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

Selective extraction methods for aluminium, iron and organic carbon from montane volcanic ash soils

Montane volcanic ash soils contain disproportionate amounts of soil organic carbon and thereby play an often underestimated role in the global carbon cycle. Given the central role of Al and Fe in stabilizing organic matter in volcanic ash soils, we assessed various extraction methods of Al, Fe, and C fractions from montane volcanic ash soils in northern Ecuador, aiming at elucidating the role of Al and Fe in stabilizing soil organic matter (SOM). We found extractions with cold sodium hydroxide, ammonium oxalate/oxalic acid, sodium pyrophosphate, and sodium tetraborate to be particularly useful. Combination of these methods yielded information about the role of the mineral phase in stabilizing organic matter and the differences in type and degree of complexation of organic matter with Al and Fe in the various horizons and soil profiles. Sodium tetraborate extraction proved the only soft extraction method that yielded simultaneous information about the Al, Fe, and C fractions extracted. It also appeared to differentiate between SOM fractions of different stability. The fractions of copper chloride- and potassium chloride-extractable Al were useful in assessing the total reactive and toxic Al fractions, respectively. The classical subdivision of organic matter into humic acids, fulvic acids, and humin added little useful information. The use of fulvic acids as a proxy for mobile organic matter as done in several model-based approaches seems invalid in the soils studied

Tephra stratification of volcanic ash soils in Nothern Ecuador

We combined proxies traditionally used in stratigraphic research (mineral assemblages, grain size distribution, and element ratios) with soil organic carbon contents and radiocarbon dating both at a high vertical resolution, to unravel the tephra stratigraphy in volcanic ash soils. Our results show that soil profiles along an altitudinal transect intersecting the upper forest line in Northern Ecuador were formed in three distinct tephra deposits. Although the deposits contained a similar assemblage of minerals, we were able to differentiate these deposits because of their characteristic organic carbon distribution, grain size distribution and typical SrO to Na2O, CaO and crystalline Al2O3 ratios. Unravelling the tephra stratigraphy improved understanding of the vertical distribution of soil organic carbon, including paleoecological proxies, in the studied soils. We demonstrated that bioturbation likely plays an important role in current pedogenesis, resulting in overprinting (merging, mixing) of the paleosol. Surprisingly, in spite of bioturbation, a linear age depth relationship exists, leading to the hypothesis that the active zone of bioturbation shifted upwards during soil formation. Therefore, we conclude that paleoecological proxies are stratified in our soils, albeit probably somewhat more crudely than in undisturbed peat bogs or lake sediments