A chronosequence of soils near the Franz Josef Glacier (1968)
AuthorsStevens, Peter R.show all
Professor Hans Jenny, in his book “Factors of Soil Formation”, wrote (p.31) “The estimation of relative age or degree of maturity of soils is universally based on horizon differentiation. In practice, it is generally maintained that the larger the number of horizons and the greater their thickness and intensity the more mature is the soil. However, it should be kept in mind that no one has ever witnessed the formation of a mature soil. In other words, our ideas about soil genesis as revealed by profile criteria are inferences. They are theories, not facts. This accounts for the great diversity of opinion as to the degree of maturity of specific soil profiles. It is well known that certain eminent pedologists take objection to the general belief that chernozems are mature soils; others consider brown forest soils and gray-brown podsolic soils merely as immature podsols. The list of controversial soil types is quite long. Whatever the correct interpretation may be, it is evident that the issues center around the factor time in soil formation.” One method of studying the effects of Time as a Soil Forming Factor is the recognition and investigation of a Chronosequence, wherein four of the five Soil Forming Factors are kept constant or ineffectively varying. 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 formation. It is helpful to assign accurate ages to the soils of such a Chronosequence, allowing the derivation of Chronofunctions - rate-equations of soil formation. In many parts of the world, intermittent alteration of the Earth's surface by mudflows, volcanism or processes of glaciations has provided areas of terrain whose relative and absolute ages can be determined by the techniques of the botanist and, geomorphologist with some degree of confidence, Such an area occurs in the vicinity of the Franz Josef Glacier, which bas at various known times deposited morainic and alluvial debris in defined places. A succession of plants has colonised these, ground-surfaces, and on them a Chronosequence of soils has developed concomitantly. Deposition of organic matter in and upon the parent material initiates depth-gradients of many soil characteristics such as reaction, bulk-density, and contents of organic carbon, nitrogen and various fractions of phosphorus. Under the influence of a warm and extremely humid climate the processes of weathering and leaching within the soil profile are responsible for striking gains, losses, transformations and redistributions of many organic and inorganic constituents. Some of these changes have been studied during the past four years, following an earlier investigation by the present author of the most youthful soils of the Chronosequence in 1962 and 1963. This initial work traced the development of six ecosystems 0, 6, 12, 25, 45 and 55 years of age. Spectacular accretion of nitrogen, especially, showed no indications of having reached any apparent steady-state in the oldest ecosystem, despite the impending elimination from the botanical succession of Carmichaelia grandiflora, a native legume. Clearly, it was important to determine nitrogen contents and the point of steady-state of nitrogen in soils older than 55 years, which lack any plants known to be capable of nitrogen fixation. In addition, the recognition of older soils, whose position on an orderly continuum of soil development could be assumed with some degree of confidence, would provide considerable information about rates of change of many other aspects of soil morphology, mineralogy and physical and chemical characteristics. This thesis is organised in the conventional manner. The breadth of investigation, involving aspects of glaciology, geomorphology and ecology as well as pedology, has necessitated a lengthy Review of Literature embracing facets of Chronosequences, geochronology, podzolisation and gleying, and podocarp successions in New Zealand, as well as a brief review of the previous study. A lengthy period of field-work has been encompassed by a few brief notes in the section on Materials and Methods, followed by a presentation of Results, and ensuing Discussion. For easy reference, the Tables, Figures, Plates, Appendices and References have been bound within a separate volume. Finally, it is relevant to comment upon the scientific and utilitarian value of such a study. Firstly, this study, conducted by what A. Rode has called the “Comparative Geography Technique”, is necessarily a “static” study. Selection of soils from various points along a Time-sequence of soils presupposes that consecutive members of the sequence have at one time passed through, and been identical with, the stages of development represented by all members preceding them in the sequence. This is an assumption which a priori cannot be proved. Secondly, the study does not deal with the “dynamic” diurnal or seasonal changes within and between ecosystems. Nevertheless, the recognition of successive ecosystems, accurately dated, has enabled the establishment of a “Natural Laboratory” in which other studies of microfauna and flora, mineralogy and weathering can be and are being conducted. Insofar as soils exhibiting varying degrees of development are often found on geomorphic surfaces of differing ages within New Zealand, the chronosequence concept could provide a useful tool in geomorphology and geochronology. The provision of information about a, group of soils little understood by agronomist or forester might also be a worthwhile result of this study. It may have thrown some light on what might be termed the “Birth, Life and Senescence” of a West Coast soil.