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Summary1. Organic pollution is widespread in coastal areas and can have profound impacts on the seabed. Coastal sediments play an important role at a global scale in the recycling of organic matter, and this process is influenced by the habitat complexity of the sediments, among other factors. Mollusc shells are produced as a waste product from a range of anthropogenic activities, but we demonstrate that they can be used to increase the habitat complexity of sediments.2. We studied the effect of mussel-shell debris (shell-hash) on the biogeochemical processes of marine sediments affected by organic pollution, using a mesocosm experiment simulating the bioturbation effects of macrofauna.3. We found that shell-hash improved the ecological status of organically-polluted sediments by reducing the accumulation of sulphide from anaerobic metabolic pathways.4. Additionally, when shell-hash was present in an organically-polluted sediment, there was a decrease in ammonium release to the water column, thus preventing the negative ecological consequences of eutrophication. Synthesis and applications.Our study indicates that shell-hash debris can be used as a potential tool to mitigate the effects of organic enrichment on marine sediments. A density of shell-hash debris of 1900 g m -2 in the sediment can diminish toxic byproducts (sulphides and ammonium) derived from the stimulation of anaerobic metabolic pathways by organic pollution, at levels that are biologically relevant. The Accepted ArticleThis article is protected by copyright. All rights reserved.mitigation effect of shell-hash is more pronounced in sediments where macrofauna is not present.

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Soil contamination is one of the main threats to ecosystem health and sustainability. Yet little is known about the extent to which soil contaminants differ between urban greenspaces and natural ecosystems. Here we show that urban greenspaces and adjacent natural areas (i.e., natural/semi-natural ecosystems) shared similar levels of multiple soil contaminants (metal(loid)s, pesticides, microplastics, and antibiotic resistance genes) across the globe. We reveal that human influence explained many forms of soil contamination worldwide. Socio-economic factors were integral to explaining the occurrence of soil contaminants worldwide. We further show that increased levels of multiple soil contaminants were linked with changes in microbial traits including genes associated with environmental stress resistance, nutrient cycling, and pathogenesis. Taken together, our work demonstrates that human-driven soil contamination in nearby natural areas mirrors that in urban greenspaces globally, and highlights that soil contaminants have the potential to cause dire consequences for ecosystem sustainability and human wellbeing.

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