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Physical Soil Properties andthe Significance of Soil PollenConcentrationsP. J. Rennie aa Department of Forestry , University of OxfordPublished online: 06 Jan 2010.

To cite this article: P. J. Rennie (1958) Physical Soil Properties and the Significanceof Soil Pollen Concentrations, Geologiska Föreningen i Stockholm Förhandlingar, 80:3,340-342, DOI: 10.1080/11035895809454894

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340 P. J. RENNIE [Maj-Okt. 1958

Physical Soil Properties and the Significance of Soil Pollen Concentrations

P. J. RENNIE

Department of Forestry, University of Oxford

Recently, G. \V. DInlBLEBY (1957) has shown how traditional methods of pollen analysis (ERDTMAN 1943), previously used mainly for aquatic organic deposits, may be successfully applied to terrestrial soils, which contain varying and frequently very high proportions of mineral material. Features of this application, relevant to the present discussion, are to estract samples at in- creasing depths from the soil surface to form a vertical sequence, to subject to pollen analysis only that part of the sample passing a 1 mm sieve, and to use as a basis of interpretation both the absolute numbers and relative proportions of pollen grains of different plant genera found within 1 gm of the oven-dried sieved sample.

The purpose of this communication is twofold. First, to demonstrate from existing data (FEUSTEL and BYERS 1930; R E ~ T N I E ’ ~ ~ ~ ~ ) on the physical properties of acid soils, that concentrations of absolute numbers of pollen grains espressed in this way are an unsuitable and potentially misleading basis of interpretation, particularly in fields (THOMPSON, ASHBEE and DInrBLEBY 1957) where the method is at present finding increasing application. Secondly, to show how a small cstra expenditure of experimental effort would enable pollen concentrations to be expressed much more relevantly and in a way likely to enhance the usefulness of the method.

Clearly, any consideration of the vertical distribution of absolute numbers of pollen grains must be based on a comparison of numbers contained in soil samples of similar volume. The inadequacy of the above weight-basis lies partly in the fact that no correction is made for the varying proportions of mineral material exceeding 1 mm in diameter which may habe been originally present, but mainly in the fact that unit oven-dry weights of different soil horizons occupy greatly differing volumes in their natural field state. Thus, to illustrate the first point: if pollen concentrations on the weight-basis in the mineral hori- zons (5-50 cm) of two Yorkshire Calluna podzols - Silpho and Harwood- dale htoors - were being compared, all concentrations for Silpho would first have to be approximately halved, because Silpho soil contains about 55 per cent of material exceeding 1 mm in diameter, but Harwood-dale soil only 2 per cent. Typical data for three distinct soil types; degraded brown earths, podzols and deep peats illustrate the second point. For the brown earth -

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nd 80 H. 31 PIIYSICAL SOIL PROPERTIES 34 1

Tubney IVood, Berkshire - because apparent density increases almost linearly from 0.95 to 1.55 gmlcc from the surface to a depth of 40 cm, all pollen concentrations on the weight-basis would have to be progressively increased throughout this depth by amounts rising to as much as 63 per cent to obtain pollen numbers representative of similar volumes of field soil.

lower bleached humus pan iron Fan uppcr subsoil loivcr subsoil

Table I. Apparent densities and corriction for the horizons of a podzol

x 7.5 1.35 0.95 1.90 1.60 1.70 x 9.4

Depth, cm

-3 - 0 0 - 8 8 -16

1G -28 28 -30 30 -30.6

43 -50 3 0 . 6 4 3

I Apparent density gm Correction I oven-dry soil/cc Factor3 Horizon

1 to convert pollen concentrations from grainslgm. of oven-dry soil to grainslcc of field soil: this profile contains less than 5 per cent coarie material (> 1 mm in diameter).

For podzols variation in apparent density among horizons can be far greater. Shown in table I is such variation in respect of a podzol under Piiiiis silucstris - Suffield Moor, Yorkshire - together with correction factors, by which pollen concentrations on the weight-basis would first have to be multiplied before valid comparative study were possible. Again for deep peats FEUSTEL and' BYERS (1930) have shown that apparent density can vary very considerably, both within the same peat profiles, e.g., 0.41-0.71 gmlcc (heath peat) ; and between different profiles, e.g., 0.06 gmlcc (Sphagnum peat) to 1.21 gmlcc (custard apple peat). Varied correction factors again appear necessary, though it must be rcmembercd that for deep peats, as distinct from essentially inorganic soils, volume itself is not an unchanging standard and may alter with time (GODWIN 1934).

Although the effect of changing pollen concentrations from a weight to volume-basis upon the significance of the data can only be assessed by the detailed examination of specific cases, a few general points can be made. Thus, interpretations based on relative proportions of different pollen genera are, of course, always unaffected, whilst those based on absolute numbcrs are likely to be very little affected when the changes within a vertical sequence are very large, e.g., from 250 x lo3 to 1 x lo3 grainslgm of soil. It is when the changes are of a smaller order, e.g. from 6.5 x lo3 to 3.7 x lo3 grainsjgm down a brown earth profile, or from 8.5 x lo3 to 0.6 x lo3 grainslgm from the raw humus to ironpan of a podzol, that recalculation onto a volume-basis and reappraisal of the data seem necessary, for it may be shown that the entire character and sometimes direction of a concentration-curve can be altered. Of espxial interest too, are cases where soil materials of greatly differing particle-

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" I L P. J. RENNIE [Maj-Okt. 1958

size range or apparent density - such as sands, humous horizons and gravels - alternate as in prehistoric barrows. I t can be demonstrated, for example, that discontinuity in pollen concentrations in a vertical sequence through such structures ( TIIOIIPSON, ASIIBEE and DIAIBLEBY 1957) merely reflects the different apparent densities of the materials: recalculation of the concentrations onto a volume-basis largely obliterates such discontinuity, but shows up with far greater clarity other features of major interest being sought, such as former soil surfaces.

T o convert pollen concentrations from a weight to volume-basis necessitates no change in the pollen analytical technique; it does require, however, a determination of apparent density and of coarse material (> 1 mm diameter) if present. This is best carried out on separate samples and for many soil types is a simple procedure. For stony soils the procedure (RENXIE 1957) is more difficult, but even here the extra work involved is very small compared with the main pollen analyses.

References

DImLEDY, G. IV., 1957: Pollen analysis of terrestrial soils. New Phytol., 56, 12 ERDTIIAN, C., 1943: An Introduction of Pollen Analysis (Chronica Botanica, Ii'altham, Xfass.

U.S.A.) FEUSTEL, I. C. and BYERS, H. G., 1940: The physical and chemical characteristics of certain

American peat profiles. United States Department of Agriculture, Tech. Bull., No. 214. GODWIN, H.; 1934: Pollen analysis. An outline of the problems and potentialities of the method.

Part I. Technique and interpretation. New Phytol., 33, 278-309. RENNIE, P. J., 1994: Some physico-chemical properties of moorland soils as related to afforesta-

tion. Thesis (University of Oxford); RENNIE, P. J., 1957: Routine determination of the solids, water and air volumes within soil

clods of natural structure. Soil Sci., 84, 351-365. TIIo\msoN, AI. IV., ASSIIBEE, P., and DIlmLmY, G. I\7., 1957: Excavation of a Barrow near the

Hardy Aionument, Black Down, Portesham, Dorset. Proc. prehist. SOC., 23, 124-1 36.

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