Nitrogen(N)inputs from anthropogenic sources are currently estimated to be 30% to 50% greater than those from natural terrestrial sources and tenfold greater than anthropogenic inputs from 100 years ago(Fang et al.，2011). In Asia，emissions of reactive N have increased dramatically，leading to deposition of 30-73 kg·hm^-2a^-1N in some subtropical forests of southern China(Fang et al.，2011). Increased nitrogen(N)deposition caused by human activities has altered ecosystem functioning and biodiversity. Soil microbial communities sustain many vital ecosystem processes，such as nutrient cycling，decomposition of organic matter and waste，nutrient availability，degradation of pesticides and contaminants，soil structure，and plant growth and health(van Elsas et al.，2006). Assessing the effect of nitrogen deposition on microbial community structure，diversity and activity in soil is therefore critical to advancing the underst and ing of the functionality，stability，and resilience of managed and natural ecosystems(Kennedy et al.，1995). So better underst and ing of microbial compositional and physiological acclimation mechanisms is critical for predicting terrestrial ecosystem responses to global change(Bi et al.，2012).

In recent years，numerous studies on the effects of N addition or simulated N deposition on soil microbes have been conducted both in the field and laboratory across different forest or plantation type，however，most of the related studies focused on the mature plantation or forest(Wallenstein et al.，2006; Cusack et al.，2011; Wu et al.，2013; Hu et al.，2010)，whereas few studies focused on the young plantations. In addition，these studies have not shown consistent effects of N addition on microbial biomass and community structure，with positive，negative，and neutral effects being reported. For example，across temperate forests，Nilsson et al.(2007)reported that N input had no effect on total fungal biomass in the soils of oak forests along a natural N deposition gradient，but Demoling et al.(2008) and Fraterrigo et al.(2006)found that fungal biomarkers were decreased in soils of N-fertilized plots. In contrast，Gallo et al.(2004) reported that fungal biomarkers increased under N addition in north temperate forest soils. In tropical forest，Cusack et al.(2011)found that soil microbial biomass increased in response to N fertilization in both distinct tropical rain forests，but Liu et al.(2013)found that nitrogen additions decreased soil microbial biomass in the short term but returned to pre-treatment levels over the long term after four years N addition in a tropical forest of subtropical China. A common element to these studies is that they were carried out on mature forests or plantation. Nitrogen deposition is increasing in non-agricultural area including forest ecosystem and many previous studies have proven that nitrogen deposition significantly affects soil and plants. In addition，amounts of newly-built plantations were widely distributed after clear-cutting. Less is known，however，about how N addition alters on chemical composition of soil organic carbon(SOC) and microbial community characteristics in a young subtropical plantation.

Ginkgo biloba(Ginkgoaceae)，a famous and widely cultivated plant，is often referred to as a living fossil(Yan et al.，2009). In addition，G. biloba which is a deciduous gymnosperm species，is the only remaining species of the once large order Ginkgoales，with geological records indicating this plant has been growing on the Earth for 150-200 million years，and it is widely used for street and l and scape trees(He et al.，2009; Ling et al.，2003). However，few studies focused on the effect of global change such as nitrogen deposition on G. biloba plantation，especially on soil microbes. In the present study，to acquire an insight into the mechanisms of soil microbial response to nitrogen deposition in subtropical young plantation of G. biloba，a one-year long field simulated nitrogen deposition experiment was established. Based on relevant expectations arising from previously reported studies on other subtropical plantations，we hypothesized that: 1)N addition would affect chemical composition of SOC. 2)High-N addition would significantly decrease soil microbial biomass; 3)N addition would significantly influence soil microbial community composition regardless of the amount of N loaded.

The study site was situated in a young G. biloba (6-year-old)plantation(5 hm²)at Xing’an county，Guangxi province(25°55′N，110°24′E)，China，and has a subtropical monsoon and humid climate. The mean annual precipitation is 1 842 mm，about 65% of which occurs from March to September. The annual average relative humidity is 76% and the mean annual temperature is 18.6℃ with the lowest and highest monthly average temperature of 14.5 ℃ and 28.5 ℃. The main ground cover species include Cibotium barometz， Blechnum orientale，Lophatherum gracile，Miscanthus floridulu and Ottochloa nodosa. The soil is classified as Red soil，which predominantly derived from the granite weathering.

Twelve plots(20 m × 20 m)were r and omly staked out with an at least 15 m buffer in the G. biloba nursery garden. The experiment included four treatments with 3 replicates for each treatment: 1)Control(without any N addition applied，C); 2)50 kg·hm^-2a^-1 N(Low-N); 3)100 kg·hm^-2a^-1 N(Medium-N); 4)150 kg·hm^-2a^-1 N(High-N). The rates of N addition referred to the relevant studies also conducted in plantations of subtropical China，especially in Dinghu Mountain Biosphere Reserve(DHSBR)(Mo et al.，2007; 2005; Zhang et al.，2008)，which was relatively near to the studied site in this paper. N additions(applied as NH₄NO₃)were initiated in May 2012 and applied bimonthly over the course of one year at the rate of 0 g(Control)，952.38 g(Low-N)，1 904.77 g(Medium-N) and 2 857.15 g(High-N)for each N application(Liu et al.，2013). Before N addition，NH₄NO₃ was weighed，dissolved in 20 L distilled water and applied to each plot below the canopy using a backpack sprayer(Mo et al.，2007). Each control plot received 20 L of water without N addition as well. During the experiment，nurse and management were not conducted in order to exclude the anthropogenic disturbance.

Soil samplings were conducted in May 2013. From each plot，10 soil cores(3.5 cm inner diameter)were collected r and omly from a 10 cm soil depth and compounded to one composite sample. After removing impurities，soils were sieved to 2 mm mesh size and divided into two parts，one part was retained for measuring soil chemical properties and the other was frozen at -20 ℃ as soon as possible for analysis of soil microbial biomass and community structure.

Soil moisture content was measured by oven-drying method using 20 g of sampled soil oven-dried at 105 ℃ for 24 h. Soil pH was measured in a 1:2.5 soil / water suspension. SOC was measured by dichromate oxidation and titration with ferrous ammonium sulfate(Lu，2000). Total N，NH₄⁺-N and NO₃^－-N in filtered 2 mol·L^-1 KCl - extracts of fresh soil sample were measured with a flow injection auto analyzer(FIA)(Lachat Quik-Chem 8000，Lachat Instruments，USA). Available P concentration was analyzed colorimetrically after acidified ammonium persulfate digestion(Anderson et al.，1993). Chemical composition of SOC was determined using solid-state ¹³C nuclear magnetic resonance(NMR)spectroscopy(Bruker BioSpin Gmbh，Karlsruhe，Germany).

Soil respiration was measured using the static chamber and gas chromatography techniques，gas samples were taken with a 100 mL plastic syringe at 0，10，20，and 30 min after chamber closure(Mo et al.，2007). CO₂ concentrations were determined using an Agilent 7890A gas chromatograph.

Soil microbial biomass carbon(MBC) and microbial biomass N(MBN)were estimated by chloroform fumigation-extraction. MBC and MBN(mg·kg^-1)were calculated according to Wu et al.(1990) and Joergensen et al.，1990)，respectively.

Soil microbial biomass and community structure were determined using phospholipid fatty acid(PLFA)analysis as described by Bossio et al.(1998). Concentrations of each PLFA were calculated based on the 19:0 internal st and ard concentrations. The relative abundance of individual fatty acid was expressed as the proportion(mol%)of the sum of all fatty acids. Gram-positive bacteria were identified by the PLFAs: i14: 0，i15: 0，i16: 0，i17: 0，a15: 0，a17: 0，Gram-negative bacteria were identified by the PLFAs: cy17:0，15:0 3OH，16:1 2OH(Liu et al.，2013; Frostegård et al.，1996). The fungi were identified by the PLFAs: 18:1ω9c(Myers et al.，2001)，and PLFAs 16:1ω5c were used as a marker for arbuscular mycorrhizal fungi(AMF)(Olsson，1999). The ratio of fungal-to-bacterial PLFAs(18:1ω9c / i14:0，i15:0，a15:0，i16:0，i17:0，a17:0，cy17:0)was used as an indicator of changes in the relative abundance of these two microbial groups(Cao et al.，2010). The actinomycetes were identified by the PLFAs 10Me 17: 0 and 10Me 18: 0(Zak et al.，1996).

One-way ANOVAs and Duncan’s test were used to determine statistical significant differences(P< 0.05)in soil chemical characteristics，soil microbial variables among different N addition treatments，which was performed using SPSS software package 19.0 for Windows. Additionally，correlations between soil chemical properties and microbial variables were determined using the Pearson’s correlation coefficients. The composition of soil microbial community was summarized using a principle component analysis(PCA)on the relative abundances(mol%)of 23 PLFAs in each sample. Furthermore，the relationship between soil microbial community composition and soil chemical properties was determined using Redundancy analysis(RDA). In addition，soil chemical properties were tested for significant contribution to the explanation of the variation in soil microbial community composition with the Monte Carlo permutation test(P< 0.05). PCA and RDA were conducted using CANOCO software for Windows 4.5(Microcomputer Power，Inc.，Ithaca，NY). The figures were plotted by Origin Pro 9.0(Origin Lab Corporation，Northampton，MA，USA).

By the time of sampling in May 2013，Medium-N and High-N addition significantly altered soil chemical properties(Tab. 1). High-N treatment significantly decreased soil pH by 12.5%，SOC concentration in High-N addition plots was significantly lower(18.1%)than that in the Control plots(P< 0.05). Moreover，concentrations of soil total N，NH₄⁺-N and NO₃^－-N significantly increased in Medium-N and High-N treatment plots in comparison with the Control plots(P<0.05). Soil available P and total P concentrations were not significantly influenced by N additions(P> 0.05)(Tab. 1).

Tab.1 Soil chemical properties after one-year N addition^①

Tab.1 Soil chemical properties after one-year N addition① T pH(H2O) SOC/(g·kg-1) Avail. P/(mg·kg-1) Total P/(g·kg-1) Total N/(g·kg-1) NH4+-N/(mg·kg-1) NO3－-N/(mg·kg-1) ①Values are mean ± standard errors for the sampling plots (n=3). T: Treatments; C: Control; L: Low-N; M: Medium-N; H: High-N. Significant differences (P<0.05, Duncan’s test) among treatments are indicated by different letters. C 4.55±0.04a 15.70±0.78a 2.04±0.24a 0.19±0.01a 0.98±0.09c 13.59±1.59c 8.72±0.81c L 4.44±0.12a 14.59±0.49ab 1.98±0.32a 0.18±0.02a 1.01±0.09bc 16.76±2.28c 11.68±1.13c M 4.33±0.43ab 13.76±2.47ab 1.70±0.37a 0.28±0.06a 1.21±0.12ab 23.97±2.29b 16.76±0.94b H 3.98±0.08b 12.86±0.92b 1.63±0.16a 0.21±0.08a 1.32±0.11a 29.04±1.76a 21.68±3.44a

SOC of all treatments included four carbon functional groups: Alkyl-C [(0-47)×10^-6]，O-alkyl C[(47-112)×10^-6]，Aromatic-C[(112-165)×10^-6] and Carbonyl-C[(165-215)×10^-6].High-N treatment led a significant decline in the proportion of O-alkyl C by 6.1% and increase in the ratio of Alkyl-C and O-alkyl C by 6.1%(P<0.05)(Tab. 2).Other variables including Alkyl-C，Aromatic-C，Hydrophobic-C，Hydrophilic-C and Hydrophobic-C/Hydrophilic-C were not significantly influenced by N addition(P>0.05)(Tab. 2).

Tab.2 Comparisons of chemical composition of SOC among treatments^①

Tab.2 Comparisons of chemical composition of SOC among treatments① Variables Control Low-N Medium-N High-N ① Hydrophobic-C=Alkyl-C + Aromatic-C, Hydrophilic-C=O-alkyl C + Carbonyl-C; Values are mean ± standard errors. Significant differences (P<0.05, Duncan’s test) among treatments are indicated by different letters. Alkyl-C (%) 22.2±1.6a 23.1±1.0a 22.5±1.8a 22.4±0.8a O-alkyl C (%) 45.6±1.3a 45.9±1.4a 43.8±0.9ab 42.8±0.6b Aromatic-C (%) 20.3±0.9a 18.9±1.1a 21.5±1.3a 20.5±0.9a Carbonyl-C (%) 11.9±1.6a 12.1±0.9a 12.2±0.8a 14.3±1.9a Alkyl-C/O-alkyl C 0.49±0.01b 0.50±0.01b 0.51±0.01ab 0.52±0.01a Hydrophobic-C(%) 42.6±2.6a 42.1±1.8a 43.6±2.1a 42.9±1.5a Hydrophilic-C(%) 57.4±2.9a 58.1±3.2a 56.1±2.1a 57.1±3.2a Hydrophobic-C/Hydrophilic-C 0.74±0.02a 0.71±0.05a 0.77±0.01a 0.76±0.02a

Medium-N and high-N treatments significantly decreased MBC concentration and increased MBN concentration relative to the Control treatment(P<0.05)(Fig. 1a，b).Soil respiration in High-N treatment plot [(43.2±1.2)mg·m^-2 h^-1CO₂-C] was 10.3% lower compared with the Control plots [(48.1±2.7)mg·m^-2 h^-1CO₂-C](Fig. 2).In addtion total microbial biomass，bacterial biomass and fungal biomass in Medium-N and High-N treatment plots decreased compared to the Control plot(P<0.05)(Fig. 3a，b，c).

	Fig. 1 Microbial biomass C (a) and Microbial biomass N (b) in soil samples after one-year N additions Significant differences (P<0.05, Duncan’s test) among treatments are indicated by different letters. Error bars show standard errors (n=3).

	Fig. 2 Comparisons of soil respiration among treatments Values are means for three months (March 2013 to May 2013). Significant differences (P<0.05, Duncan’s test) among treatments are indicated by different letters. Error bars show standard errors (n=9).

	Fig. 3 Comparisons of soil total PLFAs (a), bacterial PLFAs (b), fungal PLFAs (c), and F:B (d) among treatments after one-year N addition F:B: the ratio of fungal to bacterial PLFAs. Significant differences (P<0.05, Duncan’s test) among treatments are indicated by different letters. Error bars show standard errors (n=3).

The ratio of fungi to bacterial PLFAs was significantly increased by high-N treatment(9.5%)compared to the control plots(P<0.05)(Fig. 3d). Futhermore，Medium-N and High-N treatments significantly decreased the relative abundance of gram-positive bacterial PLFAs by an average of 4.7% and 8.5%，but significantly increased the relative abundance of actinomycetes PLFAs by an average of 8.0% and 7.0% relative to the control plots(P<0.05)(Fig. 4).

Fig. 4 Relative abundances of the individual PLFAs (mol%) in soil samples G+: The proportion of gram-positive bacterial PLFAs. G-: The proportion of gram-negative bacterial PLFAs; Fungal: The proportion of fungal PLFAs. Actino.: The proportion of actinomycetes PLFAs. AM: The proportion of AM fungal PLFAs. Significant differences (P<0.05, Duncan’s test) among treatments are indicated by different letters. Error bars show standard errors (n=3).

Principal Components Analysis(PCA)of the microbial community composition，defined by the PLFA profile using 23 individual PLFAs，demonstrated that the first two axes explained 36.6% and 19.6% of the total variation in microbial communities(Fig. 5a，5b)，respectively. According to the patterns in the PCA plot，the Control treatment samples were so close to the Low-N treatment samples，showing that Low-N treatment did not significantly affect soil microbial community compositions relative to the Control treatment(Fig. 5a). In addition，Medium-N and High-N treatment samples were located in different quadrants and clearly separated from the Control and Low-N treatment samples by PC2，indicating that Medium-N and High-N treatments significantly altered the microbial community structure，the main reason might be their lower relative abundance of gram-positive bacterial PLFAs and higher relative abundance of actinomycetes PLFAs under N addition compared to the Control samples(Fig. 5a，Fig. 4).

	Fig. 5 Phospholipid fatty acid (PLFA) pattern in soil samples from different N addition plots (a) and PC loadings of the individual PLFAs (b) Values are means (n=3) with bidirectional error bars of axis 1 and 2. For both the PCA plots, values on the PC1 and PC2 axes represent the percent variation explained by PC1 and PC2, respectiv

For individual PLFAs，Gram-positive bacterial PLFAs such as a15: 0，i15: 0 and i16: 0 got higher scores on PC1，other PLFAs such as 18: 0 2OH also got higher scores on PC1. While on PC2，Gram-negative bacterial PLFAs such as cy17: 0 and 16: 1 2OH got higher scores. Additionally，other PLFAs such as Gram-positive bacterial PLFAs(i14: 0，i17: 0 and a17: 0) and AMF PLFAs biomarker 16: 1ω5c got higher scores on PC2 as well(Fig. 5b).

The correlations between soil microbial community and soil chemical properties were analyzed by RDA and the significance of environmental variables present in the ordination was determined by Monte Carlo permutation tests(P<0.05). The results showed that seven soil variables，including soil pH，SOC，NH₄⁺，NO₃^－，TN，available P and total P，explained 48.5% of the variation in soil microbial community composition(Fig. 6). Soil microbial community composition was significantly related to NO₃^－(F=4.09，P=0.002) and total P(F=2.03，P=0.028)，with RDA axis1 explaining 33.6% of the variance and RDA axis 2 explaining 14.9%，respectively(Fig. 6). The Control plot samples were almost clearly separated from Medium-N and High-N treatment samples by RDA axis 2. In addition，NO₃^－ and total P showed a negative association with RDA axis 1(Fig. 6).

	Fig. 6 Redundancy Analysis (RDA) results of soil microbial community composition and soil chemical properties In RDA plots, values on the x and y axes represent the percent variation explained by RDA axis 1 and RDA axis 2, respectively (P<0.05). SOC and TN refer to soil organic carbon and soil total nitrogen, respectively.

The proportional abundance of gram-positive bacterial PLFAs was positively correlated with pH，SOC and C/N，and negatively correlated with Total N，NH₄⁺-N and NO₃^－-N(P<0.01)(Tab. 3). For the proportional abundance of gram-negative bacterial PLFAs，it was negatively correlated with pH，SOC，Avail. P and C/N(P<0.05)(Tab. 3). In addition，the proportional abundance of fungal PLFAs was positively correlated with C/N(P<0.05) and negatively correlated with Total P，Total N，NH₄⁺-N and NO₃^－-N(P<0.05)(Tab. 3).Moreover，the proportional abundance of actinomycetes PLFAs was positively correlated with Total N，NH₄⁺-N and NO₃^－-N(P<0.01)，but negatively correlated with pH，SOC and C/N(Tab. 3). Furthermore，the proportional abundance of AMF PLFAs was not significantly correlated with soil chemical properties(P>0.05)(Tab. 3).

Tab.3 Pearson’s correlation coefficients between the proportional abundance of main microbial communities and soil chemical properties^①

Tab.3 Pearson’s correlation coefficients between the proportional abundance of main microbial communities and soil chemical properties① Community pH SOC Avail. P Total P Total N NH4+-N NO3－-N C/N ① G+: The proportion of gram-positive bacterial PLFAs; G-: The proportion of gram-negative bacterial PLFAs; Fungal: The proportion of fungal PLFAs; Actino.: The proportion of actinomycetes PLFAs; AMF: The proportion of AM fungal PLFAs. * and ** denote significant correlations at P<0.05 and P<0.01, respectively. G+ (mol%) 0.682* 0.690* 0.279 -0.193 -0.807** -0.893** -0.882** 0.815** G- (mol%) -0.628* -0.629* -0.658* 0.319 0.445 0.344 0.406 -0.582* Fungal (mol%) 0.573 0.414 0.400 -0.638* -0.650* -0.643* -0.642* 0.599* Actino.(mol%) -0.721** -0.671* -0.480 0.260 0.859** 0.815** 0.839** -0.857** AMF (mol%) -0.391 -0.244 -0.252 0.482 0.035 0.046 -0.034 -0.094

In this study，High-N addition significantly decreased soil organic carbon content(Tab. 1). N fertilization has been observed to stimulate ecosystem C loss(Liu et al.(2009)) and such a decline was consistent with a recent report by Wei et al.(2011). A possible mechanism is that N addition stimulated SOC decomposition more than plant production，leading to a net loss of ecosystem C(Mack et al.，2004)，and another one is that higher atmospheric N deposition resulted in higher C loss by increasing heterotrophic respiration and dissolved organic carbon leaching(Bragazza et al.，2006). In addition，N addition have significant effects on C inputs processes from plants，such as amount of litterfall and litter decomposition(Knorr et al.，2005)，fine root biomass(Wallenstein et al.，2006) and root exudation(Bowden et al.，2004)，which were closely related to soil organic carbon content dynamics.

High-N treatment significantly decreased soil pH(Tab. 1)，such a finding was consistent with the result from a N addition study on larch plantation(Hu et al.，2010). A decline in soil pH may be resulted from NH₄⁺ uptake by plants，nitrification of NH₄⁺ in soils，and NO₃^－ leaching(Matthew et al.，2006). In a mature subtropical forest of China，Lu et al.(2009)reported that the ecosystem was sensitive to high N addition，and soil acidification was significantly enhanced after continuous two-year N additions(50-150 kg·hm^-2a^-1 N). Furthermore，Fan et al.(2007)found that exchangeable base cations(e.g. Ca²⁺ and Mg²⁺)reduced with increasing N addition in a subtropical Chinese fir plantation after three years of N addition(60-240 kg·hm^-2a^-1N)，the nitrate leached out was accompanied by positively charged “counter-ions”，the base cations K⁺，Ca²⁺ and Mg²⁺，resulting in the further acidification of the leached soil，or hydrogen and aluminum ions，which may cause the acidification of receiving ecosystems.

Medium-N and High-N additions significantly decreased MBC concentration in this study(Fig. 1a). Decreased soil organic carbon and pH may be the main reason of reduced MBC，according to the related studies by Sarathchandra et al.(2001) and Wallensteina et al.(2006). Additionally，Medium-N and High-N application significantly increased microbial biomass N in the present study(Fig. 1b)，mainly because soil microbial biomass serves as both an important source and sink for plant available nutrients，and N availability was increased after N addition and consequently immobilized by microbes，which led to an increase in soil microbial biomass N and a decline in nutrient loss(Garcia et al.，1994; Wang et al.，2008).

Medium-N and High-N additions significantly decreased total microbial biomass，bacterial biomass and fungal biomass，these findings are corresponding with the results obtained in field studies(Fig. 3)，where N fertilization decreased soil microbial biomass by an average of 11% to 35%(Ramirez et al.，2012). One explanation is the decrease in soil acidity，soil pH is a major factor influencing the structure of the soil microbial community and individual microbial PLFAs，the strongest influence of soil pH was on the fungal and bacterial growth(Bååth et al.，2003; Rousk et al.，2010)，so the declines of bacterial biomass and fungal biomass in this study may be due to the higher growth inhibition of Medium-N and High-N additions to bacterial and fungal communities，consequently leading a decline in soil total biomass. In addition，soil C availability and plant belowground C allocation play dominant roles in regulating soil microbial population size and community composition. The influences of N addition on soil microbial biomass and community structure are strongly linked to changes of plant production and belowground C supply(Hu et al.，2010)，so another possible mechanism to explain the reduction of soil microbial biomass is the changes of belowground C under N additions. In the present study，High-N treatment significantly increased the ratio of fungal to bacterial，such a result was consistent with a recent report from a comparable pH gradient in an arable soil，where the fungal:bacterial growth ratio also increased 50-fold between pH 8.3 and 4.0(Rousk et al.，2009). In addition，Liu et al.(2013)reported that F:B ratios were significant higher in the N-addition plots comparing to the control plots after four years N addition in a tropical forest of subtropical China.

In the present study，Medium-N and High-N additions significantly altered the soil microbial community structure(Fig. 5a). Allison et al.(2008)summarized the results of studies previously reported on the soil microbial community composition response to the mineral fertilization(N/P/K)，and the majority of these studies demonstrate that composition is sensitive to disturbance. More than 80% of the mineral fertilization(N/P/K)studies found significant effects of disturbance on microbial composition. And this study proved that soil microbial community composition in a young plantation of Ginkgo biloba was significantly sensitive to Medium-N and High-N additions.

High-N treatment led a significant reduction in soil respiration after 12 months N addition in this study(Fig. 2)，which was also found in other forest ecosystems(Mo et al.，2007; Burton et al.，2004; Chen et al.，2002). Ramirez et al.(2012)found that decreases in soil respiration by N addition ranged between 8% and 15%. The reductions in soil respiration after nitrogen additions may be possibly related to the following mechanisms: 1)N additions decreased autotrophic respiration from plant roots，decrease in soil respiration was related to the reduction in st and ing root biomass and root exudation in N-fertilized plots(Bowden et al.，2004); 2)Heterotrophic respiration from the microbial community may be reduced by N addition，which was indicated by that the decomposition rates of litter and soil organic matter were suppressed(Mo et al.，2007; Jiang et al.，2010).In the present study，soil respiration as an indices of soil microbial activity was measured for three months only to determine the difference of soil microbial activity among treatments rather than the dynamics of CO₂ emission，such a experiment design was also reported in other N addition studies(Liu et al.，2013)，so the mean soil respiration for three months as an indicator of soil microbial activity was appropriate. In contrast，several neutral effects of N addition on soil respiration have also been reported across different ecosystems. For example，Jiang et al.(2010)found that nitrogen deposition(during growing season)tended to decrease CO₂ emission，but the differences caused by nitrogen deposition were not significant in a short-term simulated nitrogen deposition experiment in an alpine meadow on the Qinghai-Tibetan Plateau. Similarly，Liu et al.(2013)reported that soil respiration did not significantly change after four years N addition in a tropical forest. So it is，therefore，that effects of N addition on soil respiration were different across ecosystems and need a better underst and ing.

The effects of N additions on soil microbial biomass and community structure in a young plantation of G.biloba were concluded that: 1)Low-N treatment did not significantly affect soil microbial biomass; 2)Medium-N and High-N treatments significantly decreased MBC(C)concentration but increased microbial biomass nitrogen(N)concentration; 3)Medium-N and High-N treatments significantly decreased soil total microbial biomass，fungal biomass，and bacterial biomass; 4)Medium-N and High-N treatments significantly decreased the relative abundance of gram-positive bacterial PLFAs，and increased fungal:bacterial ratio and the relative abundance of actinomycetes PLFAs; 5)High-N treatment significantly decreased soil pH，respiration，SOC concentration and the proportion of O-alkyl but increased Alkyl:O-alkyl. Our conclusion indicated that the responses of soil microbial biomass and community structure in a young plantation of G.biloba varied due to the amount of N loaded.