Pioneer plant Phalaris arundinacea and earthworms promote initial soil structure formation despite strong alluvial dynamics in a semi-controlled field experiment

Soil structure formation is among the most important processes in riverfloodplains which are strongly influ-enced by alluvial dynamics. In the context of river restoration projects, a better understanding of soil structureformation in habitats adjacent to the river can help to prevent damages caused by riverbank erosion. Ecosystemengineers such as pioneer herbaceous plants and earthworms likely contribute to soil structure formation evendespite less favourable environmental conditions. This study aims to assess the capacity of the herbaceousperennial and native speciesPhalaris arundinaceaand earthworm communities to promote a stable soil structurein alluvial sediments, in particular fresh alluvial deposits, in the short term. Delimited plots were set-up in arestoredfloodplain adjacent to the Thur River in NE Switzerland and exposed to natural alluvial dynamics for19 months. Four treatments were replicated in a randomised complete block design: (i) plots withPhalaris ar-undinaceaas only vegetation, (ii) plots with all vegetation constantly removed, (iii) and (iv) the earthwormcommunity reduced by mustard treatment, otherwise as (i) and (ii), respectively. Soil structure formation wasanalysed at the end of the experiment using different indicators: aggregate stability,field-saturated hydraulicconductivity and the porosity calculated from X-ray CT reconstructions of freeze cores.Phalaris arundinaceawascapable of improving the porosity and aggregate stability of both alluvial sediments present at the beginning ofthe experiment but also of sediments freshly deposited during the observation period. The latter indicates astructuring effect within only one vegetation period. Earthworm abundance was as a whole very low, most likelydue to the large proportion of sand. There was a small earthworm effect on soil structure formation, and only incombination withPhalaris.arundinacea. Ourfindings highlight the ability ofPhalaris arundinaceain efficientlystructuring sandy alluvial sediments in the short term even under strong alluvial dynamics.Phalaris arundinaceacan therefore play a key role in the early stage of river restoration projects. Thus, facilitating the colonisation bysuch native pioneer herbaceous plants is a suitable step to improve the success of river restoration projects.