Plant detritus origin and microbial-detritivore interactions affect leaf litter breakdown in a Central Apennine (Italy) cold spring

The decomposition of dead organic matter is a key process for the metabolism and functioning of lotic ecosystems. Particulate organic matter from fallen leaves is the main source of energy input also in forested springs. However, detritus processing in spring habitats has been rarely investigated. The present paper is aimed to assess, for the first time, the influence of detritus origin on leaf litter breakdown in a hydrologically, thermally and chemically stable cold spring and to evaluate the relative contribution of microorganisms and invertebrate detritivores to the decomposition process. For this purpose, we used leaves of the native black poplar (Populus nigra) and the invasive common reed (Phragmites australis) enclosed in leaf-bags of different mesh sizes. We demonstrated that leaf detritus of native black poplar decomposed twofold faster than the invasive common reed. We also found that the percentage of dry mass loss was significantly higher in medium/coarse litter bags compared to fine ones. However, microorganisms alone in fine mesh bags were able to decompose about 80% (poplar) and 60% (common reed) of the initial dry mass. No substantial differences were detected in structure, composition and functional organization of assemblages colonizing poplar and common reed leaf-bags. Therefore, differences in microbial activity and microbial/detritivore interactions rather than composition, diversity and abundance of the detritivore guild could better explain the faster breakdown of native leaves. Our results suggest that the substitution of natural riparian vegetation with invasive low-quality leaf plant species will have severe impacts on spring ecosystems. Alterations of structural and functional attributes of springs will be strictly related to the specific characteristics of plant invaders and to local conditions which may influence the detritivore/microbial contribution to leaf litter decomposition/breakdown.