Secondary compounds can reduce the soil micro-arthropod effect on lichen decomposition

• We test the interactive effect of phenolics and arthropods on lichen decomposition.
• We artificially removed phenolic compounds and excluded arthropods in litter-bags.
• Phenolic compounds and the presence of micro-arthropods accelerated decomposition.
• Phenolic removal enhanced the arthropod effect on mass loss of Usnea dasypoga.

Phenolic compounds have been shown in several studies to have important ‘carryover effects’ on litter decomposition, microbial nutrient immobilization and nutrient availability. These effects arise in part because of the adverse effect they have on the feeding activities of litter-feeding invertebrates such as micro-arthropods that drive decomposition processes. However, the interactive effects of phenolic compounds and soil micro-arthropods on litter decomposition are poorly understood. Phenolic compounds can easily be removed by acetone rinsing from living lichens, allowing us to specifically test the role that phenolic compounds (and their removal) have in controlling the effects of micro-arthropods on the decomposition of their litter. We performed a litter-bag experiment aimed at exploring how lichen litter mass loss and nutrient release during decomposition was affected by phenolics (by using acetone rinsed and non-rinsed lichen material) and micro-arthropod activity (by using different mesh sizes to allow or exclude entry by micro-arthropods) for each of six contrasting lichen species (Cladonia rangiferina, Cladonia stellaris, Evernia prunastri, Hypogymnia physodes, Pseudevernia furfuracea and Usnea dasypoga). Both the removal of phenolic compounds and the presence of micro-arthropods accelerated mass and nutrient release overall, but not for either of the two Cladonia species. Removal of phenolics also had an overall positive effect on the effects of arthropods on the loss of P, but not mass and N, from the decomposing lichens. Further, for U. dasypoga, but not the other species, natural levels of phenolic compounds deterred micro-arthropods from accelerating mass loss, and the removal of these compounds enabled micro-arthropods to enhance its decomposition. Our findings that lichen phenolic compounds can sometimes interact with micro-arthropods to influence lichen litter mass loss and nutrient release during decomposition assists our understanding of how lichens and their consumers may impact on organic matter dynamics, biochemical nutrient cycling and other related ecosystem processes.