南亚热带森林两种优势树种幼苗的元素含量对模拟氮沉降增加的响应

Effects of simulated nitrogen deposition on elemental concentrations of Schima superba and Cryptocarya concinna seedlings in subtropical China

氮沉降是当前全球变化的一个重要方面,它对各类森林生态系统和植物的影响及其机制的研究是当前国际生态学界关注的问题之一。探讨了南亚带森林两种优势树种荷木(Schimasuperba)和黄果厚壳桂(Cryptocaryaconcinna)幼苗的养分状况对模拟氮沉降增加的响应。实验随机分为对照(Control)、T5、T10、T15和T305个样方,每个样方包括3个重复。所施氮肥为NH4NO3,每次溶解在10L自来水中,5个样方喷施的浓度分别为0、0.12、0.24、0.36和0.72mol·L-1。每月喷施2次,5个样方一年喷施的总氮量分别相当于氮沉降率0、5、10、15和30g·m-2·a-1。经过11个月的处理,土壤中速效氮水平明显提高,而pH值下降。两种幼苗叶、枝干和根的氮含量随处理水平的增加而升高,但P、K、Ca和Mg的含量与对照相比普遍下降。荷木幼苗的单株氮贮量除T5处理外,所有处理样方均高于对照,但以T10样方最高;黄果厚壳桂幼苗的单株氮贮量随处理水平增加而增加,在T15样方达最大值,之后下降。氮处理引起两种幼苗体内的氮分配到枝干的比例增加,但分配到叶中的比例下降。两种幼苗叶、枝和根3器官中N与其他营养元素的比值随氮处理水平增加而显著增加。

Effects of nitrogen deposition on forest ecosystems and plants are of global concern because nitrogen deposition is an important aspect of global change. To clarify the effects of elevated nitrogen deposition on subtropical tree seedlings of Schima superba and Cryptocarya concinna, which are dominant in subtropical monsoon evergreen broadleaf forests in China, a simulated study was carried out. The experiment included five randomly chosen treatment plots, each of which was divided into three subplots. In every subplot, we transplanted 40 seedlings of each of the two species. From January of 2003, we sprayed five levels of NH4NO3 solution： 0 （control）, 0.12 （T5）, 0.24 （T10）, 0.36 （T15）, and 0.72 （T30） mol · L^-1 to the five plots respectively twice every month, equivalent to the nitrogen deposition rates of 0, 5, 10, 15 and 30 g · m^-2· a^1. Soil nitrogen availability increased greatly with increasing nitrogen additions, however, soil pH values decreased. Nitrogen addition significantly increased the nitrogen contents in leaves, shoots （including branches and stems） and roots, but decreased the contents of P, K, Ca and Mg in leaves, shoots and roots. Nitrogen storage per seedling of Schima superba in all plots except for T5 plots was higher than that in control plots, with the highest in T10 plots. Nitrogen storage per seedling of Cryptocarya concinna increased with increasing nitrogen addition for the dose levels from control to T15, followed by a decline in the higher treatment levels. Nitrogen treatment also caused increased ratios of N/other elements in leaves, shoots and roots of the two seedlings.