2.3 Calculation

The litter mass loss was calculated by comparing the initial mass and the mass after decomposition. The initial mass remaining after decomposition was calculated based on the following formula (Wu et al., 2013):
\begin{equation} X_{r}=\frac{X_{i}}{X_{t}}\times 100\nonumber \\ \end{equation}
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:
\begin{equation} \frac{X_{t}}{X_{i}}=\mathbb{e}^{-kt}\nonumber \\ \end{equation}
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):
\begin{equation} N_{r}=\left(X_{t}\times{[N}_{t}]+X_{i}\times[X_{i}]\right)\times 100\nonumber \\ \end{equation}
We also calculated the expected rate of the litter mixture after decomposition using the following formula:
\begin{equation} X_{\exp}=\left(\frac{X_{i1}}{X_{i1}+X_{i2}}\times X_{r_{1}}+\frac{X_{i2}}{X_{i1}+X_{i2}}\times X_{r_{2}}\right)\times 100\nonumber \\ \end{equation}
Where Xexp is the percentage of the expected mass remaining (MR) 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 nutrients in water between observed and expected values were analysed by independent t-test. The variation of the difference in mass remaining between observed and expected values from zero (i.e the observed equals to expected value) 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 nutrients in water and in leaves.

3 Result

Non-additive effect on litter decomposition were found in the litter mixtures from three mangrove species. Nutrients in water, the initial trait dissimilarity and CWM of litter mixtures varied with species composition.

3.1 Decomposition rate (k) and mass remaining

Non-additive effect on litter decomposition was detected in the litter mixtures from mangrove species, but not in the mixture of A. corniculatum vs. S. alterniflora (Fig. 1a, b). The observed mass remaining of the mixture of A. corniculatum vs. K obovatawas substantially higher than the expected value, whereas the observed mass remaining of the mixture of A. corniculatum vs. A. marina was lower than the expected one (p < 0.05), only the mixture of A. corniculatum vs. S. alterniflora  did not show significant difference between the observed and the expected values (p > 0.05; Fig. 1b).