Abstract
This paper reports soil development over time in different climates, on time-scales ranging from a few thousand to several hundred thousand years. Changes in soil properties over time, underlying soil-forming processes and their rates are presented. The paper is based on six soil chronosequences, i.e. sequences of soils of different age that are supposed to have developed under the similar conditions with regard to climate, vegetation and other living organisms, relief and parent material. The six soil chronosequences are from humid-temperate, Mediterranean and semi-arid climates. They are compared with regard to soil thickness increase, changes in soil pH, formation of pedogenic iron oxides (expressed as Fed/Fet ratios), clay formation, dust influx (both reflected in clay/silt ratios), and silicate weathering and leaching of base cations(expressed as (Ca+Mg+K+Na)/Al molar ratios) over time. This comparison reveals that the increase of solum thickness with time can be best described by logarithmic equations in all three types of climates. Fed/Fet ratios (proportion of pedogeniciron Fed compared to total iron Fet) reflects the transformation of iron in primary minerals into pedogeniciron. This ratio usually increases with time, except for regions, where the influx of dust (having low Fed/Fet ratios) prevails over the process of pedogeniciron oxide formation, which is the case in the Patagonian chronosequences. Dust influx has also a substantial influence on the time courses of clay/silt ratios and on element indices of silicate weathering. Using the example of a 730 kasoil chronosequence from southern Italy, the fact that soils of long chronosequences inevitably experienced major environmental changes is demonstrated, and, consequentially a modified definition of requirements for soil chronosequences is suggested. Moreover, pedogenic thresholds, feedback systems and progressive versus regressive processes identified in the soil chronosequences are discussed.
Highlights
Assessment of soil-forming processes and their rates is an essential scientific base for several reasons
Two soil chronosequences in semi-arid climate were located on Holocene beach ridges along the eastern coast of Patagonia, in southernmost South America, in the northern part of Golfo San Jorge
This paper compares soil chronosequences that were individually described in a number of publications with regard to observed changes in soil properties over time, including soil thickness increase, changes in soil pH, formation of pedogenic iron oxides, clay formation, dust influx, and silicate weathering and base leaching (expressed as (Ca+Mg+K+Na)/Al molar ratios) over time
Summary
Assessment of soil-forming processes and their rates is an essential scientific base for several reasons. Modelling soil formation, in turn, is an essential requirement to take the step from description and analysis of soil development towards prediction of future soil development Such prediction is important with regard to possible soil responses to the climatic shifts that are going on at present and expected in the near future. The few existing studies include those of Pillans [26] who investigated soils on basaltic lava flows with ages ranging from 10 ka to 5.59 Ma in the tropical climate of northern Queensland (Australia), and Muhs [27] who studied soils on Pleistocene reef terraces of Barbados These soils formed in Sahara dust, volcanic ash from the Lesser Antilles island arc, and detrital carbonate from the underlying reef limestone. Using the example of the Mediterranean soil chronosequence from southern Italy, comprising ca. 730 ka, the fact that soils of long chronosequences inevitably experienced major environmental changes is demonstrated, and, consequentially a modified definition of requirements for soil chronosequences is suggested
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