Speculations
Decomposition is a result of the activity of microbial decomposers and
invertebrate detritivores which causes mass loss (Gessner et al., 2010).
In mixtures, colonisation by microbial decomposers and invertebrate
detritivores can be a cause for and a consequence of the mixture effect.
Most studies use mass loss or breakdown rates as the endpoint to assess
the effect of mixing litter, as illustrated by the meta-analyses already
referred, with many studies also assessing effects on microbial
decomposers (e.g., Wardle et al., 1997; Chapman et al., 2013; López-Rojo
et al., 2020; Santonja et al., 2020; Abril et al., 2021), less on
detritivore-invertebrates (Abelho, 2009; Jabiol et al., 2014; López-Rojo
et al., 2018), and rarely on both (Zimmer, 2002). As shown here, the
effect of mixing litter may be additive for one of the individual
compartments (mass loss, decomposers, or detritivores) and even so
result in nonadditive effects on the process of decomposition. A measure
that includes observations on mass loss, microbial decomposers, and
invertebrate detritivores, such as the one used here, provides an
integrated assessment of the effect of mixing litter on the process of
decomposition.
Lateral inputs may constitute a significant percentage of total leaf
litter entering streams in temperate regions (Benfield, 1997). If the
obtained results hold for other streams and other mixtures with
contrasting N-concentrations, and the effect of mixing litter differs
between vertical litterfall and lateral inputs, studies on vertical
litterfall underestimate the effect of mixing litter on detritivores and
overestimate the effect on mass loss and decomposers. Moreover, if
applicable elsewhere, the obtained results may also forecast future
changes on the effect of mixing litter on decomposition in streams.
Climate change predictions are for longer warm seasons associated to a
decrease in rainfall over large parts of the subtropics, more intense
drought in many regions, and decreased stream flows (Pörtner et al.,
2022). In response to hydric stress, deciduous riparian trees are likely
to lose their leaves earlier in the season, and a higher proportion of
those leaves will fall on the riparian area due to the reduced stream
flow. Thus, the prolonged and dry summers are likely to increase the
proportion of leaf litter that enters streams through lateral movement
and at the accumulation of leaves at the riparian area for longer
periods of time before they are washed into the water.