Geobiology (2010) DOI: 10.1111/j.1472-4669.2010.00264.x 1 2 3 Twelve testable hypotheses on the geobiology of weathering 4 1 2 3 4 5 5 S. L. BRANTLEY, J. P. MEGONIGAL, F. N. SCATENA, Z. BALOGH-BRUNSTAD, R. T. BARNES, 6 M. A. BRUNS,6 P. VAN CAPPELLEN,7 K. DONTSOVA,8 H. HARTNETT,9 T. HARTSHORN,10 7 A. HEIMSATH,11 E. HERNDON,1 L. JIN,1 C. K. KELLER,12 J. R. LEAKE,13 W. H. MCDOWELL,14 8 F. C. MEINZER,15 T. J. MOZDZER,2 S. PETSCH,16 J. PETT-RIDGE,17 K. S. PREGITZER,18 P. RAY- 9 19 20 21 2 22 23 MOND, C. S. RIEBE, K. SHUMAKER, A. SUTTON-GRIER, R. WALTER AND K. YOO 10 1 11 Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA, USA 2 12 Smithsonian Environmental Research Center, Edgewater, MD, USA 3 13 Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA, USA 4 14 Hartwick College, Oneonta, NY, USA 5 15 2 University of Colorado, 3215 Marine Street, E127, Boulder, CO, USA 6Department of Crop and Soil Sciences, Pennsylvania State University, University Park, PA, USA 16 7 17 School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA 8Biosphere 2 Earthscience, University of Arizona, Tucson, AZ, USA 18 9School of Earth and Space Exploration, and Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ, USA 19 10Department of Geology and Environmental Science, James Madison University, Harrisonburg, VA, USA 20 11School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA 21 12School of Earth and Environmental Sciences, Washington State University, Pullman, WA, USA 22 13Department of Animal and Plant Sciences, University of Sheffield, Sheffield, UK 23 14Department of Natural Resources and the Environment, University of New Hampshire, Durham, New Hampshire, UK 24 15USDA Forest Service, Pacific Northwest Research Station, Corvallis, OR, USA 25 16Department of Geosciences, University of Massachusetts Amherst, Amherst, MA, USA 26 17Department of Crop and Soil Science, Oregon State University, Corvallis, OR, USA 27 3 18Dean and Thomas E. Reveley Professor, College of Natural Resources, University of Idaho, Moscow, ID, USA 28 19Yale School of Forestry and Environmental Studies, Yale University, New Haven, CT, USA 29 20Department of Geology and Geophysics, University of Wyoming, Laramie, WY, USA 30 21College of Natural Sciences and Mathematics, and Department of Biological and Environmental Sciences, The University of West 31 Alabama, Livingston, AL, USA 32 22Department of Earth and Environment, Franklin & Marshall College, Lancaster, PA, USA 23 33 Department of Soil, Water, and Climate, University of Minnesota, St Paul, MN, USA 34 35 ABSTRACT 36 37 38 Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth’s 39 surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar- 40 to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe 41 soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry 42 drives changes in mineral stoichiometry and distribution. (3) On landscapes experiencing little erosion, biology 43 drives weathering during initial succession, whereas weathering drives biology over the long term. (4) In eroding 44 landscapes, weathering-front advance is coupled to surface denudation via biotic processes. (5) Biology shapes 45 the topography of the Critical Zone. (6) The impact of climatic forcing on denudation rates can be predicted from 46 models incorporating biogeochemical reaction rates and geomorphological transport laws. (7) Rising global tem- 47 peratures will increase carbon losses from the Critical Zone. (8) Rising atmospheric PCO2 will increase rates and 48 extents of mineral weathering in soils. (9) Riverine solute fluxes will respond to changes in climate primarily due to 49 changes in water fluxes and secondarily through changes in biologically mediated weathering. (10) Land use 50 change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered 51 settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity 52 and biogeochemical processes requires longer timescales. (12) Biogeochemical properties impart thresholds or tip- 53 ping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur. 54 55 Received 12 May 2010; accepted 29 October 2010 56 4 Corresponding author: S. L. Brantley. Tel.: XXX; fax: XXX; e-mail: [email protected] Ó 2010 Blackwell Publishing Ltd 1 GBI 264 Dispatch: 24.11.10 Journal: GBI CE: Shantha Journal Name Manuscript No. B Author Received: No. of pages: 26 PE: Farhath 2 S. L. BRANTLEY et al. 1 Although some of the hypotheses have been implicit in scien- 5 INTRODUCTION 2 tific research conducted since the late 1800s, we argue that 3 The impact of humans on the Earth’s atmosphere, water, sedi- new analytical, modeling, and field opportunities now allow 4 ments, soil, and biota is thought to be of such magnitude that advances that can test these hypotheses over the next decade. 5 a new geological epoch – the Anthropocene – has been 6 proposed (Crutzen, 2002; Zalzsiewicz et al., 2008). The pace 7 EARLY DISCUSSIONS OF THE EFFECTS OF of anthropogenic change threatens sustainable use of soil and 8 BIOLOGY ON WEATHERING AND EROSION water, and impairs critical ecosystem services (Vitousek et al., 9 1997; DeFries & Eshleman, 2004; McNeil & Winiwarter, Understanding how biology interacts with earth materials to 10 2004; Foley et al., 2005; Wilkinson, 2005; Wilkinson & form the CZ has long been a subject of curiosity, scientific 11 12 McElroy, 2007; Holdren, 2008; Mann & Kump, 2008). inquiry, and social necessity. Historically, numerous proposals 13 Predicting changes in earth resources requires quantitative were advanced to explain weathering, the process that breaks 14 models to forecast – Earthcast – the behavior of the zone down and solubilizes rock components, leaving residual soil 15 extending from the vegetation canopy to groundwater (Bach- minerals and exporting solutes. Some of these early ideas 16 let et al., 2003; Yang et al., 2003; Steefel et al., 2005; Qu & match our current understanding; for example, in the early 17 Duffy, 2007; Minasny et al., 2008; Godde´ris et al., 2009; to mid-1800s, the decomposition of igneous rocks was 18 Rasmussen et al., 2010). This realm, where rocks meet life, attributed to the reactivity of water and carbonic acid (Four- 19 has been named the Critical Zone (CZ; US National Research net, 1833; Hartt, 1853) and landscape evolution was thought 20 Council Committee on Basic Research Opportunities in the to be influenced strongly by the production of physically 21 Earth Sciences, 2001). Within the CZ, biota – including mobile soils (Gilbert, 1877, 1909). Furthermore, very early 22 humans – interact with earth processes to define the chemis- on, the role of vegetation was noted: for example, Belt (1874) 23 try, texture, and topography of our surface habitat through wrote that ‘the percolation through rocks of rain water 24 weathering, element cycling, and erosion. charged with a little acid from decomposing vegetation’ accel- 25 To describe the behavior of a system as multifaceted as the erated weathering. In roughly the same time period, Charles 26 CZ, Earthcasting models need to be developed and parame- Darwin studied the effects of earthworms on soils, concluding 27 terized from observations of the atmosphere, water, surface that ‘… all the vegetable mold over the whole country has 28 Earth materials, and biota, made over a range of spatial and passed many times through, and will again pass many times 29 temporal scales. It is not sufficient, for example, to focus only through, the intestinal canals of worms’ (Darwin, 1881). By 30 on rocks at depth if the goal is to solve problems related to the 1880s, the concept of soils as natural systems that are gov- 31 human-land-air-water interactions. Likewise, if the goal is to erned by the interaction of climate, living matter, parent mate- 32 understand the long-term implications of climate change, it is rial, relief, and time had been developed (Dokuchaev, 1883). 33 not sufficient to measure the short-term responses of vegeta- Perhaps the modern era of biota-weathering research began 34 tion, because nonlinear, highly complex responses can emerge in the latter half of the 1900s as researchers began to explore 35 36 over the long term, in strong contrast to more predictable, the role of microbial communities in weathering of historic 37 shorter-term linear responses (Swetnam et al., 1999; Gunder- stone buildings (Ehrlich, 1990; Krumbein et al., 1991). 38 son, 2000; Chadwick & Chorover, 2001; Bachlet et al., Today, a wide variety of researchers from many disciplines are 39 2003). One of the best time-integrated records of the CZ is focusing on the relationships between biota and weathering 40 the soil itself: soil horizons, thicknesses, texture, structure, and erosion. Such efforts now focus on quantifying both the 41 composition, biological activity and their spatial patterns in importance of microbial activity and plants in elemental 42 landscapes integrate and record the flow of material, changes cycling and rock weathering in natural systems (e.g. Banfield 43 in climatic and tectonic forcing, and the influence of humans & Nealson, 1997; Berner et al., 2004). Furthermore, 44 and other biota on the CZ (Dokuchaev, 1883; Jenny, 1980; researchers are also modeling the effects of fauna on weather- 45 Yaalon, 1983; Retallack, 1990; Richter & Markewitz, 2001). ing and erosion (e.g. Gabet et al., 2003). Recent work on 46 Indeed, humans ultimately depend on weathering and erosion nutrient cycling and material flows within ecosystems also 47 processes to maintain the natural resources underlying society acknowledges the importance of rock weathering in structur- 48 and ecosystems (McNeil & Winiwarter, 2004; Montgomery, ing ecosystems (Banfield & Nealson, 1997; Buss et al., 2005; 49 2007).