Patterns and determinants of plant-derived lignin phenols in coastal wetlands: implications for organic C accumulation

1. As a major plant-derived soil organic carbon (SOC) component, lignin phenols are unique biomarkers that reflect biogeochemical characteristics under different vegetation compositions and climatic zones in coastal wetlands. However, the latitudinal patterns of plant-derived lignin phenols to SOC and their link with the stability and controlling mechanisms remain poorly understood.

2. A total of 156 soil samples from 39 sites along a 5000 km coastal transect, were taken to explore the effects of biological and environmental controls on the patterns of lignin phenols. Lignin phenols had contents ranging from 1.91 to 83.3 mg g−1 OC, and a positive correlation was detected in grass-dominated salt marsh, but a weakly negative correlation in mangrove. Positive correlations between SOC or lignin content and C/V or S/V (the cinnamyl- or syringyl-to-vanillyl) ratios were found, while overall negative correlations between SOC or lignin content and (Ad/Al)V or (Ad/Al)S (the acid-to-aldehyde of vanillyl or syringyl units) ratios were detected, respectively, which confirmed the validity of these lignin biomarker degradation parameters.

3. Our findings revealed that plant C inputs and monomer ratios directly influenced the capacity of lignin phenols in soils. Lignin content and stabilization was mainly controlled by soil properties (i.e., pH, EC, sand/clay). Mean annual temperature (MAT) influenced the patterns of lignin phenols both directly by increasing decomposition and indirectly by changing the vegetation and soil biogeochemistry (i.e., microbial substrate availability).

4. Coastal wetlands are characterized by high primary productivity and C burial rate, yet plant-derived lignin phenols are not as much as we thought compared to microbial residues C. Precise identification and quantification of the origin, decomposition, and determinants of lignin phenols help us understand their contribution to C sequestration and its response to climate and environmental changes.