2.1 Experimental design
Among the three mangrove species, A. corniculatum and K. obovata are among the most widely distributed mangrove species in China, and typically growing slowly (Wang et al. 2001; Liao and Zhang 2014). While, L. racemosa is generally fast-growing in natural forests (McKee 1995a, b).
The soils used to plant saplings were collected from a mangrove stand near the city of Xiamen, China. Due to the high nutrient concentrations in the initial soils (0.69 g kg-1 ± 0.10 g kg-1 of N and 0.60 g kg-1 ± 0.05 g kg-1 of P), we mixed the soils with sands at the ratio of 3: 1 and homogenized them to reduce the concentrations to the level as low as in the pristine mangrove forests like Shankou Mangrove Reserve, where the N ranges 0.24 g kg-1 - 0.58 g kg-1, and P 0.12 g kg-1 - 0.19 g kg-1 (He et al. 2006). The soil nutrient concentrations in human-disturbed mangroves can be as high as 0.8 g kg-1 - 2.70 g kg-1 of N (Alongi et al. 2001) and 0.21 g kg-1 - 0.45 g kg-1 of P (Han and Gao 2013). Based on the ranges mentioned above, we applied a slow-release fertilizer at 0 g, 111.08 g, 258.52 g, and 349.46 g on the well-mixed soils to generate a nutrient gradient spanning the range of nutrient-poor to nutrient-rich. This fertilizer contains N, P, and K with a mass ratio of 15: 9: 11, and the nutrient release can last 12 months. Two month later, we measured the soil nutrient concentrations in pots: the N were 0.40 g kg-1, 0.70 g kg-1, 1.00 g kg-1, and 1.50 g kg-1; and the corresponding concentrations of P were 0.34 g kg-1, 0.65 g kg-1, 1.08 g kg-1, and 1.25 g kg-1. The P in here has been above the natural concentrations, and only the N covered the range from nutrient poor to rich levels. Salinity was maintained at ~15‰ by mixing freshwater with salt. Then the new water was added each day to replenish water lost through transpiration.
The two-year-old saplings were provided by Quanzhou Mangrove Nursery where the saplings were cultivated with hypocotyls. The saplings with comparable size were planted in square pots and grown at natural photoperiod and light intensities in an enclosed land with a stainless fence. The pots were arranged in the center of the cultivation house to minimize the heterogeneity of light intensity. Fast- and slow-growing mangrove species were planted separately with four saplings in each pot. Five replicates (pots) were set up for each treatment of each species. To eliminate the interference from pot size (Boot and Mensink 1990), we used large containers (40 × 40 × 40 cm) according to the suggestion by Poorter et al. (2012) that plant biomass does not exceed 1 g L-1.
2.2 Sampling and measurement
After 2 years of cultivation, tree height was determined from the soil surface to the top branches of the canopy. All leaves of each individual were freeze-dried to a constant weight after been harvested and cleaned to determine leaf traits. Leaf N and P reflect the allocation of available nutrients to the photosynthetic tissues, leaf C represents the investment in structural tissues (Poorter and Bergkotte 1992), SLA is defined as the light-capturing surface area per unit of dry biomass (Poorter et al. 2009), δ13C indicates plant water use efficiency (Farquhar et al. 1989), and total leaf biomass (the dry matter of all leaves of each individual) are associated with plant growth (Violle et al. 2009). The average leaf biomass of each pot of each species represented one replicate. Around 10 to 20 healthy sun-lit leaves of each individual were selected for trait analysis and the average value of all leaves in each pot of each species was obtained to represent the trait value of each replicate. After been ground and sieved, the fine powders of leaf samples were determined for element concentrations in leaves. Leaf C and N were analyzed with a VarioMax Elemental Analyzer (FIA, Elementar, Hanau, Germany) and P with an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 5300 DV, Perkin Elmer, Waltham, MA, USA). SLA was calculated as the leaf area per mass (cm2 g-1). Leaf area was measured with a YMJ-B Portable Leaf-Area Analyzer (Zhejiang Top Cloud-Agri Technology). Leaf δ13C was analyzed in the isotope analysis lab of Tsinghua University. Leaf nitrogen resorption efficiency (NRE) is defined as the percent of N in senescent leaves and in green mature leaves (Aerts 1996). Leaf succulence is the water content on a leaf area basis. Due to the lack of adequate leaves of K. obovata, leaf δ13C, NRE and succulence were analyzed only for L. racemosa and A. corniculatum. The soils in pots were determined the nutrient concentrations with a FIA and a ICP-OES respectively following the method for leaves.
The analysis procedures of tolerance-related chemical traits were followed by the method of Lin et al. (2007). Briefly, 200 μg of dry samples were ultrasonically extracted with 4 ml of MCW (methanol: chloroform: water = 12:5:1) for 1 h, then centrifuged at 5000 rpm for 15 min to collect the supernatants. The supernatants were then mixed with 1/4 volume of deionized water and 1/6 volume of chloroform for liquid separation. The nonpolar phase and the polar phase were used for the determination of soluble phenol, soluble sugar, and lipid content, respectively. Specifically, the soluble phenol content was determined by the Folin-Ciocalteu method, the soluble sugar was determined by the dinitrosalicylic acid method, and the lipid content was determined by the gravimetric method. The remaining precipitates were washed with methanol, water, and MCW extract consecutively, then dried at 60oC. After been extracted with hydrochloric acid (10 ml of 2 M) and the supernatant was collected. The remaining precipitates were washed with hot water and then dried at 60oC after uric acid (2 ml of 72%) was added. The remaining precipitates were standing at room temperature for 3 h, and then were added with 10 ml of water standing for another 24 h. After the extraction, the supernatant was collected by centrifugation at 5000 rpm for 15 min. The remaining precipitates were washed with hot water three times and dried at 60oC before being weighed. The cellulose content was calculated by measuring the reduced sugar content in the supernatant by the dinitrosalicylic acid method, with the difference in the weight of precipitates as verification again. The remaining precipitates were burned at 550oC for 2 h in a muffle furnace. After been cooling to room temperature, the ash was weighed and the lignin content was calculated.
2.3 Statistical analysis
All data were examined for normality (with Skewness and Kurtosis) and homogeneity (with Levene’s test) in SPSS 16.0 for Windows. To analyze the variation of traits with soil nutrient availability, the interactions of traits between soil nutrient levels and trait values were tested by multivariate generalized linear models. The one-way ANOVA was used to compare the difference in traits among the treatments for each species (LSD or Games-Howell was used for Post-hoc comparison). Following the integrative analysis, we conducted principal component analysis (PCA; pair-wise correlation analyses) with all the tested traits. The correlation analysis was used to examine the pairwise relationships between traits. To unify the expression, we used the N concentrations of 0.40 g kg-1, 0.70 g kg-1, 1.00 g kg-1, and 1.50 g kg-1 to represent level 1, level 2, level 3, and level 4 of soil nutrient gradients.