Alder presence in plantations affects the characteristics of the leaf litter of non-nitrogen-fixing tree species and its subsequent decomposition by stream microbes

Nitrogen (N)-fixing species, like alder, can influence the leaf litter characteristics of neighbouring non-N-fixers, potentially affecting detritus-based ecosystems. We assessed the effects of black alder presence or absence in plantations on the leaf litter characteristics, and subsequent colonization and decomposition by stream microbes, of three non-N-fixing species: oak, chestnut, and beech. Leaf litter of non-N-fixers was collected from trees in monocultures and mixed plantations with alder (BangorDIVERSE experimental plantations) and chemically and physically characterized. Colonization and decomposition of the leaf litter by stream microbes was assessed in microcosms containing single-species (leaf litter of each non-N-fixing species and origin) and mixed-species (mixture of leaf litter of the three non-N-fixing species either from monocultures or mixed plantations with alder). Effects of alder presence or absence on non-N-fixers are species-dependent. Chestnut leaf litter from mixed plantations had lower polyphenols concentration, higher N concentration, and lower C:N ratio than that from monocultures. Oak leaf litter had higher lignin concentration, while beech leaf litter had lower lignin concentration when originated from mixed plantations than monocultures. Consequently, fungal biomass (in oak), sporulation rates (in oak and chestnut), and decomposition rates (in oak and chestnut) were higher in leaf litter from trees grown in the mixed plantations with alder. In contrast, beech leaf litter showed no effect of alder presence or absence in plantations. The relative differences in leaf litter characteristics, microbial colonization, and decomposition rates among the non-N-fixing species depended on litter origin, leading to different litter mixing effects when litter originated from mixed plantations (synergistic) and monocultures (additive). These findings highlight the potential indirect effects of alder decline or invasion on stream functioning through alterations to leaf litter characteristics for non-N-fixing species. These findings also emphasize the importance of considering litter origin in litter-mixing experiments that assess the effects of forest compositional changes on stream functioning.