Microclimate within litter bags of different mesh size: Implications for the ‘arthropod effect’ on litter decomposition

Litter bags are often used to determine the impact of soil animals on litter decomposition rates through the use of varied mesh sizes that exclude soil animals on the basis of body size. However, concerns have been raised regarding confounding factors that can co-vary with mesh size (e.g., microclimatic differences and leaching losses) and that might affect mass loss from litter bags independent of soil animal effects. To explore if these factors may interfere with the interpretation of studies that have used litter bags of varying mesh sizes, we quantified the effects of mesh size (50 μm, 0.4 mm and 1.0 mm aperture) on litter mass loss in the absence of soil animals. Further we measured potential microclimate (temperatures, light transmission and moisture) differences that may co-vary with mesh size. In addition, we quantified litter mass losses through litter fragmentation and leaching. Litter mass loss in the absence of soil animals did not differ between mesh sizes, suggesting that the use of the different meshes does not generate confounding factors that directly influence mass loss rates in the absence of animals. Microclimatic differences between litter bags of different mesh sizes were minimal and revealed slightly warmer (0.7 °C) temperatures and easier water entry for the fine than the coarse mesh bags, but no effect on litter evaporation rates. Light transmittance was largest for the fine mesh (79%) compared to the medium (60%) and coarse mesh (73%). The impact of UV on litter mass loss was 1% higher in the coarse mesh. Loss of fragmented litter was considerably higher from the coarse mesh (93%) than the fine mesh bags (17%). In conclusion, different mesh sizes can be used to reliably quantify the role of soil animals in litter mass loss from litter bags. The greater litter mass loss from litter bags with coarser mesh sizes is however caused by the soil animals both promoting litter decomposition and greater litter fragmentation (with the undecomposed fragments then being lost from the bags). Additional approaches are needed to determine the relative importance of these two effects of soil animals on litter mass loss from litter bags.