Aggregate Stability under Oak and Pine after Four Decades of Soil Development

The development of water‐stable aggregates is an important soil genesis process because it strongly influences important soil characteristics, including infiltration, aeration, and erodibility. We studied a 41‐yr‐old biosequence of lysimeter soils at the San Dimas Experimental Forest in southern California to assess water‐stable aggregates as a function of the imposed scrub oak ( Quercus dumosa Nutt.) and Coulter pine ( Pinus coulteri B. Don) communities. Significantly different aggregate stabilities developed in these initially identical and homogeneous soils. Earthworms under the oak produced a 7‐cm‐thick A horizon composed almost entirely of worm casts. Casts were also deposited within the litter of the Oi horizon. The A horizon and the Oi horizon worm casts had aggregate stabilities near 90%, ≈35 g kg −1 organic C, and abundant fungal hyphae, as observed by scanning electron microscopy. The 1‐cm‐thick A horizon under pine contained no worm casts and had 78% water‐stable aggregates, 12.9 g kg −1 organic C, and abundant very fine roots and fungal hyphae. Subsoils under both oak and pine had 43 to 51% water‐stable aggregates, attributable to inorganic binding mechanisms since organic C contents were ≤4 g kg −1 , roots were relatively few, and no fungal hyphae were observed. After 41 yr of soil formation, aggregate stability was ≈15% greater and the volume of stable aggregates was seven times larger under scrub oak than under Coulter pine.