[M-IS14] Biogeochemistry

[MIS14-P06] A direct determination of N₂O reduction potential in soil using ¹⁵N₂O tracer

*Ryo Shingubara¹, Yasuhiro Nakajima², Hikaru Uno², Koji Ito¹, Hiroaki Shimada¹, Kiwamu Minamisawa³, Rota Wagai¹ (1.Institute for Agro-Environmental Sciences, NARO, 2.Research Center for Advanced Analysis, NARO, 3.Graduate School of Life Sciences, Tohoku Univ.)

Keywords:gross N2O reduction, nitrogen isotopes, incubation, soil, pure culture

N₂O reduction is a key biogeochemical process for mitigating N₂O emission from soil. This process is the final step of denitrification and commonly quantified using the acetylene inhibition technique or ¹⁵NO₃⁻ amendment. Recently, significant N₂O production from NH₄⁺ via nitrification was reported. In addition, recent microbiological studies showed the presence of clade II nosZ bacteria, non-denitrifying N₂O reducers catalyzing reduction of N₂O but not NO₂⁻ nor NO. Thus, quantifying N₂O reduction directly is becoming more critical. Several research groups developed the ¹⁵N₂O pool dilution technique (Clough et al., 2006; Yang et al., 2011; Wen et al., 2016), which added ¹⁵N-labelled N₂O to the soil, determined the gross N₂O production from the decrease in the ¹⁵N/¹⁴N ratio of N₂O, and calculated the gross N₂O reduction from the difference between the gross and net N₂O productions. Here, we tested the applicability of a more direct method by amending the ¹⁵N₂O tracer and analyzing the ¹⁵N/¹⁴N ratio of N₂ to determine the ¹⁵N₂ production and thereby N₂O reduction potential. First, we validated ¹⁵N mass balance in the incubation experiment using pure cultures of denitrifying bacteria. Second, we applied the method to soil samples collected from the upland agricultural field and adjacent forest by the incubation under controlled moisture condition in vials. In this presentation, we will discuss the validity and applicability of this method to determine gross N₂O reduction potential toward mechanistic understanding of N₂O dynamics in soil.