Interaction between litter quality and simulated water depth on decomposition of two emergent macrophytes

Both water depth and litter quality are important factors influencing litter decomposition in wetlands, but the interactive role of these factors in regulating mass loss and nutrient dynamics is far from clear. The responses of mass loss and nutrient dynamics to simulated water depths and litter quality are investigated in leaves of Carex brevicuspis and leaves and stems of Miscanthus sacchariflorus from the Dongting Lake, China. Three litter types differing in litter quality were incubated for 210 days at three water depths (0 cm, 5 cm, and 80 cm, relative to the water surface) in a pond near the Dongting Lake. The litter mass remaining, nitrogen (N), phosphorus (P), organic carbon (organic C), cellulose, and lignin contents were analyzed during the controlled decomposition experiment. Moreover, water properties (temperature, dissolved oxygen content, and conductivity) and fungal biomass were also characterized. Initial N and P contents were highest in C. brevicuspis leaves, intermediate in M. sacchariflorus leaves and lowest in M. sacchariflorus stems, whereas the organic C, cellulose, and lignin contents exhibited an opposite trend. After a 210 days incubation, decomposition rate was highest in M. sacchariflorus leaves (0.0034–0.0090 g g^-1 DW day^-1, in exponential decay model), intermediate in C. brevicuspis leaves (0.0019–0.0041 g g^-1 DW day^-1), and lowest in M. sacchariflorus stems (0.0005–0.0011 g g^-1DW day^-1). Decomposition rate of C. brevicuspis leaves was highest at 5 cm water depth, intermediate at 80 cm, and lowest at 0 cm. Decomposition rate of M. sacchariflorus leaves was higher at 5 cm, and 80 cm than at 0 cm water depths. Water depth had no effect on decomposition of M. sacchariflorus stems. At the end of incubation, N and P mineralization was completely in leaf litters with increasing rates along with increasing water depth, while nutrients were accumulated in M. sacchariflorus stem. Organic C, cellulose, and lignin decayed quickly in both leaf litters compared to the stem litter. The fungal biomass was higher in leaf than in stem litters and changed as a response to water depth in both leaf litters rather than stem ones. These data indicate that submergence has no effect on the decomposition of refractory stem litter and shallower submergence stimulates degradation of the labile leaf litter.