Effects of litter mixing on litter decomposition and soil properties along simulated invasion gradients of non-native trees.

Aims: Plant invasions generally lead to mixtures between native and non-native litter. We assess the interactions between leaf litters from two invasive non-native trees (Robinia pseudoacia or Ailanthus altissima) and the native Populus alba on litter decomposition, nutrient release and soil properties along two gradients of invasion. Methods: Microcosms with field-collected soil covered by varying proportions of native and non-native litter simulated the two invasion gradients. We assessed the proportion of remaining litter mass and nutrient mass (N, P, C), and soil C, N-NO3−, total N, and pH, five times throughout a period of 11 months. Observed values were compared to the expected values on the assumption of no interactions. Results: Litter mass and C mass decayed slower in Robinia and faster in Ailanthus than in Populus. The three species immobilized N and P. Soil properties did not differ across pure litters. Both litter mixture gradients showed additive or antagonistic interactions on litter decomposition, whereas N and P mass were equal or higher than expected. The proportion of non-native litter in the mixture had non-linear effects on most variables, suggesting that the impact of these non-native trees on litter decay levels off or even declines as they become more abundant. Conclusion: The impacts of Ailanthus and Robinia litter on soil processes should not be derived from single species experiments, both due to non-additive effects and to non-linear responses to litter abundance. [ABSTRACT FROM AUTHOR]