| With the global economy's rapid development and population growth, fossil fuelconsumption, atmospheric nitrogen deposition increased proportionally. Because the forest isa large area directly affected by nitrogen deposition, In Europe and North America, Ecologistshave conducted a lot of research which impacts of nitrogen deposition o n structure andfunction of forest ecosystems, but this research mainly concentrated in temperate forests;china in this area is still in initial stage. Therefore, background the N deposition has increasedremarkably in many regions. A field experiment was conducted with annual nitrogenadditions at four levels; it can make up insufficient date which response of characteristics andmechanisms of subtropical regions to nitrogen deposition. There are important significancesfor the sustainable management of Chinese fir plantation and nitrogen deposition research insubtropical regions.In a 12-year-old Chinese fir plantation, a field experiment was conducted with annualnitrogen additions at four levels as NO (0 kg N hm-2·a-1 ), N1 (60 kg N hm-2·a-1 ), N2 (120 kg Nhm-2·a-1 ) and N3 (240 kg N hm-2·a-1 ), with three replicates in each treatment. This thesisstudied the effects of N deposition on nutrient dynamic in the leaves and litterfall.The results obtained showed as following (1) N treatments (N1, N2 and N3) increased Nconcentrations in the leaves by 18．25%, 11．68% and 13．14%, respectively, but decreased C/Nratios by 15．07%, 9．96% and 12．01%, respectively. Nitrogen loads generally reduced thecontents of P, K, Ca and Mg in the leaves. Although nutrient concen trations fluctuated withthe time, N treatments significantly increased the ratios of N to other macro elements to becompared with control (N0) , leading to imbalance of nutrients in the leaves. (2) High N loads(N2, N3) increased average annual increment of DBH, but low N (N1) produced nosignificant effects. Nitrogen deposition had a positive effect on tree height, but weaken with the increasing N additions. A gain of Stem wood were 28．82,28．96,32．63 and 33．68m3．hm-2 for N1,N2 and N3 treatments respectively. Tree growth responded positively to all Ntreatments. (3) The annual litterfall production was estimated to be 2238．10 kg ?hm-2 , 2286．66kg·hm-2 and 2599．50 kg·hm-2 for N1, N2 and N3 treatments, compared with 2427．51 kg·hm-2 of the control (N0) . Variance analysis showed that high levels of N deposition (N3) increasedthe litterfall amount significantly, and the low-to-medium N loads (N1 and N2) decreased thequantities to some extents but no statistically significant difference. Monthly litterfallproduction showed similar seasonal dynamics, with three peaks occurring in February, Mayand July, respectively. Fallen leaves accounted for 70．49%～73．67% of the total litterfall,branches for 19．38%～20．39%, droppings for 4．98%～7．70%, fruits for 1．11%～2．16%, andbarks for 0．29%～0．33%. Least Significant Difference (LSD) test showed that N3 treatmentproduced significant effects on the amount of fallen leaves and fruits, but not on othercomponents. (4) the annual average concentrations of four microelements in the litterfalldecreased in the following order: Fe > Mn > Zn > Cu. Nitrogen additions, especiallylow-to-medium N treatments (N1 and N2) elevated the contents of Mn and Fe in the litterfall,but reduced Zn concentration to some extent. The micronutrient flux through litterfallexhibited two peaks, the first occ urring consistently in April, but the second differently amongthe elements. Through litterfall production, the annual flux of Cu to the forest floor wasestimated at 8．69, 8．99, 9．79 and 8．77 g.hm-2 ; of Mn at 244．91, 293．95, 278．68 and 200．99g.hm-2 ; of Zn at 40．08, 42．92, 44．73 and 38．63 g.hm-2 ; of Fe at 459．00, 614．09, 598．81 and406．28 g.hm-2 ; respectively, for the above four treatments (N0, N1, N2 and N3) . The annualflux for the microelements responded positively to N1 and N2 treatments, but negativel y toN3．(5) After two years of leaf litter decomposition, the litter remaining rate was estimated tobe 24．58%, 21．99%, 15．46 and 25．17%, respectively, for N0, N1, N2, N3 treatments. Basedon Olson equations, decomposition constant k for the above treatme nts was estimated to be0．7764, 0．8076, 1．0188 and 0．7608, with 95% decomposition time of 3．99 a, 3．95 a, 3．06 a,4．11 a, respectively, suggesting that the low-to-medium N loads (N1 and N2) increased thelitter decomposition rate, but high levels of N depo sition (N3) was found to inhibit thedecomposition to some extent. (6) he average carbon contents in the decomposing littercollected bimonthly were estimated at 46．47%, 46．35%, 46．79% and 46．6%, respectively, forN0, N1, N2, and N3, showing no significant difference between the treatments but decreasing with the time consistently. The concentrations of nitrogen in leaf litter were found to increasesignificantly with the doses of nitrogen additions. For the above four treatments, thedecomposition coefficients determined by Olson equation were respectively 0．739, 0．744,0．936 and 0．708, with turnover time of 4．26, 4．26 , 3．46 and 4．41 years, for C element; thedecomposition coefficients were 0．458, 0．543, 0．776 and 0．565, with turnover time of 6．26,5．44 , 3．91 and 5．20 years for N element. N1 treatment stimulated N release from the litter,but produced no effects on C release; N2 accelerated release of both C and N, but N3exhibited inhibitory effects to some extent. The N1, N2 and N3 treatments decreased C/Nratios in leaf litter by 8．59%, 14．20% and 17．54%, respectively.
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