Soil physical and hydraulic properties affected by topsoil thickness in switchgrass and row crop management systems
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Degraded claypan landscapes can be seen as a potential resource for bioenergy production. Therefore, an important decision needs to be made to determine where claypan landscapes for grain crops and perennial grasses can be planted to increase production and to minimize economic and environmental risks. The objectives of this study were (i) to evaluate the effects of reduced topsoil thickness and perennial switchgrass (Panicum virgatum L.) vs.a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation on soil bulk density (ρb), saturated hydraulic conductivity (Ksat), soil water retention, and pore size distributions; (ii) to assess the influence of topsoil thickness on water infiltration in claypan soils for row crop and switchgrass production systems; and (iii) to determine the effect of topsoil thickness on soil thermal properties for switchgrass and row crop management systems. The experiment was carried out at the Soil Productivity Assessment for Renewable Energy and Conservation (SPARC) plots at the University of Missouri South Farm Research Center. Plots were recreated in 2009 with four levels of topsoil thickness (4, 11, 22, and 36 cm) on a Mexico silt loam (Vertic Epiaqualfs). Every plot was planted with either switchgrass or a corn-soybean rotation. The first objective results showed that soil under switchgrass had 53 and 27 percent higher macroporosity (greater than 1000 µm diameter) and coarse mesoporosity (60-1000 µm diameter), respectively, than row crop management. This caused 73 percent greater Ksat and 11 percent higher water content at saturation than row crop management. Thicker topsoil treatments (TopC and TopD) had constantly lower ρb and greater Ksat than the shallow topsoil treatments (TopA and TopB) for all sampling depths. Results from the second objective indicated that switchgrass planted on degraded soil (shallow topsoil treatment) resulted in greater Ks (estimated saturated hydraulic conductivity), S (sorptivity), qs (quasi-steady infiltration rate) and Kfs (field-saturated hydraulic conductivity) values than with row crop management for both 2014 and 2015 measurement years. Results for selected mean frequencies of 24-hour USDA-NRCS Type II storms showed that switchgrass increased estimated water infiltration, reduced estimated runoff, and decreased estimated time from water ponding to end of ponding compared with row crop management. The results of the third objective revealed that the switchgrass treatment had significantly higher SOC (soil organic carbon), greater θ (water content) and lower ρb than the row crop treatment which resulted in decreased λ (thermal conductivity) and D (thermal diffusivity) as well as increased Cv (volumetric heat capacity). The shallow topsoil thickness had greater thermal properties (λ, D and Cv) than deep topsoil thickness due to higher concentration of smectitic clay in the surface soil horizon. Results of these studies imply that establishment of switchgrass on degraded land can improve soil physical, hydraulic and thermal properties and may enhance the productivity of degraded claypan soils, while supplying biofuel feedstocks.