism seasonally ebbs and flows
regularly driving waves of mineral weathering
A
E
B
O2
A full accounting
of metabolism
indicates ecosystem
extends to the base
of the CZ itself:
Feb
CO2
May
Aug
Feldspar wthg
consumes CO2
H+ + feldspars 
Ca2+ + Na+ + HCO3-
Nov
Pine forest at Calhoun CZO, measured 0-5.5-m by Markewitz,
modeled by Oh and Richter 2005
In 2010, we cored a 70-m borehole, 30-m through
soil & weathered rock, 40-m into the Calhoun
granite
Geophysics’ seismic velocities, S. Holbrook, WyCEHG
WFront
25 m seep
Rasmussen et al.
tau
2011
Element mass balances
• Bio-generated weathering fronts
at 12-40-m!
• pH 4 down to to 12-m!
• Bacon et al. 2012 GEOLOGY
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Lessons from putting the bio- in biogeosciences?
While individual disciplines give us skills, labs,
literatures, & insights, only interdisciplinarity can
lead to a richness of understanding via the pooling
& weighing of ideas, data, & expertise from across
the disciplines
Van Gogh’s The Sower, 1888
Challenge of soil, ecosystem, & CZ sciences
• How to help these inherently interdisciplinary
sciences “go critical”?
Van Gogh’s The Sower, 1888
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