Fjölrit RALA - 15.06.2004, Síða 19

Volcanic soils: an overview and new perspectives
Randy A. Dahlgren1, Masami Nanzyo2 and Masahiko Saigusa2
1 University of Califomia - Davis, USA; 2Tohoku University - Sendai, Japan
Volcanoes are revered and feared for their awesome and devastating emptions that obliterate
terrestrial ecosystems, and often cause tremendous casualties to humans and wildlife. Yet
from these ashes of devastation arise some of the most productive soils in the world with the
capacity to sustain high human population densities. Soils formed in volcanic ejecta have
many distinctive physical, chemical and mineralogical properties that are rarely found in soils
derived from other parent materials. These distinctive properties are largely attributable to the
formation of noncrystalline materials (e.g., allophane, imogolite, ferrihydrite) containing
variable charge surfaces, and the accumulation of organic matter.
The nature and properties of volcanic soils have been intensively studied, yet the unifying
principles conceming their genesis, mineralogy, biological properties and agronomic
utilization have not been fully estabhshed. The bulk of previous research has focused on
volcanic soils formed in the humid-temperate environment. In contrast, there is a paucity of
information regarding volcanic soils formed in tropical, arid and cold regions. There is a
strong need for a comprehensive global analysis of existing data to establish thresholds in soil
genesis (especially between competing soil groups) and mineralogical transformations.
Transitions between allophanic and nonallophanic Andosols are not fully understood, nor are
transitions between dominance by noncrystalline (e.g., allophone, imogolite, ferrihydrite)
versus crystalline (e.g., halloysite, 1:1- 2:1 mixed layer clays) mineralogical assemblages. A
key to establishing threshold conditions is to understand processes regulating aqueous
aluminum and silica activities and kinetic factors regulating aqueous-solid phase interactions.
There are numerous opportunities to apply our knowledge of volcanic soils to important
environmental issues. Unique properties of volcanic soils, such as high anion exchange
capacity, provide opportunity for attenuating nitrate leaching in agricultural systems and
potential for utilization in low-level radioactive waste disposal sites to retain radioactive
anions (e.g., iodine, technetium). Similarly, the abundance of noncrystalline materials and
organic matter provides a high capacity to retain heavy metals, trace elements (cations and
anions) and organic compounds making volcanic soils a good candidate for disposal of
biosolids. Volcanism plays an important role in the global carbon cycle, representing a
primary source and sink for carbon. Soils formed in volcanic materials contain the largest
accumulations of organic carbon among the mineral soil orders. Understanding the
mechanisms by which organic matter is preserved in these soils may contribute to
management techniques to sequester carbon as soil organic matter.
Given our understanding of the nature and properties of volcanic soils, there is an
opportunity to apply this knowledge to agronomic management practices that provide for
sustainable production of food, fiber and forage. Within the concept of sustainable
management, there is a lack of knowledge conceming the interaction of biological processes
with chemical and physical soil properties of Andosols. The differences in the agricultural
productivity among Andosols are largely attributed to the colloidal composition in the rooting
zone, namely allophanic versus nonallophanic. Nitrogen and phosphoms cycling, aluminum
toxicity, acidity amelioration, mycorrhizae interactions and protection from soil pathogens
require additional research in terms of their role in soil quality and sustainable management
practices. Phosphorus is often a growth-limiting nutrient for agricultural production in
volcanic soils. In young volcanic soils, apatite plays an important role in providing
phosphorus for revegetation and crop production without addition of phosphoms fertilizers.
Under low phosphoms availability, inoculation with mycorrhizae fungi greatly enhances
8