Nutrient effects on aquatic litter decomposition of free-floating plants are species dependent

Among the most important ecosystem processes in wetlands is plant litter decomposition, which provides the detrital food chains with matter and energy and consequently could affect ecosystem health and integrity. Varying degrees of anthropogenic eutrophication mostly due to urbanization, are prevalent problems in many wetlands and aquatic ecosystems. Free-floating plants are widely distributed in (particularly, tropical and subtropical) still water bodies in wetlands and sensitive to water nutrient enrichment. However, little is known about how water nutrient affect litter decomposition of free-floating plants. In a subtropical wetland, we conducted an in-situ experiment in which litters of free-floating ferns (Azolla imbricata and Salvinia natans) and an angiosperm (Lemna minor) were incubated beneath water in 13 ponds with broadly similar size and shape but different levels of eutrophication. Litters of L. minor decomposed faster than those of the two fern species while its decomposition rate did not have a relationship to water nutrient content. However, we did find the decomposition rate of two ferns decreased significantly with water total phosphate content, which might be attributed to decline of microbial decomposers. Such interspecific variation in decomposition response to eutrophication could be explained by the functional traits of the species, e.g., SLA, leaf phosphorus content. These findings suggest water eutrophication would generally decrease plant litter decomposition rate but such effect is species- and/or trait-dependent. Thus, community composition and plant functional traits should be considered in analyses of ecosystem processes under water eutrophication