Salt Marsh Blueprints: Using Carbon Isotopes to Understand Soil Organic Matter Legacies

Abstract Salt marshes are important blue carbon sinks and dynamic, connective ecosystems that experience change across a wide range of temporal scales, from daily tidal inundation to decadal and century‐scale human modifications to sea level rise over millennia. However, whether short‐term (∼100‐year) ecosystem shifts can be detected in soil carbon records remains unclear. Here, we use carbon isotopes ( δ 13 C) to reconstruct the recent vegetation and land‐use changes in a well‐studied salt marsh in Connecticut, USA. Leveraging the distinct isotopic signatures of dominant species (C 3 Phragmites australis and C 4 Spartina alterniflora ), we analyzed bulk δ 13 C, n ‐alkane δ 13 C, and soil CO 2 , CH 4 δ 13 C to (a) reconstruct plant community shifts associated with historical farming, ditching, tidal restrictions, and subsequent restoration, (b) evaluate the sensitivity of bulk versus leaf wax δ 13 C to ecological change, and (c) compare soil carbon legacies with soil CO 2 and CH 4 emissions. Both bulk sediment and leaf wax δ 13 C records capture the imprint of historical ditching and draining practices but do not clearly record vegetation changes associated with tidal restrictions and restoration over the past 60 years. n ‐Alkane sediment δ 13 C preserves evidence of C 4 vegetation from 90 years ago that are not resolved in bulk sediment δ 13 C values. Soil CO 2 δ 13 C reflects modern vegetation inputs combined with isotopic enrichment associated with microbial processing. These results demonstrate that carbon isotopes can effectively reconstruct ecosystem history where C 3 and C 4 vegetation are distinct, and leaf wax δ 13 C provides a more sensitive archive of plant‐derived inputs to marsh soil organic matter than bulk δ 13 C.