中亚热带森林转换对土壤可溶性有机质数量与光谱学特征的影响

选取中亚热带福建三明格氏栲天然林及其转换而成的木荷、锥栗及福建柏等3种人工林表层土壤(0—10 cm)可溶性有机质(DOM)为对象,对其数量和光谱学特征进行了研究,以探讨森林转换对土壤DOM的影响。结果表明,天然林转换成上述3种人工林后,0—5 cm土壤可溶性有机碳(DOC)浓度显著降低(P0.05),降低程度分别为66.1%,69.9%及29.4%,可溶性有机氮(DON)浓度也有所下降;除福建柏外,其余两种人工林5—10 cm土壤DOC及DON浓度均低于天然林。各林分0—5 cm土壤DOC及DON浓度均高于5—10 cm土层。两个土层中,天然林土壤DOM的芳香化及腐殖化程度均显著高于人工林(P0.05),但荧光效率值低于人工林;荧光光谱图显示,天然林土壤DOM在芳香性脂肪族及木质素类复杂结构荧光基团处的吸收大于人工林;各林分土壤DOM傅里叶红外光谱出现吸收谱带的位置相似,其中吸收强度最大的为形成氢键的—OH的伸缩振动,此外还有芳香性CC伸缩振动、有机羧酸盐COO-反对称伸缩振动、碳水化合物中烷氧基C—O的振动等,人工林土壤DOM中碳水化合物的比例增加是其结构简单的主要原因。土壤DOM中结构复杂、分子量大的组分不易向下迁移;天然林与人工林间土壤DOM数量及光谱学特征的差异主要与凋落物输入及营林措施的干扰有关;本研究所涉及的3种人工林中,福建柏更有利于土壤养分的累积。

Effects of forest conversion on quantities and spectroscopic characteristics of soil dissolved organic matter in subtropical China

Dissolved organic matter (DOM) is a crucial component of soil organic matter and is attracting more attention. It plays an important role in global C/N cycling, and mediates many soil processes. As a small but active organic matter pool, the structural and chemical characteristics of DOM have recently been considered to be sensitive indicators of soil fertility. In many areas of southern China in the 1960''s, many natural broad-leafed forests were transformed into productive plantations, in which soil fertility recovery is now of great concern in the context of global climate change. However, little is known about how these conversions have affected the quantity and quality of soil DOM.In contrast to conventional methods, spectroscopic methods can provide a lot of useful information on the structural characteristics of DOM. Therefore, in this study we investigated the quantity and quality of soil DOM in a natural forest of Castanopsis kawakamii (NF) and three adjacent plantations of Schima superba (SCS), Castanea henryi (CAH), and Fokienia hodginsii (FOH) in Sanming Fujian province in subtropical China. We evaluated dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations, special ultraviolet-visible absorption (SUVA), humification index by emission fluorescence spectrum (HIX_em), humification index by synchronous fluorescence spectrum (HIX_syn), fluorescence efficiency (F_eff), and Fourier transformed infrared (FTIR). In the 0-5 cm soil layer, soil DOC concentrations decreased significantly (P < 0.05) after NF was transformed into SCS, CAH, and FOH, the relative proportions were 66.1%, 69.9%, and 29.4% respectively. The concentration of soil DON also decreased;in the 5-10 cm layer, concentrations of soil DOC and DON in all plantation types except FOH were lower than those in NF. Concentrations of soil DOC and DON in the 0-5 cm layer were higher than those in the 5-10 cm in all cases. Aromaticity and humification indices (HIX_em, HIX_syn) of soil DOM in NF were significantly (P < 0.05) higher than those of plantations, and they all decreased with depth. F_eff of soil DOM increased after forest conversion. The fluorescence spectrum showed that the relative absorption intensities of aromatic-aliphatic and lignin-derived fluorophores in soil DOM in NF were stronger than that of plantations. FTIR spectra of soil DOM from all forests featured -OH, aromatic C C, -COO^-, C-O carbohydrates, but with varying relative intensities. Higher abundances of carbohydrate rendered soil DOM in plantations less complex. In comparison with plantations, soil DOM in natural forests was nutrient richer and contained more complex and highly condensed compounds. Therefore, it was better for the accumulation of organic matter because molecules with more complex structures were more likely to be absorbed onto the soil surface instead of being leached out of the ecosystem. The differences in soil DOM between natural forests and plantations might be attributed to changed litter input, forest management practices, and human activity, and FOH is the most suitable plantation type for soil fertility recovery.