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