N₂O and N₂ emissions from denitrification respond differentially to soil temperatures and nitrogen supply

Abstract

Purpose: 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. Conclusions: 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.