Impacts Of Nitrogen Deposition On Physical-chemical Properties And Enzyme Activities Of Soil In Chinese-fir Plantation Forest

The amount of nitrogen compounds in atmosphere is increasing fiercely because of human activities, causing the increasing of the amount of nitrogen deposition in proportion in the near decades. Nitrogen deposition can lead to a series of serious ecological problems, such as over-nutrition in water bodies, forest decline, soil acidification, biodiversity decreasing. As the main accepter of nitrogen deposition, forest ecosystem plays an important role in maintaining the ecological balance of the earth. So the countries of Europe and America pay important attention to the researches about the effects of nitrogen deposition on the structure and function of forest ecosystem in popular, and it becomes one of the spotlight researches on ecology.China has become the third biggest district of nitrogen deposition in the world, following Europe and America. And the condition of nitrogen deposition will get more serious in the next decades, but we have few researches on it. Chinese fir is one of the most important timber tree species in southern China, which distributes in nature and is planted broadly. Fujian, which locates in the southeast of China, with subtropical seasonal climate, has a large area of forest. So study on the nitogen deposition in Fujian not only makes up the deficiency in the researches about the effects of nitrogen deposition on forest ecosystem in our country, but also makes a base for study in depth in our country and even subtropical district, and simultaneously provides theoretical instruction for the sustainable management of Chinese fir plantation.The background level of Chinese fir artificial forest was investigated at the end of 2003．Nitrogen addition experiments were initiated in January 2004．Four N addition treatments in three replicates (three plots) were established in middle-age forest:N0(Control 0 kg N hm^-2 .·a^-1 ),N1(60 kg N hm^-2·.a^-1 ),N2 (120 kg N hm^-2 .·a^-1 ),N3(240 kg N hm^-2 .·a^-1 ).For this purpose 12 plots (20m×20m) were set up.All plots and treatment were laid out randomly. CO(NH₂) ₂ solution was sprayed monthly by hand onto the floor of these plots beginning in January 2004．This paper studies the effects of nitrogen deposition on the physical-chemical properties and the activities of soil enzymes.The results obtained showed as the following: 1 Effects on physical-chemical properties of soil under different levels of nitrogen deposition1) The volume of soil,pH,the amount of the total and exchangeable Ca,exchangeable base,base saturation present of soil and the total amount of availale nutrient increase because of nitrogen deposition, but the increasing amounts of them have no obvious relation with the amount of nitrogen deposition.2) The amount of soil organic matter,the ration of carbon to nitrogen,available P,K and exchangeable Mg increase at first and then decrease with the increasing of nitrogen deposition.3) The amount of al the total N,NH₄-N,NO₃-N and availale N response positively with the amount of nitrogen depositon.4) There is on obvious effects of nitrogen deposition on capilary porosity,the amount of soil moisture,total P,K and Mg,cation exchangeable capacity of soil.5) Nitrogen deposition leads to the decreasing of the amount of exchangeable Al with acid and the ration of them,and the ratio of Al/(Ca+Mg),but the effects are not prominence.2 Effects on the activities of soil enzymes under different levels of nitrogen d eposition1) The activities of all the three types of enzymes decrease with the increasing soil depth at the same level of nitrogen deposition. It is different significantly in comparing the activities of the three enzymes between the treatments of nitrogen deposition.2) There is no the same changeable trend of the activities of the three enzymes along with time.The invertase activites change obvious comparing with urease and catalase, both of the later two enzymes have the same change trend.