1. Soil Hydraulic Properties as Influenced by Grass and Agroforestry Buffer Strips
Tshepiso Seobi
Graduate Adviser: Dr. Stephen Anderson
Department of Soil, Environmental &
Atmospheric Sciences

2. Rationale
Significant concerns regarding soil erosion, water runoff and yield loss from row crop production still persist.
Conservation tillage has increased over the past few decades which has improved soil conservation.
Other methods exist for controlling soil erosion such as agroforestry and grass buffer strips.

4. Rationale
Recently, agroforesty has been suggested as an alternative to traditional row crop production to stabilize against variable economics.
Work has not been done in temperate climatic zones until the past decade.
Effects of agroforestry practices on soil hydraulic properties has received little attention.

5. Rationale Research by Udawatta et al. (2002) indicated that
grass/agroforestry buffer strips reduce
water runoff (by about 9%) and soil loss
(by about 12%) in small watersheds compared to a control watershed.
This study will attempt to evaluate the effects of agroforestry and grass buffer strips on soil hydraulic properties.

6. Project Objectives
This study evaluated the effects of grass buffers, agroforestry buffers, and row crop areas on the following properties:
- ponded infiltration
- saturated hydraulic conductivity
- soil water retention
- pore size distributions
- dry bulk density, and
- CT-determined pore characteristics
(CT = computed tomography)

7. Additional Measurements
Soil water content was continuously monitored at selected locations in the watershed to allow comparisons between the agroforestry buffers and the row crop areas.

8. Project Hypotheses
Soil infiltration properties are not influenced by agroforestry and grass buffer strips.
Saturated hydraulic conductivity is not influenced by buffer strips.
Soil water retention, pore size distributions and dry bulk density are not modified by buffer strips.
Soil water content throughout the year is not influenced by buffer strips.
CT-measured macropore characteristics are not affected by buffer strips.

9. Materials and Methods Study Site
Three paired watersheds were established at the Greenley Research Center near Novelty, Missouri.
These North-facing watersheds were demarcated in 1991 and corn and soybeans were grown in rotation.
No-till management was used with contour planting average yields were 8.5 t/ha for corn and 2.8 t/ha for soybeans.
Treatments were established in Contour buffer strips are 4.5m wide and 36.5m apart (22.8m at the lower slope positions).

10. Study Site
Grass species planted in the grass buffers included redtop, brome grass and birdsfoot trefoil.
Tree species planted in the agroforestry buffers included pin oak, swamp white oak and bur oak.
Soils are mapped as Putnam silt loam (upslope, 0-1% slope) and Kilwinning silt loam (downslope, 2-5% slope) with a water restrictive argillic B horizon that occurs at a 4-37cm depth.

13. Field Methods
Treatments were agroforestry buffers, grass buffers and row crop areas.
Six replicate locations were selected in the Agroforestry Watershed for comparison. Three pin oak trees were selected on each of the second and third agroforestry buffers for measurement. Grass buffer areas midway between trees and row crop areas midway between buffers were also selected for measurement.

14. Field Methods Ponded Infiltration
Ponded infiltration with single rings (25 cm diam.) were conducted in June Rings were driven 18 cm into the soil which was 5 cm into the claypan horizon.
Infiltration rings were placed on the west side of the tree, 20 cm from the trunk for agroforestry measurements, midway between two trees for grass buffer measurements, and midway between buffers north of the selected trees for row crop measurements. Row crop measurements were made in non-trafficked interrows.

17. Field Methods Soil Cores
Undisturbed cores (76 mm diam. x 76 mm long) were removed to determine saturated hydraulic conductivity, water retention, pore size distributions and bulk density. These cores were taken 20 cm southwest from the trees for the agroforestry treatment, midway between two trees for the grass buffer treatment, and midway between buffers north of the selected tree for the row crop treatment.
Cores were taken at four depths: 0-10, 10-20, and cm.

19. Field Methods CT (computed tomography) Soil Cores
Undisturbed soil cores in plexiglas rings were taken 15 cm east from the trees for the agroforestry treatment, midway between trees for the grass buffer treatment, and midway between buffers for the row crop treatment.
Six replicate cores were at one depth, 0-10 cm.

20. Field Methods Soil Water Monitoring
Campbell CS-616 TDR (time domain reflectometry) automated samplers were placed in the agroforestry and row crop treatments in May 2003.
Sensors were installed at the 5, 10, 20 and 40 cm depths.
Calibration data were collected during wet and dry water content periods.

21. Saturated Hydraulic Conductivity
Laboratory Methods
Saturated Hydraulic Conductivity
Soil cores were covered with nylon mesh at the bottom end and an empty ring was connected with a rubber band to the top of the sample core.
Cores were slowly saturated over 48 hours in tubs.
Cores were removed from the tubs and hydraulic conductivity was measured with the constant head method. Some samples were evaluated using the falling head method.

22. Laboratory Methods Water Retention
Water retention was conducted with Buchner funnels for higher soil water pressures (-0.4, -1.0,
-2.5, -5.0, -10.0, and kPa).
Pressure chambers were used for lower soil water pressures (-33, -100, and kPa).

24. Laboratory Methods CT-Measurements
Core samples were drained at –3.5 kPa water pressure.
Samples were scanned using a medical Siemens X-ray CT scanner.
Image software (Image-J) was used to evaluate macropore characteristics.

25. Results and Discussion
Geometric means of ponded infiltration parameters
Treatment
Parameter RC GB AG
Green-Ampt
K (mm h-1) 1.05a 3.42a 3.16a
S (mm h-1/2) 16.7a 17.6a 12.8a
Quasi-steady
rate (mm h-1) 10.2a 13.9a 17.0a

26. P>F values for saturated hydraulic conductivity (Ksat) and bulk density
Source of variance Ksat Bulk density
P > F
Treatment <
RC vs. Others <
GB vs. AG
Depth < <0.01
Treatment*Depth <

27. Geometric means of saturated hydraulic conductivity (Ksat).
Treatment
Depth RC GB AG
-- cm Ksat (mm h-1)
a 21.0a 31.1b
ab 6.0a 16.2b
a 13.2a 31.8b
a 1.9a 28.1b

28. Saturated hydraulic conductivity

29. Means of bulk density Treatment Depth RC GB AG
- cm g cm
b 1.215a 1.266a
b 1.437b 1.347a
a 1.356a 1.335a
a 1.241a 1.230a

30. Dry bulk density

31. Soil water retention
Table: Probability values from analysis of variance for soil water retention (0 to -5.0 kPa).
Soil water pressure (kPa)
Source of variance
P > F
Treatment
RC vs. Others
GB vs. AG
Depth <0.01 <0.01 <0.01 <0.01 <0.01
Treatment*Depth <0.01 < <0.01 <

32. Soil water pressure (kPa)
Water retention
Table: Probability values from analysis of variance for soil water retention (-5.0 to kPa).
Soil water pressure (kPa)
Source of variance
P > F
Treatment
RC vs. Others
GB vs. AG
Depth <0.01 <0.01 <0.01 <0.01 <0.01
Treatment*Depth

33. Water retention
Table: Volumetric water content as affected by soil water pressure and treatment at 0-10cm depth.
Treatment
Soil water RC GB AG
pressure Volumetric water content
--- kPa cm3 cm
a b b
a b b
a b b
a a a
a a a

34. Water retention
Table: Volumetric water content as affected by soil water pressure and treatment at 0-10 cm depth.
Treatment
Soil water RC GB AG
pressure Volumetric water content
--- kPa cm3 cm
a a a
a a a
a a a
a a a
a a a

35. Water retention

36. Water retention

37. Water retention

38. Water retention

39. Soil porosity
Table: Probability values from analysis of variance for soil porosity.
Source of variance Total Macro- C. meso- F. meso- Micro-
por. por por. por por (> ( ( (<10 µm) µm) µm) µm) P > F
Treatment <
RC vs. Others <
GB vs. AG
Depth < <0.01 < <0.01
Treatment*Depth < <

40. Pore size distribution
Table: Macropores (>1000 µm diam.) as affected by treatment and depth.
Treatment
Depth RC GB AG
- cm cm3 cm
a 0.029a 0.032a
a 0.026a 0.030a
a 0.023a 0.024a
a 0.023a 0.024a

41. Macroporosity distribution

42. Pore size distribution
Table: Coarse mesoporosity (60 to 1000 µm diam.) as affected by treatment and depth.
Treatment
Depth RC GB AG
- cm cm3 cm
a 0.107b 0.103b
a 0.076ab 0.091b
a 0.082ab 0.099b
a 0.077b 0.092b

43. Coarse mesoporosity distribution

44. Pore size distribution
Table: Fine mesopores (10 to 60 µm diam.) as affected by treatment and depth.
Treatment
Depth RC GB AG
- cm cm3 cm
a 0.116b 0.079a
a 0.053ab 0.059b
a 0.040a 0.047a
b 0.038a 0.050ab

45. Fine mesoporosity distribution

46. Pore size distribution
Table: Micropores (<10 µm diam.) as affected by treatment and depth.
Treatment
Depth RC GB AG
- cm cm3 cm
a 0.319a 0.345a
a 0.365a 0.347a
a 0.385a 0.380a
b 0.435a 0.428a

47. Microporosity distribution

48. CT-Image
Row Crop

49. CT-Image
Grass Buffer

50. CT-Image
Agroforestry Buffer

51. Row Crop
Grass Buffer
Agroforestry
Buffer

52. Table: CT-measured properties
Treatment
Parameter RC GB AG
Number of macropores
Max size (mm)
Ave. size (mm)
SD (mm)
Area fraction (%)

53. CT Results
Preliminary results indicate that soil macropore numbers and area fraction are increased under grass and agroforestry buffer strips.

54. Soil Water Monitoring
A - 5 cm depth

55. Soil Water Monitoring
B - 10 cm depth

56. Soil Water Monitoring
C - 20 cm depth

57. Soil Water Monitoring
D - 40 cm depth

58. Soil Water Monitoring
Soil water evapotranspiration is high on the agroforestry at the beginning of the season, when compared to row crop areas.

59. Conclusions
Ponded infiltration parameters were not significantly different between treatments.
Saturated hydraulic conductivity was significantly higher in the agroforestry treatment compared to the others. Soil bulk density for the row crop treatment was significantly higher than the other treatments for the first two depths.
Water content at high water potentials (0, -0.4,
-1.0 kPa) was significantly higher in the agroforestry and grass buffers for the surface depth only.

60. Conclusions
Coarse mesoporosity was significantly higher in the agroforestry buffer treatment compared to the row crop treatment for all depths.
Reduced antecedent soil moisture in agroforestry allows water to be stored in the profile before runoff is generated.

61. Acknowledgements Dr. Anderson for his patient guidance.
Other graduate committee members, including Drs. Gantzer and Thompson, for offering their time when needed.
Fulbright-SA and IIE for making my studies possible and the Center for Agroforestry for support for travel and supplies.
Dr. Udawatta for the water content data and other background support.
Dr. Rachman and Pieter Los for their field and laboratory assistance.