Nutrient enrichment in water more than in leaves affects aquatic microbial litter processing
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Nutrient enrichment of soils and water will intensify in the future and has the potential to alter fundamental ecosystem processes, such as litter decomposition. We tested the direct (via water nutrient enrichment) and indirect (via changes in leaf chemistry) effects of nutrient enrichment on microbial activity and decomposability of Quercus robur L. (oak) leaves in laboratory microcosms simulating streams. Senescent leaves of oak trees grown without and with fertilization were incubated under ambient and elevated water nutrient [nitrogen (N) and phosphorus (P)] concentrations for 60 days. Soil fertilization led to an increase in leaf (3.4×) and leaf litter (2.3×) N concentration. Increased water-dissolved nutrients concentrations stimulated microbial activity (N uptake, microbial respiration, fungal biomass buildup and conidia production by aquatic hyphomycetes) that translated into accelerated litter decomposition (2.1× for unfertilized and 1.6× for fertilized trees). Leaves from fertilized trees had higher microbial activity and decomposition rates than leaves from unfertilized trees only at low dissolved nutrient availability. When both litter and water nutrients concentration increased, microbial activity and leaf decomposition were stimulated, but the effects were additive and direct effects from increased dissolved nutrient availability were stronger than those mediated by increases in litter N concentration (indirect effects). Our results suggest that increases in water nutrient availability (within the range used in this study) may exert a stronger control on microbial activity and litter decomposition than litter nutrient enrichment.
KeywordsEutrophication Fertilization Litter Aquatic hyphomycetes Decomposition Streams
This study was supported by the Portuguese Foundation for Science and Technology (FCT) through the strategic project UID/MAR/04292/2013 granted to MARE. We thank the FCT for the financial support granted to Verónica Ferreira (IF/00129/2014), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for the scholarship granted to Cristiane Biasi (Process No.: 99999.006609/2015-05) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the productivity grant granted to Sandro Santos (Process No.: 311142/2014-I).
Author contribution statement
MG and VF conceived and designed the experiment. CB and VF performed the experiment and analysed the data. CB, VF, MG and SS drafted the original manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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