Mitigating acid sulfate soil development in sediment addition projects through application of sediment amendments

Sediment addition is an increasingly common strategy to promote coastal resilience where sediment is added to salt marsh surfaces to increase elevation and prevent marsh loss. Added sediments are typically dredged materials from marine environments that may become acidic when exposed to air because of their high reduced sulfide levels, known as potential acid sulfate soils (PASS). Low soil pH inhibits plant growth and can delay ecosystem recovery after sediment addition. We used a laboratory soil core experiment to evaluate how a range of amendments altered pH (mulch, crushed shells, pelletized lime, and recycled concrete), and a field study to examine the effects of recycled concrete. We found that concrete amended sediment (laboratory: 8.24 ± 0.32; field: 8.15 ± 0.55) increased pH relative to adding unamended sediment (laboratory: 7.33 ± 0.35; field: 7.21 ± 0.45). Environmental context is likely important for sediment additions and even PASS may not become acidic in flooded, low marsh environments. Under more oxidized conditions, particularly in higher-elevation marsh or when deeper layers of sediment are added, recycled concrete is a potential amendment to neutralize acidity. Sediment amendments may also affect other ecosystem responses, including water chemistry and greenhouse gas emissions. We found that concrete amendments lowered ferrous iron concentrations and decreased carbon dioxide emissions compared to adding unamended sediment. Additional testing of amendments under diverse environmental conditions and at field scales would further our understanding of the effectiveness and feasibility of amending soils during sediment additions to prevent acid sulfate soil development.