Human activities such as fossil fuel burning, forest disturbance and land-use conversion have increased the global atmospheric concentration of carbon dioxide CO₂ as well as atmospheric deposition of nitrogen (IPCC, 2007; Matson et al., 2002). Increasing temperature can bring prolonged drought and increase the availability of N in many terrestrial ecosystems (Aber et al., 2003; IPCC, 2007). For such cases, how to response the impact by the drought and increasing N deposition for Pinus tabulaeformis become the focus of many researchers.

N is one of the most important nutrients affecting the growth, development, yield, fruit quality, and physiology of plants (Fernandes et al., 1995; Gerendás et al., 1997; Lebauer et al., 2008; Vitousek et al., 1991). It is required in the greatest quantities at each stage of plant growth during which N level markedly affects the amount of Rubisco content, and therefore photosynthesis (Evans, 1989; Evans et al., 1988). When the amount of available N in the soil cannot meet plant requirements, an increase in input of soil N may stimulate plant growth and photosynthesis (Hossain et al., 2010) by increasing leaf area and plant biomass (Liu et al., 1992), shoot / root ratio (Pregitzer et al., 1990), net photosynthetic rates (Mitchell et al., 1993; Van Hove et al., 1989), leaf N content (Mitchell et al., 1993; Mulligan, 1989), chlorophyll content (Chandler et al., 1995; Ripullone et al., 2003), and stomatal conductance (Dickmann et al., 1992). Similarly, N addition, such as atmospheric deposition, can enhance the plant tolerance to abiotic stresses such as water deficit, salt, and high temperature stresses (Lauter et al., 1981; Wu et al., 2008a). Conversely, an excess of soil nitrogen may limit plant growth and photosynthetic capacity, due to accelerated soil acidification, reducing the absorption of other mineral elements (Hossain et al., 2010).

Soil water is considered another key environmental factor that strongly affects plant growth and photosynthesis (Nemani et al., 2003), particularly so for soil water stress (Lawlor, 1995a). It is well known that one of the primary physiological consequences of water deficit is photosynthesis inhibition(Brestic et al., 1995; Lawlor, 1995b), attributed mainly to stomatal closure, reduced mesophyll conductance, and inhibition of Rubisco activity (Chaves et al., 2004; Earl et al., 2005; Flexas et al., 2004; Foyer et al., 1998; Grassi et al., 2005). While the major effects of water deficit on plant function are decreased shoot growth due to decreased leaf biomass and leaf area allocation, and increased leaf N content (Alves et al., 2000). Soil water affects plant growth and photosynthesis directly on the one hand by influencing leaf phenology (Peñuelas et al., 2009) and photosynthetic rate (Patrick et al., 2009), and on the other hand indirectly by influencing the absorption of nutrients (Otsus et al., 2004; Peñuelas et al., 2009), by providing the medium for nutrients uptake by roots (Ibrahim et al., 1998). So the effects of soil water and N nutrition on plant photosynthesis and growth are highly linked (Hu et al., 2005).

Soil N availability can be affected by soil water availability (Engelbrecht et al., 2007; Quaye et al., 2009; Song et al., 2010) via several microbialmediated pathways, such as litter decomposition (Liu et al., 2006) and N mineralization (Wang et al., 2006). Adequate soil water positively affects N input, decomposition, mineralization, and physical transport, thereby increasing N availability to plant growth and photosynthesis (Burke et al., 1997), while water deficit reduces N uptake and plant growth (Misra et al., 2000). Many studies have shown that, under well-watered conditions, additional N supply significantly promoted plant growth and photosynthesis, but under water stress conditions, N addition had negative effects on plants (Song et al., 2010; Sun et al., 2011). Therefore, appropriate N supply under drought may stimulate plant growth and alleviate the effects of water stress by preventing cell membrane damage and enhancing osmoregulation (Brueck et al., 2010; Liu et al., 2012), whereas, excess N application reduced biomass allocation to root (Patterson et al., 1997), increased leaf sensitivity to water stress (Tan et al., 1997), and decreased plant growth (Liu et al., 2012). So appropriate N supply is recommended to improve plant growth and photosynthetic efficiency under water stress (Shangguan et al., 2000). However, some other studies found no significant interactions between N and soil water to plant photosynthesis and growth (Eghball et al., 1993; Song et al., 2010; Wang et al., 2012; Wu et al., 2008b). Therefore, soil water and N co-act to regulate plant growth and photosynthesis may show different response to each other for diverse species.

P. tabulaeformis, an endemic evergreen coniferous species in China, is a key species of coniferous forests in arid, semi-arid and semi-humid regions of China, and widely used as afforestation and reforestation tree species in ecological restoration and soil conservation programs (Zheng et al., 1978). These P. tabulaeformis forests spread naturally from northeast to north and from central to west of China, across 10 provinces (Liaoning, Neimeng, Hebei, Shandong, Shanxi, Gansu, Shanxi, Qinghai, Henan and Sichuan) and 1 metropolis (Beijing), between 103°20'E to 124°45'E, 31°00'N to 43°33'N, and 100 to 2 600 m, a. s. l, with its distribution central in Shanxi (Liu, 2002; Zheng et al., 1978). There is a large difference over these regions in climatic conditions, as the average annual rainfall range from 400 mm to 1 000 mm and the average annual temperature range from 1 to 16 ℃. Therefore, P. tabulaeformis is assumed insensitive to water conditions. And as reported by Zheng et al. (1978) P. tabulaeformis was a poor resistance plant. Thus, we hypothesize that the co-act of soil water and N will change the responses of growth, biomass production and photosynthesis for P. tabulaeformis to soil water and N.

The objective of this study was to investigate the effects of different soil water content and N addition on the growth and photosynthesis of P. tabulaeformis seedlings by measuring whole-plant growth, leaf area, biomass production and allocation, leaf photosynthesis, and chlorophyll fluorescence. Better understanding of the interactions between soil water and N on P. tabulaeformis may provide critical insights on the potential responses of the P. tabulaeformis forest to climate change associated with increasing drought and atmospheric N deposition and therefore improve the management of P. tabulaeformis plantations.

1 Materials and methods 1.1 Experimental design

The experiment was conducted in Xiaotangshan Experimental Site of Beijing Forestry University, located in the northern suburb of Beijing. A nested design was used with soil water as the primary factor and N addition as the secondary one. The experiment involved four adjacent greenhouse compartments, each 3. 0 m × 3. 0 m × 2. 0 m (W × L × H) in size. Each greenhouse compartment was subjected to one of four soil water regimes: 1) 8 % of soil water content (W1); 2) 12 % soil water content (W2); 3) 16 % of soil water content (W3); 4) 20 % of soil water content (W4). And then each of the four greenhouse compartments was divided into four plots, each plot was subjected to one of four N addition levels: 1) 0 mg·kg ^-1 (soil) (N₁); 2) 31. 25 mg·kg ^-1 (soil) (N₂); 3) 62. 50 mg·kg ^-1 (soil) (N₃); 4) 93. 75 mg·kg ^-1 (soil) (N₄). Each subplot replicate had 20 seedling pots, and the experiment 320 pots in all.

1.2 Plant culture and growth measurements

Two-year-old P. tabulaeformis seedlings (average of 18 cm in height with about 50 new needles unfolded) were obtained from Container Tree Seedling Nursery in Luanping County, Heibei Province. In midMarch 2012, seedlings were transplanted into plastic cylindrical pots (25 cm diameter × 30 cm depth) filled with field soil, with one seedling per pot. Field soil was collected from the Xiaotangshan Experimental Station, and was a mixture of sand and peat (1 : 1 volume) with medium fertility (pH 7. 8, N 19. 6 mg· kg ^-1, P 4. 6 mg·kg ^-1, K 135 mg·kg ^-1). The soil was air dried and 8 kg of the field soil was added to each pot.

In late May 2012, 320 healthy and similarly sized seedlings were selected from all seedlings and were randomly divided into 16 groups of 20 seedlings (one group per each water regime / N level / replication). An additional 20 seedlings were used to determine the average initial dry mass. Soil water was monitored by weighting, supplementing water as needed (W1, 8%; W2, 12%; W3, 16%; W4, 20% soil water content) every two days. The four N levels were controlled by inputting urea (purity ≥ 99. 5% of CH₄N₂O, Urea Amresco0568, USA) 10 times to the potted soil and once every 10 days during the experiment after watering. Every time inputted N to the four N levels was: N1, 0 mg·kg ^-1; N2, 3. 125 mg·kg ^-1; N3, 6. 250 mg·kg ^-1; N4, 9. 375 mg·kg ^-1. Treatments began on 15 June 2012, when seedlings were about 20 cm in height, and ended on 25 October 2012. Seedlings grew under each treatment for 130 days. The average growing season temperatures and relative humidity in the greenhouse was kept as ambient (20 -36 ℃, 30% -76%). The experimental layout was surrounded with a single row of border plants to protect the experimental seedlings from external influences, and all subplots and main pots were rotated weekly to provide for random distribution.

One destructive harvest was conducted at the end of the experiment on 10 seedlings from each nitrogen supply treatment. During the harvest, we measured the heiqht of the main stem, diameter at stem base, and separated the seedling into roots (washed free of soil) and shoots by severing at the root collar, and the shoots were then further divided in stem (including branches and petioles) and leaf components. All harvested samples were oven-dried at 80 ℃ for 48 h, then weighed. During the experiment, we collected all dead leaves from each plant and the leaf mass was added to the final harvest data (Anten et al., 2001). For each plant, total leaf area was calculated as mean leaf area per leaf multiplied by the number of leaves per plant, the number of leaves per plant were counted at harvest and the mean leaf area per leaf was calculated as described by Li et al. (2007).

1.3 Leaf gas exchange measurements and chlorophyll fluorescence emission

Spot measurements was carried out in the experimental field on four similarly clear days of 2 July, 23 July, 17 August and 21 September between 09:00 and 11:30 (the mean values of the four days used), using a portable photosynthesis analyzer (LI-6400, Li-Cor, Lincoln, NE, USA) supplying photosynthetic photon flux density by an red and blue leaf chamber. Net photosynthesis at saturating light (500 ± 50 μmol · m ^-2 s ^-1) (A_sat), stomatal conductance (G_s), transpiration (Tr), intercellular CO₂ concentration (C_i), and ambient CO₂ concentration (C_a) were measured. Using A_sat and Tr, the specific leaf water use efficiency (WUE_L, defined as the ratio of net photosynthesis to transpiration) was calculated.

Chlorophyll fluorescence emissions were measured immediately following spot measurements using the portable photosynthesis analyzer fitted with fluorometer chamber. Steady-state fluorescence from lightacclimated needles (F_s), maximal fluorescence from light-acclimated leaves during transient exposure to supersaturating light intensities (F_m'), minimal fluorescence from light-acclimated leaves upon transient exposure to weak far-red illumination (F_o'), maximal fluorescence from dark-adapted needles during transient exposure to super-saturating light intensities (F_m), and minimal fluorescence from dark-adapted needles during transient exposure to weak far-red illumination (F_o) were recorded. These measured variables were used to determine PSII actual efficiency (ΦPSⅡ = F_v'/F_m', where F_v'= F_m'-F_s), which is equivalent to the PS Ⅱ quantum yield under ambient light conditions; PS Ⅱ maximum efficiency (F_v /F_m, where F_v= F_m-F_o), which represents the intrinsic efficiency of PSII in the fully-oxidized state, and the estimated PSII photochemical quenching (q_P = (F_m' -F_s) / (F_m'-F_o')) (Baker, 2008).

All above measurements were conducted on attached fully expanded, sunlit leaves exposed to ambient atmospheric pressure (110 ± 0. 3 kPa), temperature (30 ± 1 ℃), and CO₂ concentration (360 μL·L ^-1). Three sample seedlings of P. tabulaeformis for each soil water regime / N level / replication treatment were used. And for each sample seedlings, three single attached leaves were used. Regularly, before measurements were recorded, each needle was allowed 5 -10 min to equilibrate to chamber conditions, when readings were stable and the coefficient of variation was < 1%.

1.4 Statistical analyses

Statistical analyses were performed with the Statistical Software Package for the Social Science (SPSS, version 13. 0). A two-way analysis of variance (ANOVA) was used to determine the differences of soil water treatments, nitrogen supply treatments and their interactions on mean variables (i. e. n = 4). Means were compared using Duncan' s test. In all analyses, test results were considered significant if P < 0. 05 and highly significant if P < 0. 01.

2 Results 2.1 Plant biomass production and growth

An increase in soil water from W1 to W2 resulted in significant increases for both stem height and diameter (Fig. 1a, b). The increase in soil water from W2 to W3 resulted in no significant effect on stem height and diameter (Fig. 1a, b). When soil water increased to W4, stem height and diameter both dropped significantly (Fig. 1a, b). Under lower soil water treatments (W1 and W2), N supply decreased both the main stem height and diameter, and the range of decrease increased with decreasing N supply; but under the highest soil water treatment (W4), N supply increased both the main stem height and diameter, and the range of increase with increasing N supply (Fig. 1a, b). The treatment W3N2 was markedly stimulated both main stem height and diameter larger than the other treatments (Fig. 1a, b). Additionally, under W3 soil water condition, N3 and N4 treatments had positive effects in main stem diameter, but had adverse effects in main stem height (Fig. 1a, b).

The soil water effects on plant biomass increased with the increasing of soil water content from W1 to W3 then decreased with the further increase of soil water content (Fig. 2a). However, W3 treatment markedly increased both plant biomass and leaf area than other soil water treatments, but the effects of W1, W2, and W4 on leaf area were not significantly different (Fig. 2a, b). Furthermore, under lower soil water treatments (W1, W2), N supply decreased both plant biomass and leaf area (Fig. 2a, b). While under the highest soil water treatment (W4), N supply increased both plant biomass and leaf area (Fig. 2a, b). Seedlings grown under the W3N2 treatment had the highest biomass and leaf area (Tab. 1 and Fig. 2a, b).

2.2 Biomass partitioning

The decrease in soil water from W4 to W2 caused a slight increase in aboveground biomass (leaf biomass and stem biomass) but a decrease in root biomass, which led to the decrease in the ratio of root to shoot biomass (R / S) (Fig. 3a-d). A further decrease in soil water to W1 caused a decrease in aboveground biomass and an increase in root biomass, resulting in increased R / S (Fig. 3a-d). N treatments had different effects on leaf biomass, stem biomass, root biomass, and R / S under different soil water treatments. For the lowest soil water treatment (W1), N2, compared with N1, caused a significant decrease in R / S, but further increase in N supply improved R / S (Fig. 3d). Under W2 soil water condition, N4 addition resulted in a significant (P < 0. 05) decrease in R / S, whereas no significant difference in R / S was observed between N2 and N3 addition (Fig. 3d). Under W3 and W4 soil water treatments, N1 produced highest R / S over the other three N treatments and no significant difference in R / S was found among N2, N3 and N4 treatments (Fig. 3d).

2.3 Leaf gas exchange

The combination of lower soil water content (W1, W2) with different N supply didn' t result in significant changes to leaf A_sat, but in higher soil water treatments (W3, W4), N supply caused significant difference in leaf A_sat (Fig. 4a). In high soil water treatment (W4), A_sat increased with increasing of N supply (Fig. 4a). Leaf A_sat markedly increased by N2 under W3 but by N4 addition under W4 treatments (Fig. 4a). And the W3N2 treatment had the highest value of leaf A_sat as 4. 57 μmol· m ^-2 s ^-1, significantly higher than the other treatments (Fig. 4a).

Similarly to the responses of leaf A_sat, G_s was significantly enhanced by N2 and N4 addition under W3 and W4 treatments (Fig. 4b). And under W3N2 treatment, G_s exhibited the highest as 0. 032 mol·m ^-2 s ^-1 (Fig. 4b). However, under lower soil water conditions (W1, W2), the effects of different N addition on G_s were not significantly different, but with the increase of soil water content, the effects of different N addition on G_s were increasingly significant (Fig. 4b).

The responses of WUE_L to the interactive effects of soil water and N treatments were different to A_sat and G_s. With an increase in soil water, differences among the effects of the four N treatments increased (Fig. 4c). N addition didn 't change WUE_L under lower soil water condition (W1). But a significant increase in WUE_L was found in W2N1 and W2N2 treatments, whereas N3 and N4 addition didn' t affect WUE_L in W2 treatment. However, under high soil water conditions (W3, W4), N3 and N4 addition markedly enhanced WUE_L, while N1 and N2 addition didn't affect WUE_L. Seedlings grown in the W3N4 and W4N4 treatments had the highest WUE_L at 3. 572 mmol·mol ^-1 s ^-1.

2.4 Chlorophyll fluorescence

Actual PS Ⅱ efficiency and PS Ⅱ photochemical quenching were significantly influenced by soil water treatments in P. tabulaeformis seedlings (P < 0. 05) (Fig. 5a, c). With the increase in soil water from W1 to W3, the values of actual PSⅡ efficiency tended to increase slightly, but the values of PSⅡ photochemical quenching increased slightly from treatments W1 to W2, and then decreased significantly at the highest water content (W4) (Fig. 5a, c). Additional N significantly decreased actual PSⅡ efficiency and PSⅡ photochemical quenching under lower soil water conditions (W1, W2), but increased them under the highest soil water treatment (W4) (Fig. 5a, c). In addition, markedly interactive effects of W3 and N2 were detected on actual PS Ⅱ efficiencies and PS Ⅱ photochemical quenching as their values were the highest (Fig. 5a, c). However, PS Ⅱ maximum efficiency was not significantly affected by both soil water and N treatments, although it was also highest for the W3N2 treatment (Fig. 5b).

3 Discussion 3.1 Plant growth, biomass production and partitioning

In this study, we demonstrated significant interactive effects of N addition and soil water on the plant growth and biomass production of P.tabulaeformis seedlings. We showed that the lowest and the highest soil water treatments (W1, W2 and W4) all decreased seedlings growth relative to the W3 treatments in terms of plant main stem height and diameter, plant total biomass, and leaf area. Stem height was reduced more than stem diameter under lower water conditions, suggesting that height growth is more sensitive to soil water than diameter growth. This agree with the result for Sophora davidii seedlings (Wu et al., 2008a). The reduction of the seedlings growth induced by low soil water (W1, W2) was aggravated by N addition. This response of P. tabulaeformis is contrary to Fraxinus mandschurica and Sophora davidii seedlings, annual grass and wheat (Cabrera-Bosquet et al., 2007; Wang et al., 2012; Wu et al., 2008b; Zhou et al., 2011). However, P. tabulaeformis' s response to the highest soil water treatment (W4), is consistent with report for F. mandschurica seedlings, apple trees, annual grass and wheat, that the reduction of the plant growth induced by high soil water supply was significantly attenuated by N addition and this tendency was partially diminished by N addition (Cabrera-Bosquet et al., 2007; Liu et al., 2012; Reich et al., 2006; Wang et al., 2012; Zhou et al., 2011). Similar result was also obtained by Ibrahim et al. (1998) in poplar. The growth responses of the seedlings to the interactive effects of soil water and N addition suggest that the effect of N supply is closely related to the soil water availability (Hu et al., 2005; Reich et al., 2006; Wang et al., 2012), and N addition could amplify the negative effects of lower soil water (W1, W2) but could alleviate the negative effects of highest soil water (W4) manipulation on P. tabulaeformis seedlings growth. This result agrees with the hypothesis that there is co-act of soil water and N to the growth of P. tabulaeformis, and it changed the responses of P. tabulaeformis to soil water and N. In addition, the W3N2 treatment resulted in the greatest growth indicating an appropriate N supply and optimum soil water conditions to the growth of P. tabulaeformis seedlings (Reich et al., 2006; Wang et al., 2012).

The shifts in biomass allocation also had an important impact on tree growth in the acclimation to changes of soil nutrient and water content (Domisch et al., 2002; Reich et al., 1995). So the ratio of root to shoot biomass (R / S) is an indicator that represents the changes in belowground biomass and aboveground biomass allocation attributed by soil nutrient and water content changes (Lambers et al., 1998). Previous studies of Arndt et al. (2001),Marron et al. (2002), Li et al. (2003), Yin et al. (2005), Ma et al. (2009) and Wang et al. (2012) found N limitation and drought stress increased carbon translocation from the leaves to the roots, thereby increased the R / S ratio. According to the resource depletion model of competition processes, increased root allocation is an adaptive response to belowground resource limitation, when belowground resources are the limiting factors, so the relative growth rates of all the individuals are not reduced by the same proportion, it would be more likely that plants will adopt different resource allocation strategies and alert the relative proportion of biomass allocation in order to allocate more resource to belowground components (Munson et al., 1990; Newton et al., 1993; Nilsson et al., 1993; Weiner et al., 1986). But our result contrary to theirs, as soil water decrease not obviously increased carbon allocation from the leaves to the roots thereby increasing the R / S ratio. That may be because W1, W2, W3 and W4 have not obviously below the critical point of water stress.

It is also shown that N addition did not drive an alternation in the ratio of the aboveground and belowground biomass in P. tabulaeformis seedlings in W2, W3, and W4 treatments. This may be because N nutrition is not the limitation factor in W2, W3, and W4 treatments, as N addition had alleviated the N nutrient shortage in a short term, leading to nonsignificant differences in biomass allocation in different tree components. According to the resource depletion model of competition processes, when resources are adequate, the relative proportion of biomass allocation to different tree components might be unchanged (Chang et al., 1996). However, under lower soil water condition (W1), N addition increasing stimulated an alternation in the ratio of the aboveground to belowground biomass, which indicated that biomass allocation of P. tabulaeformis seedlings might be limited by N addition in lower soil water conditions. That may be because water deficit limited the availability of N, and soil water and N both became limiting factors, so plants allocated more biomass to belowground components as a way of response to belowground resource deficiencies (Chang et al., 1996).

3.2 Leaf gas exchange

The leaf gas exchange study provides insight into the mechanism of the interactive effects of soil water and N addition on the photosynthesis of P. tabulaeformis seedlings. Similar results were found for F. mandschurica seedlings (Wang et al., 2012), durum wheat (Cabrera-Bosquet et al., 2007), and hybridized species (Campbell et al., 2010). Under the low soil water treatments (W1, W2) there was no significant difference between the effects of different N addition on leaf A_sat, G_s, and WUE_L, but under high soil water treatments (W3, W4) the difference was significant. The photosynthetic responses to soil N and water availability indicated that the effects of N on the photosynthetic rate of the seedlings strongly depend on soil water content (Liu et al., 2012; Ma et al., 2009; Nakaji et al., 2001). In addition, N2 addition significantly enhanced the effect of W3 regime on leaf A_sat. That indicates the photosynthetic rate of P. tabulaeformis seedlings might be dependent on soil N availability in high soil water conditions. Similar results also have been found in some hardwood tree species (Tyree et al., 2009; Wang et al., 2012; Wang et al., 1998; Wendler et al., 1996).

We also investigated the changes of G_s response to different soil water and N addition to explain the potential mechanism in leaf photosynthesis. The results showed under lower soil water conditions (W1, W2), N addition had no significant effects on G_s, but under high soil water conditions (W3, W4), N addition led to significantly enhancement of G_s for P. tabulaeformis seedlings. Similar results had been found in wheat, F. mandschurica seedlings, and nine boreal tree species (Cabrera-Bosquet et al., 2007; Reich et al., 1998; Wang et al., 2012). In addition, N2 and N4 addition triggered a significant increase in G_s of seedlings under W3 and W4 soil water treatment. It's likely that N addition accelerate the transport of photosynthetic CO₂ in the leaves, leading to enhanced A_sat of the seedlings (Wang et al., 2012).

Furthermore, we investigated the shifts of WUE_L response to different soil water and N addition, as WUE_L is a functional indicator strongly related to plant growth and health under water deficit condition, and is mostly dependent on the amount of water used for growth and biomass production (Liu et al., 2004; Monclus et al., 2006). Some studies reported WUE_L was improved under water limitation (Liu et al., 2005), but some others have found the inverse case (Clavel et al., 2005; Wu et al., 2008a). In this study, P. tabulaeformis seedlings employed both none of the above two strategies neglect N effects, it 's WUE_L declined with the increase of soil water under high soil water conditions (W3, W4), but increased with the increase of soil water under low soil water conditions (W1, W2). That might be attributed to the different soil water regimes applied in the experiments. In addition, we found the changes of WUE_L indicated that the effect of N addition on plant water use strongly depends on the availability of soil water, as the effects of N addition on WUE_L increased with soil water increasing. This is likely because the increasing biomass production simulated by N addition under high soil water conditions. But this result consistent to that reported by Liu et al. (2012), Ma et al. (2009), and Nakaji et al. (2001), who found the effects of N addition on WUE_L increased with soil water decreasing.

3.3 Chlorophyll fluorescence

The chlorophyll fluorescence parameters provide basic information on the function of the photosynthetic apparatus and on the capacity and performance of photosynthesis. In the present study, PS Ⅱ actual efficiency and PS Ⅱ photochemical quenching were obviously decreased by N addition in lower soil water regimes (W1, W2), but increased by N addition in the highest soil water regime (W4), which agree with previous study in S. davidii seedlings (Wu et al., 2008b). These results once again indicate that the effects of soil water and N addition on the photosynthesis of P. tabulaeformis seedlings highly interactive, as the N addition effects strongly depend on the soil water availability. Moreover, the W3N2 treatment markedly stimulated PS Ⅱ actual efficiency and photochemical quenching for P. tabulaeformis seedlings than other treatments. This indicate N2 and W3 treatments were the appropriate N supply and optimum soil water conditions to the photosynthetic capacity of P. tabulaeformis seedlings. However, our result also shown PS Ⅱ maximum efficiency was not significantly affected by soil water and N addition. It' s likely that the efficiency of harvesting light by P. tabulaeformis seedlings isn' t affected by soil water and N availability but other environmental factors or the internal factors. But further research is needed to reveal what factors affect the efficiency of harvesting light by P. tabulaeformis.

4 Conclusion

In conclusion, this study evaluated the interactive effects of N addition and soil water on the growth and photosynthetic responses of P. tabulaeformis seedlings, the forest of which widely distributed in the temperate ecosystem in China' s central and northern regions. We demonstrated P. tabulaeformis had different growth and photosynthetic responses to N addition in different soil water conditions. N addition significantly enhanced the growth and biomass production of the seedlings under plentiful soil water conditions (W3, W4), but aggregated the negative effect of low soil water treatments (W1, W2) on plant growth. Moreover, N addition could lead to an increase in the photosynthetic capacity under high soil water conditions (W3, W4), but a decrease in the low soil water treatments (W1, W2), which was paralleled with the shifts of PS Ⅱ actual efficiency and PS Ⅱ photochemical quenching. Furthermore, W3N2 treatment was the appropriate N supply and optimum soil water conditions to the growth, biomass production, and photosynthetic capacity of P. tabulaeformis seedlings. Our data provide evidence that N deposition might be beneficial to biomass production and photosynthesis of P. tabulaeformis forest in the central and northeast rainfall areas in China, but harmful to P. tabulaeformis forest in the northwest arid and semi-arid regions. So in the northwest arid and semi-arid regions in China, P. tabulaeformis should no longer be used as afforestation and reforestation tree species in ecological restoration and soil conservation programs.