Morphologic Development and Clay Redistribution in Lysimeter Soils under Chaparral and Pine

Lysimeter soils under 41‐yr‐old stands of chaparral species and a pine ( Pinus coulteri B. Don) were studied to determine the biotic influence on soil morphologic development. The large (5.3 by 5.3 by 2.1 m) unconfined lysimeters at the San Dimas Experimental Forest in southern California were filled in 1937 with homogenized fine sandy loam soil material derived from diorite. After a 9‐yr stabilization period, the lysimeters were planted with monocultures of species common to California chaparral ecosystems. The species represented, each on a separate lysimeter and its surrounding buffer zone, are scrub oak ( Quercus dumosa Nutt.), ceanothus ( Ceanothus crassifolia Torr.), chamise ( Adenostoma fasciculatum Hook. & Arn.), and Coulter pine. The soils were described and sampled in triplicate in 1987. Differences in earthworm (including Aporrectodea trapezoides Dugés and Allolobophora chlorotica Savigny) activity were related to plant species and played a major role in differentiating the soils. Earthworm activity under scrub oak and ceanothus produced A horizons (7 and 2 cm thick, respectively), composed largely of worm casts, that were darker and enriched in clay compared with underlying C horizons and archived fill material. Earthworm activity was minimal under chamise, and relatively little morphologic development was apparent. The soil under pine, where earthworms were absent, had a clay‐depleted A horizon (1 cm thick) and Bt horizons with sufficient clay increase, illuviation argillans, and thickness to qualify as an argillic horizon. Mixing and sorting of soil by earthworms has acted against argillic horizon development under the other plant species.