Edafología. Volumen 7-1. Abril 2000. pág 1-29.


Mountain mires of Northwestern Spain

Departamento de Edafología y Química Agrícola. Facultad de Biología. Campus Sur s/n. 15706 Santiago de Compostela. España.

The Northwestern Iberian Peninsula is a mid-latitude oceanic region with areas where different types of mires are frequent or abundant. Formation of the mires dates back to 11,000-10,000 years B.P. in gla-ciated areas and to Holocene times elsewhere, the principal phases occurring between 9,000-8,000 years B.P., 5,000-4,000 years B.P. and 3,000-2,000 years B.P. Nevertheless, increased superficial runoff linked to local episodes of erosion also triggered minerotrophic peat (fens) formation at different times after 5,000 years B.P.

Mean maximum peat depth is 2-3 m and most frequent rates of peat accumulation vary between 0.2 and 0.7 mm yr -1 . Dry mass addition ranges between 40 and 120 g m -2 y -1 and tends to be higher in blan-ket bogs than in fens. The lower values in fens are due, to some extent, to erosion, compaction and regrowth of the peat. Bulk density ranges from 0.06 to 0.60 Mg m -3 , total porosity between 85-95% and organic matter content between 25 and 95%. These are mainly acid peats (pH 4.0-5.5). Exceptionally, some fens affected by waters draining limestone levels have a pH of 6. Effective cation exchange capa-city varies from 4.5 to 20.5 cmol c kg -1 depending on the degree of organic matter decomposition and pH.

The order of cations is Mg,Ca > Al > Na > K in blanket bogs and Al > Ca > Mg > Na > K in fens, alt-hough there is a relative heterogeneity.

Using FAO framework these bogs are classified as Histosols, mainly as fibric and terric Histosols, but thionic, terric-thionic and fibric-thionic are also represented. Using the Soil Survey Staff soil taxonomy Fibrists (boro, sphagno and medifibrist) are the most abundant, but Hemists (boro, sulfi and
medihemists) and Saprists (boro, sulfi, and medisaprist) can also be found. From a habitat point of view, based on the Interpretation Manual of European Union Habitats of the network Natura 2000, the types represented are: blanket bogs (7130) -almost exclusive of the northern sector-, transition mires and qua-king bogs (7140) ­mostly fen formations-, and active and degraded raised bogs (7110-7120). Ombrotrophic peat bogs are ideal archives for reconstructing environmental changes ocurred during the late Quaternary. Among them are vegetation changes, climate change, atmospheric metal deposition and human induced erosion.


Key words: bogs, fens, soil properties, natural habitats, archives of environmental changes



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