Phosphorus Concentration and Light Availability Differentially Affect Microbial-Mediated Leaf Litter Conditioning

Anthropogenic activities such as agriculture and urbanization can alter phosphorus and other biogeochemical cycles and modify riparian cover of stream ecosystems worldwide.  The effects of these global changes, and their interactions in particular, on detrital-based ecosystems has received little attention.  Our objective was to quantify microbial-mediated leaf litter conditioning in laboratory microcosms not only with increasing P availability, but also with increased light intensity, simulating the effect of both nutrient enrichment and canopy loss.  Phosphorus concentrations and light availability differentially affected algal biomass (as chlorophyll a), microbial respiration rates, and litter stoichiometry.  Algal biomass responded to increased P enrichment only when coupled with greater light intensity.  Respiration rates increased with P enrichment in both light levels and this response was magnified in the high light treatment.  Litter carbon (C): P ratios decreased significantly with P enrichment with a differential response across light intensities.  Our results demonstrate that changes in P concentration and light availability can alter microbial metabolism and litter stoichiometry.  This shift in food quality can have important implications for consumer biodiversity and ecosystem functioning.