Nutrients and temperature additively increase stream microbial respiration.

Rising temperatures and nutrient enrichment are co-occurring global-change drivers that stimulate microbial respiration of detrital carbon, but nutrient effects on the temperature dependence of respiration in aquatic ecosystems remain uncertain. We measured respiration rates associated with leaf litter, wood, and fine benthic organic matter (FBOM) across seasonal temperature gradients before (PRE) and after (ENR1, ENR2) experimental nutrient (nitrogen [N] and phosphorus [P]) additions to five forest streams. Nitrogen and phosphorus were added at different N:P ratios using increasing concentrations of N (~80-650 μg/L) and corresponding decreasing concentrations of P (~90-11 μg/L). We assessed the temperature dependence, and microbial (i.e., fungal) drivers of detrital mass-specific respiration rates using the metabolic theory of ecology, before vs. after nutrient enrichment, and across N and P concentrations. Detrital mass-specific respiration rates increased with temperature, exhibiting comparable activation energies (E, electronvolts [eV]) for all substrates (FBOM E = 0.43 [95% CI = 0.18-0.69] eV, leaf litter E = 0.30 [95% CI = 0.072-0.54] eV, wood E = 0.41 [95% CI = 0.18-0.64] eV) close to predicted MTE values. There was evidence that temperature-driven increased respiration occurred via increased fungal biomass (wood) or increased fungal biomass-specific respiration (leaf litter). Respiration rates increased under nutrient-enriched conditions on leaves (1.32×) and wood (1.38×), but not FBOM. Respiration rates responded weakly to gradients in N or P concentrations, except for positive effects of P on wood respiration. The temperature dependence of respiration was comparable among years and across N or P concentration for all substrates. Responses of leaf litter and wood respiration to temperature and the combined effects of N and P were similar in magnitude. Our data suggest that the temperature dependence of stream microbial respiration is unchanged by nutrient enrichment, and that increased temperature and N + P availability have additive and comparable effects on microbial respiration rates.