Saltmarshes are vital coastal blue carbon ecosystems, known for storing substantial organic carbon in their sediments. However, this carbon does not originate from a single source: some is produced locally by saltmarsh plants, while the rest is carried in from terrestrial or marine systems via rivers and tides. Distinguishing between these sources is crucial for credible blue carbon accounting and for avoiding potential double counting in carbon credit programs.

A new study proposes an integrated biomarker-isotope method for credible saltmarsh blue carbon accounting. It reveals that plant-derived organic carbon dominates saltmarsh sedimentary organic carbon, accounting for more than 75% of the total in two Chinese saltmarsh systems.

The study, led by scientists at the South China Botanical Garden (SCBG) of the Chinese Academy of Sciences (CAS), was published in CATENA on June 26.

In the study, researchers investigated sedimentary organic carbon sources in two representative Chinese saltmarsh systems—the Yellow River Delta and the Yancheng saltmarshes—across dominant vegetation communities, including Spartina alterniflora, Suaeda salsa, and Phragmites australis. They developed an integrated framework that combines molecular biomarkers with stable isotope validation.

The framework uses n-alkanes to identify plant-derived carbon sources, amino sugars to quantify microbial necromass carbon, and δ¹³C-N/C isotope models to validate source partitioning. With this approach, the researchers were able to quantify five major organic carbon sources: autochthonous halophyte carbon, allochthonous terrestrial xylophyte carbon, allochthonous marine algae carbon, fungal necromass carbon, and bacterial necromass carbon.

The results show that plant-derived organic carbon dominates saltmarsh sedimentary organic carbon, contributing more than 75% of the total. Local halophyte-derived carbon was the largest individual component, accounting for 36.9% to 58.2%.

By contrast, microbial-derived carbon generally accounted for no more than 20%, pointing to a carbon accumulation pathway that differs markedly from that seen in many terrestrial ecosystems. Soil texture and nutrient conditions emerged as key regulators of plant- and microbial-derived carbon, but they operate through distinct mechanisms.

The protocol for quantifying organic carbon sources in saltmarsh sediments. (Image by WANG Faming)

Stable isotope validation backed the reliability of the biomarker-based results. The Yellow River Delta saltmarshes were dominated by autochthonous carbon inputs, whereas the Yancheng saltmarshes showed stronger allochthonous contributions. These regional differences suggest that blue carbon accounting should be tailored to local hydrodynamic conditions, sediment sources, and vegetation types rather than relying on a single one-size-fits-all set of parameters.

According to the researchers, the study provides a methodological foundation for improving source-resolved blue carbon accounting in saltmarshes. In systems where local plant inputs prevail, vegetation conservation and restoration may strengthen carbon sequestration potential. In systems with greater external inputs, management may need to strike a balance between local production and sediment stabilization and carbon retention.

While this framework provides a more traceable way for linking saltmarsh conservation with climate mitigation, broader application will require local endmember calibration and uncertainty assessment, the researchers said.

ZHOU Jinge, a doctoral student at the SCBG, is the first author of the paper. Professor WANG Faming from the SCBG is the corresponding author.

This work was supported by the National Natural Science Foundation of China, the National Key R&D Program of China, the CAS Project for Young Scientists in Basic Research, the Guangdong Basic and Applied Basic Research Foundation, the CAS Youth Innovation Promotion Association, the National Forestry and Grassland Administration Youth Talent Support Program, and ANSO.