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.