Non-additive effects of water availability and litter quality on decomposition of litter mixtures

Non-additive (synergistic or antagonistic) effect, a common phenomenon for the decomposition of mixed litter in nature, is usually regulated by litter quality and environmental factors. In this study, we investigated decomposition rates and nutrient (C, N, and P) dynamics in response to water availability in six litter treatments using plant material from Dongting Lake, China. Three single-litter treatments (leaves of Carex brevicuspis, leaves of Miscanthus sacchariflorus, and stems of M. sacchariflorus) and three mixed-litter treatments (1:1 mixtures of single litters) were incubated at three levels of water availability (20%, 46%, and 100% saturation) for 120 days in a mesocosm experiment. Decomposition rates for single-litter treatments were ranked: M. sacchariflorus leaves > C. brevicuspis leaves > M. sacchariflorus stems. Decomposition rates generally increased with increasing water availability. Antagonistic or additive interactions occurred in the M. sacchariflorus leaves C M. sacchariflorus stems treatment, and synergistic interactions occurred in the other two mixed-litter treatments. N content and lignin loss rate of M. sacchariflorus leaves and M. sacchariflorus stems were increased by mixing with C. brevicuspis leaves. The magnitude of synergistic interactions increased with increasing water availability and the opposite was true for antagonistic interactions. These data suggest that the direction of non-additive effects is dependent on litter quality, while the magnitude is regulated by water availability.