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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
ψ=ρω2 (r2-r12)/2g
Column
Water
Sand+biochar
Leachate
4 cm
h
25 cm
MeshL
r1
r
Water
Sand+biochar
Nylon Filter
ω
Reservoir tube
Filter tube
Adding up to 6 wt% Biochar, K↓ by 78%
0 1 2 3 4 5 610
-6
10-5
10-4
10-3
Flush #
K (
m/s
)
NB2 wt% BC4 wt% BC6 wt% BC8 wt% BC10 wt% BC
0 2 4 6 8 1010
-6
10-5
10-4
10-3
Biochar amendment (wt%)
K (
m/s
)
Flush # 0Flush # 1Flush # 2Flush # 3Flush # 4Flush # 5Flush # 6
K↓ with Flushes
0 1 2 3 4 5 610
-6
10-5
10-4
10-3
Flush #
K (
m/s
)
NB2 wt% BC4 wt% BC6 wt% BC8 wt% BC10 wt% BC
0 2 4 6 8 1010
-6
10-5
10-4
10-3
Biochar amendment (wt%)
K (
m/s
)
Flush # 0Flush # 1Flush # 2Flush # 3Flush # 4Flush # 5Flush # 6
Biochar Particles Smaller than Sand Decrease K at 6 wt% amendment
0 1 2 3 4 5 610
-5
10-4
10-3
Flush #
K (
m/s
)
NB<0.251 mm BC0.853-2.00 mm BC0.251-0.853 mm BC
Potential Mechanisms Grain size effect: pore throat size and tortuosity
K is mainly controlled by pore space between biochar and sand.
+
+
r
L
r
L
r
L
0.01 0.1 1 10 1000
0.1
0.2
0.3
0.4
0.5
Wat
er c
onte
nt (
m3/m
3)
Soil suction (-bar)
FC PWP
0 wt% BC exp-data2 wt% BC exp-data4 wt% BC exp-data6 wt% BC exp-data8 wt% BC exp-data10 wt% BC exp-data
Soil Water Retention Curves
More Biochar, Higher Water Content
0.1 1 10 1000
0.05
0.1
0.15
0.2
Wat
er c
onte
nt (
m3/m
3)
Soil suction (-bar)
FC
PWP
0 wt% BC exp-data2 wt% BC exp-data4 wt% BC exp-data6 wt% BC exp-data8 wt% BC exp-data10 wt% BC exp-data
More Biochar, Higher Plant Available Water
Field capacity, permanent wilting point and plant available water content increase with biochar amendment rate.
0 2 4 6 8 10
0.03
0.06
0.09
0.12
0.15
Wat
er c
onte
nt (
m3/m
3)
Biochar amendment (wt%)
field capacitypermanent wilting pointplant available water
2.9 ± 0.4%
11.8 ± 0.9%
1.7 ± 0.4%
8.1 ± 0.9%
4 ± 1%
1.2 ± 0.5%
Pyrolysis T and Biochar Grain Size Have NO effect on Available Water Content at 6 wt%
Most of water in biochar-amended sand is not available to plants.
0.03
0.06
0.09
0.12
0.15
Wat
er c
onte
nt (
m3/m
3)
BC size:BC Temperature:
NB <0.251 mm
400 oC
0.251-0.853 mm
400 oC
0.853-2.00 mm
400 oC
<0.251 mm
700 oC
0.251-0.853 mm
700 oC
0.853-2.00 mm
700 oC
field capacitypermanent wilting pointplant available water
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
mn
rsr 1
Where
1log
log0
1
1
cd
d
d
c
c
d
if
if
if
Zhang, Z. F., 2011
1mZnnZnnrs
mnnrsr
1dc
cc 10110nm101ZZ
ψc is solved by:
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
