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Vegetation, which is generally considered a co-variable controlled by climate in studies of pedogenesis, can itself alter the balance of soil-forming processes by modifying the microclimate. The chaparral and pine biosequence at the San Dimas Experimental Forest (SDEF) in southern California offers an opportunity to determine the effect of individual species on soil microclimate and related soil properties. Soil temperature and moisture were monitored under pure stands of Coulter pine (Pinus coulteri B. Don), chamise (Adenostoma fasciculatum Hook. and Arn.), and scrub oak (Quercus dumosa Nutt.). Despite the appreciable differences in soil morphology that have evolved under pine compared with oak, the microclimates created by the dense canopies and thick litter layers (7-10 cm) of these two vegetation types were similar. Average monthly soil temperatures within the top 65 cm ranged from about 8 to 20[degrees]C under pine and 7 In 18[degrees]C under oak. Average monthly water contents ranged from 8 to 32% (by volume) under pine and 6 to 32% under oak. In contrast, average monthly soil temperatures within the top 65 cm under chamise ranged from 6 to 23[degrees]C and average monthly water contents were 6 to 36%. Diurnal variation in soil surface temperature under chamise was much more pronounced than under oak and pine because of the relatively open canopy and thin litter layer. Microclimate created by chamise may play an important role in the decomposition rate of litter, but appears to have little influence on soil processes occurring within the mineral soil. Soil macrofaunal communities appear to have a greater effect than microclimate in causing the divergent soil morphologies under oak and pine.

Andisols support [approximately equal to]200 000 ha of mid-elevation grand fir (Abies grandis [Dougl. ex D.Don] Lindl.) forests in the Pacific Northwest region that are characterized by little or no natural conifer regeneration following removal of the forest canopy. Previous work suggests that the properties of these Andisols have been altered as a result of the establishment of successional communities dominated by bracken fern (Pteridium aquilinum [L.] Kuhn) in deforested areas. In this study, we compared soil properties in a 30-yr-old bracken fern site (clear-cut in 1965), a natural bracken fern site that is estimated to be centuries old, and an adjacent undisturbed forest in the Clearwater National Forest of northern Idaho. Results indicate that changes in chemical properties have accompanied establishment of successional communities. Mean weighted pH within the ash cap of the 30-yr-old bracken fern site (4.6) is significantly lower than that of the undisturbed forest (5.2). Mean values for Al saturation range from 27% in the undisturbed forest to 52% in the 30-yr-old bracken fern site; organic C is also lower in the undisturbed forest (37 g/kg) than in the 30-yr-old bracken fern site (54 g/kg). The dominant secondary mineralogical component of soils of the undisturbed forest is inorganic, short-range-order Al-Fe minerals, while metal-humus complexes are dominant in the bracken-fern-influenced soils. Data indicate that bracken fern successional communities are responsible for a shift from allophanic to nonallophanic properties in these soils, probably due to increased levels of soil organic C associated with bracken fern and a subsequent increase in formation of Al-humus complexes. Furthermore, such a mineralogical shift may contribute to the observed problems with conifer regeneration.

Abstract: Earthworms are often referred to as ecosystem engineers due to their ability to alter the soil environment. Since earthworms influence a wide range of critical chemical and physical soil properties it is important to understand how their populations are impacted by soil management. Earthworms were sampled during the spring and summer of 2001, 2002, and 2003 from conventional tillage (CT) and no-till (NT) plots established in 2000. Although there was a strong trend for higher earthworm density in NT plots in 2001 (p =0.08) and 2002 (p =0.19), statistically significant differences were not detected between tillage treatments until 2003 (p =0.04) when mean earthworm density was 37.7individualsm−2 in CT and 149.9individualsm−2 in NT during spring and 17.1individualsm−2 in CT and 58.4individualsm−2 in NT in summer. A high mortality rate between spring and summer, combined with greater cocoon production under NT suggests that the earthworm population turns over rapidly in NT plots. Data also suggest that adverse soil environmental conditions will limit earthworm density in these dryland agroecosystems. Despite significantly higher earthworm density after three years of NT management, soil bulk density, saturated hydraulic conductivity, and aggregate stability of the 0.5- to 1-mm size fraction were not different between the two tillage treatments. The apparent lack of impact of reduced disturbance and increased earthworm density on soil physical properties may be due to the short time this soil has been under NT management, limited seasonal earthworm activity due to environmental conditions, or differences in the scale at which soil physical properties have been affected after three years of NT management and the scale at which our measurements were made. [Copyright &y& Elsevier]