Molecular Fingerprints of Soil Organic Carbon in Wetlands Covered by Native and Non-native Plants in the Yellow River Delta

This study compared soil organic carbon (SOC) in wetlands dominated by native and one invasive plant specie to better understand how short-termSpartina alternifloracolonization affected carbon circulation in the Yellow River Delta (YRD). Freshwater marsh dominated byPhragmites australis(FM) had the highest SOC, total nitrogen (TN), and water-extractable organic carbon (WEOC) contents, whereas SOC contents varied only slightly among salt marshes covered bySuaeda salsa(SM2),S. alterniflora(SM3) and bare flat (MD). Invasion byS. alterniflorasubstantially changed the molecular characteristics of SOC. The spectral characteristics of Fourier-transform infrared spectroscopy (FTIR) of humic acids (HAs) were similar from 4000 to 1800 cm(-1)but differed greatly from 1800 to 1000 cm(-1)among four wetlands. Pyrolysis-gas chromatography/mass spectrometry technology (Py-GC/MS) was used to characterize molecular fingerprints of HAs. Aliphatics (27.38%), lignin (16.64%), nitrogen-containing compounds (Nc) (16.16%), polysaccharides (16.93%), and phenol (13.42%) were dominant inFM, and aliphatics, alkyl, and Nc were primary in HAs fromMD,SM2, andSM3.Lignin moieties were only found in HAs fromFMandSM3, which accounted for about 16.64% and 1.6% of the total ion current, respectively. The absorption bands of the FTIR spectrum around 3340 and 1650 cm(-1)inFMsamples were much larger than those in the other three wetlands. However, the ratio of the peak areas at 1620 and 2930 cm(-1)(R-1620/2930) inSM3, was lower than that inMDandSM2, and it meant carbon inSM3was less stable. Proportions of lignin and phenol moieties to total ion counts (TIC) inMDandSM2were 3.3% and 3.4%, while these proportions in FM and SM were 30% and 7.5%, respectively. It showedS. alterniflorainvasion into salt marsh would increase SOC contents and its stability, while it will inverse if freshwater marsh was supplanted byS. alterniflorain the YRD.