EDAFOLOGIA, Vol. 11 (1), pp.67-102, 2004



Dpto de Edafología y Geología, Facultad de Biología, Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez s/n, 38204 La Laguna, Tenerife, Islas Canarias, Spain


In this paper we show the first results of soil organic carbon content and forms in andosols and andic soils under evergreen forest vegetation, as well as the potential for carbon supply through litter and its mineralization and incorporation to soil. This study has been carried out in the Garajonay National Park (La Gomera, Canary Islands), where the average annual rainfall ranges between 650 - 900 mm, the average potential evapotranspiration is comprised between 600 and 800 mm, and the average annual temperature is 14oC. All of this determines an udic soil moisture regime. The predominant climacic vegetation in the area is the ever green forest, made up of laurel (laurisilva) and heather (fayal-brezal) mature forests. Dominant soils in the park include Andosols (Melanudands, Fulvudands and Hapludands), together with Leptosols (Udorthents) and, in a lesser extension, Luvisols (Palehumults and Paleudalfs). Umbrisols (Dystrudepts) and Cambisols (Distrustepts) are confined to degraded areas. However, it is worth pointing out that all soils show a more or less marked andic character in their surface, due to further incoming of volcanic ash, from more recent eruptive episodes in the neighbour islands.
We focus this paper mainly on those soils having a marked andic character and especially on Andosols. The seven subgroups for Andosols have been grouped for our purposes into two main categories: allophanic andosols and non-allophanic (or organomineral) andosols, being the former assigned to Melanudands and the latter to Fulvudands or Hapludands (Soil Survey Staff, 1999). This division has been applied according to the "binary composition" coined by Mizota and Van Reeuwijk (1989), by which the Alp/Alo ratio indicates the occurrence of allophanic or non-allophanic properties, in such a way that Alp/Alo values lesser than 0.5 point a soil mineralogy dominated by allophanic minerals, otherwise occurring the active Al mainly as Al-humus complexes.
This study evidences the high organic C content in Garajonay soils (most values comprised between 150 and 200 tm.ha-1 up to a depth of 30-40 cm), although these values decrease in degraded areas (less than 150 tm.ha-1). Organic C accumulates to a higher degree and in stable forms (lower C/N ratio) under mature laurel and heather forests. The accumulation and stabilization of organic C relates basically to the ecosystem maturity and stability, rather than the vegetation type.
Despite the fact of the high levels of organic C all over the park, it accumulates in stable forms mainly in soils having andic properties. Among these soils, non-allophanic (organomineral) andosols tend to accumulate more organic C (with a prevalence of stable adsorbed forms), related to allophanic andosols (that have higher amounts of readily oxidizable organic C). The existence of metal/carbon ratios higher than 0.12 indicates the occurrence of active Fe and Al forms, capable to form complexes with (and therefore to sequester) organic C.
The biomass respiratory activity and dehydrogenase activity are higher in allophanic andosols under laurel forests, reaching the maximum values when soil moisture is also the highest.
All these results seem to point out that allophanic andosols are the soils with the highest mineralizing activity, as well as the highest levels of readily oxidizable organic C and the lowest metal:carbon ratios, whereby organomineral andosols under mature ericaceae communities have a greater capability to sequester organic C in stable forms.