Changes in chemistry and aggregation of a california forest soil worked by the earthworm Argilophilus papillifer eisen (megascolecidae)

Abstract

Little is known about the functional significance of earthworms in California forest ecosystems. A microcosm study of Argilophilus papillifer Eisen (Megascolecidae), a species indigenous to California, was conducted to ascertain its influence on chemical and physical properties of a California forest soil. Earthworms were incubated for 6 months in microcosms consisting of A-horizon soil and a surface layer of one of two types of litter: conifer needles or a mixture of conifer, oak, and understory leaf residues. Chemical properties, percentage and distribution of water-stable aggregates, and micromorphological characteristics of casts and worm-worked soil were compared with soil incubated without worms. Carbon, N, exchangeable cations and cation exchange capacity (CEC) of casts and worm-worked soils from both litter treatments were enhanced compared to soils from microcosms without worms. Patterns of nutrient enrichment generally followed nutrient concentrations in the litter substrates. The major exception was N, which was significantly greater in casts from the conifer–oak treatment although concentrations of N in the two litter substrates were comparable. Most (90%) of the aggregates in casts were water-stable, with a mean-weight-diameter (MWD) of 2.8 mm, compared to 60% water-stable aggregates with a MWD of 0.6 mm in bulk soil. Scanning electron microscopy and energy-dispersive spectrometry of casts and bulk soil revealed marked differences in micromorphological characteristics. The interiors of casts were heterogeneous and contained numerous voids, many filled with foliate and tube-like microstructures formed by exposed kaolinite domains oriented around filaments, probably desiccated fungal hyphae. Interior surfaces of bulk soil aggregates were dense and featureless, with mineral grains embedded in a plasma of clay- and silt-sized particles. Kaolinite domains were coated with Fe-oxides and Al-hydroxides and were not discernible. Rearrangements of mineral colloids in casts may have contributed to the observed increases in CEC and aggregate stability.