Agroforestry and grass buffers for improving soil hydraulic properties and reducing runoff and sediment losses from grazed pastures
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Agroforestry buffers, a system of land use in which harvestable trees or shrubs are grown among or around crops or on pastureland, have been proposed for improving water quality in watersheds. The objectives of this study were (i) to evaluate saturated hydraulic conductivity ([kappa]sat) and water retention for soils managed under rotationally-grazed pasture (RG), continuously grazed pasture (CG), grass buffers (GB), and agroforestry buffers (AgB); (ii) to compare differences in computed tomography (CT)-measured macropore ([greater than]1000-[mu]m diam.) and coarse mesopore (200- to 1000-[mu]m diam.) parameters for AgB, GB, RG and CG treatments, and to examine relationships between CT-measured pore parameters and [kappa] sat ; (iii) to compare the influence of AgB and GB systems under rotationally stocked (RG) and continuously stocked (CG) pasture systems on water infiltration measured using ponded infiltration and tension infiltration methods; (iv) to evaluate differences in root length density (RLD) and root and soil carbon content within GB, AgB, RG and CG treatments; and (v) to model runoff and sediment losses for grazed pasture watersheds with and without AgB buffers. Pasture and GB areas included red clover (Trifolium pretense L.) and lespedeza (Kummerowia stipulacea Maxim.) planted into fescue (Festuca arundinacea Schreb.) while AgB included Eastern cottonwood trees (Populus deltoids Bortr. Ex Marsh.) planted into fescue. Soil bulk density was 12.6% higher for the pasture treatments compared to buffer treatments. Soil water content at high soil water potentials (0 and -0.4 [kappa] [pi]a) was greater in the buffer treatments relative to pasture treatments for the 0-10 cm soil depth. Soil macroporosity ([greater than]1000 [mu] m diam.) was 5.7, 4.5, and 3.9 times higher, respectively, for the AgB, GB, and RG treatments compared to the CG treatment for the 0-10 cm soil depth. Buffer treatments had greater macroporosity ([greater than]1000-[mu] m diam.), coarse (60- to 1000-[mu]m diam.) and fine mesoporosity (10 to 60 [mu]m diam.) but lower microporosity ([lesser than] 10 [mu]m diam.) compared to pasture treatments. The [kappa]sat for the buffer treatments was 16.7 times higher compared with pasture treatments. The CT-measured soil macroporosity was 13 times higher (0.053 m³m⁻³) for the buffer treatments compared to pasture (0.0045 m³m⁻³) treatments. The CT-measured pore parameters (except macropore circularity) were positively correlated with [kappa]sat. Quasi-steady state infiltration rates (qs) and field-saturated hydraulic conductivity ([kappa]fs) for buffers were about 30 and 40 times higher compared to pasture treatments, respectively. Green-Ampt and Parlange models appeared to fit measured data with [rho]² values ranging from 0.91 to 0.98. The infiltration rate in 2007 for the GB treatment was the highest (221 mm h⁻¹) and for the CG treatment was the lowest (3.7 mm h⁻¹). Estimated sorptivity (S) and saturated hydraulic conductivity ([kappa]s) parameters were higher for buffer areas compared to the stocked pasture areas. Grazing reduced the infiltration rate for the pasture treatments. Buffer treatments had 4.5 times higher RLD as compared to pasture treatments. The AgB treatment had the highest (173.5 cm/100 cm³) and CG had the lowest (10.8 cm/100 cm³) RLD. Root carbon was about 3% higher for the buffers compared to RG treatment. Soil carbon was about 115% higher for the buffers compared to pasture treatments. This study illustrates that agroforestry and grass buffers maintained higher values for soil hydraulic properties compared to grazed pasture systems. The CT-study illustrates the benefits of agroforestry and grass buffers for maintaining soil pore parameters critical for soil water transport. Results from the infiltration study conclude that the buffer areas have higher infiltration rates which imply lower runoff compared to pasture areas. The root study implies that establishment of agroforestry and grass buffers on grazed pasture watersheds improves soil carbon accumulation and root parameters which enhance soil physical and chemical properties, thus improving the environmental quality of the landscape. The Agricultural Policy Extender (APEX) model was used to simulate runoff and sediment losses from the AgB watersheds and control (CW) watersheds. The model was calibrated from 2002 to 2005 and was validated from 2005 to 2008. The [rho]² and NSE values for the calibration and validation period of the runoff varied from 0.52 to 0.78 and 0.51 to 0.74, respectively. The model did not predict sediment loss very well (NSE values were less than 0.19) because of insufficient measured events. The measured runoff was 36% lower for AgB watersheds compared to CW watersheds. The measured sediment loss for the AgB watersheds was about 49% lower compared to CW watersheds. The model was run for long-term scenario analyses from 1999 to 2008. The runoff decreased 24% when the buffer width was doubled. The runoff from the AgB watersheds was 9.8% lower with double stocking densities compared to CW watersheds with double stocking densities. Results of these studies indicate that establishment of agroforestry and grass buffers on grazed pasture watersheds improve soil hydraulic³² properties, pore parameters, soil carbon sequestration and water quality indices and thus contribute to enhance overall environmental quality.