High soil temperatures alter the rates of nitrification, denitrification and associated N₂O emissions

Abstract

Purpose: The responses of nitrification and denitrification are not well characterised at temperatures above 35 °C, which is the focus of our study. Materials and methods: Soils collected from two dairy pastures (Victoria, Australia) were incubated at 10 to 45 °C in the dark for 5 to 10 days following amendment with 100 μg N g−1 either as NH4NO3, 14NH415NO3 or 15NH415NO3 (10 atom% 15N excess) at 50% water-filled pore space. To detect N2O from heterotrophic nitrification, acetylene (0.01% v/v) was used in a subset of samples amended with 15NH415NO3. Atom% 15N enrichments of NO3ˉ, N2O and N2 were measured during the experiment to evaluate the responses of nitrification and denitrification to temperature. Results and discussion: N2O production from the two soils increased with rising temperature and peaked between 35 and 40 °C. N2O production from nitrification and denitrification both had similar thermal responses, which were different to N2 production. The N2O/N2 ratio decreased from > 4 at 35–40 °C to 0.5 at 45 °C, due to greater N2 than N2O production in the Dermosol. Heterotrophic nitrifiers oxidised NH4+ and released N2O at 35–40 °C, suggesting a role for heterotrophs in N cycling under warm climates. Topt for nitrification was between 35 and 40 °C, which is higher than reported previously. A short-term effect of high temperatures could provide NH4+ for the growth of crops but may also decrease soil C pools. Conclusions: Increasing temperature above 35 °C altered the rates of nitrification, denitrification associated N2O and N2 production. Nitrification and denitrification peaked at 35–40 °C in the Chromosol and Dermosol. The production of N2 increased rapidly above 40 °C, which may be related to high soil respiration rates that likely decreased O2 availability, thus expanding the anaerobic microsites; such circumstances increased the reduction of N2O to N2 production from the Dermosol.