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Soil Biogeochemical Processes within the Critical Zone

¹ Department of Soil, Water and Environmental Science
University of Arizona, Tucson, AZ 85721, USA
E-mail: chorover{at}cals.arizona.edu
² Institute of Biogeochemistry and Pollutant Dynamics
Department of Environmental Sciences
ETH Zurich, CHN, CH-8092 Zurich, Switzerland
E-mail: kretzschmar{at}env.ethz.ch
³ School of Life Sciences, Arizona State University
Tempe, AZ 85287, USA
E-mail: ferran{at}asu.edu
⁴ Department of Plant and Soil Science and Center for Critical Zone Research, University of Delaware
Newark, DE 19717, USA
E-mail: dlsparks{at}udel.edu

View larger version (50K): [in this window] [in a new window]   FIGURE 1 Solid-fluid interface character varies over all scales. For a given lithology, climate, and landscape position, interface composition depends on biogeochemical conditions at the pore scale. Within a typical soil aggregate (center, right), reactive surfaces include (A) natural organic matter, (B) nanoporous silicate minerals, (C) mineral-microbe complexes, (D) secondary aluminosilicate clays and their surface organic coatings, and (E) oxide and/or carbonate coatings.

View larger version (15K): [in this window] [in a new window]   FIGURE 2 Chemical gradients in an active (wet) biological soil crust. Microelectrodes reveal microbial metabolic activity. Photoautotrophic cyanobacteria close to the surface consume CO2, driving pH up and creating an internal O2 supply, which is quickly respired by heterotrophs leading to anoxia 2-3 mm below surface. Leakage of cellular NH3 produced by cyanobacteria during N2 fixation creates a thin layer with both free NH3 and O2, which provides a habitat for chemolithoautotrophic ammonia-oxidizing bacteria. These provide nitrate through metabolism. These microscale processes provide the mechanistic basis for the fertilizing role of soil crust communities in arid lands at landscape scale (Garcia-Pichel et al. 2003).

View larger version (57K): [in this window] [in a new window]   FIGURE 3 Incongruent dissolution and accretion on interfaces. (A) Backscatter electron image of surface coatings of poly-crystalline Fe and Al oxyhydroxide on quartz from Cape Cod aquifer sand (Coston et al. 1995); (B) weathering of primary biotite grains and formation of secondary halloysite in a granitic gneiss saprolite and (C) close-up of (B) showing tubular halloysite on edge surfaces of the weathering biotite (Kretzschmar et al. 1997).

View larger version (63K): [in this window] [in a new window]   FIGURE 4 Weathering of NiO(s) (at arrow) to form Ni-Al LDH in a smelter-impacted soil: (left) µ-SXRF tricolor map of the Welland loam unlimed soil and (right) µ-XAFS spectra from selected points within the map that show an increasing predominance of Ni-Al LDH relative to NiO(s) resulting from progressive weathering (numbers 1-3). Solid lines represent the k3 weighted -spectra, and the dotted lines are best fits obtained using linear combinations fitting with NiO and Ni-Al LDH references (top and bottom spectra) (McNear et al. 2007).

View larger version (53K): [in this window] [in a new window]   FIGURE 5 A periodically water-logged soil showing the characteristic "redox-imorphic" features of iron cycling between oxidized (reddish brown) and reduced (pale, iron-depleted) environments in close proximity to each other. Representative Fe oxidation and reduction reactions are depicted, along with bacterial strains known to catalyze the transformations. A diversity of iron-oxidizing bacteria likely carry out their reactions at circumneutral pH in soil, but their identity is not well documented. Some of them are proteobacteria closely related to Marinobacter and Hyphomomas, whereas Gallionella and Leptothrix genera identified in freshwater systems may also be present in soils. Scale numbers in dm