Characterization of soil organic matter in different particle-size fractions in humid subalpine soils by CP/MAS 13C NMR

Abstract

The particle-size separation of subalpine soils was used for delineating the different functional C pools of soil organic matter (SOM) by solid-state cross-polarization/magic-angle-spinning (CP/MAS) 13C NMR spectroscopy. The soils collected from hemlock forest and dwarfed bamboo grassland along the slope positions at elevations from 2550 to 3200 m in central Taiwan, received high precipitation (>3000 mm). The obtained data show that organic matter fractions differ according to particle-size distribution and vegetations. Being more aliphatic, clay-size fractions were significantly different from bulk soils. Sand-size fractions generally gave very similar results to bulk soils. Alkyl-C (mainly polymethylene) increased from coarse sand (>250 μm) to clay (<2 μm) particles size in all sites, suggesting an accumulation of recalcitrant material in fine particle-size fraction. O-Alkyl-C (mainly carbohydrate) content decreased from coarse to fine particle size under the hemlock forest, whereas no consistent trend was found in dwarfed bamboo grassland soils. High amounts (>35%) of O-alkyl-C in all particle-size fractions in dwarfed bamboo grassland soils were attributed to that of the linkage of phenolic acids to cell-wall polysaccharides in bamboo litter. Based on a humification index of alkyl/ O-alkyl-C, the humification degrees in particle-size fractions are in the order: clay>fine silt>coarse silt>fine sand>coarse sand in both of hemlock forest and dwarfed bamboo grassland soil. The alkyl/ O-alkyl-C ratios in hemlock forest soil were consistently higher than those in dwarfed bamboo grassland soils, suggesting that humification of SOM is higher in the forest soil than in the dwarfed bamboo grassland soils. The 13C NMR analysis and the change of C/N ratio of the particle-size fractions indicate that the accumulation of recalcitrant material in fine particle size favors alkyl-C rather than aromatic-C in these subalpine soils. The low content of aromatic-C in soil might be due to the high precipitation in this subalpine area.