Variability of soil hydraulic properties and estimation of plant-available water on claypan-soil landscapes
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Soil hydraulic properties and plant-available water (PAW) capacity are critical properties in evaluating land management and soil productivity. This study investigated these properties for claypan-soil landscapes in Central Missouri. The objectives were: (1) to evaluate the effects and interactions of conservation management, landscape position and the claypan layer on hydraulic properties including saturated hydraulic conductivity (K[subscript sat]), water retention, pore-size distributions and bulk density; (2) to investigate direct relationships between measured PAW capacity and apparent soil electrical conductivity (EC[subscript a]), and to test the hypothesis that soil PAW capacity can be approximated at a field scale using a theoretical two-layer soil profile, i.e., a silt-loam topsoil layer and a siltyclay subsoil layer; and (3) to evaluate relationships of measured PAW capacity with measured corn grain yield as well as to estimated PAW capacity using a previouslydeveloped Simple Inverse Yield Model (SIYM). Soil series on study sites included Mexico (fine, smectitic, mesic Aeric Vertic Epiaqulfs), Adco (fine, smectitic, mesic Aeric Vertic Albaqualfs), and Leonard (fine, smectitic, mesic, Vertic Epiaqualfs). For objective 1, management treatments were mulch tillage with a corn (Zea mays L.)�soybean [Glycine max (L.) Merr.] rotation (MTCS); no-till with a corn�soybean�wheat (Triticum aestivum L.) rotation (NTCSW) with a red clover (Trifolium pretense L.) cover crop following wheat; a Conservation Reserve Program system (CRP); and a hay crop system (HAY). Landscape positions were summit, backslope and footslope. At each combination of management treatment and landscape position, intact soil core samples (76 by 76 mm) were taken from the 0- to 10-, 10- to 20-, and 20- to 30-cm soil depths. For objective 2, soil profile samples were taken at 18 to 19 locations from each of two close-by fields and PAW capacity was determined for a 1.2-m soil profile (PAW[subscript 1.2]). Plant-available water fraction values of 0.23 and 0.12 were used for the topsoil layer (assumed to be silt loam) and subsoil layer (assumed to be silty clay) textures, and the boundary between the two layers was determined by EC[subscript a]. For objective 3, the same two fields were used. Inputs for SIYM simulation were growing season mean maximum and minimum daily air temperature, daily precipitation, hourly vapor pressure deficit, and soil PAW. Nine siteyears of corn grain yield data were used to estimate PAW. Results for Objective 1 showed management effects were only statistically evident in the surface 0 to 10-cm depth. The CRP was the best management for the soil properties studied with decreased bulk density, increased K[subscript sat], increased water retention, and increased fraction of larger pores. At the backslope, K[subscript sat] values for CRP and HAY were 16 and 10 times higher, respectively, than values for MTCS. The use of perennial grasses in rotation will benefit soil hydraulic properties, particularly at landscape positions most vulnerable to degradation. Results for Objective 2 showed significant relationships between the reciprocal function of EC[subscript a] (EC[subscript a][superscript -1]) and the lower limit of PAW[subscript 1.2] Relationships between EC[subscript a] -1 and PAW[subscript 1.2] were also found to be significant with an average regression r[superscript 2] of 0.76. The two-layer-soil hypothesis was proven to provide reasonably accurate PAW[subscript 1.2] estimates with root mean square error (RMSE) of 16 mm, compared with the measured PAW[subscript 1.2] Large underestimates of PAW[subscript 1.2] were a result of underestimation of topsoil thickness, whereas large overestimates were attributed to a few soil horizons at less than field capacity. Soil EC[subscript a] can be used as a quick and cost-efficient tool to quantify and map PAW capacity for similar soil landscapes. The relationships between the measured PAW1.2 and the SIYM PAW estimates were weak. The r[superscript 2] values were 0.43 and 0.31 for the two study fields. Claypan soil characteristics (i.e., low hydraulic conductivity, slow recharge, poor drainage, and high soil resistance for water movement to roots) introduced additional yield variability, which SIYM was not designed to handle. The results of this study were useful for designing best management systems and for guiding site-specific management for claypan-soil landscapes.
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