# _Vallisneria spiralis_ Promotes P and Fe Retention via Radial Oxygen Loss in Contaminated Sediments
## 3 Results and Discussion
### Pore Water Features in Bare and Vegetated Sediments  
The one-way analysis of variance revealed that pore water pH and ORP values and DIP, Fe\({}^{2+}\) and Mn\({}^{2+}\) concentrations measured in bare sediments (S) and sediments with transplanted _V. spiralis_ (S + V) were all significantly different (Figure 6 and Table 2). In particular, the presence of _V. spiralis_ resulted in lower pore water values of pH, DIP, and reduced Fe\({}^{2+}\) and Mn\({}^{2+}\) concentrations and in less negative ORP as compared to bare sediments. All these different parameters in the two treatments indicate a significant effect of the macrophyte roots in the organic-enriched sediments of the study area, likely due to a combination of radial oxygen loss, favoring aerobic processes and the oxidation of reduced chemical species, and assimilation of pore water solutes, reducing their concentration [42]. Lower pH in S + V treatment can be due to the release of exudates by the roots, stimulating heterotrophic microbial activity and CO\({}_{2}\) production [38; 62]. The halved pore water concentration of DIP in S + V as compared to S results in a halved concentration gradient with bottom water and much lower DIP theoretical diffusive efflux. This agrees with the results of dark DIP fluxes in S that exceed by a factor of 3 dark DIP fluxes in S + V (Figure 5). Lower nutrient and chemically reduced species in vegetated than in bare sediments are widely reported in the literature [52; 63; 64]. In the present study, the pore water to bottom water ratio of DIP concentration in S + V was nearly 10:1. According to the empirical model of [61], this ratio would suggest a dominance of DIP uptake from _V. spiralis_ roots, whereas from flux calculations, we speculated that nearly 50% of DIP uptake was from the leaves. However, in this study, DIP assimilation was estimated from measured oxygen fluxes using a conservative ROL value, which might lead, as already discussed, to underestimation. An increase in ROL would lead to an increase in net production and uptake, with the unaccounted P fraction being assimilated by sediment, adhering more closely to the model proposed by [61]. Another possible explanation could be the much lower DIP concentration in the proximity of root hair because, with Rhiz, an integrated water sample is collected, including sediment far from uptake zones. Figure 5: Light (gray bars) and dark (black bars) DIP fluxes measured in reconstructed microcosms containing bare sediment (S) (**a**) and bare sediment with transplanted individuals of _V. spiralis_ (S + V) (**b**). Incubations were carried out over 3 different days. Averages \(\pm\) standard errors (n = 6) are reported. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Parameter** & **Source of Variation** & **DF** & **SS** & **MS** & **F** & \(p\) \\ \hline \multirow{3}{*}{\(\mathrm{DP}\)} & Between groups & 1 & 2969.69 & 2968.69 & 637.736 & **<0.001** \\  & Residual & 8 & 37.253 & 4.657 & & \\  & Total & 9 & 3006.943 & & & \\ \hline \multirow{3}{*}{\(\mathrm{Fe}^{2+}\)} & Between groups & 1 & 5326.652 & 5326.652 & 113.655 & **<0.001** \\  & Residual & 8 & 374.936 & 46.867 & & \\ \cline{1-1}  & Total & 9 & 5701.587 & & & \\ \hline \multirow{3}{*}{\(\mathrm{Mn}^{2+}\)} & Between groups & 1 & 29.872 & 29.872 & 126.319 & **<0.001** \\  & Residual & 8 & 1.892 & 0.236 & & \\ \cline{1-1}  & Total & 9 & 31.764 & & & \\ \hline \end{tabular}
\end{table} 
Table 2: Results of the one-way analysis of variance on pore water features measured in the two treatments: bare sediments and sediments with _V. spiralis_. Significant values are printed in bold.