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

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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. Rationale

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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. Rationale

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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. Rationale

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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)

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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.

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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.

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Materials and Methods 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. 1992-2000 average yields were 8.5 t/ha for corn and 2.8 t/ha for soybeans. Treatments were established in 1997. Contour buffer strips are 4.5m wide and 36.5m apart (22.8m at the lower slope positions). Study Site

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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.

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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.

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Ponded infiltration with single rings (25 cm diam.) were conducted in June 2003. 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. Field Methods Ponded Infiltration

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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, 20- 30 and 30-40 cm. Field Methods Soil Cores

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Field Methods 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. CT (computed tomography) Soil Cores

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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. Field Methods

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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. Laboratory Methods

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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 -20.0 kPa). Pressure chambers were used for lower soil water pressures (-33, -100, and -1500 kPa). Laboratory Methods

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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. Laboratory Methods

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

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P>F values for saturated hydraulic conductivity (Ksat) and bulk density Source of varianceKsatBulk density ----------- P > F ----------- Treatment