Fjölrit RALA - 15.06.2004, Blaðsíða 100

Micromorphology of an Icelandic Histosol
Thorsteinn Gudmundsson1 and E.A FitzPatrick2
‘LBH Hvanneyi IS; 2Aberdeen GB
Introduction
Histosols and Andosols are agriculturally the most important soil types in Iceland. The
Histosols preserve remains that document the natural history and they store large amounts of
carbon and nitrogen. A Histosol profile in North Iceland was selected for intensive studies,
including the determination of the C and N reserves and micromorphological investigations
using large (5x10 cm) thin sections. The thin sections were made after replacement of the soil
water with acetone and impregnation with a polyester resin (FitzPatrick and Gudmundsson
1978).
The soil profile has three distinct rhyolitic tephra layers from eruptions of the volcano
Hekla. The top tephra layer ‘Hl’ is from the eruption of 1104 and is thus 870 years old at the
time of profile description in 1974. The age of the other tephra layers has been determined
through radiocarbon dating as follows; the middle layer ‘H3’ about 2800 BP and the bottom
layer ‘H4’ about 3800 BP. These dates allow the calculation of the rate of deposition at
different time intervals. The peat formation is assumed to have started about 9.000 years ago.
The soil organic C content is 19 - 27 % in the top 22 cm increasing to 40 - 48 % below 94
cm. In the top 22 cm the mineral material is about 50% but decreases to about 20% below 62
cm. The yearly input of C and N is highest in the top 22 cm due to living roots and slow rate
of decomposition whereas the decomposition has gone further at greater depth. Thus the
average input of C and N during the 9.000 years period is 126 and 6 kg/ha respectively.
The yearly input of mineral material was 2-3 times higher above the top tephra layer than
previously. This is associated with the period of habitation and the well documented wind
erosion and aeolian input after the settlement of the country.
Depth cm Period/ Number of years Soil Mineral material Organic C Total N C/N
total t/ha Yearly increase kq/ha total t/ha Yearly input kg/ha Total t/ha Yearly input Kg/ha total t/ha Yearly input kq/ha
0-22 Surface to Tephra H1 870 vears 563 616 247 284 129 148 8,3 10 15-17
28-60 Between tephra H1 and H3 1930 years 610 316 253 131 171 89 8,9 5 18-26
62-90 Between tephra H3 and H4 1000 years 506 506 113 113 199 199 8,2 8 23- 27
171 - 235 H4 to Bottom 5200 years 1804 347 550 106 635 122 24 5 26-36
0- 235 Whole profile 9.000 3456 384 1163 129 1134 126 50 6
Micromorphology
Structure and pores. Generally the structure is massive with laminar arrangement (fabric) of
the organic residues. In the top 20-30 cm the laminar arrangement is undulating due to
intensive cryoturbation and the formation of thúfur at the surface. The cryoturbation has also
distorted the top tephra layer especially in the thúfur. Throughout the profile there are
frequent vertical channels associated with larger roots. Below 40 cm woody fragments
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