Linking Soil Physical Parameters Along a Density Gradient in a Loess-Soil Long-Term Experiment

It is important to understand the impact of texture and organic carbon (OC) on soil structure development. Only few studies investigated this for silt-dominated soils. In this study, soil physical properties were determined on samples from a controlled experiment (Static Fertilization Experiment, Bad Lauchstadt, Germany) on a loess soil that started more than 100 years ago with six different combinations of organic and mineral fertilizers. The parameters measured include soil texture, water retention curve, air-connected porosity, gas diffusion coefficient, air permeability, and saturated hydraulic conductivity. The management resulted in a distinct gradient in OC. A bulk density gradient developed from differences in amount of clay not complexed with OC. This gradient in bulk density mainly affected content of pores larger than 3 Km. The air-connected porosity measured by a pycnometer was highly similar to the total air-filled porosity calculated from gravimetric water content. For all six treatments, diffusivities and permeabilities were quite similar; both suggested that air-filled pore space was inactive for gas transport for air saturation below 0.1, but became highly connected around 0.2 to 0.25. Furthermore, diffusion data from intact cores compared well with data from repacked samples measured at low air-filled porosities and another high-silt soil (Yolo silt loam, USA) measured at higher air-filled porosities. A two-parameter fitting model was used to analyze gas diffusion coefficient data; the model pore-connectivity factor was fairly constant, whereas the water blockage factor was markedly different. Water and air parameters both implied that change in bulk density was the major driver for diffusive and convective parameters in the experiment.