Macrophyte litter diversity affects microbial respiration but not litter mass loss

Abstract Litter mixing effects (LME) may alter microbial-mediated decomposition in aquatic ecosystems, but it remains unclear whether these effects are related to the chemical diversity of litter and leachates or whether they change over time. Furthermore, most aquatic studies of LME have focused on terrestrial litter decomposition in streams, where fine particles and leachates are readily transported downstream, neglecting interactions between particulate and dissolved organic matter derived from autochthonous plant litter. We investigated the effects of species richness and identity of eight aquatic macrophytes on microbial-mediated catabolic processes in microcosms mimicking lentic conditions. We incubated all possible combinations of 1, 2, 4 and 6 species from an eight-species pool for 48 days and quantified microbial respiration, litter dry-weight loss, metabolic efficiency of decomposition, and chemical diversity of both litter and leachates. Macrophyte litter species richness reduced cumulative microbial respiration but did not affect dry-weight loss. Increasing species richness and incubation time led to stronger negative non-additive effects on microbial respiration, whereas mass-loss deviations were weak and unsystematic. Species identity also influenced mixture responses, indicating that richness effects were not solely driven by the number of species. Chemical diversity increased asymptotically with richness and partially explained the magnitude of non-additive effects on microbial respiration. Species richness also increased metabolic efficiency of decomposition, indicating lower respiratory carbon loss per unit of litter mass lost. Our results suggest that macrophyte litter diversity can reduce carbon mineralization without slowing detritus mass loss, potentially altering energy transfer and carbon fate in macrophyte-dominated lentic food webs.