Hydrogeology, Chemical Weathering, and Soil Formation. Allen Hunt

funding, and it has led to the increased inclusion of geochemical reaction models and concepts in soil science.

      1.5.2. New Pedologies

      Biopedology, the oldest of the new pedologies, considers interactions between soils and life. Its origins lie with Charles Darwin and his work on earthworms (e.g. Darwin, 1881). Darwin was also the first to recognize the importance of faunal mixing in soil formation and the textural sorting it can produce (Johnson, 2002; Brevik & Hartemink, 2010). Biopedology has made a strong comeback in the last few decades with research into bioturbation and biomantle theory (e.g. Hole, 1961; Johnson, 1990; Peacock & Fant, 2002; Johnson et al., 2005a, 2005b; Saco & Moreno‐de las Heras, 2013; Gabet et al., 2014; Fleming et al., 2014; Johnson & Schaetzl, 2015); it is also benefitting from research in biogeomorphology (e.g. Verboom & Pate, 2013; Pawlika & Šamonil, 2018; see also Huggett, 2017).

The second oldest of the new pedologies is geopedology, which is short for geomorphopedology and focuses on the interdependency between soils and geomorphology. The argument is that soils and vegetation seldom develop in a totally inactive geomorphological environment: the landscapes in which soils and vegetation develop normally change. Thus, the development of terrestrial ecosystems and the geomorphological development of landscapes take place at the same time and influence one another. Pedologists were alerted to this fact by Shaw (1930), Robinson (1936), and Milne (1936), who discussed the role of erosion and deposition in soil evolution (see above). Indeed, soil geomorphology began in the USA in the 1930s with a spate of soil erosion studies made under the auspices of the USDA (Effland & Effland, 1992). In 1952, the USDA set up a Soil‐Geomorphology Group that included Robert V. Ruhe, who championed the process dimension of Milne’s catena concept (e.g. Ruhe, 1960), arguing that it is the clearest general systems statement about soil geomorphology, integrating the factors in explaining soil differences while focusing on past history of the land surface, geohydrology, erosion, sediment transport, and pedogenic processes (Ruhe, 1975; Birkeland, 1990; Johnson & Hole, 1994). Geopedology has blossomed over the last couple of decades (see Zinck et al., 2016) and the advent of sophisticated soilscape–landscape deterministic models has given it a new impetus (e.g. Willgoose, 2018).

      Hydropedology is one of the latest pedologies. It is “an emerging intertwined branch of soil science and hydrology that studies interactive pedologic and hydrologic processes and properties in the Earth’s Critical Zone [and that] aims to bridge disciplines, scales, and data, connect soils with the landscape, link fast and slow processes, and integrate mapping with monitoring and modeling to provide a holistic understanding of the interactions between the pedosphere and the hydrosphere” (Ma et al., 2017). Unlike conventional soil science, it emphasizes in situ soils in the landscape, which have distinct pedogenic features and varying environmental settings (Lin et al., 2012). In doing so, it yields a more realistic and integrated understanding of real‐world soil and hydrological processes.

      Anthropopedology is the study of the human impact on soils. It has a long history. Dan Yaalon and Bruno Yaron (1966) used the term metapedogenesis to cover human‐made soil changes; Ron Amundson and Jenny (1991) considered the place of the human species in the state‐factor theory of ecosystems. But two‐way interactions between soils and the human sphere (anthroposphere) have been the subject of increased research over the last few decades owing to the recognition of soil as an indispensable resource (e.g. Richter & Yaalon, 2011). Topics investigated include coupled human–natural landscapes (Barton et al., 2016) and the evolution of technosols (soils subjected to a strong human influence and containing significant amounts of artefacts, characteristic of the Anthropocene; Leguédois et al., 2016). Research in anthropopedology becomes ever‐more pressing as human security comes to rely increasingly on Earth’s diverse soil resources. As Amundson et al. (2015) point out:

      Soil is the living epidermis of the planet. Globally, soil is the medium through which a number of atmospheric gases are biologically cycled and through which waters are filtered and stored as they pass through the global hydrological cycle. Soil is a large and dynamic reservoir of carbon and the physical substrate for most of our food production. Profound changes are on the horizon for these interconnected functions—particularly sparked by changes to climate and food production—that will likely reverberate through society this century.

1.6. PROSPECT

      Big gains have come from viewing the soil as a system. From the early ideas on state factors, inputs and outputs, and transfers and transformation have evolved sophisticated models of soil landscapes and soils as a key component of the Earth system, and in particular of its critical zone. Of course, challenging questions in pedology remain, and it may take fresh approaches to answer them, but a systems approach still offers a powerful method of investigation. Indeed, Henry Lin (2011) demonstrates the value of such an approach in understanding spatial and temporal changes in soil systems and advancing forecasts and plans for changes related to critical societal needs. To do so, he brings together three general principles of soil change and pedogenesis in time and space (especially time):

      First is the principle of conservation plus evolution, which provides the reconciliation of fast and slow changes in multiphase soil systems. Incomplete closure and partial irreversibility of many cyclic processes involved in soil functioning produce a range of residual solid products that accumulate over time, meaning that soil profiles record their own history, at least to an extent; this idea was first put forward by Aleksei A. Rode (1947) and later called “soil memory” (Targulian & Sokolova, 1996). Fast and slow changes in complex soil systems thus require an evolutionary and holistic approach to account for their connections and to quantify structural and informational accumulation alongside energy and matter conservation.

      Second is the principle of dissipation plus organization, which explains the simultaneous processes of dissipation (that create the soil matrix) and organization (that create soil structure) that occur in the formation and evolution of natural soil systems. The idea is consistent with the theory of dissipative structure and self‐organization: soil entropy changes provide potential indices for the degree of soil weathering (residuals) and soil structural development (fluxes) once appropriate quantification is made. In addition to energy and mass changes, entropy change and its link to a complex system’s orderliness and information need to be quantified to gain a fuller understanding of soil complexity.

      Third is the principle of space plus time, which highlights the fundamental differences and intimate links between time and space. Space is reversible (you can return to a location already visited), conservative in the sense that energy and matter cannot be created or destroyed but can be transformed and transferred, and structured in three dimensions; time is irreversible, evolutionary, and nonstructured (always moving in one direction); however, time and space both share the common characteristics of preferentiality (preference for different spatial locations and preference for different stable states through time) and thresholds (abrupt changes) that govern soil functions and soil evolution.

Lin admits that there are many unknowns in the workings of the soil system, but he argues that his three principles offer useful perspectives for modeling and predicting soil change and pedogenesis. They are certainly areas where the systems approach may provide new pedological insights that build upon the foundations laid by Dokuchaev, Jenny, Simonson, and the other giants of the discipline.

ACKNOWLEDGMENTS

      I would like to thank Allen G. Hunt for inviting me to write this chapter and two anonymous reviewers for their helpful and encouraging comments. My doctoral thesis dealt with soil‐landscape systems, but my interests have shifted to other areas during my career, mainly to aspects of ecology and geomorphology, so revisiting the old soil literature (including my own!) and updating myself on the latest research has proved to be at once a nostalgic, educational, and rewarding adventure.

REFERENCES

      1 Aandahl, A. R. (1948). The characterization of slope positions and their influence on the total nitrogen content of a few virgin soils of western Iowa. Soil Science Society