Effects of experimental warming and CO2 concentration doubling on 13C CPMAS NMR spectra of humin in coniferous forest ecosystems of the eastern Tibetan Plateau in China
Despite the perceived importance of humin to forest ecosystem C sequestration, few studies have attempted to examine the responses to elevated temperature (ET, +2.5ºC) and elevated atmospheric CO2 concentration (EC, 700 ppm) on the chemical structures of humin. Solid-state 13C cross-polarization magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) spectroscopy was applied to evaluate the 6-year effect of ET and EC on the chemical structure of humin under the coniferous forest ecosystems in the eastern Tibetan Plateau of China. Results showed that ET treatment decreased soil organic carbon (SOC), whereas EC and ETC treatment increased SOC. ET treatment decreased aromatic C and carbonyl C of humin while increasing alkyl C, the ratios of alkyl C/O-alkyl C, aliphatic C/aromatic C, and hydrophobic C/hydrophilic C. Compared with ET treatment, the ETC treatment had a similar but reduced impact on the chemical structure of humin, while EC had slightly and undetectable impact on the chemical structure of humin. The 6-year exposure to ET decreased SOC and changed the molecular structure of soil humin to be more alkyline and hydrophobic, and it was a protection mechanism to the sequestration and stability of organic C in soil.
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