Introduction
Leaf litter from forests supports complex detritus-based food webs in
forest floors and streams through decomposition by the activity of
microbial decomposers and invertebrate detritivores (Gessner et al.,
2010). While the decomposition process has been mostly studied on
monocultures, Seastedt (1984) suggested that litter mixtures might
decompose at different rates from predicted from single species (i.e.,
showing nonadditive effects) due to the effect of resource heterogeneity
on microbial decomposers and invertebrate detritivores. This catapulted
research on the effect of mixing litter. Nonadditive effects may be due
to leaf litter chemical characteristics, such as compounds which
translocate from one leaf species and may enhance (e.g., nitrogen) or
inhibit (e.g., polyphenols) decomposition of other species (Schimel &
Hättenschwiler, 2007; Sanpera-Calbet et al., 2009; Lummer et al., 2012).
Nonadditive effects may also be due to leaf litter physical
characteristics, such as higher leaf toughness of one species which may
act as an armouring effect against physical abrasion thus decreasing
decomposition of other more fragile species (Wardle et al., 1997; Swan
et al., 2008), and synergistic effects may also arise from the higher
habitat/resource heterogeneity of mixtures when compared to single
species (Epps et al., 2007), which enhances colonisation by biota and
eventually results in higher overall processing rates by leaf consumers
(Bastian et al., 2008).
The effect of mixing leaf species may reveal important consequences of
tree biodiversity loss for the functioning and biodiversity of the
recipient ecosystem (e.g., Gessner et al., 2010), but the subject is
interesting per se , as similar studies, either in terrestrial,
aquatic, or both ecosystems, often present strikingly contrasting
results. This is well illustrated by five recent meta-analyses. Kou et
al. (2020), using a data set from 65 studies in forest ecosystems across
biomes concluded that N release (but not P) and mass loss were higher
when litter was composed of two-three species. In contrast, Porre et al.
(2020) found, in 78 studies across terrestrial ecosystems worldwide,
that most litter mixtures had close to additive mass loss and only 15%
of the data showed significant nonadditive mass loss in mixture, thus
concluding that interactive effects among litter species are contextual
and cannot be generalized beyond the context in which the results are
obtained. Similarly, Chen & Chen (2021) found that the C:N:P ratios of
plants, soil organic matter, microbial biomass and enzymes did not
differ significantly between mixtures and monocultures in 169 studies in
terrestrial ecosystems. Across biomes and ecosystems, Liu et al. (2020)
compared 69 studies and concluded that the effect of mixing litter
demonstrates nonadditive effects on decomposition rates in terrestrial
and aquatic ecosystems, and that those effects are most frequently
synergistic. Finally, Mori et al. (2020), using a data set from 151
studies across biomes and ecosystems found synergistic litter
species-mixing effects for most ecosystems, including aquatic, but no
effect in streams. Thus, the existence of a general, nonadditive
relationship between litter mixing and the process of decomposition,
both within and across terrestrial and aquatic ecosystems, remains
elusive.
The flow of organic matter from the terrestrial to the aquatic habitats
is the basis of energy and matter for many stream ecosystems (Wallace et
al., 1997; Marks, 2019), with leaf litter falling vertically or entering
the water by lateral movement through the forest floor (Benfield, 1997;
Abelho, 2001). Abelho and Descals (2019) assessed leaf decomposition of
alder, poplar and a 1:1 mixture of the two species in three scenarios
across a gradient of terrestrial:aquatic exposures to determine if
decomposition of leaf litter from lateral litter inputs to streams was
similar to that of vertical inputs and concluded that decomposition
rates, microbial and invertebrate colonisation differed significantly
among exposure scenarios. Does the effect of mixing litter on
decomposition differ between lateral and vertical inputs? Using the data
set from Abelho & Descals (2019), the objective of this work was to
assess (1) if the effect of mixing leaf litter on the process of
decomposition (mass loss, associated decomposers and detritivores)
differs between the stream ecosystem (under fully aquatic exposure) and
when litter is exposed to a period of terrestrial exposure prior to
immersion in the stream and (2) the global effect of the mixture across
exposure scenarios. Based on the meta-analyses cited above, the tested
hypotheses were: (a) the effect of the mixture is additive under fully
aquatic exposure, given the results of Mori et al. (2020) for streams;
(b) the effect of the mixture is synergistic for leaf litter with a
period of terrestrial exposure, given the predominance of synergistic
effects found by Liu et al. (2020) across ecosystems; and (c)
synergistic effects are due to
the enhancement of the decomposition process of poplar by the presence
of alder, given the stimulating effects of N-fixing species on the
decomposition of N-poor species (Hättenschwiler & Gasser, 2005).