The influence of bioturbation on the vertical distribution of soil organic matter in volcanic ash soils: a case study in northern Ecuador

Soil faunal bioturbation ('bioturbation') is often cited as a major process influencing the vertical distribution of soil organic matter (SOM). The influence of bioturbation on vertical SOM transport is complex because it is the result of interaction between different groups of soil faunal species that redistribute SOM through the soil profile in distinct ways. We performed a semi-quantitative micromorphological analysis of soil faunal pedofeatures and related their occurrence to the vertical distribution of SOM and high-resolution radiocarbon dating in volcanic ash soils under montane forest and grassland (paramo) vegetation in the northern Ecuadorian Andes. The paramo soil data suggest that bioturbation was largely responsible for the vertical distribution of SOM, while illuviation and root input were of minor importance. Bioturbation was caused by endogeic species, which typically mix the soil only over short vertical distances. Short vertical distance mixing was apparently enhanced by the upward shifting of bioturbation as a result of soil thickening due to SOM accumulation. A change from paramo to forest vegetation was accompanied by a change from endogeic to epigeic species. As these latter species do not redistribute material vertically, this eventually resulted in the formation of thick ectorganic horizons in the forest.