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-term Spartina alterniflora colonization affected carbon circulation in the Yellow River Delta (YRD). Freshwater marsh dominated by Phragmites 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 by Suaeda salsa (SM2), S. alterniflora (SM3) and bare flat (MD). Invasion by S. alterniflora substantially 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 in FM, and aliphatics, alkyl, and Nc were primary in HAs from MD, SM2, and SM3. Lignin moieties were only found in HAs from FM and SM3, 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 in FM samples were much larger than those in the other three wetlands. However, the ratio of the peak areas at 1620 and 2930 cm-1 (R1620/2930) in SM3, was lower than that in MD and SM2, and it meant carbon in SM3 was less stable. Proportions of lignin and phenol moieties to total ion counts (TIC) in MD and SM2 were 3.3% and 3.4%, while these proportions in FM and SM were 30% and 7.5%, respectively. It showed S. alterniflora invasion into salt marsh would increase SOC contents and its stability, while it will inverse if freshwater marsh was supplanted by S. alterniflora in the YRD.