Tree, Grass, and Crop
Root Length Densities and
Soil Water Content
Within an
Agroforestry Buffer System
Ranjith Udawatta1, Stephen Anderson2, and Harold Garrett1
Center for Agroforestry1,
Department of Soil, Environmental and Atmospheric Sciences2
School of Natural Resources, University of Missouri-Columbia.
Rationale
Despite improvements in the use of soil
conservation practices, crop rotations, and
nutrient management programs, significant
concerns still exist regarding soil erosion and
nutrient losses in runoff from row crop
production.
In the US, states are required to implement
water quality standards based on USEPA
guidelines or other scientifically defensible
methods (Ice and Binkley, 2003).
Such requirements result in increasing pressure
for the development of economically and
environmentally suitable guidelines to reduce
NPSP from agricultural watersheds.
Recently, agroforesty has been suggested as an
alternative to traditional row crop production
for conservation and a source of additional
income.
Effects of agroforestry practices on root
distributions and changes in soil water content
have received little attention in the temperate
climatic zone.
This study will evaluate root length densities and
changes in seasonal volumetric soil water content
in agroforestry and grass buffer cropping
systems.
Hypothesis:
Permanent vegetation with deep roots
and an extended transpiration period will
change root distribution patterns and soil
water content changes over time compared
to row crops.
Study Design:
Two paired watersheds in a corn-soybean rotation in
northeast Missouri.
Grass buffers consist of brome, redtop, and birdsfoot
trefoil and were established in 1997.
Pin oak, swamp white oak, and bur oak trees were
planted in the center of grass buffers on the
agroforestry watershed in 1997.
Data Collected
Root length densities: Tree, Grass, and
Corn areas to a 1 m depth in
2004.
Volumetric soil water content in tree,
grass and corn areas in 2004.
Study site
location and
0.5 m interval
contour lines.
Sampling Areas for Roots
Gray bands
indicate
grass buffers
and
agroforestry
buffers and
grass
waterways.
At 5000 feet altitude in August 2002
At 5000 feet altitude in August 2002
2003
2005
Study
Pin oak
Design
Data logger
for
Soil Water
Monitoring
Campbell Soil Water
Content Reflectrometer
sensors were installed on
two transects.
Senor depths
5 cm
10 cm
20 cm
40 cm
Buffer
Sensor locations
Analysis Procedures
Roots length densities by treatment were compared
to examine differences among the three treatments.
Roots length densities by depth and treatment were
compared to examine differences in depth
distribution of roots among the three treatments.
Calibrated volumetric soil water content among the
three treatments and depths were compared to
evaluate treatment effects on seasonal soil water
content.
RESULTS
Root Length Density from 0 to 1.0 m Depth by Treatment
Root length (m m -2)
3500
3000
2500
2000
1500
1000
500
0
Tree
Bur
oak
SW
oak
Pin grass corn
oak
Crop Root Length Density Between Two Buffers
a
a
-2
Root length (m m )
1200
a
900
a
600
300
0
1
2
3
Position
4
Root Length Density Within Grass Buffers
-2
Root length (m m )
3500
3000
2500
2000
1500
a
b
a
South
Middle
North
1000
500
0
Position
Root length (cm 100 cm -3)
0 20 40 60 80 100
0
20
Depth (cm)
Vertical
distribution
of root
length for
corn, tree
and grass
treatments.
40
60
80
100
Crop
Tree
Grass
Root length (cm 100 cm -3)
0
10 20 30 40 50
Vertical distribution
of root length for
pin oak, swamp white
oak and bur oak
0
20
40
Proximal root area (cm2)
35
0.60
30
25
Horizontal
20
Vertical
Depth (cm)
0.44
0.51
40
60
15
10
80
5
0
Pin oak
Swamp white
oak
Species
Bur oak
100
Pin
Swamp
white
bur
Monthly Precipitation and Corn Growing Period
Precipitation (mm)
225
670 mm
Corn
5/22/04
2004
150
Long-term
precipitation
75
0
J
F
M
A
M
J
J
A
S
O
N
D
Soil Water Content for Tree, Grass, and Crop
Areas from June 14 to November 30, 2004
AGF-corn
CGS-corn
Tree
Grass
3
-3
Water Content (cm cm )
0.6
0.4
0.2
5 cm Depth
29-Nov
15-Nov
1-Nov
18-Oct
4-Oct
20-Sep
6-Sep
23-Aug
9-Aug
26-Jul
12-Jul
28-Jun
14-Jun
0.0
29-Nov
15-Nov
20 cm
1-Nov
0.0
29-Nov
15-Nov
1-Nov
18-Oct
4-Oct
20-Sep
6-Sep
23-Aug
9-Aug
26-Jul
5 cm
18-Oct
0.2
4-Oct
0.2
20-Sep
0.4
6-Sep
0.4
23-Aug
0.6
9-Aug
0.6
26-Jul
0.2
12-Jul
0.2
12-Jul
0.4
28-Jun
0.4
28-Jun
0.0
14-Jun
29-Nov
15-Nov
1-Nov
18-Oct
4-Oct
0.6
14-Jun
29-Nov
15-Nov
1-Nov
18-Oct
4-Oct
20-Sep
6-Sep
23-Aug
9-Aug
26-Jul
12-Jul
28-Jun
14-Jun
Water Content (cm3 cm-3)
AGF-corn
Tree
20-Sep
6-Sep
23-Aug
9-Aug
26-Jul
12-Jul
28-Jun
14-Jun
Water Content (cm3 cm-3)
Soil Water Content for Tree, Grass, and Crop Areas 6-14 to 11-30
CGS-corn
Grass
0.6
10 cm
0.0
40 cm
0.0
Daily Precipitation During October
2004 Recharge Period
Precipitation (mm)
40
30
20
10
0
1-Oct
6-Oct
11-Oct
16-Oct
Date
21-Oct
26-Oct
31-Oct
Soil Water Recharge (5 and 10 cm depths)
0.60
VWC (cm cm-3)
0.50
0.40
0.30
Corn
0.20
5 cm Depth
Tree
0.10
0.00
Oct 7,
6:00
Oct 8,
6:00
Oct 9,
6:00
Oct
10,
6:00
Oct
11,
6:00
Oct
12,
6:00
Oct
13,
6:00
Oct
14,
6:00
Oct
15,
6:00
Oct
16,
6:00
Oct
17,
6:00
Oct
18,
6:00
Oct
19,
6:00
Oct
20,
6:00
Oct
21,
6:00
Oct
22,
6:00
Date and Time
0.50
0.45
0.40
VWC (cm cm-3)
0.35
0.30
0.25
Corn
0.20
0.15
10 cm Depth
0.10
Tree
0.05
0.00
Oct 7,
6:00
Oct 8,
6:00
Oct 9,
6:00
Oct
10,
6:00
Oct
11,
6:00
Oct
12,
6:00
Oct
13,
6:00
Oct
14,
6:00
Oct
15,
6:00
Date and Time
Oct
16,
6:00
Oct
17,
6:00
Oct
18,
6:00
Oct
19,
6:00
Oct
20,
6:00
Oct
21,
6:00
Oct
22,
6:00
Soil Water Recharge (20 and 40 cm depths)
0.50
0.45
0.40
VWC (cm cm-3)
0.35
0.30
0.25
Corn
0.20
0.15
Tree
20 cm Depth
0.10
0.05
0.00
Oct 7,
6:00
Oct 8,
6:00
Oct 9,
6:00
Oct 10, Oct 11, Oct 12, Oct 13, Oct 14, Oct 15, Oct 16, Oct 17, Oct 18, Oct 19, Oct 20, Oct 21, Oct 22,
6:00
6:00
6:00
6:00
6:00
6:00
6:00
6:00
6:00
6:00
6:00
6:00
6:00
Date and Time
0.45
0.40
VWC (cm cm-3)
0.35
0.30
0.25
0.20
Corn
0.15
Tree
40 cm Depth
0.10
0.05
0.00
Oct 7,
6:00
Oct 8,
6:00
Oct 9,
6:00
Oct
10,
6:00
Oct
11,
6:00
Oct
12,
6:00
Oct
13,
6:00
Oct
14,
6:00
Oct
15,
6:00
Date and Time
Oct
16,
6:00
Oct
17,
6:00
Oct
18,
6:00
Oct
19,
6:00
Oct
20,
6:00
Oct
21,
6:00
Oct
22,
6:00
Summary
1. Tree and grass areas had more root length
compared to the crop areas although the
differences were not significant.
2. Trees and grass had more roots in the
subsurface horizons relative to the corn areas.
3. Volumetric soil water content was significantly
lower in the tree and grass areas compared to
crop areas. This was attributed to the
extended transpiration period for the trees
and grass compared to corn.
4. The results of the study show that agroforestry
buffers maintained lower seasonal soil water
content allowing more water recharge during
storm events.
5. During recharge periods, soil under trees
stored more water compared to soils under
corn. This could be attributed to greater
differences in antecedent water content and
possibly improved infiltration due to better soil
physical properties such as porosity.
6. The differences in soil water content were
smaller at the 20 and 40 cm depths.
7. The 2004 average Missouri corn yield was
7379 kg ha-1. The study watersheds had an
average corn yield of 9050 kg ha-1.
8. Incorporation of tree and grass buffers did not
reduce crop yields at this site in 2004 but
improved soil water storage.
CONCLUSIONS
Results indicate that agroforestry and
grass buffer treatments can effectively
reduce runoff and nutrient losses
when incorporated directly into a
corn-soybean rotation.
Acknowledgements
We gratefully acknowledge
the Missouri Department of Natural Resources,
the Missouri Agricultural Experiment Station,
the USDA-Agricultural Research Service, and
the US-Environmental Protection Agency.
Thank You