Hydrogeology, Chemical Weathering, and Soil Formation. Allen Hunt

2010). To be sure, his categorical statement of the factors of soil formation provided a basis for exploring in a formal way the connections between ecosystems and their environmental influences. The terrestrial spheres are not mentioned specifically in this formulation, but they are there by implication: climate involves the atmosphere and hydrosphere; animals and plants (plus the three kingdoms of micro‐organisms) are the biosphere; parent material is connected to the lithosphere; and relief is part of the toposphere.

      Vladimir I. Vernadsky, a follower of Dokuchaev, made explicit the interconnectedness of the terrestrial spheres in his concept of the biosphere, a term he adopted from Eduard Suess after having read Die Antlitz der Erde (Suess, 1883–1909). Vernadsky (1926, 1929, 1998) developed original ideas on biogeochemistry and promulgated his own take on the biosphere, suggesting that living organisms and all life and life‐support systems (living organisms and their planetary environment) evolve together and form the media in which they live (air, water, soil, sediment). Some later workers argued that the biosphere should be confined to living things and the totality of life and life‐support systems be called the ecosphere (Cole, 1958; Gillard, 1969), a view to which the present author subscribes (see Huggett, 1999).

      In 1938, Sante Mattson (1938) considered all possible interactions between the lithosphere, atmosphere, hydrosphere, pedosphere, and biosphere (Figure 1.2). Three years later, Jenny listed the components of the ecosphere in his CLORPT equation, but his focus was the influence of environmental factors on soils and ecosystem properties rather than on the interrelationships between the environmental factors themselves.

With the advent of Earth systems science and the notion that “everything is connected to everything else,” a new integrative ecosystem approach emerged that addressed the interdependence of life, soils, climate, rocks, and relief (Figure 1.3). The BRASH model was an attempt to express these interdependencies as a general dynamic systems equation (Huggett, 1991, 1995):

where x is a vector of all state variables describing the system considered, f(x) is a matrix defining interactions between the state variables, z is a vector of driving variables, and t is time. For the whole ecosphere, the state variables are biosphere, b, relief, r, atmosphere, a, hydrosphere, h, and z is external driving forces (geological or cosmic). Adding the five state variables to the general interaction matrix gives the BRASH equation:

Figure 1.2 Terrestrial spheres and their interaction as envisioned by Sante Mattson. The shaded portion is the ecosphere, a term unknown to Mattson.

      Source: Adapted from Mattson (1938).

      Huggett (1995) argued that this approach reformulates the factorial model into mathematically solvable equations and models soil properties as a function of processes. Applications of these system equations can be found in Phillips (1993b), where it was shown in a numerical example that changes in the initial condition and parameter values can trigger the creation of chaotic behavior of soil development (see also Phillips, 1998).

The rise of Earth system science has led to an evaluation of the pedosphere’s role in the global system, both as a vital component of what has become known as the Earth’s critical zone and as a two‐way interactor with the other terrestrial spheres, the study of which has given rise to some new pedologies (Figure 1.4). These topics will conclude the chapter.

Figure 1.3 A schema for the terrestrial spheres: their interactions and external influences. Before 1875, the only terrestrial sphere given a special name was the atmosphere. Then the Austrian geologist Eduard Suess (1875), in the last and most general chapter of a slim volume titled Die Entstehung der Alpen (The Origin of the Alps), invented the eminently helpful terms hydrosphere, lithosphere, and biosphere, with the Swedish agricultural chemist Sante Mattson adding the term pedosphere in 1938. The present author suggested the word toposphere as substitute for Julius Büdel’s (1982) relief sphere, which Büdel used to describe the totality of the Earth’s topography (Huggett, 1995, 1997; Huggett & Cheesman, 2002). The toposphere sits at the interfaces of the pedosphere and atmosphere and the pedosphere and hydrosphere; it is a complex surface separating the predominantly solid body of the Earth from its mainly gaseous and liquid outer envelopes.

      Source: Adapted from Huggett (1995).

Figure 1.4 Research areas straddling the pedosphere and individual components of the Earth system. The word topopedology is suggested here, although there is a precedence for its use (Brillante et al., 2017).

      1.5.1. The Critical Zone

      As defined by the National Research Council (2001, 37), the critical zone is

      a dynamic interface between the solid Earth and its fluid envelopes, governed by complex linkages and feedbacks among a vast range of physical, chemical, and biological processes. These processes can be organized into four main categories: (1) tectonics driven by energy in the mantle, which modifies the surface by magmatism, faulting, uplift, and subsidence; (2) weathering driven by the dynamics of the atmosphere and hydrosphere, which controls soil development, erosion, and the chemical mobilization of near‐surface rocks; (3) fluid transport driven by pressure gradients, which shapes landscapes and redistributes materials; and (4) biological activity driven by the need for nutrients, which controls many aspects of the chemical cycling among soil, rock, air, and water. [italics in original]

      Henry Lin (2011) rightly pointed out that soils can be literally called the critical component of the Earth’s critical zone (see also Wilding & Lin, 2006).

      A key feature of critical zone research is its integrative nature. The multifarious components of the critical zone have engaged scientists from distinct and often isolated disciplines: vegetation by botanists, soils by soil scientists, groundwater by hydrogeologists, and substrate by geologists. Important though such separate studies be, predicting the overall behavior of the critical zone demands a combined effort, not least because the functional, emergent properties of such a complex system are the result not only of its various parts but also of the interactions among its parts (Chorover et al., 2007). Recent publications point to the value of integrative modeling (e.g. Banwart et al., 2017). Critical zone research has gained