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.