Modification of soil structural and hydraulic properties after 50 years of imposed chaparral and pine vegetation

Although biotic communities have long been recognized as important factors in soil development, especially of A horizons, few studies have addressed their influence on soil physical properties in nonagricultural settings. A biosequence of 50-year-old soils supporting near monocultures of Coulter pine (Pinus coulteri), scrub oak (Quercus dumosa), and chamise (Adenostoma fasciculatum) was used to determine the relative influence of vegetation type and associated soil organisms on the development of soil structural characteristics and water flow. Total porosity ranged from a high of 51% in the heavily worm-worked A horizon under oak to a low of 39% within the 35- to 50-cm depth under pine, where earthworms were absent. Macroporosity (pores with diameters >300 μm) was highest in the A horizon under oak (15.6%) and lowest under pine (9.5%). Saturated hydraulic conductivity of surface soils ranged from 10.8 cm h−1 under oak to 3.2 cm h−1 under pine. Soil under chamise, which had fewer earthworms than that under oak, had Ksat and bulk density values intermediate between oak and pine. Root and macrofauna distributions suggest that roots are the dominant factor in the development of macroporosity under pine, while earthworms have had the greatest effect under oak. Porosity has increased at an average rate of 0.22% per year in the 0- to 7-cm depth under oak (from 41% to 56%), but has not been significantly altered within the same depth under pine. Below the 7-cm depth, porosity values are similar for each vegetation type and the original parent material. Available water capacity (AWC) within the first 0- to 7-cm depth has increased from the original values (about 0.11 m3 m−3) to 0.17 m3 m−3 under oak, 0.16 m3 m−3 under chamise, and 0.13 m3 m−3 under pine. The data show that the presence of burrowing macrofauna, which is determined by litter palatability and therefore indirectly controlled by vegetation, can significantly influence porosity, increasing the water-holding capacity of a soil.