Ecosystem engineers are closely linked to ecosystem services they may provide to fulfill human needs. Understanding when and where specific ecosystem services are produced has become recently of great interest in the environmental management of natural and restored ecosystems. In this context, and focusing on soils, research is still needed to better characterize the "soil natural capital" and the ecosystem services it provides. Several soils functions are closely linked to soil structuring processes and increasing attention has been paid to interactions between organic matter and mineral particles leading to structure formation and stabilization. Soil structure formation is driven by different processes: (i.1) the plasma stability, which depends on the plasma constituents, i.e. organic carbon content and mineral textures on short time scales, (i.2) the plasma fabric, which is largely determined by soil organisms, (i.3) the soil structuration processes (i.e. structural pores forming processes) depending on shrink-swell processes. However, the specific mechanisms and time scale of structure formation and stabilization have scarcely been described, especially in floodplains. Indeed, floodplains, including the restored ones, are relevant models for studying the early stages of soil structuring processes because these floodplains are characterized by i) the co-existence of different pedogenesis durations, i.e. from a new sediment deposition to stable soils after several hundred years due predominantly to fluvial dynamics, ii) the occurrence of different sediment composition (texture), iii) an huge variability in space and time of organic matter inputs (allochtonous and autochtonous). Moreover, no research has focused on the soil structure as an indicator of ecosystem engineers’ influence in young soils recovered in alluvial ecosystems.
Our main objectives are to understand how activities of ecosystem engineers, especially plants and earthworms, contribute to (re)create one of the main soil functions and ecosystem services, i.e. soil structural stability. Hence, we will study the building of burrows and macroaggregates through ecosystem engineers’ bioturbation activities in a context of a restored floodplain.
In this research project, several questions are raised: 1) Do soil engineers, i.e. plants and earthworms, contribute concomitantly and/or independently to burrows and aggregates building up? How discriminating plants and earthworm implication in soil aggregates and burrows network formation? 2) Are mineral textures and organic matter key factors in these processes? How long does it take to obtain stable aggregates that could improve significantly soil structure? The high-level hypothesis governing our research project is that, in a context of restored floodplains, ecosystem engineers initiate the first steps of soil structure formation and stabilization through their bioturbation activities. To reach our objectives and test our hypotheses, the research plan is divided into three sections built up at different scales, from field studies to microcosm’s experiments (15 cm diameter) then mesocosms (40 cm diameter) in the field. This triple approach will be helpful to have a holistic overview in order to better understand the structuring processes of a restored floodplain. At the field scale, we propose to identify and to describe the engineer communities and their habitat characteristics (soil, vegetation and water dynamics) in order to better understand their spatial distribution within a restored floodplain. At the microscosm scale, that constitutes the main part of the project, we will study fine processes involved in burrows and macroaggregates formation through ecosystem engineers’ bioturbation activities. Controlled conditions will allow testing the efficiency of each biological agent, independently and concomitantly (plants, earthworms separately, plants and earthworms mixed), as well as variables such as sediment texture (sand, silt), the superposition of different soil layers according to facies models (sand above silt, silt above silt; existence of a pre-existing soil layer), and time (5 and 10 weeks). The mesocosm scale will allow combining field work and laboratory experiments through an intermediate situation putting mesocosms directly in the field that will match semi-controlled variables, namely experimental designs in natural environmental conditions.