Effects of simulated nitrogen deposition on soil chemical properties of forests

The effects of the increasing atmospheric nitrogen deposition on 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 forest(called broad-leaved forest for short in the following), a Pinus massoniana forest, and a coniferous and 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 of different types. The results showed that,(1) the simulated nitrogen deposition appeared the similar tendency towards the effects on soil pH values of the forest, Pinus massoniana forest, and 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 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 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 forest, Pinus massoniana forest, and in Dinghushan Mountain, were no significant difference, but the effects on contents of soil exchangeable ion Na+ in these three 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 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 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.