Changes in Leaf-Litter Chemistry and Microbial Communities Drive Leaf-Litter Decomposition Across River Terrestrial–Aquatic Habitats

Abstract

River networks are meta-ecosystems in which resources, such as leaf-litter, and their consumers are exchanged across riparian and instream ecosystems. Consumers and leaf-litter quality and decomposition vary depending on riparian land use and instream hydrological conditions, including intermittent drying. However, limited evidence of the mechanisms driving leaf-litter decomposition across aquatic–terrestrial ecosystems hinders our understanding of carbon transfer across river networks. We exposed alder (Alnus glutinosa) leaves to seven preconditioning treatments, including different riparian land uses (i.e., coniferous and deciduous forest, cattle-grazed grassland and urban) and instream habitats (i.e., buried in sediments, on a dry riverbed, in an anoxic pool), mimicking environmental conditions litter can be exposed to before entering flowing waters. We determined leaf-litter chemical composition, decomposition rates and microbial community composition in each preconditioning treatment. Then, the same litter was incubated in two flowing rivers with different flow regimes (i.e., perennial and intermittent) to monitor how previous preconditioning affected subsequent microbial succession and decomposition dynamics. During preconditioning, leaf chemical diversification, decomposer community composition and leaf-litter decomposition differed among preconditioning treatments and were mostly influenced by the presence of water, with stronger responses observed in instream than in land-use habitats. Preconditioning mediated subsequent decomposition in the flowing rivers through the alteration of leaf chemical composition—for example, depletion of carbon compounds in litter exposed to forested environments—and the turnover of bacterial and fungal taxa, likely driven by priority effects. The effects of preconditioning on aquatic decomposition differed among flow regimes, with changes in microbial community composition explaining a greater proportion of the variance in decomposition rates in the perennial river. Invertebrate-driven decomposition was two to four times faster in the intermittent than in the perennial river, reflecting the context-dependent effects of flow regimes on shredder communities. Our results demonstrate how riparian land uses and instream conditions affect river leaf-litter decomposition through cascading effects of leaf preconditioning on microbial communities and their activity. Understanding the dynamics of leaf-litter decomposition across terrestrial–aquatic boundaries is key to better predicting how global changes, including hydrological and land-use changes, may affect ecosystem functioning at the river-network meta-ecosystem scale. As the proportion of intermittent rivers is increasing globally and riparian land use is changing quickly, our conclusions indicate that the transition from perennial to intermittent flow regimes, along with riparian forest loss, could significantly alter carbon cycling across river networks.