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.