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).