4.2 Relationships of trait dissimilarity and CWM to the decomposition rate

A strong non-additive effect generally occurs in a litter mixture with divergent traits (Kuebbing and Bradford 2019). For example, deciduous subordinates significantly impacted litter decomposition in an evergreen-dominated forest where the trait values were contrasting between deciduous and evergreen species (Guo et al. 2020). However, we found no the relation of trait dissimilarity with the litter decomposition and the strength of non-additive effect. Instead, the CWM of litter C was related to the decomposition rate of litter mixtures in our study (Fig. 7). These findings agreed with the results derived from European tree species. There was no significant relationship between the decomposition rate and the litter trait dissimilarity of 28 litter mixtures (Frainer et al. 2015), but the trait CWM exerted the strongest effect on the mass loss of the litter mixtures in another study that the litter mixtures were derived from 14 European tree species (Finerty et al. 2016).
Trait CWM is related to mass-ratio hypothesis and trait dissimilarity related to niche complementarity hypothesis (Handa et al. 2014; Finerty et al. 2016; García-Palacios et al. 2017). The core point of these two mechanistic hypotheses is the interaction between litters (Tardif and Shipley 2013; Handa et al. 2014; Finerty et al. 2016; García-Palacios et al. 2017). Mass-ratio hypothesis proposes that there is no interaction between litters, or the positive and negative interactions cancel out each other (Tardif and Shipley 2013). While niche complementarity hypothesis emphasizes a positive interaction between litters (Schimel and Hättenschwiler 2007; Berglund et al. 2013; Handa et al. 2014). Although we did not directly examine the interactions between litters, the strong correlation of the trait CWM and the decomposition rate indicated a non-interaction between litters. This is consistent with the weak interaction between these alive species in our previous study (Ndayambaje et al. 2021).
Among litter mixtures, A. corniculatum vs. K. obovata had the highest CWM of leaf C, and the strongest non-additive effect but the lowest rate of litter decomposition (Fig. 6). The CWM of leaf C of A. corniculatum vs. A. marina and A. corniculatum vs. S. alterniflora were similar, but A. corniculatum vs. A. marina had higher CWM of leaf N and P and concurrently a faster decomposition rate than A. corniculatum vs. S. alterniflora (Fig. 6). Taken together, these results indicated that the C concentration of litters was a key trait strongly controlling litter decomposition rate, with adjusted by the N and P concentrations in litters.
The litters of mangrove species are generally characterized by low-quality, such as high in C and phenolics (Kraus et al. 2003; Lin et al. 2006, 2007, 2010; Prescott 2010). Lignin-derived phenols can be lost at a lower rate during decomposition than the total neutral sugars and the bulk organic C (Marchand et al. 2005). Condensed tannins tend to bind more strongly to protein than to fibre, which enable to sequester nitrogenous materials and conserved N in leaf litter rather than providing to soil microbe, such mechanism could retard litter decomposition rate (Lin et al. 2006; Zhou et al. 2012). Among the tested coastal species, A. corniculatum and K. obovata had profoundly greater concentrations in leaf condensed tannin than A. marina and S. alterniflora. Some other studies also found that A. corniculatum had highest total polyphenols, and A. marina had very low or even undetectable levels in total polyphenols (Zhou et al. 2010; Wang et al. 2014). These observations provide additional support to explain why the litter mixture of A. corniculatum vs. K. obovata showed lower decomposition rate than A. corniculatum vs. S. alterniflora and A. corniculatum vs. A. marina.