# _Vallisneria spiralis_ Promotes P and Fe Retention via Radial Oxygen Loss in Contaminated Sediments
## 1 Introduction
This plant can grow in organic, heavily contaminated substratum [33; 34; 35; 36] and has a demonstrated capacity to release variable O\({}_{2}\) amounts to sediments, depending on the season (i.e., larger amounts during summer, when heterotrophic microbial activity is higher) or on the sedimentary organic content (i.e., larger amounts along organic matter sedimentary gradients) [37; 38]. The effects produced by _V. spiralis_ on benthic biogeochemistry addressed mostly nitrogen removal via denitrification in fluvial sediments [33; 39; 40], whereas the effects of this macrophyte on phosphorus benthic cycling in organic contaminated sediments were comparatively poorly investigated [41]. Rooted macrophytes may promote phosphorus retention both directly through root uptake and indirectly through the reoxidation of redox-sensitive compounds, which can bind or co-precipitate phosphorus [42]. In this work, individuals of _V. spiralis_ were transplanted in reconstructed microcosms containing organic sediments collected from a contaminated wetland area in northern Italy. The main aim of the work was to understand the effects of _V. spiralis_ transplant on mitigation of phosphorus and phosphorus-associated metals (iron and manganese) recycling. Microcosms with bare sediments and with sediments transplanted with _V. spiralis_ were conditioned for 3 weeks in large aquaria to check whether the macrophyte was growing and to allow the development of steady chemical gradients and stable microbial communities in the two treatments [43]. Bare and vegetated sediments were incubated in the light and in the dark to measure net fluxes of O\({}_{2}\) and dissolved inorganic phosphorus (DIP) and then characterized for different pore water features. At the end of the experiment, the roots of _V. spiralis_ were analyzed for the amount of Fe, Mn, and P immobilized on their plaques and inside their biomass, in order to speculate about the direct (i.e., uptake) and indirect (i.e., via ROL) effects of the macrophyte on DIP retention. It was hypothesized that: (1) _V. spiralis_, through its plasticity, can adapt and survive in organic sediments likely due to increased release of O\({}_{2}\) to the rhizosphere, at the expense of the O\({}_{2}\) evolved to the water column; (2) the large amounts of O\({}_{2}\) diffusing to the sediments from the roots can oxidize and precipitate Fe\({}^{2+}\) and Mn\({}^{2+}\) dissolved in pore water and form plaques on the root surface that in turn co-precipitate DIP; (3) the amount of DIP precipitated on the roots surface exceeds the amount present in the roots biomass, suggesting that indirect DIP retention mechanisms promoted by the plant should be considered to be a biogeochemical service as direct DIP uptake. These issues are discussedwith two main perspectives: the potential use of macrophytes as nature-based solutions to contrast eutrophication and internal DIP recycling and the negative consequences of macrophye removal, a common practice in irrigation and drainage canals as well as in the littoral zone of many lakes, for sediment biogeochemistry and water quality.