N2O and N2 emissions from denitrification respond differently to temperature and nitrogen supply

The reduction of nitrate (NO(3)Ee) to nitrous oxide (N2O) and eventually to dinitrogen (N-2) during denitrification in soil has rarely been studied at temperatures above 30 A degrees C. The aim of this study was to understand the impact of high temperatures on denitrification and associated N2O/N-2 ratios in soil with different nitrogen (N) availability. The study was conducted on a Dermosol collected from a dairy farm from south west Victoria, Australia (38A degrees 10' S, 142A degrees 58' E). Soil samples were wetted to 60% water holding capacity then pre-incubated at 25 A degrees C for 7 days. Re-packed soil cores were supplied with different amounts of N (equivalent to 0, 50,100, and 150 kg N ha(-1)) as (NH4)-N-14 (NO3,)-N-15 10 atom% excess N-15, and incubated at 25, 35, and 45 A degrees C for 10 days. Gas samples were taken during the experiment to assess the reduction of NO(3)Ee to N2O and eventually to N-2. The majority of soil N losses during denitrification were from N2O emissions, which were influenced by an interaction between temperature and N availability. The highest rate of N2O emission occurred at 35 A degrees C, in soils provided with N equivalent to 100 to 150 kg N ha(-1). A decrease in N2O emissions above 35 A degrees C was partially attributed to an increase in N2O reduction, e.g., N-2 production, between 35 and 45 A degrees C. Increased N-2 production at 45 A degrees C decreased N2O/N-2 ratios by 33 to 85%, resulting in ratios of 0.3 to 1.2. Temperature may have a direct effect on the reduction of NO(3)Ee to N2O due to decreased oxygen availability with increasing soil respiration rates, thus enhancing the use of NO(3)Ee as a terminal electron acceptor by denitrifiers. Temperature interacted with soil N availability to control N2O emission from denitrification, while the reduction of N2O to N-2 also increased with temperature. Significant conversion of N2O to N-2 above 35 A degrees C decreased the N2O/N-2 ratios from denitrification. Depletion of oxygen in soil microsites with higher temperatures appeared to influence N2O production through selection of more NO(3)Ee acting as a terminal electron acceptor during denitrification.