模拟氮沉降对杉木幼苗养分平衡的影响

为研究氮沉降对植物养分平衡的影响，对1a生杉木（Cunninghamia lanceolata（Lamb.）Hook.）幼苗进行了室内模拟试验。以NH4NO3作为外加氮源，设计了N0（0 g N m-2?a-1）、N1（6 g N m-2?a-1）、N2（12 g N m-2?a-1）、N3（24 g N m-2?a-1）和N4（48g N m-2?a-1）等5种氮沉降水平，每处理重复6次。通过1a的试验发现，杉木幼苗叶、茎、粗根和细根中的N、K、Mg含量随氮处理水平的增加而上升，但Ca在各器官中的含量则呈下降趋势；中低氮（N1，N2）对叶、茎和粗根中P的含量表现为促进作用，而高氮（N3，N4）则表现为抑制作用。幼苗各器官中的N与其他养分元素的比值随氮处理水平的增加而普遍升高，但粗根中的N/K、N/Mg则表现为下降。与对照（N0）相比，在N1、N2、N3、N4处理中，幼苗对外加氮素的表观利用率分别为60.7%、57.9%、43.3%和27.9%，随氮处理水平增加，利用率呈明显下降趋势。随着氮处理水平的增加，幼苗体内的氮分配到叶和细根中的比例增加，而分配到茎和粗根中的比例下降。因此，氮沉降明显增加了杉木幼苗各器官的氮含量，影响了幼苗的养分平衡。

Effects of simulated nitrogen deposition on nutrient balance of Chinese fir (Cunninghamia lanceolata) seedlings

Human activities such as fossil fuel combustion, production and use of chemical fertilizers, and livestock ranching have dramatically altered the global nitrogen cycle. A potential concern of chronic anthropogenic N emitted to atmosphere is altered nutrient cycling in forest ecosystems. Most studies reporting effects of N additions on forest ecosystems have been primarily carried out in temperate regions of Europe and North America. Importantly, however, there is a strong need to understand how nitrogen interacts with forests in the tropical and subtropical regions because rates of nitrogen deposition in these areas are projected to increase rapidly in upcoming decades. To assess the impact of simulated nitrogen deposition on plant nutrient balance, one-year-old Chinese fir seedlings (Cunninghamia lanceolata (Lamb.) Hook.) from a nursery were transplanted at the beginning of March 2007 before budbreak to plastic pots containing 17.5 kg of forest soil (red earth). The seedlings were kept in natural daylight in a greenhouse and irrigated by hand with distilled water before treatment. The experiment started in April 2007 and lasted for 12 months. Ammonium nitrate solutions were sprayed on the seedlings every three days at four doses, N1(6 g N m^-2 · a^-1), N2(12 g N m^-2 · a^-1), N3(24 g N m^-2 · a^-1) and N4(g N m^-2 · a^-1), with N0 (0 g N m^-2 · a^-1) as the control and six replicates in each treatment. Totally 30 Chinese fir seedlings were available for destructive sampling to monitor biomass and nutrient concentration at the end of experiment. The amount of water solution sprayed to the seedlings each month was equivalent to the average monthly precipitation recorded over the last 30 years. The results indicated that nitrogen addition increased N, K and Mg contents in leaves, shoots (including branches and stems), coarse roots and fine roots of the seedlings, but reduced Ca concentration. Phosphorus contents in the seedlings responded positively to low-to-medium N treatments (N1, N2), but negatively to high N treatments (N3, N4). Nitrogen loads generally resulted in increased ratios of N relative to other elements in the seedlings, but decreased N/K and N/Mg in coarse roots. Linear regression correlations between nitrogen contents in leaves or fine roots (independent variables) and other element contents in leaves or fine roots (dependent variables) were developed for varying N levels. The N contents in leaves were positively correlated with K and Mg contents, but negatively with P and Ca contents. For fine roots, correlation between N contents and P and Mg contents was positive, while correlation between N contents and P and Mg was negative. The apparent N-use efficiency by the seedlings was estimated at 60.7%, 57.9%, 43.3% and 27.9% in the N1, N2, N3 and N4 treatments, respectively. Nitrogen addition increased N allocation to leaves and fine roots, but lowered N to shoots and coarse roots. Fine roots are more sensitive to N addition than any other seedling component. In general, the present simulated N deposition in greenhouse has been found to change the nutrient balance due to significant enhanced N concentration in the seedlings.