Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Abstract Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Amundson, R. Articles by Gaillardet, J. Search for Related Content GeoRef GeoRef Citation

Coupling between Biota and Earth Materials in the Critical Zone

¹ Division of Ecosystem Sciences, 137 Mulford Hall
University of California, Berkeley, CA 94720, USA
E-mail: earthy{at}nature.berkeley.edu
² Nicholas School, Division of Environmental Sciences and Policy
Duke University, A207B LSRC, Box 90328, Durham, NC 27708, USA
³ Department of Physical Geography, Macquarie University
NSW 2109, Australia. Deceased
⁴ Grupo de Estudios Ambientales-IMASL, Universidad Nacional
de San Luis & CONICET, Avenida Ejercito de los Andes 950
D5700HHW San Luis, Argentina
⁵ Laboratoire de Géochimie-Cosmochimie, Institut de Physique du
Globe de Paris, CNRS UMR 7579, Université Paris 7, 4 Place Jussieu
75252 Paris Cédex 05, France

View larger version (54K): [in this window] [in a new window]   FIGURE 1 Cartoon illustrating hypothesized changes in Critical Zone hydrology, biogeochemical processes, and landscape evolution with time. The sediment column on the left represents a fluvial deposit on a floodplain at time = 0. Following river incision or meandering, the floodplain is abandoned, vegetation is established, and the landscape is physically stabilized. After river incision, long-term soil formation processes ensue, and after 105 or more years, chemical weathering (enhanced by the presence of life) and advective transport help to form element-depleted layers near the surface and soil horizons enriched in clay (red layer in right column). Biological mixing stirs organic matter into the soil surface creating a darkened surface horizon. Biota also move soil laterally, creating low mounds, as shown in the photograph on the right. The rates of all processes may change with time. This schematic is representative of soil biogeochemical processes in California's San Joaquin Valley.

View larger version (41K): [in this window] [in a new window]   FIGURE 2 Diagram illustrating plant and fungal controls on soil mineral weathering and element cycling. Silicate dust and aerosols from local and distant sources are deposited on leaves and the land surface. Water partially dissolves them and transports the material into the soil. The reactivity of the resulting solution with soil minerals generally declines with depth as waters approach equilibrium, represented by red arrows. Roots passively and actively (via ion exchange) take up dissolved ions from the soil water (indicated by the two-way red arrows). Additionally, roots enhance water-mineral reactions through release of organic molecules capable of increasing mineral solubility. Mycorrhizal fungi, through symbiosis with roots, provide additional avenues of mineral weathering by releasing organic acids and chelating agents that significantly increase the solubility of important nutrients such as P and Ca. Mineral-derived elements accumulate in plant tissue in various forms, serving both structural and physiological functions. Some elements, such as Ca and Si, may form biologically mediated minerals within the plant (carbonates, oxalates, amorphous silica), which are released to the soil following death of the tissue (MODIFIED FROM BERNER ET AL. 2004).

View larger version (59K): [in this window] [in a new window]   FIGURE 3 Schematic diagram illustrating soil production and slope-dependent transport on a hillslope dominated by biotically driven diffusive sediment transport (modified from Heimsath et al. 1997). A soil biomantle (commonly the dark A horizon, rich in organic matter) is mixed by burrowing animals, insects, and plants on timescales of 102 yrs. This random mixing, combined with a gravity gradient, drives net downslope transport. This is quantified by a "diffusion coefficient," K, multiplied by slope. The physical mixing of fine particles by burrowing animals, worms, etc. commonly causes stones and archeological artifacts to "settle" at the base of the mixed layer, forming "stone lines." On landscapes at or near steady state, as the soil mantle thins through erosion, biological and physical processes release material from the underlying rock (soil production), which then become part of the overlying soil. The rate of soil production commonly decreases with increasing soil thickness primarily because biological and physical mixing processes decline with soil depth.

View larger version (29K): [in this window] [in a new window]   FIGURE 4 Schematic diagram of trends in the variation of total soil mass with rainfall for soils of comparable age (106 y) and temperature (16°C). Where humidity is high, soils undergo large loss of most rock-forming elements through biotically mediated weathering and element cycling. As rainfall declines, net loss also decreases. At the broad boundary between arid (plant-supporting) and hyperarid (plant-inhospitable) climates, soil formation no longer includes element loss, and there is a net soil gain via atmospheric deposition. The rate of gain increases with increasing aridity (Ewing et al. 2006).