We carried out a litter-bag experiment in a glasshouse in the Institute of Urban Environment, Chinese Academy of Science. The leaves were sampled from Quanzhou Bay Mangrove Reserve (24◦57’24’’ N - 118◦41’25’’ E), the south of Fujian, China (Hu et al. 2011). This site is characterized by an oceanic monsoon climate with a warm wet winter and hot rainy summer. The annual mean temperature is 20.4 ◦C and the average annual precipitation is 1095.4 mm (Lu et al. 2018). The dominant mangrove species encompassed Kandelia obovata, Aegiceras corniculatum, and Avicennia marina, with an alien saltmarsh species Spartina alterniflora emerged (Zhang et al. 2012). Three litter mixtures were set up based on species distributions in the field (Ndayambaje et al 2021): A. corniculatum vs. A. marina, A. corniculatum vs. K. obovata, and A. corniculatum vs. S. alterniflora. The soil was collected from the mangroves in Xiamen city: electronic conductivity (EC 0.46 ± 0.05 ms cm−1), organic matter (0.32 ± 0.04 mg g−1), carbon (C 20.09 ± 0.67 mg g−1), total nitrogen (TN 2.02 ± 0.08 mg g−1), total phosphorus (TP 0.62 ± 0.03 mg g−1), sulphate (S 0.72 ± 0.07 mg g−1), potassium (K 13.73 ± 3.33 mg g−1), calcium (Ca 8.47 ± 2.39 mg g−1), magnesium (Mg 9.96 ± 0.22 mg g−1) and pH (8.11 ± 0.06).
2.1 Experimental design
Healthy green leaves were sampled from the trees of K. obovata, A. corniculatum, A. marina, and S. alterniflora in the same forest. Thus, all the leaves were exposed to similar climatic conditions, facilitated interspecies comparison of litter mass loss. After being taken to the laboratory, the leaves were gently washed and all dirt particles were then removed by using a soft brush followed by rinsing in distilled water. Five replicates were set up for each litter or litter mixture.
All leaf litters were air-dried to constant weight, 5 g of litter for each single species and 5 g of each litter mixture (2.5 g per species) were placed in each 75 × 75 mm nylon bag with 1 mm2 mesh size. There were 40 bags in total, including 15 bags of litter mixture (3 pairs) and 25 for single species (5 single species). All the litter bags were placed on the top of the soil in the plastic containers, all the containers (500 ml) consisted of 200 g soil and 200 ml water. Based on the previous observation that the decomposition of mangrove leaves generally becomes slow and steady after 7 weeks (Chanda et al. 2016), the duration of 90 days is often representing a short-term decomposition experiment (Keuskamp et al. 2013).
2.2 Element analysis
After 90 days of decomposition, litter and water were collected to the laboratory for mass and nutrient analyses. The water on the soil surface was collected with a syringe and then stored in a polyethylene bottle. Litters were determined C and N concentrations with an elemental analyzer (Vario MAX, Vario MACRO, Germany Elementar) after being grounded through 60 mesh sieves. Phosphorus (P) in litters was quantified with an ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer, Optima 7000DV, PerkinElmer USA) after being digested with nitric and perchloric acid. The ammonium–nitrogen (NH4-N), nitrate-nitrogen (NO3-N), and nitrite-nitrogen (NO2-N) were quantified by an ultraviolet spectrophotometer (UV6100, mapada instrument, China) following the standard methods (Wei et al. 2002). Digestion of the water by alkaline potassium persulfate and potassium persulfate were carried out after filtration of the water sample, and then the total N and total P were determined respectively according to the standard methods (Wei et al. 2002), NH4-N, NO3-N, and NO2-N then analyzed by an ultraviolet spectrophotometer (UV6100, mapada instruments, China). The condensed tannin of the leaves was also determined before the decomposition experiment was conducted. The total condensed tannin was the sum of the extractable, protein-bound, and fibre-bound concentrations. The extraction process was followed by the method of Lin et al. (2007).
2.3 Calculation
Litter mass loss was calculated by comparing the initial mass and the mass after decomposition based on the following formula (Wu et al. 2013):
$${\text{X}}_{\text{r}}=\frac{{\text{X}}_{\text{i}}}{{\text{X}}_{\text{t}}}\times 100$$
Where Xr is the percentage (%) of mass remaining after decomposition, Xi is the initial litter mass, Xt is the mass of the remained litter in the litter bags after a given time period (t) of decomposition.
The decomposition rate (k) was calculated by exponential decay model, the litter mass remaining after decomposition and initial litter mass:
$$\frac{{X}_{t}}{{X}_{i}}={e}^{-kt}$$
Where k is the decomposition rate coefficient; t is the time duration of decomposition.
The percentage of the initial litter nutrient remaining (Nr) during decomposition was calculated by the following equation (Zhang et al. 2014):
$${N}_{r}=\left({X}_{t}\times {[N}_{t}]+{X}_{i}\times [{X}_{i}]\right)\times 100$$
We also calculated the expected rate of the litter mixture after decomposition using the following formula:
$${X}_{exp}=\left(\frac{{X}_{1}}{{X}_{1}+{X}_{2}}\times {X}_{{r}_{1}}+\frac{{X}_{2}}{{X}_{1}+{X}_{2}}\times {X}_{{r}_{2}}\right)\times 100$$
Where Xexp is the expected mass remaining after decomposition, X1 and X2 are the initial dry masses in single species which was 2.5 g, and Xr1 and Xr2 are the mass remaining from the single decomposition species. The effect strength was calculated by the difference between the observed mass remaining (%) and the expected mass remaining (%) (O-E): additive (no significant difference between observed and expected values), synergistic non-additive (negative value, meaning an acceleration of litter decomposition), antagonistic non-additive (positive value, meaning a deceleration of litter decomposition) (Lecerf et al. 2011).
The community-weighted mean value of traits (CWM) was calculated as the mean value of each species in the mixture because the two litters mixed as 1: 1 of mass in this study (Roscher et al. 2018).
2.4 Statistical analysis
Statistical analysis was performed with IBM SPSS Statistics 23. All data were removed outlier and checked for the normal distribution and homogeneity of variances before the statistical analysis was carried out. One-way ANOVA was used to detect the differences in decomposition rate and water nutrient content among litters, as well as in trait dissimilarity and CWM among each trait. The differences in mass remaining and water nutrient content between the observed and the expected values were analyzed by an independent t-test. The variation of the difference in mass remaining between the observed and the expected values from zero was detected by one-sample t-test analysis. The linear correlations of decomposition rate or mass remaining with leaf trait dissimilarity or CWM were assessed through the correlation process. This process was also used to identify the correlation between water nutrient contents and leaf nutrient concentrations.