氮沉降对杉木人工林土壤可溶性有机质数量和结构的影响

氮沉降作为现在乃至未来气候变化的趋势之一，其可能深刻影响土壤可溶性有机质的数量和结构。选取我国中亚热带杉木人工林不同深度土壤（0-10 cm和10-20 cm）进行氮沉降试验，利用光谱技术研究氮沉降对土壤可溶性有机质数量和结构的影响。试验设对照（CT，0 kg hm^-2 a^-1）、高氮（HN：80 kg hm^-2 a^-1）、低氮（LN：40 kg hm^-2 a^-1）3种处理。结果表明：（1）在0-10 cm和10-20 cm土层，HN、LN处理的土壤可溶性有机碳和可溶性有机氮含量显著高于CT。（2）在0-10 cm和10-20 cm土层，1月时HN、LN处理的芳香性指数和腐殖化程度都显著高于CT，而4月时HN、LN处理的芳香性指数和腐殖化程度都显著低于CT。除了氮含量的直接影响外，RDA （冗余分析）表明，两土层中土壤含水量、pH和土壤有机碳是驱动氮沉降对土壤DOM数量和结构的重要环境因子。因此，氮沉降对土壤DOM的影响是复杂的，未来尤其应该注重氮沉降对生态系统影响的季节模式。

Effects of nitrogen deposition on the quantity and structure of soil dissolved organic matter of Cunninghamia lanceolata plantations with different depths

Nitrogen deposition, in relation with cliamte change, is anticipated to increase in the following decades. It is a potential factor that might profoundly affect ecosystem functioning. Enhanced nitrogen deposition might result in stoichiometric imbalance or other changed soil environment, and thereby affect plant growth and soil processes. Soil DOM (dissolved organic matter) plays an important role in ecosystem biogeochemical cycles. It is central to the questions of carbon sequestration in soils and nutrient availability to microorganisms. For example, in the early stage of plant litter decomposition, non-structural compounds are lost to soils mainly by the vehicle of DOM, which in return forms mineral-stabilized soil organic matter. Soil DOM is thus often considered to be a senstitive indicator of the interaction among plant, soil and microorganisms. Although many experiments on nitrogen deposition have been reported, inconsistency still remains about how soil DOM will respond to nitrogen deposition, especially in subtropical China.. Natural evergreen broad-leaved trees make up the typical vegetation in the subtropical zone of China, but large areas of natural forests have been transformed to Cunninghamia lanceolata plantations. Therefore, it is critical that we should explore the effects of nitrogen deposition on the dynamic changes of soil DOM in these plantations. In this study, we conducted a mesocosm field experiment in a Cunninghamia lanceolata plantations that stimulated nitrogen deposition. DOM from soils of 0-10 cm and 10-20 cm depth were examined by spectroscopic characterization, to explore the effect of nitrogen deposition on the quantity and structure of soil DOM. Three treatments (three replicates per treatment) were assigned to 9 plots: CT (control), HN (80 kg hm^-2 a^-1) and LN (40 kg hm^-2 a^-1). The results showed that (1) compared with CT, nitrogen deposition significantly increased the quantity of soil DOC (dissolved organic carbon) and soil DON (dissolved organic nitrogen) for both 0-10 cm and 10-20 cm soil layers. (2) The aromaticity index and humification index of DOM from both 0-10 cm and 10-20 cm soil layers were significantly enhanced by nitrogen deposition treaments in January, whereas an opposite pattern were observed in April. Besides the direct impact of nitrogen content, RDA (redundancy analysis) revealed that soil moisture, pH and soil organic carbon content were also main drivers regulating soil DOM. Our study suggests that nitrogen deposition might regulate soil DOM in either a direct way or an indirect way. More attention should be paid on the seasonal pattern of the influence of nitrogen deposition on ecosytem functioning.