| Carbon is one of the most important elements in all life forms, and the carbon cycle is one of the most important processes in all life forms. Forest are the main terrestrial ecosystems which loading 46.3% of the total carbon pool in the earth land, and forest vegetation carbon pools maintained as high as 77.0% of the terrestrial vegetation carbon pools, so the cycle of the forest carbon are the world's most important carbon cycle, and it plays an important role in maintaining global ecological security and the sustainable development of human society. The global carbon cycle has been impacted by human society through three main ways: first, large-scale deforestation to forest land into agricultural land and agricultural farming methods change; second, large-scale using of ore fuel by human society, the third are chemical substance emissions produced by human life. The first two directly led to the loss of the global carbon pool and become carbon source, and the third is usually mainly referring to the acid deposition including deposition of sulfur and nitrogen deposition. The role of the both acid deposition at the global carbon cycle are relevant to the geographical and the ecological environment. With the development of human society, the negative effects of growing nitrogen deposition on global carbon cycle became more and more serious, so it is concerned by the international community, especially in the temperate regions of Europe and North America. High nitrogen deposition also occurred in some areas of china, even some scholars pointed out that China has become one of the world's three major focus areas on nitrogen deposition. Some researching projects related to nitrogen deposition have been started.Chinese fir, which natural distribution and artificial cultivation is very vast, is one of important timber species in southern China. Chinese Fir grows fast, and its absorption of carbon is also fast, so that it has an obvious effect on reducing atmospheric CO2 concentration. From this perspective, fir cultivation has a very important role on mitigating the global change. Researching the effects of nitrogen deposition on the Chinese fir plantation ecosystems, not only can make up China's deficiencies in the areas of the study, as well as provides theoretical guidance for the sustainable management of Chinese fir plantation, and furthermore makes a base for study the global change. In this paper, to investigate the response of forest ecosystem to increased nitrogen deposition, a field experiment was conducted in a 12-year-old Chinese fir plantation forest in Sanming, northwestern Fujian. Nitrogen loadings were designed at four levels as N0, N1, N2, and N3, at the doses of 0, 60, 120 and 240 kg N hm-2 yr-1, respectively, with three replicates in each treatment. Based on three or five years of manipulation, the research results responding to nitrogen loading are as follows:1. Effects of nitrogen deposition on carbon stock of Chinese fir plantation1.1 Effects of nitrogen deposition on biomass and carbon reservation in forest tree layerAfter five years of nitrogen deposition simulation, the growth of tree layer biomass in plot stands was as follow order: N1> N0> N2> N3. The growth of tree layer biomass were up to 53.690, 60.663, 43.402 and 40.051 t·hm-2 respectively in plot stands treated by N0, N1, N2 and N3, and compared to the control treatment, respectively, the growth of tree layer biomass increase 12.99%, -19.16% and -23.54% more than that of the plot stands treated by N0; Variance analysis showed that, there was significantly different between N1 and the control treatment, and N1 promote the Chinese fir to increase tree layer biomass; Also N2, N3 and the control treatment reached the level of the difference (p <0.10), but the N2, N3 treatment cause fir tree layer biomass increasing less than that in the control treatment.; After five years of nitrogen deposition in the Chinese fir plantation, the carbon storage of tree layer in treated plot increased in the following order:N1>N0>N2>N3, and the carbon storage of tree layer in the plot stands treated with N0, N1, N2 and N3 increased respectively by 24.048, 26.810, 20.215 and 18.746 t·hm-2, and compared to the control treatment, respectively, the growth of tree layer carbon reserves increase more 11.49, -15.94% and -22.05% than that of the plot stands treated by N0. Variance analysis showed that there is significant difference between N1, N3 and N0, but no obvious difference between N2 and N3. The results showed that after five years of nitrogen deposition, compared with the control treatment, N1 promoted the carbon storage growth of tree layer in the plot stands of Chinese fir, and N2,N3 inhibited the carbon storage growth of tree layer in Chinese fir stands.1.2 Impaction of nitrogen deposition on year carbon flux in forest litter After three years ( in 2006.12), the carbon flux of the forest litter in stands treated by N0,N1,N2 and N3 increased by 52.9, 114.4, 157.2 and 92.6 kg ? hm-2, respectively, compared to its in 2004.12, when the stands were treated for just one year, and respectively, had an increase of 4.89%, 11.53%, 15.38% and 8.46%. Compared with the control, the litter carbon flux in stands treated by N1,N2 and N3 were increased by 6.64%, 10.49 and 3.57%, respectively. So we could conclude that the litter carbon flux in stands treated by N1, N2 and N3 were increased by 65.9, 107.2 and 39.0 kg?hm-2 respectively because of nitrogen deposition. If the above data were converted to CO2, the forest in stands treated by N1, N2, N3 stored another 120.8, 196.6 kg.hm-2 and 71.6 kg.hm-2 of CO2 every year by the forms of litter compared to the control (N0) treatment. 1.3 Effects of nitrogen deposition on vegetation carbon storage of understory in Chinese fir plantationAfter four years of nitrogen deposition simulation, the carbon storage of understory herbs in stands treated by N1, N2, N3 had reduced respectively by 0.072, 0.136, 0.167 t/hm2 compared to the control (N0) treatment, and the carbon storage of undergrowth in stands treated by N1, N2, N3 had reduced respectively by 0.092, 0.237, 0.314 t·hm-2 compared to the control (N0) treatment. General view, after four years of nitrogen deposition simulation, the carbon storage size of understory vegetation in the stands treated with nitrogen deposition are as follows: N0> N1> N2> N3, and compared to the control treatment, the carbon storage of understory vegetation in the stands treated by N1, N2 and N3 reduced respectively by 0.164, 0.373 and 0.480 t·hm-2, and respectively again, about annual reduced by 0.041, 0.093 and 0.120 t·hm-2 in average. When we converted the carbon storage into the amount of CO2, the reduced carbon storage of the understory vegetation in stands which treated by N1, N2 and N3 is equal to release CO2 0.151, 0.342 and 0.441 t·hm-2 respectively every year. The results showed that the higher level of the nitrogen deposition is, the more vegetation carbon stock reduced in the understory of Chinese fir.1.4 Effects of nitrogen deposition on forest soil carbon poolsThe carbon content of surface soil was impacted the most greatly by nitrogen deposition, along with continued nitrogen deposition, the carbon content of surface soil decreased, but the decreasing speed reduced gradually; the higher level of nitrogen deposition, the more the carbon content of surface soil reduced; In the first two years of nitrogen deposition, the carbon content of middle and bottom soil reduced gradually, but rising in the subsequent two years. Taking together, the carbon content of soil layer in 060cm depth had a trend to lower in first two years of simulation testing, and then gradually increased. After four years of tests, it was found that N1, N2, and N3 reduced slightly the carbon content of stand soils, and N2 reduced mostly, followed by the N3, N1 reduced the minimum, so it seems that there is no law in the impaction of all nitrogen deposition levels on the soil carbon pools, and there is non-linear relationship between the volume of nitrogen deposition and its impaction on soil carbon pools. Although nitrogen deposition reduced the soil carbon content, at the same time it had increased the density of the soil for more, and instead of reducing the soil carbon content, nitrogen deposition increased the carbon reserves in soil pools. After a four- year simulation of nitrogen deposition, the forest soil carbon stock increased by 5.79, 7.33, 6.42 and 14.89 t·hm-2 respectively in soil pools treated with N0, N1, N2 and N3. Compared to the control treatment, respectively, the soil carbon stock increased by 1.36, 0.45 and 4.84 t·hm-2 in the soil treated by N1, N2, and N3 after four years. This results show that the difference nitrogen deposition will significantly affect the forest soil carbon reserves, but there is no apparent regularity on soil carbon reserves affected by the increasing level of nitrogen deposition. If converting the data into the amount of CO2, respectively, the stands soil pools treated by N1, N2, and N3 absorbed 1.25, 0.410 and 4.44 t·hm-2 of CO2 every year, compared to the control treatment.2. Effects of nitrogen deposition on process of forest chemical ecology2.1 Effects of nitrogen deposition on pH value of forest soilsThe following characters are presented in the effects of nitrogen deposition on different layers of forest soil: the soil in depth of 020 cm is more sensitive than that in depth of 2040cm and 4060cm responding to nitrogen deposition, and the upper layer of soil is easier to be acidified than the lower layer of soil; Descending degree of pH value is positively correlated with the amount of nitrogen deposition, the longer nitrogen deposition, the greater extent of soil pH values descending and the more obvious the soil acidification. During the experimental process of nitrogen deposition, the soil pH values of all plots decrease, however, the descending degree of soil pH value at the beginning of the nitrogen deposition simulation is significantly lower than that two years later. In particular, for soil in depth of 2060cm treated with N1, the decline of pH value is not very obvious at the beginning.2.2 Effects of nitrogen deposition on effective nutrient content in forest soil During the whole process of nitrogen deposition, effective nitrogen content (ammonium and nitrate) increase in the plot soil treated with nitrogen deposition compared to the control treatment, and the higher level of nitrogen deposition, the greater extent of its growth. With time passing, there is a growing gap between the effective nitrogen content in plot soil treated with nitrogen deposition and that in control treated soil, so we can conclude that nitrogen deposition have an accumulative effect on the effective nitrogen content in soil. The nitrogen deposition has the following effects on soil available phosphorus content in Chinese fir plantation. The nitrogen deposition causes attenuation of available phosphorus content in soil; At the beginning of nitrogen deposition, the higher the level of nitrogen deposition, the greater extent of attenuation of available phosphorus in soil; however, the attenuation of available phosphorus in the soil does not speed up with the increasing amount of nitrogen deposition. Nitrogen deposition result in a greater extent of available potassium leaching from the soil at the depth of 0-20cm , but in soil at the depth of 20—40cm and 40—60cm , the change of available potassium content is lower than that in contrast, on the contrary, it increased with the nitrogen deposition. Generally speaking, the greater extent of nitrogen deposition causes a greater degree of leaching of available potassium; The nitrogen deposition aggravates the migration of available potassium to the deeper layer of soil, and the leaching of available potassium strengthens with nitrogen deposition continuous; The exchangeable Ca2+ and Mg2+ contents in the same layer of soil are different due to the different levels of nitrogen deposition. The high-to-low order of exchangeable Ca2+ and Mg2+ contents in soils treated with different levels of nitrogen deposition is N0>N1>N3>N2, moreover, the difference of treatment levels between N0,N1,N3 and N2 is very obvious(p<0.05), so it can be that every layer of soil is more sensitive to N2 . The exchangeable Ca2+ and Mg2+ contents in the same layer of soil with the same level of nitrogen deposition decline with time passing; the exchangeable Ca2+ and Mg2+ in soil leach fast in the initial of nitrogen deposition, but with continued nitrogen deposition and accompanying soil acidification, the soil will also release some of the base mineral ions, and thus the leaching degree of soil exchangeable Ca2+ and Mg2+reduced after about two years of nitrogen deposition.2.3 Effects of nitrogen deposition on nutritional status of treesCompared to N0, N1, N2 and N3 improve the nitrogen content of fir needles to some extent after four years of nitrogen deposition simulation, and the nitrogen content of fir needles in stands treated with nitrogen deposition increased 18.25 % , 11.68 % and 13.14 % respectively, compare to that in the contrast treatment., however, the effect declines with time passing. At the third year of this experiment, even if nitrogen input is increased, increment rate of nitrogen in fir needles can not be improved continuously. In this study experiment, although phosphorus content in Chinese fir needles treated by N1, N2, and N3 showed an upward trend, the increasing amplitude in fir needles treated by N1 or N2 was significantly lower than that of the contrast treatment, which displayed that N1 or N3 nitrogen deposition has an inhibitory effect on phosphorus absorption in the Chinese fir. At most of four years, the nitrogen deposition showed a certain extent of inhibitory effect on the increase of K and Mg content in needles. The influence of nitrogen deposition on ratios of N and C, K, P, Mg content in Chinese fir needles presents similar law, which ascending in first and descending at last, but among 3 years of the study, the ratios are all inside the pale of optimal values needed by the growth of firs, and it illuminates that Chinese fir has not yet shown a nutritional imbalance problem after three years of treatment; The influence of nitrogen deposition on trace element in Chinese fir needles has the following characters: mid-and-long term nitrogen deposition inhibits the increase of Mn content in Chinese fir needles, moreover, the higher the level of nitrogen deposition, the more obvious the effect; The Zn content of needles in Chinese fir through N1 and N2 treatment ascended in first and descended at last; The Zn content of needle in Chinese fir through N3 treatment showed an increasing trend year by year; Nitrogen deposition inhibits the increase of Fe content of needle in Chinese fir, but with time passing, the inhibitory effect is more and more inconspicuous. 2.4 Effects of nitrogen deposition on soil respiration and activity of soil enzymes in forestBased on three years of manipulation, nitrogen loading was found to inhibit soil respiration rate in the 0-20 cm depth, but promot respiration rate in the 20-60 cm horizon. N1, N2 and N3 treatments decreased surface soil respiration rates by 28.34%, 2.04% and 15.31%, respectively, but increased soil respiration rate by 53.44%, 62.22% and 20.20% within the 20-40 cm depth, respectively, and by 117.46%, 42.72% and 72.86% at the soil depth of 40-60 cm, respectively.In the first two years of treatment, nitrogen deposition was observed to elevate soil cellulose enzyme activity, but in the third year the magnitude of enhancement declined in N1 and N2, and significant inhibition was detected for N3. Significant positive linear relationship was developed between soil respiration rate and for the treatments of N1 and N2, but failed for N3. The effect of nitrogen deposition on catalase activity was monitored continually for four years in forest soil treated by N0, N1, N2, and N3. The results show that little change was found for catalase activity in soil after treated with N0, but the catalase activity upgraded year by year in the soil treated with N1 and N2, as for N3, the catalase activity step up 32.58% in first year and gradually step down in the following three years. After four-year simulation of nitrogen deposition, December in 2007, catalase activity in the forest soils treated with N0, N1, N2 and N3,,were raised about 4.34%, 41.21%, 55.56% and -20.13%, respectively, compared with the activity in 2003 respectively. Compared to control treatment, the treatment of N1, N2, N3 enhanced the forest soil catalase activity about 33.38%, 53.13% and -20.31% respectively. The variance analysis show that, there is significant difference between various treatments (p<0.05), and it was obvious that the catalase activity be raised in forest soil after treated with N1 and N2, while N3 made the activity be lower. Among three years of monitor, the activity of urea enzyme in forest soil treated by N0 changed little , but the activity of urea enzyme in forest soil treated with N1 and N2 raised year by year; and the activity of urea enzyme in forest soil treated with N3 was increased about 32.58% in the first year of simulation, but gradually step down in the following two years. After 3-year simulation of nitrogen deposition, December in 2006, we detected that the activity of urea enzyme in soil from forest was raised about 6.01%, 99.82%, 86.08% and -14.29%, after treated with N0, N1, N2 and N3 respectively, compared with the activity in 2003. Compared to the control treatment, the activity of urea enzyme in soil were raised remarkably after treated with N1 and N2, while N3 made the activity be lower.2.5 Effects of nitrogen deposition on litter fall composition and its decomposition process Nitrogen deposition increased the nitrogen content of deciduous leaf markedly. Comparing with the control treatment, the treatment of N1, N2, N3 made the nitrogen content in deciduous leaf increase 32.5%, 19.3% and 10.2% respectively. Although nitrogen content in other components of litter fall showed different response, but there is no significant difference on statistical. There is obvious positive relationship between nitrogen deposition levels and the return of volume of nitrogen in ecological system, that is, the higher nitrogen deposition level accompanying the higher total amount of nitrogen restitution in litter fall. Compared with control treatment (N0), the total amount of nitrogen restitution in litter fall increased about 10.9% and 32.6% respectively in forest treated by N2 and N3, while the effect of N1 on it was non-significant. The content of microelements in compositions of litter fall showed a sequence of Fe>Mn>Zn>Cu. In this research period, the nitrogen deposition didn't significantly change the main characteristic of trace nutrient recycle in fir wood. The magnitude of copper content of litter fall in stands after treatment is N3>N0>N1>N2, for zinc content is N0>N3>N2>N1, and for manganese content is N1>N2>N3≈N0, and for ferrum content is N1>N2>N3>N0.The treatment of N1 and N2 promoted the decomposition of litter fall with different extent. N2 promoted the decomposition most significantly, The effect of N1 on decomposition was positive too, but N3 appeared some little depressant effect on it. The turnover time for litter fall decomposition treated by N0, N1, N2, N3 was 3.99, 3.95, 3.06, 4.11a, respectively. The carbon release rates of litter falls in plots treated by four levels of nitrogen deposition has comparatively large difference in the different decomposition phase. In initial phase of decomposition, the carbon release rate of litter fall treated with N1 was only 6.78%, the least among all treatments, and just being 61.94% as much as that in control treatment. In intermediate stage of the decomposing experiment, the carbon release rate of litter fall treated with N2 and N3 were obviously smaller than that in control treatment. In the later stage of decomposition, the carbon release rate of litter fall treated with N1,N2, N3 were 16.11%, 19.73%,16.30% respectively, all of which were larger than 12.60%, the data of control treatment. Overall, with treatment of different nitrogen deposition, the carbon release rate of litter fall has a sequence of N2>N0>N1>N3.
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