Phytoremediation by rooted macrophyte Vallisneria spiralis, not biochar amendment, reduces eutrophication from nutrient release in organic-rich sediment of a polluted wetland

Wetlands sediments are often contaminated by macro and micropollutants. Combined traditional and biochar assisted phytoremediation techniques allow contrasting of both pollutants. However, the interacting effects of different techniques are seldom analysed, assuming that they are null or positive. We investigated phytoremediation by the macrophyte Vallisneria spiralis and biochar addition in polluted sediments. The aim was to assess if biochar affects the biogeochemical services provided by the macrophyte via radial oxygen loss and nutrient uptake. Sedimentary fluxes and properties, and microbial activity and diversity were analysed in four conditions: control sediments, sediments with V. spiralis, sediments with biochar and with both V. spiralis and biochar. Sediments with V. spiralis had higher redox and Fe³⁺ solid pools and had lower pore water concentrations of NH₄⁺ and PO₄^3- than control sediments. Sediment with macrophytes had lower denitrification, higher nitrification and similar nitrate ammonification potential than bare sediment. The screening of microbial diversity suggests distinct communities in surface and subsurface sediments, regardless of the presence of macrophytes or biochar. The presence of V. spiralis roots stimulated slow growing groups of microbes as nitrifiers, iron and sulphide oxidizers or promoted the switch to aerobic metabolism without affecting microbial diversity and abundance. Biochar addition, likely due to low reactivity and 30 days incubation time, did not affect pore water nutrients, microbial activity and diversity, suggesting limited remediation in the organic-polluted sediments considered. Biochar also did not affect the biogeochemical ecosystem services provided by V. spiralis, suggesting the absence of negative interactions between the two treatments.