氮添加对亚热带常绿阔叶林土壤溶液化学特性的影响

土壤溶液被称作"土壤的血液",是土壤中各种生物化学反应的中介物质,在外界环境发生变化时,土壤溶液化学成分能在其他土壤指标尚无变化之前对环境变化做出迅速响应。为了探索持续增加的大气氮（N）沉降对森林生态系统的影响,以华西雨屏区亚热带常绿阔叶林为对象,设置对照（CK,0 g m^-2 a^-1）,低N（LN,5 g m^-2 a^-1）,高N（HN,15 g m^-2 a^-1）三种N处理,通过人工施加硝酸铵（2017年9月起改施硝酸钠及氯化铵）的方法模拟N沉降增加情景,N处理42个月后,使用负压土壤溶液采样器定位收集A层（37-45 cm）及B层（52-60 cm）土壤溶液,并进行分析（每月1次,为期1 a）。结果表明：对照处理中A、B两层土壤溶液NO₃^-浓度达（3.94±0.77）mg/L、（4.27±1.13）mg/L,N添加显著提高两层土壤溶液NO₃^-浓度和B层NH₄⁺浓度;N添加显著降低土壤溶液pH,且显著增加Al³⁺浓度,Ca²⁺和Mg²⁺含量有增加趋势,但影响不显著;N处理使A层土壤溶液可溶性有机碳（DOC）浓度显著降低,对两层土壤溶液芳香化指数（AI）无显著影响;两层土壤溶液电导率（EC）及氧化还原电位（Eh）显著增加;此外,两层土壤溶液中许多化学成分均呈现极显著相关,特别是NO₃^-与EC相关系数达到了0.855。本研究中,对照处理极高的NO₃^-含量以及B层土壤溶液硝酸盐浓度高于A层,表明该亚热带常绿阔叶林N的有效性超过植物和微生物的总营养需求而发生淋溶,该生态系统已达到氮饱和状态,此外N添加会显著促进土壤酸化和铝离子活化,表层土壤溶液DOC的降低一定程度反映了凋落物分解受N添加的抑制作用,显著升高的氧化还原电位可能导致土壤中某些金属元素的迁移率降低。

Effects of nitrogen addition on soil solution chemistry in a subtropical evergreen broad-leaved forest

Soil solution is called "soil blood", which is the medium of various biochemical reactions in the soil. When the external environment changes, the chemical composition of the soil solution can quickly respond to the environmental changes before other soil indicators have changed. In order to explore the impact of increasing atmospheric nitrogen (N) deposition on forest ecosystems, we set up three N treatments including the control (CK, 0 g m^-2 a^-1), low N (LN, 5 g m^-2 a^-1), and high N (HN, 15 g m^-2 a^-1) in a subtropical evergreen broad-leaved forest in the raining area of west China, artificially applied ammonium nitrate to simulate the increase of N deposition (replaced with sodium nitrate and ammonium chloride since September 2017). After 42 months of N treatment, soil solution from two soil layers (layer A:37-45 cm, layer B:52-60 cm) was collected using negative pressure soil solution sampler and analyzed chemical composition once a month for a year. In CK, soil solution NO₃^- concentration reached (3.94±0.77) mg/L (layer A) and (4.27±1.13) mg/L (layer B). Nitrogen additions significantly increased soil solution NO₃^- and NH₄⁺ concentration in layer B. Nitrogen additions significantly reduced soil solution pH value and increased Al³⁺ concentration, and the concent of Ca²⁺ and Mg²⁺ increased with no significant in statistics. Nitrogen treatment significantly reduced the dissolved organic carbon (DOC) concentration in layer A, and had no significant effect on the aromatic index (AI) of soil solution in the two layers. The electrical conductivity (EC) and the redox potential (Eh) of soil solution increased significantly in two soil layers. In addition, many chemical components in the soil solution of two layers had extremely significant correlation, especially the correlation coefficient between NO₃^- and EC reached 0.855. In this study, the extremely high NO₃^- concentration in CK and higher NO₃^- concentration in layer B than in layer A indicated that the nitrogen availability of the subtropical evergreen broad-leaved forest exceeded the total nutrient requirements of plants and microorganisms, and the ecosystem had reached the state of nitrogen saturation. In addition, nitrogen addition significantly promoted soil acidification and aluminum activation. The decrease of DOC in surface soil solution to some extent reflected the inhibition of litter decomposition by nitrogen addition. The significant increase of redox potential may lead to the decrease of the mobility of some metal elements in the soil.