The release of arsenic is a hidden risk during the in-situ decomposition of landed sargassum litter

Low molecular organic compounds and nutrients are released during the natural breakdown of plant tissues. However, if the organisms accumulate potentially toxic elements (PTE), these can be released into the environment; this is a latent risk during the decomposition of sargassum litter. This topic has been studied only in ex-situ conditions. We quantified the PTE concentration in fresh and in situ naturally decomposed sargassum litter. The results indicated that only arsenic demonstrates a high concentration in fresh biomass and litter across different stages of decomposition, exceeding 65 % initially, but decreasing to less than 20 % in the later stage concerning total As in each decomposition phase. These findings offer valuable insights into the release of As in leachates during natural decay and the residual As in partially decomposed sargassum litter. • PTEs were detectable in sargassum decomposed tissues. • Only As and Cu were detectable in sargassum decomposed lixiviates. • The concentration of As exceeds limits for direct use as soil organic amendment. • Around 90 % of As was environmentally released along sargassum in-situ decomposition.