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

Purpose: The responses of nitrification and denitrification are not well characterised at temperatures above 35 degrees 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 degrees C in the dark for 5 to 10 days following amendment with 100 mu g N g(-1) either as NH4NO3, (NH4NO3)-N-14-N-15 or (NH4NO3)-N-15-N-15 (10 atom% N-15 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 (NH4NO3)-N-15-N-15. Atom% N-15 enrichments of NO3-, N2O and N-2 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 degrees C. N2O production from nitrification and denitrification both had similar thermal responses, which were different to N-2 production. The N2O/N-2 ratio decreased from >4 at 35-40 degrees C to 0.5 at 45 degrees C, due to greater N-2 than N2O production in the Dermosol. Heterotrophic nitrifiers oxidised NH4+ and released N2O at 35-40 degrees C, suggesting a role for heterotrophs in N cycling under warm climates. T-opt for nitrification was between 35 and 40 degrees 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 degrees C altered the rates of nitrification, denitrification associated N2O and N-2 production. Nitrification and denitrification peaked at 35-40 degrees C in the Chromosol and Dermosol. The production of N-2 increased rapidly above 40 degrees C, which may be related to high soil respiration rates that likely decreased O-2 availability, thus expanding the anaerobic microsites; such circumstances increased the reduction of N2O to N-2 production from the Dermosol.