Temperature Sensitivity of Freshwater Denitrification and N2O Emission-A Meta-Analysis

Freshwater denitrification removes a considerable amount of nitrogen from inland waters, which are under pressure from eutrophication and warming. However, incomplete denitrification can lead to the formation of N2O, a potent greenhouse gas, which can amplify climatic warming. Although temperature effects on denitrification are well studied in individual habitats and experiments, global patterns in temperature-responses of denitrification and N2O emissions remain to be elucidated. Here, we investigated the temperature sensitivity (Q10) of denitrification and N2O emissions in freshwater ecosystems worldwide, using a meta-analytic approach. To this end, Q10 values from in-situ and temperature manipulation studies were related to environmental nutrient conditions, O-2, pH, sediment organic matter (SOM), and geographic location. Temperature sensitivity of denitrification displayed a strong positive correlation with environmental nitrogen concentrations, pH and O-2. Significant correlations with SOM and SOM:N ratios were observed as well, but the direction of the effect differed between in-situ and temperature manipulation studies. Surprisingly, temperature sensitivity of N2O emissions did not correlate with pH, SOM, nutrient or O-2 conditions. Temperature sensitivity of the ratio between N2O emission and NO3- concentration (adapted EF5 values) was 6.6 times higher in Australia and New Zealand compared to other geographic regions. As global temperatures and nitrogen deposition in freshwater ecosystems are expected to increase over the coming decades, our results suggest enhanced future denitrification, which may present a natural way to balance eutrophication. The observed temperature sensitivity of N2O emission factors, however, may indicate enhanced denitrification-derived N2O emissions from freshwater ecosystems in a future warmer world.