This project is focused on the development of microstructural properties and the pore systems they define. We analyzed the information gained through the use of a unique combination of time-dependent in situ diffuse reflectance infrared spectroscopy and porosity characterizations, using both N₂ and CO₂ as sorptives. All of the porosimetric data were analyzed using density functional theory, which allows a uniform conceptual basis to be applied across the entire range of microstructural porosity. We used 1,2-dichloroethane (DCA) as the probe molecule in the diffuse reflectance experiments. Rather than simply providing static characterizations of porosity, this approach provides a dynamic view of the pore system of soil particles as it functions to control the movement of molecular species. Soil samples collected from the San Dimas Experimental Forest show differences in microaggregate development over a 50-year period under four types of vegetation natural to California. During this past year, soil samples from the pine lysimeter were investigated. N₂ porosity distributions for these samples suggest that, as with the oak samples, the accumulation of organic matter has significantly diminished the mesoporosity of the surface soil, as compared to the soil at a depth of 50-65 cm. The oak and pine samples were similar in greater CO₂-measurable microporosity in the surface soil. The spectroscopic results indicated that, in both profiles, the amount and rate of desorption for sorbed liquid phase DCA decreased because of the organic matter accumulation in the surface soil. The horizon immediately below the surface soil under pine sorbed unexpectedly high amounts of liquid, and it sorbed chemical persistently. We believe that this is due to surface clay eluviation and subsequent deposition in the subsurface layers. Intense earthworm activity prevented this phenomenon from occurring under oak. No earthworm casts were found under pine.