Forms of eroded soil organic carbon in andosols of the Canary Islands (Spain)

A first assessment of soil organic carbon (SOC) loss mechanisms due to water erosion in Canary Islands andosols has been carried out. The study has been performed in experimental erosion plots. The soils are classified as Fulvic Andosols or Ultic Fulvudands. The mean soil loss by interrill erosion is 9.6 t ha−1 year−1, with a runoff percentage of 13% and a rain erosivity index, according to USLEʹs R factor of 640 MJ mm ha−1 h−1. The andosols studied here show organic carbon (OC) contents of 120±1.5 g kg−1 in the first 5 cm of surface soil, equivalent to a surface content of 2.4×103 g m−2 OC. A 73.3±0.7% of this carbon occurs as allophane–Al–humus or Al–humus complexes (Cpy), 2.6±0.1% as Cps, 0.1% as dissolved soil organic carbon (DSOC) and the remainder in other nonextractable forms. The measured values of eroded OC in sediments range 116±18.1 g kg−1, implying losses of OC bonded to the solid phase of 114.4 g m−2 year−1 C, whereas the concentration of OC in runoff water was 12±13.6 mg l−1, involving flows of DSOC equal to 1.03 g m−2 year−1 C. Regardless of the aggregate size fraction eroded, the lowest enrichment ratios (ERs) are produced for the Cpy (ER=0.70–0.74), whereas the highest ERs have been found in the smallest-sized aggregate fractions (ER=0.74–1.22) for all forms of eroded OC. The chemical stabilization of pyrophosphate-extractable forms (Cpy) in the smallest aggregates seems to be the main mechanism of carbon sequestration in these andosols. Moreover, the predominance of nonpyrophosphate-extractable forms in the sediments leads to the assumption that the eroded SOC might be a source of atmospheric CO2, although further research is necessary about the susceptibility of these OC forms to mineralization in areas that are ecologically different from those in which they originated.