N2O and N2 emissions from denitrification respond differently to temperature and nitrogen supply
The reduction of nitrate (NO3ˉ) to nitrous oxide (N2O) and eventually to dinitrogen (N2) during denitrification in soil has rarely been studied at temperatures above 30 °C. The aim of this study was to understand the impact of high temperatures on denitrification and associated N2O/N2 ratios in soil with different nitrogen (N) availability.
Materials and methods
The study was conducted on a Dermosol collected from a dairy farm from south west Victoria, Australia (38° 10′ S, 142° 58′ E). Soil samples were wetted to 60% water holding capacity then pre-incubated at 25 °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 14NH4 15NO3, 10 atom% excess 15N, and incubated at 25, 35, and 45 °C for 10 days. Gas samples were taken during the experiment to assess the reduction of NO3ˉ to N2O and eventually to N2.
Results and discussion
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 °C, in soils provided with N equivalent to 100 to 150 kg N ha−1. A decrease in N2O emissions above 35 °C was partially attributed to an increase in N2O reduction, e.g., N2 production, between 35 and 45 °C. Increased N2 production at 45 °C decreased N2O/N2 ratios by 33 to 85%, resulting in ratios of 0.3 to 1.2. Temperature may have a direct effect on the reduction of NO3ˉ to N2O due to decreased oxygen availability with increasing soil respiration rates, thus enhancing the use of NO3ˉ 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 N2 also increased with temperature. Significant conversion of N2O to N2 above 35 °C decreased the N2O/N2 ratios from denitrification. Depletion of oxygen in soil microsites with higher temperatures appeared to influence N2O production through selection of more NO3ˉ acting as a terminal electron acceptor during denitrification.
KeywordsDinitrogen N2O/N2 ratio Nitrogen transformation Nitrous oxide Oxygen Reduction of N2O
This study was funded by the Vietnam International Education Development (VIED), The University of Adelaide, and Tim Healy Memorial Scholarship (Future Farm Industries CRC). We acknowledge the assistance of Ryan Farquharson (CSIRO), Murray Unkovich (University of Adelaide) and Nanthi Bolan (University of Newcastle) in the development of methods, Kevin Kelly (Department of Economic Development, VIC, Australia) in providing site access for soil collection and environmental data, Nigel Charman for assistance with soil sampling, and Ann McNeill, Philippa Tansing, and Sean Mason for technical assistance on mineral nitrogen analysis and soil physical measurements.
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