Coastal wetlands are among the most carbon-rich ecosystems in the world. Litter decomposition is a major process controlling soil carbon input. Litter mixing has shown a non-additive effect on the litter decomposition of terrestrial plants particularly of those species having contrasting litter quality. But the non-additive effect has been rarely tested in coastal plants which generally having low-quality litters. We selected three common mangrove species and one saltmarsh species, co-occurring in subtropical coasts, to test whether the non-additive effect occurs when the litters of these coastal species mixing together. We are also concerned whether the changes in the decomposition rate of litter will affect the nutrient contents in waters. A litter-bag experiment was carried out in a glasshouse with single and mixed leaf litters. A non-additive effect was observed in the litter mixtures of mangrove species Aegiceras corniculatum vs. Kandelia obovata (antagonistic) and A. corniculatum vs. Avicennia marina (synergistic). Whereas, the mixture of A. corniculatum (mangrove species) and Spartina alterniflora (saltmarsh species) showed an additive effect. The strength of the non-additive effect was unrelated to the initial trait dissimilarity of litters. Instead, the decomposition rate and mass remaining of litter mixtures were strongly related to the carbon concentrations in litters. Nutrient content in waters was dependent on the decomposition rate of litter mixtures but not on the initial nutrient concentrations in litters. Despite the behind mechanisms were not yet revealed by the current study, these findings have improved our understanding of the litter decomposition of coastal species and the consequent nutrient release.

Aims The impact of nutrient increase on nutrient resorption from senescent leaves is an important topic, which is not completely understood. Mangrove species are characterized by nutrient conservation mechanisms as a consequence of adaptation to nutrient-poor environments. The understanding of the response of the traits related to nutrient conservation strategies to nutrient availability is limited. This study aims to address whether nutrient resorption of mangrove species is responsive to long-term nutrient enrichment. Methods A field-based study was carried out in a Kandelia obovata dominated mangrove stand where a nutrient gradient was generated by the long-term discharge of aquaculture wastewater. Seasonal variations of nutrient resorption and nutrient availability were assessed. Important findings The data from summer and winter showed the same results: 1) resorption proficiency or the nutrient concentrations (N and P) in senescent leaves did not varied with nutrient increases except the plot that the N availability was extremely high; 2) the resorption efficiency of both N and P was positively correlated with nutrient availability, and N resorption efficiency was also positively correlated to leaf N: P ratio; 3) N resorption was complete while the resorption of P was incomplete. These results indicate that resorption proficiency of K. obovata is less sensitive than resorption efficiency to nutrient availability, and the positive response of N resorption efficiency is related to the imbalance of nutrient availability, while the positive response of P resorption efficiency is unrelated to plant nutrient status. We suggest that caution should be taken in using resorption efficiency of mangrove species to predict plant nutrient status.