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.