1 Introduction

Coastal wetlands are among the most  important carbon storage ecosystems (Osland  et al. 2018). Plant litters contribute greatly to the soil organic carbon of  these wetlands through decomposition. Leaf litters in nature are usually mixed together and the litter decomposition rate of single species may not represent the real decomposition rate, because litter mixing could cause litter decomposition rates to be different from the mean rate of the component species, known as the non-additive effect (Lecerf et al. 2011; Handa et al. 2014). The alteration of litter decomposition due to the non-additive effect will alter the amount of C input to soils and the nutrient content in waters, and consequently impact the soil carbon stock and the nutrient purification capacity of the coastal wetlands.
The non-additive effect on litter decomposition has been observed in terrestrial plants, but has been rarely tested in coastal species. Kandelia obovata, Aegiceras corniculatum (black mangrove), and Avicennia marina (white mangrove) are among the most common species in subtropical mangrove forests. An alien salt marsh species Spartina alterniflora has emerged in some of these mangrove stands along the southeast coasts in China (Zhang et al. 2012). It is unknown whether the non-additive effect on litter decomposition occurs in these coastal communities.
Litter decomposition rate has been confirmed to be related to litter traits, including chemical traits, especially nitrogen (N) or C: N ratio and lignin concentrations, as well as some morphological traits such as leaf mass per area (LMA), specific leaf area (SLA) and leaf dry-matter content (LDMC) (Osono and Takeda 2004; Cornwell et al. 2008; Fortunel et al. 2009; Wickings et al. 2012; Coleman et al. 2020). Mass-ratio hypothesis and niche complementarity hypothesis are the main trait-based theories to explain the non-additive effect on litter decomposition (Finerty et al. 2016; García-Palacios et al. 2017). The mass-ratio hypothesis states that the dominant trait values have the greatest impact on ecological processes including litter decomposition, and the dominant trait values are reflected by the community-weighted mean of trait value (CWM) (Finerty et al. 2016; García-Palacios et al. 2017). In contrast, the niche complementarity hypothesis emphasizes the role of litter trait dissimilarity (Schimel and Hättenschwiler 2007; Berglund et al. 2013; Handa et al. 2014). Although both mass-ratio and niche complementarity hypotheses could explain litter decomposition and nutrient release (García-Palacios et al. 2017), their relative contributions varied with plant communities (Chapman and Koch 2007).
Unlike most land deciduous plants, coastal plants generally have low-quality litters as the consequence of adaptation to high salinity and nutrient-poor environments (Feller et al. 2003). In particular, their tissues are rich in the C concentrations and recalcitrant compounds such as phenolic compounds, condensed tannins, and lignin, which are highly resistant to decay (Kraus et al. 2003; Lin et al. 2006, 2007, 2010; Prescott 2010). Little is known whether the non-additive effect on decomposition occurs on these low-quality litters. In specific, this study aims to test: i) whether litter mixing affects litter decomposition rate and nutrient release; ii) if so, how trait dissimilarity and CWM of litter mixture control the non-additive effect of litter decomposition.