ORGANIC CARBON POOLS AND FORMS IN FOREST ANDOSOLS OF THE CANARY ISLANDS : SOIL ERODIBILITY AND CARBON LOSSES BY EROSION

The objective of this paper is: -to know the content and forms of soi1 organic carbon (SOC) in andosols and andic soils under evergreen forest vegetation (laurel and heather forest) -to know the role of organic matter in aggregate stability and resistance of andosols to water erosion, and -to know the losses of SOC that are produced by water erosion in Andosols after deforestation and the forms of organic carbon most affected by these losses. The study has been performed in a sampling area of some 40 km2 in the Garajonay National Park (La Gomera, Canary 1s.). Predominant andosols in this area (Melanudands, Fulvudands and Hapludands) have been grouped in two main categories: allophanic and non-allophanic (or organomineral) andosols. A total of 163 sampling sites were studied and soi1 samples were collected at each in the first 30-40 cms at 3 randomly distributed sampling points. In the soi1 samples sieved through 2mm-mesh, were determined: Walkley-Black soi1 organic carbon (SOC), pyrophosphate extractable SOC, Fe and Al, potassium sulphate extractable SOC, dissolved SOC and acid oxalate extractable Fe, Al and Si. In addition, the following analyses were performed: fulvic and humic acids, K-factor of USLE and aggregate stability by disaggregation and water-drop tests. The studied andosols present a high capacity to accumulate carbon (up to 14.5k5.9 kgCme2, 113 gCkg-‘), a high percentage of which (57.2 per cent) is found in pyrophosphate extractable forms (8.3+3kgCme2), that is, complexed with the active forms of Fe and Al. The dissolved organic carbon in these soils is around 112.41344. 7 gL-‘. 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 non-pyrophosphate extractable organic C). Raindrop impact seems to be the only important disaggregation mechanism in non allophanic andosols, although it is frequent in both types of soils and related to the content in pyrophosphate extractable organic carbon. The disaggregation due to aggregate humectation is absent from non allophanic andosols and is characteristic of allophanic andosols stabilized by allophane-humus complexes. In order to assess the rates of soi1 organic carbon loss by water erosion in forest allophanic andosols, a study was performed in experimental erosion plots. Soils occurring on them are classified as Ultic Melanudands (allophanic Andosols). The mean soi1 loss by interrill erosion in these soils is 9.6 t ha-’ yi’, with a runoff percentage of 13.0 per cent and a rain erosivity index, according to USLE’s R factor of 640 Mjmmha-‘h“. Soi1 samples have been collected from the top layer (O-S cms) on the bare soils. In the sample of soi1 sieved through 2-mm mesh, were determined the same parameters as above. These determinations have also have made in sediment samples. Important losses of organic carbon in these soils are caused by erosion (114.4 gC mm2yi’), which are fundamentally bonded to the solid phase since DOC losses in runoff water are negligible (1.03 gC m‘2yrM’). Although complexed forms of organic carbon predominate at the soi1 surface as allophane-Al-humus or Al.-humus complexes (73.3 per cent of total soi1 organic carbon), the predominating form in the sediments corresponds to non pyrophosphate extractable and non complexed forms of organic carbon (53.4 per cent).