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Soils have developed on an enlarging meander loop in the River Rheidol valley following a period of severe heavy metal pollution in the nineteenth century. Successive zones of the alluvium have been identified according to age of deposition. Soils in these zones comprise a normal chronosequence with regard to humus and free iron oxide content. Heavy metal contamination is worst in the oldest section, contemporaneous with maximum lead mining activity, but is appreciable even in the most recent section. The relevance of these results to other alluvial regions is discussed.

One method of studying the effects of time as a soil-forming factor is to recognize and investigate a chronosequence, wherein four out of five soil-forming factors are constant or vary ineffectively. Thus, observed differences between soils of different ages forming a sequence are deemed to be the result of the lapse of varying intervals of time since the initiation of soil formnation. In this paper theoretical considerations underlying the chronosequence concept are examined and soil development is discussed. Several non-strict chronosequences are reviewed, and a number of chronosequence studies are examined. The significance of such studies is assessed, and general desiderata for future investigations set out.

Summary The rate of transformation of inorganic phosphorus fractions and forms was investigated in a chronosequence of weakly weathered soils developed on windblown sand. The net decline of acid-extractable Ca-P during 104 years of soil development was approximately 2600 kg/ha m profile. This fraction was progressively removed from surface horizons and had virtually disappeared from the oldest soil. Residual inorganic P, considered to be included apatite which is not extracted by 0·5 and 1N HC1, was relatively constant in profiles of the chronosequence but decreased in the oldest soil. In the younger soils, the NH4F-P fraction was low but, with increasing time, the weight of this fraction increased, initially in surface horizons. The net accumulation of non-occluded secondary inorganic P during 104 years was 360 kg/ha m profile and this was largely due to an increase in the NH4F-P, rather than the 1st NaOH-P fraction. There was no net shift from non-occluded to occluded secondary inorganic P. Although apatite is initially present as inclusions within primary minerals in the wind-blown sand, an extensive loss of P has occurred during 104 years.

CEC and oxidisable carbon content were highly correlated (r=0.96) whereas a lower coefficient was obtained for a correlation of CEC and clay content (r=0.57) in a chronosequence of sandy soils from New Zealand.

Summary The particle-size distribution of the whole sample and the content, particle-size distribution, and oxygen-isotope abundance (δ18O) in quartz were employed to determine the extent to which aeolian materials such as aerosolic dust (10 to I /μm) from Australia, loess (50 to 10μm) in New Zealand, and flottsand (250 to 20 μm) in Australia have been accreted into soils and sediments in the South Pacific area. Although aeolian material from both local and more distant sources has been deposited on the Franz Josef, Fox, and Tasman glaciers, the extent to which aeolian materials have been added to soils in South Westland, New Zealand, could not be determined because the oxygen-isotope abundance (12.9 to 13.5 0/00 in quartz from the soils was similar to that (13.1 to 13.9 0/00) in quartz from the dust samples. The quartz from loess added to two basaltic soils in North Auckland, New Zealand, had an oxygen-isotope abundance (12.9 to 13.9 0/00) slightly lower than that (15.0 to 15.4 0/00) in quartz from a greywacke-derived soil and the underlying rock also in North Auckland. The oxygen-isotope abundance (13.6 to 15.4 0/00) in quartz from a chronosequence of soils developed in basalt in Victoria, Australia, indicates that the quartz did not originate from the parent basalt, but probably was transported by aeolian processes from stranded beach ridges and dunes and added to the soils in the form of flottsand, an aeolian material coarser than loess and finer than dune sand. Most particles of the pelagic sediments fell within the aerosolic dust size range. The oxygen-isotope abundance (12.1 to 15.1 0/00) in quartz isolated from several pelagic sediments between latitudes 35 and 45° S. was similar to that (12.9 to 15.4 0/00) in quartz from New Zealand and Australian soils located between these latitudes. An aeolian source of quartz explains the above relationship and also the decreasing delta values of quartz from sediments and soils with increasing latitude in the South Pacific area. Oxygen-isotope abundance in quartz refutes a volcanic origin of quartz in pelagic sediments but supports the deduction of Griffin et al. (1968) from mineralogical analyses that the quartz and associated minerals were transported from lands to seas.

Summary Fourteen samples, representative in terms of their clay mineralogical composition, of a small area in a Hercynic massif, illustrate the use of a densitometer in geomorphological and clay-mineralogical work. These fourteen samples are from weathering residues and sediments belonging to four landscape-units, distinguished on physic-graphical criteria. The clay-mineralogical composition of the alteration products in the area is considered to be dependent on three factors: the parent material factor, the time factor and the profile drainage factor. The influence of these factors is shown by arranging the samples in three sequences: (a) a lithosequence, showing the differences in clay-mineralogical composition between the basic and acid parent materials, (b) a chronosequence, showing the increase of weathering degree of the clays with increasing importance of the time factor, and (c) a toposequence, showing the influence of the profile drainage factor. Reflections of Guinier-de Wolff powder camera X-ray photographs of the fractions

Three distinct profile types of soil found on alluvial deposits appeared to represent a chronosequence in soil formation. The chemical, morphological and mineralogical characteristics of these soils, classified as Alluvial, Brown Wooded and Grey Wooded, are given and soil development is discussed.

Eight sites of known age were sampled in Glacier Bay National Monument, Alaska, to examine the changes in plant diversity during primary succession in that region. Four strata–trees, tall shrubs, low shrubs—herbs, and bryoid—thalloids–were sampled independently. Data suggested a sequence of wave—like invasions on sites by strata, largely in order of increasing size. Bryoid—thalloids were exceptional in demonstrating a late peak in cover values. In general, diversity of a particular stratum declined during the period in which the stratum dominated the community in terms of foliar cover. Richness (species number) of communities increased rapidly in the first 100 years, then more gradually to reach a maximum in the muskeg steady state. Equitability (evenness of distribution of foliar cover among species) was erratic, but tended to increase with age. After initial rises, three diversity indices showed nearly flat curves with two exceptions: marked decreases in the 30— to 50—year period; and a rise to maximum levels in the final steady state. Total information per unit area calculated from diversity and foliar cover increased in a sigmoid manner with peak information in the final muskeg stage.

Summary A study was made of the amounts and distribution of C, N, S, and organic P accumulated over a period of 10 000 years of soil development on wind-blown sand. After initial rapid rates of accumulation of C, N, and organic P during the first 1000 years, subsequent rates of gain were slower, but steady states for C, N, and S had still not been reached after 10 000 years. Sulphur declined during the first 50 years of soil development but then increased in a parallel fashion to organic C. During 10 000 years, gains of organic C, total N, total S, and organic P were 204 000, 9800, 1180, and 1170 kg/ha m profile, respectively. Gains in total N and S in the older soils are readily accounted for by returns in precipitation. Pronounced changes in soil organic matter composition during 10 000 years are reflected in the widening of the C/N ratio (8 to 20) and the C/organic P ratio (39 to 159).

It is inevitable that a great deal of ecology must be descriptive, yet it is surprising that when interest changed from the description of the structural aspects of vegetation to the examination of the changes which occur, the essential parameter of any changing system, time, has so seldom been mentioned. Succession has formed the subject of innumerable papers in the last forty years, yet in only a handful has the rate of the process been discussed. Even where the evidence was clearly available in the form of historical data or in the annual rings of the woody plants, the idea of time as an essential part of the process did not seem to occur to, or perhaps seemed unimportant to, the authors. In a random selection of over thirty papers on succession written between 1930 and 1940 none refers to the rate at which the process was taking place, and, although there were occasional papers in which the time element was discussed, these were very much the exception. In fact, as reference to the combined index of the Journal of Ecology shows, there is not a single entry for time, or for any other heading which suggests that the rate of the process was important. Yet these papers were written in a world in which speed was almost deified. So divorced has succession become from its legitimate spouse, time, that some ecologists have felt the necessity to invent a new term which leaves no opportunity for the time element to be overlooked; we no longer have a succession but a chronosequence! The study of soils has in some respects been more fortunate than other aspects of ecology, although even here interest in time has been very restricted. Like plant ecology it passed through a descriptive phase and then became concerned with the developmental processes which lead to the formation of a mature soil more or less in equilibrium with its environment; nevertheless an emphasis was placed much earlier on the time factor. Dokachaev in his classical work on soils recognized the importance of the time element as early as 1881, but only occasionally did other workers give much attention to this aspect of soil development. Although almost every general account of soils describes the weathering of rocks, the formation of the soil and the subsequent development of a profile, the time scale of these changes is usually ignored. The papers dealing with the time aspect of soil development are so few that it would not be difficult to list them all. One of the earliest studies was by Dokachaev on the soil development on the ruins of the Staro Ladoga Fortress. The fortress was built in 111.6 from slabs of Silurian limestone and granite boulders and when examined in 1880 after a period of 764 years there was a covering of 10-12 cm of soil. A similar investigation was made by Akimsev of the fourteenth-century fortress of Kamanetz in the Ukraine. The 273