Discussion
The main objective of this study was to assess if a period of terrestrial exposure prior to immersion in the stream altered the effect of mixing litter when compared to fully aquatic exposure. While the results, obtained for one mixture at one site, may not extrapolate a trend on a broader context, the mixture used here was the best combination to assess possible nonadditive effects. The number of species in a mixture may not be a good a predictor of litter-quality-dependent processes and the inclusion of another species may even contribute to the homogenization of the substrate (Epps et al., 2007). Moreover, functional diversity has been considered a better predictor than species diversity (e.g., Bonanomi et al. 2014; Grossman et al., 2020) and highest functional diversity has been found in mixtures of species with contrasting N-concentrations and C:N ratio (Santonja et al., 2020), which also more frequently show nonadditive synergistic interactions (Bonanomi et al. 2014).
The obtained results provide a cross-boundary perspective on the effect of mixing litter, showing a legacy effect of exposure to terrestrial decomposition on the fate of plant litter in aquatic ecosystems, and highlighting the importance of assessing the effect on microbial decomposers and detritivores and not only on mass loss. The manipulation of the length of time that leaf litter was exposed to terrestrial and aquatic conditions allowed assessing the effect of a mixture of two leaf species with contrasting N-concentrations and C:N ratios on litter decomposition during movement across the riparian-stream meta-ecosystem.
The hypothesis that the effect of the mixture is additive under fully aquatic exposure was rejected, as an additive effect occurred only on detritivores while the effect on mass loss and on decomposers was synergistic. The hypothesis that the effect of the mixture is synergistic when exposed in the terrestrial and the aquatic habitats was only partially accepted, as the synergistic effect occurred only on decomposers and detritivores while the effect on mass loss was additive. The hypothesis that the synergistic effects are due to an enhancement of the decomposition process of poplar by the presence of alder was only partially accepted. The presence of alder affected poplar only when there was a period of terrestrial exposure, with increased sporulation rates and fungal richness in exposure 25:75, and increased mass loss in exposure 50:50. The presence of poplar affected alder only under fully aquatic exposure, with increased mass loss of alder. Partitioning the effects on both species in the mixture allowed distinguishing if additive effects were due to no change on mass loss and fungi of both species or to opposite nonadditive effects of each species (Hui & Jackson, 2009). Without both assessments – on the mixture and on its component species - nonadditive effects may be masked and interpreted as additive. This could partially explain the apparent contrasting results of Mori et al. (2020) and Liu et al. (2020) across terrestrial and aquatic ecosystems, where the latter also assessed the species-specific effects of the mixture and found a predominance of synergistic effects on all ecosystems while the former found the effect to be additive in streams.