The effect of mixtures on colonisation of leaf litter decomposing in a stream and at its riparian zone

The effect of mixing litter on decomposition and colonisation has been the focus of many studies carried independently in terrestrial and aquatic ecosystems. Those studies are carried out in different regions, use different experimental protocols and methodologies for the assessment of additive or non-additive effects, and the conclusions on the effect of mixtures vary accordingly. In this study I tested the hypothesis, via a short-term decomposition experiment, that, when using the same experimental protocol, mixtures have similar additive effects on decomposition and associated biota in a stream and at its riparian zone. The effect of mixing litter was assessed by comparing values in mixtures with the average of single species, alder and poplar (method 1); comparing the value of a species in mixture with the value of that species alone (method 2); and by a graphical analysis of the average difference between observed and expected values and the 95% confidence intervals (method 3). Method 1 was the most conservative, detecting non-additive effects on macroinvertebrate abundance only; method 2 detected non-additive effects on both dry mass remaining and ergosterol, with differential responses of the leaf species and the habitat; and method 3 detected non-additive effects on all variables except ergosterol and percentage detritivore abundance and also identified different responses of the leaves exposed in the terrestrial and the aquatic habitats. These results show that (i) the methodology used to detect the effects of mixtures deeply influences the results obtained, and may partially explain the diversity of responses available in literature; and (ii) the effect of mixtures may differ in the stream and at the riparian area. However, the findings of the present work should be assessed in a larger-scale experiment in order to generalise the effects of mixing litter on terrestrial and aquatic habitats.