Characterization of Soil Organic Matter in Different Particle-Size Fractions by CP/MAS 13C NMR along a Slope in Subtropical Montane Forest System.

Mountain regions are known to be especially gradient changes of alpine forest; however, information on the particle size fraction of soil organic matter is still scarce to date. This study examined the characteristics and functional groups of the soil organic matter (SOM) by cross-polarization and magic angle spinning ¹³C-nuclear magnetic resonance spectroscopy (CP/MAS ¹³C NMR) at different slope positions in a subtropical, perhumid montane ecosystem. The sampling site, Yuanyang lake, is featured with high annual rainfall (>4000 mm) and elevation from 1670 to 2169 m. The collected soils were fractionated into different particle sizes in order to obtain more biological significant insides into SOM. The results of ¹³C NMR suggest that extent of decomposition was higher at summit (SM) than that at lakeshore (LS) or footslope (FS). Based on a humification index (HI) of (alkyl)/ ((N-alkyl-C)+(O-alkyl-C)+(acetal-C)), the degree of humification increased as the particle size decreased. In addition, results from humification index supported that degree of decomposition was higher at SM. Combining the physical fractionation with chemical analysis tools could provide valuable information in SOM characterization.