氮沉降对华北落叶松叶特性和林下土壤特性的短期影响

以50 a生华北落叶松天然次生林为研究对象，采用全生长季野外多水平林地施氮对比试验的方法，结合树叶特性和林下土壤理化性质测定，设置对照（CK，0 g·m-2·a-1）、低氮（LN，8 g·m-2·a-1）、高氮（HN，15 g·m-2·a-1）3个施氮水平，进行短期氮沉降模拟试验，分析了短期氮沉降对华北落叶松针叶及林下0~10、10~20 cm两层土壤中土壤营养动态的影响，为华北落叶松林的群落结构优化调整、可持续经营管理提供理论依据。研究结果表明：为期一个生长季的短期施氮显著增加了华北落叶松林下0~20 cm土壤层中TOC、NH4＋-N、NO3–-N的含量，却未影响土壤pH值和TN含量，且对表层（0~10 cm）土壤的影响更明显，但整个生长季内月份间的变化较大。短期施氮也导致50 a生华北落叶松针叶的宽度（W）、长度（L）、氮含量（LN）、比叶面积（SLA）和投影面积（PA）显著增加，但未影响叶生物量（LM）和叶有机碳含量（LC）。同时，叶氮含量与0~20 cm层、10~20 cm层土壤中全氮含量以及10~20 cm层土壤中NH4＋-N含量显著正相关，叶有机碳含量则与0~20 cm层土壤有机碳含量和0~10 cm层土壤全氮含量亦呈正相关关系。以上结果说明，华北落叶松天然次生林处于氮限制状态，短期氮沉降可能会提高土壤肥力并促进树木的生长，有利于华北落叶松天然次生林生产力的提高。

Short Term Effect of Nitrogen Deposition on Needle ofLarixand Forest Soil

In a natural secondary forest ofLarix principis-rupprechtii （~50 years old）, 3 nitrogen addition levels （0 g·m-2·a-1, 8 g·m-2·a-1, 15 g·m-2·a-1） were setup and applied in one growing season to simulate the short term nitrogen deposition. Our objectives were to determine the effects of increased nitrogen deposition on soil nutrient dynamics in such stand and the needle responses of Larix principis-rupprechtii and their relationships by continuous monitoring of soil physicochemical properties and measuring leaf traits of current year needles. The results showed： the short-term nitrogen addition significantly increased NH4＋-N, NO3–-N, and Total Organic Carbon （TOC） contents in the 0~20 cm horizon and more pronounced in the 0~10 cm horizon, but not the soil pH and soil total nitrogen content （TN）, while significant monthly variations were present. Nitrogen addition also increased the needle width （W）, length （L）, nitrogen content （LN）, specific leaf area （SLA） and projected area （PA）, but had no effects on needle biomass （LM） and needle carbon content （LC）. Furthermore, LN was positively correlated with TN in the 0~20 cm and 10~20 cm horizons as well as NH4＋-N in the 10~20 cm horizon respectively, while LC was positively related to TOC in the 10~20 cm horizon and TN in the 0~10 cm horizon respectively. In conclusion, the results suggested that the natural secondaryLarixprincipis-rupprechtiiforest is currently N-limited, even the short-term nitrogen input may improve the soil fertility and be beneficial to the absorption and utilization of trees, thus may potentially promote the productivity of such forest.