模拟氮沉降对森林土壤化学性质的影响

大气氮沉降的增加对森林土壤的影响是近来生态学研究的重要课题。以鼎湖山季风常绿阔叶林（以下简称为“阔叶林”）、马尾松（Pinus massoniana）林、针阔叶混交林（以下简称为“混交林”）和增城木荷（Schima superba）人工幼林等4种林型土壤为研究对象，采用野外原位模拟大气氮沉降的方法，设置3种模拟氮沉降量，即N0（对照，N：0 g·m-2·a-1）、N5（N：5 g·m-2·a-1）、N10（N：10 g·m-2·a-1），在模拟氮沉降时间分别为42个月（阔叶林）、31个月（马尾松林）、50个月（混交林）、20个月（人工幼林）后，采集0~20 cm土层的林地土壤，分析土壤的化学性质，探讨不同氮沉降量对不同林型土壤化学性质的影响。结果表明，（1）模拟氮沉降对鼎湖山阔叶林、马尾松林、混交林土壤pH值的影响基本一致，均使pH值下降。其中，当氮沉降量达到N10时，阔叶林土壤pH值降为3.97，与对照相比下降了0.11 pH单位，差异达显著性水平（p〈0.05）。而人工幼林土壤pH值未随着氮沉降量的不同而有明显的变化。（2）模拟氮沉降在近2年至4年的时间内，对阔叶林、混交林、人工幼林的土壤有机质、全氮、全磷、全钾、水解性氮、速效磷、速效钾含量的影响均不明显，马尾松林土壤有机质、全氮、全磷、速效磷、速效钾含量也没有明显变化，但模拟氮沉降导致了马尾松林土壤水解性氮含量明显下降，从95.12 mg·kg-1降至84.39 mg·kg-1，差异达显著性水平（p〈0.05）。（3）模拟氮沉降对鼎湖山阔叶林、马尾松林、混交林等3种林型土壤盐基饱和度、盐基离子Ca2＋、Mg2＋、K＋含量的影响未达显著水平，而对这3种林型土壤交换性Na＋含量的影响则较明显且影响趋势基本一致，即氮沉降的增加导致了土壤交换性Na＋含量明显下降。在N10处理下，与对照相比，这3种林型的土壤交换性Na＋含量分别下降了40.0%、68.4%、50.0%，差异达显著性水平（p〈0.05）。氮沉降对人工幼林土壤盐基离子含量无明显的影响。由此可得出结论：在近2年至4年的时间内，氮沉降的增加能引起鼎湖山3种林型土壤尤其是阔叶林土壤加速酸化，引起交换性Na＋明显淋失，以及马尾松林土壤水解性氮含量明显下降；但氮沉降的增加对木荷人工幼林土壤化学性质暂无明显的影响。后者可能与该林型模拟氮沉降时间较短、林龄较轻而处于快速生长期等因素有关。

Effects of simulated nitrogen deposition on soil chemical properties of forests

The effects of the increasing atmospheric nitrogen deposition on forest soils have become one of the major issues in ecological research recently. The authors used the method of simulating atmospheric nitrogen deposition in situ, set up three levels of nitrogen deposition：N0 （CK, N：0 g·m-2·a-1）, N5（N：5 g·m-2·a-1）, and N10（N：10 g·m-2·a-1）for a monsoon evergreen broad-leaved forest （called“broad-leaved forest”for short in the following）, a Pinus massoniana forest, and a coniferous and broad-leaved mixed forest （called “mixed forest” for short in the following） in Dinghushan Mountain, and for a young Schima superba plantation in Zengcheng, Guangdong province, China, collected the soil samples of 0~20 cm soil layer after simulating atmospheric nitrogen deposition for 42 months （broad-leaved forest）, 31 months （Pinus massoniana forest）, 50 months （mixed forest）, and 20 months （the young plantation）, and analysed the soil properties, to discuss the effects of different nitrogen deposition levels on the soil properties＆amp;nbsp;of different forest types. The results showed that, （1） the simulated nitrogen deposition appeared the similar tendency towards the effects on soil pH values of the broad-leaved forest, Pinus massoniana forest, and mixed forest in Dinghushan Mountain, forcing the pH values to drop. Among them, when the nitrogen deposition was at N10 level, the soil pH value of the broad-leaved forest decreased to 3.97 by falling off 0.11 pH value unit compared with CK, and the difference reached significant level （p〈0.05）. But there was no evident change of pH value with the nitrogen deposition levels in the young plantation. （2） Within the nearly 2 years to 4 years, the simulated nitrogen deposition did not evidently affect the contents of soil organic matter, total nitrogen, total phosphorus, total potassium, hydrolysable nitrogen, rapidly available phosphorus and rapidly available potassium in the broad-leaved forest, mixed forest and young plantation, but led to a marked decrease in contents of soil hydrolysable nitrogen in Pinus massoniana forest, from 95.12 mg·kg-1 to 84.39 mg·kg-1, with a significance level of 0.05. （3） The effects of the simulated nitrogen deposition on soil base-saturation percentage, and contents of base-exchangeable ion Ca2＋, Mg2＋and K＋in the broad-leaved forest, Pinus massoniana forest, and mixed forest in Dinghushan Mountain, were no significant difference, but the effects on contents of soil exchangeable ion Na＋in these three forest types were evident and appeared the similar tends, i. e., the increase in nitrogen deposition brought about the evident decrease in contents of soil exchangeable ion Na＋. At N10 level and compared with CK, contents of soil exchangeable ion Na＋in these three forest types decreased by 40.0%, 68.4%and 50.0%, respectively, with a significance level of 0.05. There was no evident effect on contents of soil exchangeable ions in the young plantation under nitrogen deposition. It is concluded that within the nearly 2 years to 4 years, the increasing nitrogen deposition could lead to accelerating soil acidification and evident leaching of soil exchangeable ion Na＋ in the three forest types in Dinghushan Mountain, and lead to evident decrease in the content of soil hydrolysable nitrogen in Pinus massoniana forest, but it could not evidently affect soil chemical properties of young Schima superba plantation. And the latter might be related to that there was less time for simulating nitrogen deposition in the plantation, and the plantation was at the stage of rapid growth, and so on.