This project Flood Strubio was funded by the SNF .
Leader: Claire LE BAYON
PhD student: Clémence SALOME

Floodplains are known to be areas of extraordinary biodiversity with a mosaic of shifting habitats with high interdependency. Nowadays, these ecosystems are subject to conservation and protection. Regarding pedology, floodplains contain a wide pattern of all steps of soil evolution, i.e. from a new sediment deposition to stable soils after several hundred years. However, less attention has been paid in this context to pedogenesis, especially the very first steps of soil structuring in the youngest and the least developed soils. In the present project, we aim to understand the processes and the mechanisms of soil structuring, in particular the role of organic matter and soil fauna.

Then, two types of questions are raised, concerning at the same time basic mechanisms and external regulation:

• In soils, how is organic matter involved in soil structure? Internal mechanisms in relation to soil biota are needed to better understand physical and biochemical processes implied in organic matter decomposition, incorporation and its further role in soil stabilization processes.
• Which external constraints may regulate soil structuring? Fluvial dynamics and particularly punctual floods have to be taken into consideration. A detailed knowledge of the historical context and the use of flood simulations could be useful to understand the resilience of soils consecutive to erosion and/or supply of materials. In addition, vegetation (reflecting mesoclimate), humus forms and soil types appear to be relevant.

Within the framework of a functional approach, two main hypotheses could be stated:

• at the alpine level, pedogenesis processes are assumed to be slower than at the plain level. So, the pioneer stages would be clearly visible as intermediate steps near the headwaters at the opposite from the plains where the soil structural stability is more strongly established.
• the vegetation and the soil diversity as well as biological activities, are supposed to differ in relation to climatic constraints ; separated effects at the alpine level could be combined at the plain level and the same tendency could be observed at a lateral distance from the river.

To assess these assumptions, we propose to study physico-chemical and biological variables:

• along the river, from the headwaters to the plains liable to flooding.
• perpendicular to the river, from the river itself to far away from it, until reaching the climax stage of the vegetation.

We choose to combine manipulated laboratory and field studies, the first ones giving information about key processes, the second ones allowing the understanding of the whole ecosystem and its regulation factors. 
 

Key-Words: Soil structuring, soil biota, organic matter, european floodplains, field and laboratory experiments.