Responses of microbially driven leaf litter decomposition to stream nutrients depend on litter quality

The present study aims to understand how microbial decomposition of leaf litter from two riparian tree species differing in their quality varies among streams covering a gradient of nutrient concentrations. We incubated leaf litter from alder (Alnus glutinosa) and sycamore (Platanus × hispanica) in 3 streams with low human pressure and 2 streams influenced by wastewater treatment plant effluents. We quantified leaf litter decomposition rates (k) and examined the temporal changes in the leaf litter concentrations of carbon (C) and nitrogen (N) throughout the incubation period. We measured the extracellular enzyme activities involved in degradation of C (i.e., cellobiohydrolase) and organic phosphorus (i.e., phosphatase). Results showed that alder k decreased with increasing nutrient concentrations, while sycamore decomposed similarly among streams. For both species, leaf litter N concentrations were positively related to in-stream dissolved N concentrations. However, we found different temporal patterns of leaf litter N concentrations between species. Finally, we found relevant differences in the enzymatic activities associated to each leaf litter species across the nutrient gradient. These results suggest that the intrinsic characteristics of the leaf litter resources may play a relevant role on the microbially driven leaf litter decomposition and mediate its response to dissolved nutrient concentrations across streams.