Effects of afforestation on soil organic matter characteristics under subtropical forests with low elevation

We selected sites of natural broad-leaved forests and adjacent coniferous plantations (Cunninghamia konishii and Calocedrus formosana of 30 and 80 years old, respec- tively) in central Taiwan to evaluate the effects of planta- tions on soil organic matter (SOM) characteristics and composition. SOM was characterized by chemical analy- ses, solid-state 13C cross-polarization magic-angle nuclear magnetic resonance (CPMAS 13C NMR), and optical measurements. Semiquantitative analysis of CPMAS 13C NMR spectra showed the litter of broad-leaved forests to be less resistant to decomposition than that of coniferous for- ests. The humification degree of SOM was higher under broad-leaved than coniferous forests because of the rela- tively high percentage of aromatic C and carboxyl C in the humic acids (HAs) of A horizons under broad-leaved for- ests. Additionally, the E4/E6 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.

本研究选取台湾中部的天然阔叶林及其邻近针叶人工林（分别为树龄30年的台湾杉木（Cunninghamia konishii）与80年的台湾翠柏（Calocedrus formosana）），以评估人工林对土壤有机质（SOM）特征与组成的影响。本研究通过化学分析、固态13C交叉极化魔角旋转核磁共振（CPMAS 13C NMR）及光学测定，对土壤有机质的特征与组成进行表征。对CPMAS 13C NMR光谱的半定量分析结果显示，阔叶林的凋落物相较于针叶林更易分解。阔叶林地表层土壤（A层）的腐殖酸（HAs）中芳香碳与羧基碳占比较高，因此其土壤有机质的腐殖化程度高于针叶林。此外，阔叶林A层腐殖酸的E4/E6比值低于针叶林，这表明其土壤有机质的缩合程度更高。针叶林土壤中较高的烷基碳含量，可归因于针叶凋落物中蜡质与脂类物质占比较高的特性。针叶树人工造林会促使凋落物层积累，逐步提升表层土壤有机质浓度，并改变表层土壤有机质的组成结构。