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We selected sites of natural broad-leaved forests and adjacent coniferous plantations (Cunninghamia konishii and Calocedrus formosana of 30 and 80 years old, respectively) in central Taiwan to evaluate the effects of plantations on soil organic matter (SOM) characteristics and composition. SOM was characterized by chemical analyses, solid-state 13 C cross-polarization magic-angle nuclear magnetic resonance (CPMAS 13 C NMR), and optical measurements. Semiquantitative analysis of CPMAS 13 C NMR spectra showed the litter of broad-leaved forests to be less resistant to decomposition than that of coniferous forests. The humification degree of SOM was higher under broadleaved than coniferous forests because of the relatively high percentage of aromatic C and carboxyl C in the humic acids (HAs) of A horizons under broad-leaved forests. Additionally, the E 4 /E 6 ratio of HAs was lower in the A horizon under broad-leaved than coniferous forests, which reflected more condensation of SOM. High alkyl C content under coniferous forests could be attributed to needle litter quality, which has a high content of waxes or lipids. Afforestation with conifers induced accumulation of the litterfall layer, gradually increased SOM concentrations, and changed the composition structures of SOM in the topsoils.