Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2002
Publication Date: 8/23/2002
Citation: Johnson-Maynard, J.J., Graham, R.C., Shouse, P.J. 2002. Modification of soil structural and hydraulic properties after 50 years of imposed chaparral and pine vegetation. Geoderma. Vol 110:227-240 Interpretive Summary: Soil biotic communities are known to significantly impact soil development, including soil structure. For example, roots contribute to the formation of aggregates directly by encapsulating soil particles, and indirectly through the production of exudates, which act as glue in soils. Plants alter wetting and drying cycles that are important to the stabilization of aggregates and the formation of soil structure. Root channels with live or decaying roots can increase the permeability by serving as conduits for preferential flow. The litter layer indirectly promotes formation of macropores by serving as a food source for burrowing soil fauna. Due to dietary requirements and preferences, these effects are known to be plant species-dependent. Much research has concentrated also on the importance of earthworms on soil hydraulic properties. Their casts are generally more stable than other soil aggregates, thus often providing the majority of structural aggregates in the upper 10-20 cm of a soil. In this study, a biosequence within the San Dimas Experimental Forest (SDEF) in southern California was used to determine the relative influence of chaparral species (scrub oak and chamise) and pine, and their associated macrofaunal populations, on the development of soil physical and hydraulic properties. Previous studies at the SDEF site revealed differences in soil morphology, clay distribution, and aggregate stability between soils forming under oak and pine after only 50 years. The overall goal was to determine if changes in bulk density, pore size distribution, aggregate stability, and soil structure were large enough to alter water flow and water retention within each soil. We found that, depending upon the palatability of each vegetation type, different macrofauna populations (in terms of size and composition) developed under each vegetation type. Indirectly, through their ability to support earthworms, the chaparral species significantly increased soil structure, total porosity, and the pore size distribution. Over the 50-year period of soil formation, the alteration of these physical properties resulted in measurable differences in the unsaturated hydraulic properties as demonstrated by higher permeability to water flow and increased water holding capacity. Results contribute to a better understanding of the effects of soil flora and fauna on water flow processes and leaching in natural soils.
Technical Abstract: 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 percent in the heavily worm-worked A horizon under oak to a low of 39 percent within the 35- to 50-cm depth under pine, where earthworms were absent. Macroporosity (pores with diameters >300 Am) was highest in the A horizon under oak (15.6 percent) and lowest under pine (9.5 percent). Saturated hydraulic conductivity of surface soils ranged from 10.8 cm/h under oak to 3.2 cm/h 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 percent per year in the 0- to 7-cm depth under oak (from 41 percent to 56 percent), 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/m3 under chamise, and 0.13 m3/m3 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.