Diversity and Complexity of Soil Organic Carbon in Typical Wetlands of Louisiana, USA

Coastal wetlands store large amounts of soil organic carbon (SOC), and have assumed key roles in mitigating increasing CO₂ in the atmosphere. The ongoing debate about SOC stabilization mechanisms stems partly from our incomplete understanding of its complex chemical architecture at the molecular scale. Deciphering the molecular composition of soil organic matter is crucial for revealing mechanisms that govern SOC persistence. This study utilized the field sampling data from 2016 and aimed to characterize molecular composition of SOC in typical salt marsh (SM) and freshwater marsh (FM) in Louisiana coastal regions, USA by extending the application of graph networks with pyrolysis-gas chromatography-mass spectrometry, and then to quantify potential links between SOC persistence and molecular diversity and network complexity. The results revealed that SOC predominantly consisted of alkyl compounds (Alkyl), phenol (Ph), lignin (Lg), and aliphatic compounds, constituting 23.21% and 27.85%, 17.84% and 21.55%, 16.94% and 15.49%, 17.20% and 15.93% of total ion chromatogram (TIC) in SM and FM wetlands, respectively. Molecular diversity in SM was higher than that in FM, while the network graph exhibited greater complexity in FM, featuring 167 and 123 nodes, and 1935 and 1982 edges in the network graphs of SOC from SM and FM, respectively. Correlation analysis confirmed positive relations between molecular diversity indices, network complexity, and abundance of stable carbon isotopes (δ¹³C). The variance partitioning analysis (VPA) supplied that soil nutrients exerted the most significant control on SOC persistence. Molecular diversity and network complexity, when combined with soil nutrients, could explain 34% of the variances in SOC persistence.