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Cultivation of marshes (Ma) to arable like pasture (Pa) and sugarcane (Sa) usually causes soil organic carbon (SOC) pool depletion within a short time. However, there are some uncertainties about which molecular composition of soil organic matter (SOM) is sensitive to land use change (LUC). In the present work, molecular components of SOM were investigated and compared to better understand the impacts of LUC on the carbon cycle from Ma to Pa or Sa in Louisiana and Florida. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) analysis indicated that LUC greatly altered the molecular composition of SOM. More lignin, polysaccharide, and phonetic compounds were founded from Ma, and more nitrogen-containing compounds were identified from Sa. Lignin and phenolic compounds had unexpectedly the most decrease from native marsh-sugarcane/pasture transitions, showing the same trend as SOC. This meant that lignin and phenol were not as stable as expected when undergoing LUC. LUC significantly yield more molecular moieties and then resulted in higher complexities and diversities of molecular components in Pa or Sa than those in Ma. Principal component analysis implied higher contributions of old carbon to SOM in Ma, and fresh biomass input contributed more SOM in Sa. Our results implied that human activities such as LUC could not only alter carbon fluxes but also simultaneously change molecular mechanisms that drive the carbon cycle. • Land use change has greatly altered the molecular composition of SOM. • Reclamation of marshes to sugarcane increased the chemical complexity of SOM. • Lignin and phonetic compounds were depleted when marsh changed to sugarcane. [ABSTRACT FROM AUTHOR]