The effect of the mixture differs among exposure scenarios
Under the fully aquatic exposure, the mixture had a synergistic effect on mass loss and decomposers (sporulation rates and fungal richness), while the effect on detritivores was additive. The synergistic effect on mass loss of the mixture was due to a species-specific effect on mass loss of alder in the presence of poplar (16% more, on average). Fungi (abundance, activity and/or diversity) may demonstrate evidence for interactive effects of mixtures in streams (López-Rojo et al., 2020; Santonja et al., 2020; but see Sanpera-Calbet et al. (2009) and Abril et al. (2021) for contrasting results). Fungi colonizing the mixture were more diverse, probably due to a better substrate for conidia attachment and germination (Dang et al., 2007; Kearns & Bärlocher, 2008) and invested more in reproduction than in biomass when compared to the average of the single species. Aquatic hyphomycetes invest on average 50% of their biomass in sporulation, and the conidia produced may correspond to 10% of the daily litter mass loss (Gessner, 1997). Thus, the increased sporulation rates of the mixture and possibly also positive complementarity among fungal species may explain the increased mass loss (López-Rojo et al., 2020). Additionally, shredders prefer high-quality in detriment of poor-quality leaves (Swan & Palmer, 2006) and may strongly increase the decomposition rates of alder (Bruder et al., 2011). Thus, their feeding activity probably also contributed to the increased mass loss of alder in the presence of poplar and to the synergistic effect of the mixture.
When the period of terrestrial exposure was shorter than the period of aquatic exposure, the mixture had a synergistic effect on detritivores, stronger for shredder biomass and abundance, but not on mass loss or decomposers, although fungal richness was also higher-than-expected. On average, the mixture had 63% more fungal species than the average of the monocultures, but the increased richness did not translate into higher mass loss, possibly due to functional redundancy among aquatic hyphomycete species (Bärlocher, 2005; López-Rojo et al., 2020). On the other hand, invertebrate detritivores have been shown to play a fundamental role on the synergistic effects of mixtures on mass loss in terrestrial and aquatic habitats (Hättenschwiler & Gasser, 2005; Schädler & Brandl, 2005, Vos et al., 2013; Jabiol & Chauvet, 2015, Liu et al., 2020; Santonja et al., 2020). But increased colonisation may also be a consequence of the mixture effect, as shredders may benefit from the structured habitat created by recalcitrant species which can also provide shelter against predators or stream flow (Hättenschwiler et al., 2005; Sanpera-Calbet et al., 2009; Jabiol et al., 2014). Although the mixture had on average 29% more biomass, 16% more abundance and 10% more shredder taxa than the average of the monocultures, mass loss of the mixture was not higher-than-expected. Functional redundancy among shredder taxa may be due to dominance by food generalists (Hättenschwiler et al., 2005). Furthermore, the more complex habitat may have also attracted more predators and shredders might have reduced consumption rates as a behavioural response to their presence (Jabiol et al., 2014). However, it is more plausible that there was a similar consumption of both species during decomposition. When given a choice, shredders consume preferentially the highest quality leaves available (Swan & Palmer, 2006). Given that the availability of each species in the mixture changed during decomposition due to the faster breakdown rates of alder, and that N-concentrations of the two species converged during decomposition (Abelho & Descals, 2019), shredders probably started by consuming alder, but when mass remaining of alder was already low and the quality of poplar had increased, shifted consumption from alder to poplar.
When the period of terrestrial and aquatic exposures was equal, the process of decomposition of the mixture was enhanced but there was a ´dilution’ of the effects on each compartment: the effect of the mixture was additive on mass loss, decomposers, and detritivores. However, the partitioning of the contributions of each species within the mixture showed a synergistic effect of the presence of alder on mass loss of poplar. While exposed in the terrestrial habitat, the water buffering effect of the mixture might have increased moisture persistence in the litter layer (Wardle et al., 2003) thus favoring fungal colonisation. In fact, although the effect was nonsignificant (but barely), fungal biomass of poplar was elevated in the presence of alder. The conditions in terrestrial habitats are more likely to favor N-transfer than in streams (Gessner et al., 2010) and it is possible that there was a transference of nitrogen, either passively or actively by fungal hyphae, from alder to poplar (Schimel & Hättenschwiler, 2007; Lummer et al. 2012). But the effect could also have occurred during stream immersion, given that the increase in decomposition rates of nutrient-poor species by the presence of N-fixing species has also been shown in aquatic ecosystems (Migliorini & Romero, 2020). However, this species-specific effect was small and did not translate in a synergistic effect on mass loss of the mixture.