Effect of expanded shrub litter on decomposition of graminoid litter in a temperate freshwater marsh

Background and aims Shrub encroachment can alter litter decomposition in temperate graminoid-dominated wetlands, yet its influence on the direction of litter mixing effects is still elusive. Methods We collected senesced aboveground tissues of two graminoid species (Deyeuxia angustifolia and Carex schmidtii) and leaf litter of two shrub species (Betula fruticosa and Salix floderusii) in a freshwater marsh of Northeast China, and measured litter mass loss dynamics in monocultures and shrub-graminoid mixtures (in mass ratios of 1:2 and 2:1) using a 730-day incubation experiment. Results In the monocultures, shrub litter had greater mass loss than graminoid litter presumably due to the relatively higher N, and dissolved organic C and N concentrations. Additive effects were observed in all shrub-graminoid mixtures after 182 days of decomposition, while nonadditive effects gradually became more prominent as litter decomposition proceeded. Salix floderusii leaves usually promoted decomposition of neighboring graminoid litter, especially when S. floderusii dominated, leading to a synergistic effect on mass loss of the whole litter mixtures. However, the effects of B. fruticosa on decomposition of graminoid litter varied with incubation time, species composition and species proportion. At the end of incubation, synergistic effects were observed in B. fruticosa-D. angustifolia mixtures, while additive effects occurred in B. fruticosa-C. schmidtii mixtures. Conclusions The effects of shrub expansion on the direction of litter mixing effects are dependent on incubation time, shrub species and dominant graminoid types in this freshwater marsh. These findings help to predict the consequence of altered species composition on community-level litter decomposition in temperate graminoid-dominated wetlands.