Following the middle of the twentieth century, Earth scientists increasingly became aware of the significant role played by the incorporation of dust in the formation of soils. Dust plays an especially important role in the development of aridland soils, given the usually abundant supply of dust and generally limited magnitude of chemical weathering that characterizes aridland soils. The recognition that virtually all calcium in pedogenic carbonate is derived from calcium-bearing dust that primarily accumulates in a very common horizon of aridland soils, the calcic horizon, was critical in convincing scientists that soil formation in aridlands differs in fundamental respects from that in more humid settings. Research conducted by Leland Gile and his colleagues in the Desert Soil Geomorphology Project in southern New Mexico produced one of the most important bodies of published research in the 1960s to the early 1980s, that demonstrated both the significant impact of dust on aridland soils and their geomorphic significance. Subsequent research in the Cima volcanic field of the Mojave Desert, southwestern United States, showed that the nature of soil development associated with the formation and evolution of desert pavements is markedly different from that proscribed by the “canonical” A/B/C model of soil profile development described by V. Dokuchaev, the profile model most familiar to the majority of Earth scientists. The soils that develop beneath desert pavements form in parent materials increasingly composed of entrapped dust that is subsequently translocated below the pavement.
Vesicular horizons, the key surface horizon of desert pavement soils, and subjacent, thickening clay and calcium carbonate–enriched B horizons form mainly through accretionary and inflationary profile (AIP) development, a type of cumulative soil development associated with rising desert pavements. The “upward growth” model of desert pavement formation profoundly contrasts with the “deflation” model. The behavior of the vesicular A horizon is important in maintaining a balance between rates of dust entrapment and translocation that enables continued AIP, while maintaining desert pavement at the surface. Soil chronosequence studies show that AIP and desert pavement formation are favored on other landforms (e.g., alluvial fans), providing that a weathering-resistant rock type is available to form a persistent pavement. Once desert pavements and soils have formed, most have likely survived exposure to “glacial” periods, despite effectively greater precipitation and locally significant changes in the nature of biotic communities that are characteristic of such periods.
AIP is also an important mode of soil profile development in semiarid regions where desert pavements cannot form and vesicular horizons are weakly developed or absent. Increased plant density continues to favor the dust entrapment rates necessary to maintain AIP, as well as to provide a surface stabilizing function. Given the ecologically limiting role of water in aridlands, the nature of and time-dependent changes in soils forming by AIP strongly influence patterns of recruitment and composition of plants in aridland landscapes and their response to environmental change. The hillslopes of aridland hills and mountains are excellent dust traps, and in favorable circumstances, weathering and dust entrapment promotes the development of thick, vegetated and smooth soil-mantled hillslopes. Toposequence studies in selected areas of the southwestern United States indicate that the presence of dust-entrapping colluvium is probably necessary to maintain AIP on hillslopes over long periods of time. Vegetation also promotes entrapment of dust, but vegetation on hillslopes is highly sensitive to episodic drought and/or fires that temporarily eliminate or drastically reduce vegetation cover. Episodically increased erosion potential prevents sustained AIP, and accumulated dust in soils is quickly transported from the hillslopes. Certain types of bedrock in aridland hillslopes weather rapidly and favor the development of soil-mantled, transport-limited hillslopes, but the typically thin, weakly developed soils are prone to rapid erosion, especially when subject to a combination of extreme drought followed by large increases in rainfall. Glacial-to-interglacial changes in climate also cause substantial changes in biotic communities, rates and processes of weathering, and soil development that influence the behavior of aridland hillslopes.
Models that employ the soil production function have been developed recently to enable calculation of rates of soil production on soil- and vegetation-mantled upland hillslopes. The results of a half century of soil geomorphological research in aridlands, relying largely on the “CLORPT”-based approach, complemented by studies of the behavior of biotic communities in aridlands, suggest that maintaining the necessary conditions fundamental in the application of soil production functions in studies of hillslopes (transport solely by diffusive processes maintaining steady-state soil thickness and a spatially and temporally constant diffusion coefficient) for thousands to tens of thousands of years is highly unlikely in aridlands. Changing the word soil, in the term soil production function to the term mobile regolith will reduce confusion concerning the actual meanings of these different terms and facilitate a greater degree of integration of very different approaches in the study of soil landscape evolution.
