| Because of the development of industrial , agricultural production, transportation, and the acceleration of urbanization, a large number of nitrides and sulfides were emissioned into the atmosphere by human activities, which caused an increased in acid deposition. The increasing in acid deposition has been concerned as a global environmental issue. As the main recipients of acid deposition, the forest ecosystems are playing an important role in maintaining the world's ecological balance.Fujian has become one of the most serious regions hit by acid rain pollution. Eucalyptus, as the main introduced species in Fujian Province, with the characteristics of cold hardiness and fast-growing is now showing good momentum of development. Therefore carried out the researches on the Eucalyptus plantations in response to acid deposition, has an important theoretical guidance role in further expanding the cultivated area and formulating measures to cope with global acid deposition, which can not only provide a scientific basis to the China's fast-growing Eucalyptus forests of sustainable management, but also can provide a theoretical foundation for the management of the sub-tropical broad-leaved forests at acid deposition increased environment.The experiment about the effect of similated nitrogen (N) and sulphur(S) combined deposition on young Eucalyptus dunnii plantation was arranged by the method of quadratic regression revolution design. We sprayed (CO(NH2)2)as additional nitrogen source, (Na2SO4) as additional sulfur source once a month from the end of July 2007. This paper studies the response of the chemical properties of soil, the nutrient of forest and the growth of 3-year-old young E. dunni trees to similated N and S combined deposition and eliminated the mathematical models corresponding. The single effect, marginal effect as well as combined effect between nitrogen (N)and sulphur(S) were analyzed.The results obtained showed as the following:1. Response of the chemical properties of soil of young E.dunnii forest to N and S combined depositionThe amount of soil organic matter,total N,P decreased at first and then increased with the increasing of time,until up to a certain extent, the amount of them have obvious relation with the settlement of simulated N and S combined deposition.On the whole soil,simulation of N,S combined deposition for one year, the average content of soil organic matter declined 4.7% in the upper soil(0-20cm), 3.9% in the middle soil(20-40cm), 3.8% in the lower soil(40-60cm). Two seasons after simulated N and S combined deposition,the regression of mathematical models about the response of the organic matter in the upper soil of the young E.dunnii forest to simulated N and S combined deposition is significant. The interaction effect of N deposition and S deposition on soil organic matter is insignificant. The marginal effect of N deposition and S deposition is significantly, As the N and S deposition processing, the content of soil organic matter of young E.dunnii forest increased first and then decreased with the increasing of both N and S deposition, which complyed with the law of diminishing returns.Simulated N and S combined deposition promoted the content of total soil N of young E.dunnii forest,the average content of total soil N in the three soil samples increased 19.5%,15.7%,9.3% respectively. The regression of mathematical models about the response of the content of total soil N in the upper soil to simulating N and S combined deposition is significant after a year. Independent effect of N deposition is significant. The content of total soil N increased with the increasing of the encoding value. The interaction effect of nitrogen deposition and sulfur deposition is significant.The average content of total soil P in the three soil samples declined 1.61%, 0.63%, 1.47% respectively. The regression of mathematical models about the response of the content of total soil P in the upper soil (0-20cm) to N and S combined deposition is significantly after one year. The response of the content of total P of upper soil to N deposition comply with the law of diminishing returns, the interaction effect of N deposition and S deposition on the upper soil total P is significant(p<0.05).N and S combined deposition leads to the increasing of the amount of soil total K,but there is no obvious effect on it., the regression of mathematical models about the response of the content of total soil K in the upper soil of the young E.dunnii forest to simulated N and S combined deposition is insignificant after a year. The amount of exchangeable Al and exchangeable acid in the young E.dunnii forest soil increased gradually, but the majority of analysis of variance were insignificant, the regression of mathematical models about the response of the amount of exchangeable Al and exchangeable acid in the young E.dunnii forest soil to simulating N and S combined deposition is not significant after a year. Simulated N and S combined deposition leads to the decreasing of the ratio of carbon(C) and nitrogen (N), the content of available P as well as available K,and caused the content of soil S increased at first and then decreased, but only a part of analysis of variance is significant, the regression of the response models is not significant. Simulating N and S compound settlement a year latter, Response of the content of the available P in the upper soil of young E.dunnii forest to sulfur deposition complyed with the law of diminishing returns. The interaction effect of N deposition and S deposition on the upper soil availablel P of young E.dunnii forest is significant (p<0.05).2. Response of the nutrient of young E.dunnii forest broad-leaves to N and S combined depositionN and S combined deposition leads to the amount of nitrogen of leaf increasing at first ,then decreasing when the nitrogen of leaf reaches a certain extent will be reduced.The effect on nitrogen is obvious prominence at the late. The amount of P,K of leaf is improved at early time,inhibited at the later time ,the effects of the combined deposition is obvious; the combined deposition caused the leaching of the amount of Mg in leaf,and impacts the amount of organic matter of leaf,but the effects are not obvious prominence. N and S combined deposition leads to the value of N/K increasing at first and then decreasing,at last less then the optimum range,the nutrient of leaf lose the balance,leads to the value of N/Mg and N/P decreasing at first ,then increasing and maintaining in the optimum range.Simulation of N and S compound settlement a year later, the regression of response modles of simulating N and S combined deposition on the content of organic matter,N,P,Mg in the broad-leaves of young E.dunnii forest trees except the content of K were significant (p<0.05). The independent effects of N and S deposition on the organic matter in the broad-leaves of young E.dunnii forest were significant (p<0.05). The response modles of the content of N in the broad-leaves were signifigent. the marginal effect of N and S deposition on the content of N in broad-leaves reached significant levels (p <0.05). The regression of mathematical models about the response of the content of P in broad-leaves of the young E.dunnii forest to simulated N and S combined deposition is insignificant. The independent effect of N deposition achieved a significant level (p <0.05), the marginal effect of N deposition and S deposition achieved a significant level (p <0.05). The regression of the modle about the response of the content of Mg in broad-leaves of the young E.dunnii forest to N and S deposition is significant. The independent effect of nitrogen deposition on the content of Mg in broad-leaves is significant, the marginal effect of nitrogen deposition is significant.3. Response of the growth of young E.dunnii forest trees to nitrogen(N) and sulphur(S) combined deposition The experiment about the effect of similated nitrogen (N) andsulphur(S) combined deposition on the growth of diameter at breast height(DBH)of young Eucalyptus dunnii plantation was arranged by the method of quadratic regression revolution design. The response of DBH in E. dunnii layer to nitrogen (N) and sulphur(S) combined deposition was studied. Eliminated the growth of age, the mathematical models: 2= 0.7763+0.1935x1 ?0.1256x2?0.5100x1x2?0.2450x1?0.5975x were established to describe the above relations. As for the growth of DBH, their single effect, marginal effect and combined effect between sulphur(S) and nitrogen (N) were analyzed. The study showed that simulate of N and S combined deposition can promote the growth of tree diameter at breast height, the effect of the combined deposition is significant;the promotion comply with the law of diminishing returns,inputing the amount of the nitrogen and sulfur appropriately, can promote the growth of trees.When the pure sulfur up to 28.20 kg ? hm-2 ? a-1 S(encoding value is 0.394 9), nitrogen up to 64.78 kg ? hm-2 ? a-1 N(encoding value of -0.105 1), E.dunnii diameter growth at breast height is expected to reach the largest amount of 0.84 cm. Either the amount of nitrogen or sulfur deposition over critical loads values will inhibit the growth of trees.
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