Hydraulic Conductivity and Soil Water
Retention of Soil-Biochar Mixtures
Z. Liu, B. Dugan, C.A. Masiello, and H. Gonnermann,
Rice University
Motivation
 CO2-driven acceleration of hydrologic cycle
will result in both increasing drought and
more intense precipitation events;
 Biochar may improve crop productivity by:
 Reducing speed of infiltration, holding water on
the landscape longer;
 Increasing plant available water.
 GOAL: test the effect of biochar on these
properties and determine controlling
mechanisms in sandy soils.
Key Points: Adding Biochar to Sand
 Adding up to 6 wt% biochar can decrease
hydraulic conductivity (K) by up to 78%;
 Adding up to 10 wt% biochar can increase
field capacity (from 3-12%), permanent wilting
point (from 2-8%) and plant available water
(from 1-4%);
 Biochar grain size alters K; however, neither
biochar grain size nor pyrolysis temperature
have a large effect on plant available water.
K and Soil Water Retention Methods
4 cm
ω
Column
Water
r
25 cm
Sand+biochar
Filter tube
Nylon Filter
r1
h
Reservoir tube
L
Sand+biochar
Mesh
Water
Leachate
ψ=ρω2 (r2-r12)/2g
Adding up to 6 wt% Biochar, K↓ by 78%
-3
10
-4
K (m/s)
10
-5
10
-6
10
Flush # 0
Flush # 1
Flush # 2
Flush # 3
Flush # 4
Flush # 5
Flush # 6
0
2
4
6
8
10
Biochar amendment (wt%)
K↓ with Flushes
-3
-3
10
10
-4
-4
K (m/s)
10
K (m/s)
10
-5
10
-6
10
-5
NB
2 wt% BC
4 wt% BC
6 wt% BC
8 wt% BC
10 wt% BC
0
1
10
-6
2
3
4
Flush #
5
6
10
Biochar Particles Smaller than Sand
Decrease K at 6 wt% amendment
-3
10
K (m/s)
NB
<0.251 mm BC
0.853-2.00 mm BC
0.251-0.853 mm BC
-4
10
-5
10
0
1
2
3
4
Flush #
5
6
Potential Mechanisms
Grain size effect: pore throat size and tortuosity
r
L
r
+
L
+
K is mainly controlled by pore space
between biochar and sand.
r
L
0 wt% BC exp-data
2 wt% BC exp-data
4 wt% BC exp-data
6 wt% BC exp-data
8 wt% BC exp-data
10 wt% BC exp-data
0.5
0.2
3
3
Water content (m /m )
0.4
Soil Water Retention
Curves
0.15
FC
3
3
Water content (m /m )
0.3
0.2
FC
PWP
0.1
0
0.01
0.1
1
Soil suction (-bar)
10
100
0.1
0 wt% BC exp-data
2 wt% BC exp-data
4 wt% BC exp-data
6 wt% BC exp-data
8 wt% BC exp-data
10 wt% BC exp-data
PWP
0.05
0
0.1
More Biochar, Higher Water Content
10
1
Soil suction (-bar)
10
More Biochar, Higher Plant
Available Water
Water content (m3/m3)
0.15
0.12
field capacity
permanent wilting point
plant available water
11.8 ± 0.9%
8.1 ± 0.9%
0.09
0.06
2.9 ± 0.4%
4 ± 1%
0.03
1.7 ± 0.4%
0
2
4
6
8
Biochar amendment (wt%)
10
1.2 ± 0.5%
Field capacity, permanent wilting point and plant available water
content increase with biochar amendment rate.
Pyrolysis T and Biochar Grain Size Have NO
effect on Available Water Content at 6 wt%
Water content (m3/m3)
0.15
0.12
field capacity
permanent wilting point
plant available water
0.09
0.06
0.03
NB
BC size:
BC Temperature:
<0.251 mm
o
400 C
0.251-0.853 mm
o
400 C
0.853-2.00 mm
o
400 C
<0.251 mm
o
700 C
0.251-0.853 mm
o
700 C
Most of water in biochar-amended sand is
not available to plants.
0.853-2.00 mm
700 oC
Conclusions
 Adding up to 6 wt% biochar can decrease hydraulic
conductivity by up to 78%;
 Biochar particles smaller than sand decrease K;
 Adding up to 10 wt% biochar can increase field
capacity (from 2.9 ± 0.4% to 11.8 ± 0.9%),
permanent wilting point (from 1.7 ± 0.4% to 8.1 ±
0.9%) and plant available water (1.2 ± 0.5% to 4 ±
1%);
 Biochar grain size and pyrolysis temperature do not
have large effect on plant available water content;
 Most of water in biochar-amended sand is not
available to plants.
Extended Van Genuchten Model

   r    s   r  1   

n m
Where

0
1

log



d
    
1
 log  d c

1




if  c     d
if    c

ψc is solved by:
1
n
n m
Zc  Zd   r  s   r 1   10 

if    d



 m 1


n
n Zc
 nm s   r  10
1   10


Zhang, Z. F., 2011
n
n Zc
Biochar Migration
Particle Size
(mm)
Biochar Skeletal Density
(g/cc)
<0.251
1.59 ± 0.01
0.251-0.853
1.497 ± 0.009
0.853-2.00
1.47 ± 0.01