Standard vs. Natural: Assessing the impact of environmental variables on organic matter decomposition in streams using three substrates.

The decomposition of allochthonous organic matter, such as leaves, is a crucial ecosystem process in low-order streams. Microbial communities, including fungi and bacteria, colonise allochthonous organic material, break up large molecules and increase the nutritional value for macroinvertebrates. Environmental variables are known to affect microbial as well as macroinvertebrate communities and alter their ability to decompose organic matter. Studying the relationship between environmental variables and decomposition has mainly been realised using leaves, with the drawbacks of differing substrate composition and consequently between-study variability. To overcome these drawbacks, artificial substrates have been developed, serving as standardisable surrogates. Here, we compared microbial and total decomposition of leaves to the standardised substrates of decotabs and, only for microbial decomposition, of cotton strips, across 70 stream sites in a Germany-wide study. Furthermore, we identified the most influential environmental variables for the decomposition of each substrate from a range of 26 variables including pesticide toxicity, concentrations of nutrients and trace elements using stability selection. The microbial as well as total decomposition of the standardised substrates (i.e., cotton strips and decotabs) were weak or not associated with that of the natural substrate (i.e., leaves, r² < 0.01 to r² = 0.04). The decomposition of the two standardised substrates, however, showed a moderate association (r² = 0.21), which is probably driven by their similar composition, with both being made of cellulose. Different environmental variables were identified as most influential for each of the substrates and the directions of these relationships contrasted between the substrates. Our results imply that these standardised substrates are unsuitable surrogates when investigating the decomposition of allochthonous organic matter in streams. This article is protected by copyright. All rights reserved. Environ Toxicol Chem 2023;00:0-0. © 2023 SETAC.