Soils of Volcanic Regions in Europe

In continental France, volcanic soils cover about 6000 km

2

, mostly in the Massif Central (>5000 km

2

), but also in small area of Vosges and Morvan. Most of these soils are Andosols, although there are also Cambisols (Brunisols) and exceptionally Podzols.

The volcanic soils distribution is fairly well related to the present climato-topo-sequence, as well on recent pyroclasts as on old volcanic or basic metamorphic formations.

The key factor for Andosols occurrence is the climate, and notably udic or perudic moisture regimes. The chemical composition of parent rock has rather weak influences on the soil genesis, except rhyolitic materials, which favour Podzol formation. However, the age of parent rock is important. The Silandosols are related to recent pyroclastics on the Chaîne des Puys. Aluandosols have formed on older compact lavas, but their effective parent material is often a loose pre-weathered volcanic or basic metamorphic material. Their final development occurred during Holocene. The anthropic effects on soil transformation, except erosion or alteration of soil surface properties, are not obvious.

There are limited occurrences of vitric (at the Windsborn site, data not shown), silandic and aluandic Andosols in Germany. Also, the rare soil type of an andic Luvisol can be found in the Vogelsberg — region near Frankfurt. We observe that, in Germany, this and other soils containing poorly crystalline aluminosilicate phases (as reflected by their Al

o

and Si

o

contents) have been regularly mistaken for Cambisols of low bulk density. We propose that appropriate measures should be taken to protect and conserve these soils, because they are valuable as archives of natural history and provide opportunities to study unique soil processes.

A variety of soils occur on volcanic parent materials in Greece, spanning diverse soil groups. Some of the parent materials are quite old (>7 M y), too old for maintaining Andosol characteristics, and none of 2363 profiles of the national database classifies as Andosols. Young volcanism is found on Santorini where relatively poorly developed vitric soils predominate.

The Carpathian Basin, with diverse volcanic parent materials, age of rocks, type of vegetation and climatic conditions, is an excellent field within a small region to study the main influences of soil development on volcanic material. At ustic/xeric moisture regime, independently from the type of the rock (ignimbrite, andesite, basalt) Phaeozems and Cambisols develop. At ustic and udic moisture regimes, both on pyroxene andesite and basalt, Umbrisols develop. Under udic moisture regime, above 1000 m elevation, on pyroxene andesite in Romania and Slovakia, Andosols can develop. In a topo-sequence on pyroxene andesite, under udic moisture regime, between 700–1000 m, Cambisols or Luvisols dominate, but Andosols, sometimes with spodic features from 1000 to 1800 m elevation.

Thermally assisted Hydrolysis and Methylation was used to analyse in more detail the aromatic and aliphatic building blocks of the organic matter of two soil profiles from the Azores. Both Ah horizons showed significant contributions of lignin, tannins, cutin and suberin, which were virtually absent from the underlying horizons. Alkanoic acids were present throughout both soil profiles, with long-chain acids (>C

20

) decreasing with depth in comparison with short-chain acids in EUR05, whereas having a relative minimum in AB2 and 2Bw1 horizons in EUR06. Despite the huge potential of Andosols to store carbon, the studied soils did not exhibit a great preservation potential of

plant

-derived molecules (lignin, cutin/suberin-derived lipids, C

26

alkanol) in comparison with other soils as studied by THM. By contrast, bacterial-derived branched alkanoic acids are better preserved than the plant-derived alkanoic acids, which confirms the high degree of microbial and insect-derived OM in the Andosols studied.

Volcanic soils exhibit a high to very high sorption capacity for heavy metals at different pH-values. In particular, they exhibit strong sorption of Pb and Cr at all tested pH values. Sorption of Cd and Ni was found to be strongly pH dependent. A clear relation of adsorption and desorption to the content of allophane was not found, but with increasing allophane contents, Pb-sorption decreased weakly and Cd- and Ni-sorption decreased strongly from pH 7 to pH 4. It appears that, at ambient pH and at pH 4, Cd desorbs more completely with increasing allophane content. The general high sorption capacity of the studied soils, thus, appears to be caused mainly by organic matter.

For the range of volcanic soils investigated, water retention analysis showed that near-saturation and field capacity water contents are more representative of the whole water retention behavior than the most commonly used “wilting point” water content. The Integral Retention Index,

IRI

, showed a strong relationship with the different environments; the index also showed a clear relationship with the andic properties (as estimated by Al

o

+1/2Fe

o

;

r

=0.84), the soils with the lowest Al

o

+1/2Fe

o

and also a high content in pumice departing from this linear relationship. This is especially relevant because

IRI

, has a very important physical meaning with respect to ecosystem fertility.

The fraction of the pore space occupied by macroporosity, inferred from the parameter

φ

1

, was relevant for half the soils investigated; however, it was not possible to detect any correlation with other environmental variables.

The

IRI

index, related to static hydrological properties, was approximately normally distributed. The distribution of the

φ

1

parameters, related to dynamic hydrological properties, was far from normality as it was better approximated by others distributions (i.e. Cauchy) than the log-normal.

Image analysis showed the great complexity of the pore system of these volcanic soils which generally present multimodal pore size distributions.

Comparison between the results from these two techniques suggests that hydrological pore analysis is more suitable to represent pore systems in the size range lower than 500 µm while a much more detailed description is provided by image analysis for pores larger than 500 µm. Specifically, the water retention approach is feasible in describing only simple pore size distribution while fails in describing more complex porous systems with micro ad macropores.

The overall results reported in the present chapter confirm the necessity of different approaches in order to thoroughly understand the soil pore system, especially when complex volcanic material is examined.

Most catastrophic mudslides in Campania are related to soils developing upon pyroclastic materials covering carbonatic relieves of the Campanian Apennines. The soil observed in the detachment crowns of the investigated fast mudflow consisted of fertile Andosols (generally Hapludands and Udivitrands) that were very sensitive to anthropic or natural changes. This is due to the unique chemical, physical, morphological and hydrological behaviour of these soils, including (i) high water retention near saturation, (ii) high hydraulic conductivity, (iii) high smeariness of most horizons, and (iv) high vertical and lateral variability of the properties of the soil horizons. Aspect is important in determining soil susceptibility to landslide triggering and landslide susceptibility is related to hydrological properties that induce higher water storage especially in northern aspect soils. It is clear that stability of these soils requires conservation, implemented through appropriate forest planning and management. Improved risk management policies are necessary to preserve soil continuity and to decrease the risk both of surface soil erosion and to human lives and infrastructure.

The Santorini Islands represent a fragile volcanic area with poorly developed soils due to dry climatic conditions. The soils are vitric in character and many are influenced by accelerated erosion. However, these soils have in the past supported thriving agriculture, including the production of famous vines. Currently agriculture is being replaced by other land uses due to rapid socio-economic changes.